TS Lightweight crankshaft pulley - Alfa Romeo Forum
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TS Lightweight crankshaft pulley

I tried posting in Modified section but no response to original thread. I would like a TS Lightweight crankshaft pulley asap but have been unable to find Sotos on Facebook or Ebay ?

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Contact the seller on Spanos Engineering at Facebook. Got a pulley for my spider last month !
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Do you mean a pulley that lessens or deletes the rubber mounted heavy metal mass? If yes, then it's probably not a good idea. The stock pulley is not just a pulley that needs to go on a diet. It's also a 'torsional harmonic oscillation damper' that assists in lessening damaging torsional flexure in the crankshaft, particularly at higher rpm (though the harmonic resonant frequency that damagingly affects a given crank might not occur at particularly high rpm). The crank torsionally twists in an elastic mode, with a harmonic frequency that can be exponentially self energising due to harmonic oscillation at and around a specific rpm, and lead to failure from metal fatigue. It's this oscillation that the damper 'dampens', and prevents the crankshaft being destroyed, either in the short term, or sometime down the road.

They are ubiquitously fitted by probably all car manufacturers and have been for many years (at least the last time I can recall seeing an engine not so fitted was an A series BMC motor in a Mini, but they were fitted to the Cooper S engines...). They are fitted for a reason, not because the manufacturer likes to spend money on needlessly complex components, and not to reduce externally felt engine vibration.

Removing the damper (fitting a lightweight pulley in it's place) will reduce rotating mass, but the affect is quite minor because the mass of the damper is not far away from the axis of the crank. If you want to reduce rotating mass, then look at the flywheel, there is much more potential there. Another reason to delete the damper might be a misguided attempt to eliminate the potential for disaster were the damper to fail (which can happen), but this is at a significant risk to the crank.

If the engine is habitually revved hard (and even if it isn't), then the damper should be periodically inspected for cracking in the rubber insert, and for the damper mass rotationally 'creeping' around the pulley hub. Either one and the damper may depart company with the hub sooner or later. Harmonic dampers are generally considered to be 'lifed' components with serious racing engines. The energy absorbed by the dampers' rubber insert is considerable, espcially at higher rpm. This means that the rubber can internally generate substantial heat, which eventually degrades and weakens the insert.

Regards,
John.
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Last edited by johnlear; 28-10-18 at 23:14.
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Thank you John for the enlightenment! I will keep to standard specification!

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Originally Posted by johnlear View Post
Do you mean a pulley that lessens or deletes the rubber mounted heavy metal mass? If yes, then it's probably not a good idea. The stock pulley is not just a pulley that needs to go on a diet. It's also a 'torsional harmonic oscillation damper' that assists in lessening damaging torsional flexure in the crankshaft, particularly at higher rpm (though the harmonic resonant frequency that damagingly affects a given crank might not occur at particularly high rpm). The crank torsionally twists in an elastic mode, with a harmonic frequency that can be exponentially self energising due to harmonic oscillation at and around a specific rpm, and lead to failure from metal fatigue. It's this oscillation that the damper 'dampens', and prevents the crankshaft being destroyed, either in the short term, or sometime down the road.

They are ubiquitously fitted by probably all car manufacturers and have been for many years (at least the last time I can recall seeing an engine not so fitted was an A series BMC motor in a Mini, but they were fitted to the Cooper S engines...). They are fitted for a reason, not because the manufacturer likes to spend money on needlessly complex components, and not to reduce externally felt engine vibration.

Removing the damper (fitting a lightweight pulley in it's place) will reduce rotating mass, but the affect is quite minor because the mass of the damper is not far away from the axis of the crank. If you want to reduce rotating mass, then look at the flywheel, there is much more potential there. Another reason to delete the damper might be a misguided attempt to eliminate the potential for disaster were the damper to fail (which can happen), but this is at a significant risk to the crank.

If the engine is habitually revved hard (and even if it isn't), then the damper should be periodically inspected for cracking in the rubber insert, and for the damper mass rotationally 'creeping' around the pulley hub. Either one and the damper may depart company with the hub sooner or later. Harmonic dampers are generally considered to be 'lifed' components with serious racing engines. The energy absorbed by the dampers' rubber insert is considerable, espcially at higher rpm. This means that the rubber can internally generate substantial heat, which eventually degrades and weakens the insert.

Regards,
John.
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Are you on about the auxiliary pulley that sits on top of the crank pulley? Changing that pulley for a lighter weight alloy one in a smaller diameter is one of the best bang for buck mods you can do on the Twin sparks.
It may only give a few bhp extra but In real on the road terms the car feels like youíve left a passenger behind.
There is a kit on eBay at the moment with shorter belt included for 175 pound I think.
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The degree to which crankshafts elastically twist may seem quite small, and all else being equal is less the the shorter the crank is. That the degree of twist is fairly small doesn't mean the metal isn't being substantially stressed, it takes a lot of energy / force to even slightly twist something as seemingly rigid as a crankshaft, and even a small amount of twisting is bad when it happens thousands or hundreds of thousands of times.

You might delete the damper and possibly get away with it, if you are lucky...

The issue tends to be greater the longer the crank is, so straight 5, straight 6, V10, V12, straight 8, and V16 etc. engines are likely to be more affected. Straight 2s, 3s, 4s, V6s, V8s, most 'boxer' engines, anything with a fairly short crank is somewhat less affected, but this doesn't mean no significant risk (well maybe straight 2 and 3 cylnder engines don't really need a damper, perhaps).

The crank will have more than one rpm at (and near) which it is being harmonically 'excited' by the forces acting on it, and the resonances will be greater and lesser at these different rpm points. The harmonic resonances are multiple and not the same, with some resonances being minor, others major. Some resonances act to partially cancel out other resonaces, others add to other resonaces and are cumulative, this is a complex dynamic (that I only have a superficial understanding of).

The crank might be unaffected (much) if the rpm rises or falls quickly through and past the rpm point at which it is being harmonically excited, because it is only briefly at the dangerous rpm and there isn't time for the self energising resonance to reach catastrophic proportions. That is, as the revs approach and then reach the critical rpm the crank begins to torsionally oscillate, but as the rpm rise above or fall below the critical rpm the oscillation dimishes before it can reach a damaging degree.

On the other hand, if the rpm were to be held more or less stable at or near or roundabout the critical rpm for any significant length of time (piece of string) then the self energising oscillation has more time to build, and build, and build...

Note that crank rotational speed is not ever constant, even if rpm are steady. With a 4 cylinder 4/ engine the crank rotational speed cyclically increases and decreases twice during each revolution of the crank (speeds up twice, slows down twice). The effect is masked / damped by the flywheel and other things, but it still exists. This is normal, but when the crank starts to harmonically oscillate the changes in 'momentary' speeds at which the front end of the crank rotates becomes more severe and more 'erratic' (it isn't really erratic as in it's not really random, but it can look that way).

The crank oscillation is lesser nearer the flywheel, and greater nearer the front of the engine. Or to look at it another way, the flywheel significantly stabilises crank speed, so any oscillation occurring within the crankshaft will be more evident further away from the flywheel than closer to it.

The damper works by allowing the mass of the damper ring to rotationally move on the rubber insert as the crank abruptly changes its' momentary rotational speed. As a torsional oscillation starts to uccur at the front of the crank, then each time the the crank momentarily rotates faster it tries to also speed up the damper ring. But because the ring is mounted on rubber and has mass and inertia, the ring momentarily 'lags' behind the rotational speed of the crank. Much the same occurs when the crank momentarily slows down, but in reverse.

So the momentary rotational speed of the damper ring mass lags behind the momentary rotational speed of the crank itself, both as the crank speeds up and as it slows down, within each crank rotation. This sets up an inertial resistance which 'interrupts' the resonant harmonic oscillation of the crank, preventing it from becoming a self reinforcing feedback loop to destruction.

The damper mass might feel fairly rigidly attached to it's hub through the rubber, but it's my understanding that it does move significantly enough as the rubber flexes. The forces acting on the damper are quite 'energetic', and the rubber flexes significantly, so it absorbs quite a lot of energy and can get reasonably hot. If it was doing this constantly then it likely wouldn't last long, but it probably isn't working all that hard most of the time, only when the rpm is at or near a critical crank speed.

Regards,
John.

Last edited by johnlear; 29-10-18 at 23:51.
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Quote:
Originally Posted by christiana View Post
Are you on about the auxiliary pulley that sits on top of the crank pulley? Changing that pulley for a lighter weight alloy one in a smaller diameter is one of the best bang for buck mods you can do on the Twin sparks.
It may only give a few bhp extra but In real on the road terms the car feels like youíve left a passenger behind.
There is a kit on eBay at the moment with shorter belt included for 175 pound I think.
Perhaps it is this one as I actually don't know! I couldn't find it on Ebay or Google. Could you post a link?
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Originally Posted by johnlear View Post
The degree to which crankshafts elastically twist may seem quite small, and all else being equal is less the the shorter the crank is. That the degree of twist is fairly small doesn't mean the metal isn't being substantially stressed, it takes a lot of energy / force to even slightly twist something as seemingly rigid as a crankshaft, and even a small amount of twisting is bad when it happens thousands or hundreds of thousands of times.

You might delete the damper and possibly get away with it, if you are lucky...

The issue tends to be greater the longer the crank is, so straight 5, straight 6, V10, V12, straight 8, and V16 etc. engines are likely to be more affected. Straight 2s, 3s, 4s, V6s, V8s, most 'boxer' engines, anything with a fairly short crank is somewhat less affected, but this doesn't mean no significant risk (well maybe straight 2 and 3 cylnder engines don't really need a damper, perhaps).

The crank will have more than one rpm at (and near) which it is being harmonically 'excited' by the forces acting on it, and the resonances will be greater and lesser at these different rpm points. The harmonic resonances are multiple and not the same, with some resonances being minor, others major. Some resonances act to partially cancel out other resonaces, others add to other resonaces and are cumulative, this is a complex dynamic (that I only have a superficial understanding of).

The crank might be unaffected (much) if the rpm rises or falls quickly through and past the rpm point at which it is being harmonically excited, because it is only briefly at the dangerous rpm and there isn't time for the self energising resonance to reach catastrophic proportions. That is, as the revs approach and then reach the critical rpm the crank begins to torsionally oscillate, but as the rpm rise above or fall below the critical rpm the oscillation dimishes before it can reach a damaging degree.

On the other hand, if the rpm were to be held more or less stable at or near or roundabout the critical rpm for any significant length of time (piece of string) then the self energising oscillation has more time to build, and build, and build...

Note that crank rotational speed is not ever constant, even if rpm are steady. With a 4 cylinder 4/ engine the crank rotational speed cyclically increases and decreases twice during each revolution of the crank (speeds up twice, slows down twice). The effect is masked / damped by the flywheel and other things, but it still exists. This is normal, but when the crank starts to harmonically oscillate the changes in 'momentary' speeds at which the front end of the crank rotates becomes more severe and more 'erratic' (it isn't really erratic as in it's not really random, but it can look that way).

The crank oscillation is lesser nearer the flywheel, and greater nearer the front of the engine. Or to look at it another way, the flywheel significantly stabilises crank speed, so any oscillation occurring within the crankshaft will be more evident further away from the flywheel than closer to it.

The damper works by allowing the mass of the damper ring to rotationally move on the rubber insert as the crank abruptly changes its' momentary rotational speed. As a torsional oscillation starts to uccur at the front of the crank, then each time the the crank momentarily rotates faster it tries to also speed up the damper ring. But because the ring is mounted on rubber and has mass and inertia, the ring momentarily 'lags' behind the rotational speed of the crank. Much the same occurs when the crank momentarily slows down, but in reverse.

So the momentary rotational speed of the damper ring mass lags behind the momentary rotational speed of the crank itself, both as the crank speeds up and as it slows down, within each crank rotation. This sets up an inertial resistance which 'interrupts' the resonant harmonic oscillation of the crank, preventing it from becoming a self reinforcing feedback loop to destruction.

The damper mass might feel fairly rigidly attached to it's hub through the rubber, but it's my understanding that it does move significantly enough as the rubber flexes. The forces acting on the damper are quite 'energetic', and the rubber flexes significantly, so it absorbs quite a lot of energy and can get reasonably hot. If it was doing this constantly then it likely wouldn't last long, but it probably isn't working all that hard most of the time, only when the rpm is at or near a critical crank speed.

Regards,
John.
Thank you again John. Would the same apply to the auxiliary pulley that Christiana mentions above?
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Thank you again John. Would the same apply to the auxiliary pulley that Christiana mentions above?
I don't know which specific product Christiana might have meant, but looking at the images of lightweight TS crank pulleys that came up in a search, I'd say not.

The stock pulley / damper consists of a steel hub, onto which is bonded the rubber insert, then the damper mass bonded to the outside of the rubber. The grooved surface on which the belt runs is machined directly into the outside surface of the damper mass, so the belt isn't directly and solidly 'attached' to the crankshaft, as it seems to be with the lightweight pulleys I saw in the photos.

In the images of lightweight TS pulleys that I viewed, there is no rubber insert, just one machined billet of steel, with the belt grooves directly machined into the outside of the pulley. The OD of the lightweight pulley is significantly smaller than the OD of the stock pulley / damper. There is no possible movement between the hub of the pulley and the rest of it, so the pulley can't act to dampen harmonic crank resonances (not in a meaningful way).

Ignoring the very slight reduction in rotational mass, the power 'gain' from using such a pulley comes from it being of smaller diameter than the stock pulley. As a result, at any given crank rpm the smaller pulley drives the alternator and PS pump at a slower speed than would the stock sized pulley (we can include the AC if it's switched on). The theory is that since the auxilliaries are now rotating more slowly, then they are less parasitic on the engine itself. I'm not particularly convinced.

Previous experience removing the belt with another car (Accord, where the belt only drove the PS, another belt drove the other things), was that there was a barely perceptible increase in apparent power, so small that I may have been imagining that I could actually feel it. This was with a complete disabling of the PS pump, but the lightweight pulley (with its' smaller diameter) only partially 'disables' the pump, so any power 'gain' would be less than than the slight one I experienced with the Accord.

You might find that with a smaller pulley at lower engine rpm, if the steering wheel is turned quickly then you may hit the limit of the now lower capacity of the PS system. If so, then the steering would suddenly become very heavy as the PS effect would 'hit the wall', albeit only briefly. That is, running the pump at a slower speed won't make the steering heavier otherwise, it won't get heavier at any time until the system can no longer cope, at which point it will suddenly get heavier, possibly a lot heavier.

I'm no expert on things auto electrical, but I suspect it is the case that the electrical parasitic load on the engine is less to do with the rpm at which the alternator is rotating than it is to do with how many electrical auxilliaries are in operation, especailly the heavier loads like headlights, wipers, fans etc. If the electrical load is low then the alternator won't be working hard, so the parasitic drag won't be high, regardless of the speed at which the alternator is turning.

On the other hand, if the engine rpm is low (and thus the alternator is turning more slowly than the designers ever intended it to due to an undersized pulley), and you're in slow traffic, at night, in the rain, then the alternator may not be able to work hard enough to keep up with the electrical demands, and voltages may drop (as the battery drains...).

So, all things considered, I wouldn't fit one of these pulleys to my engine...

Regards,
John.

P.S. It's my understanding (only, right or wrong, or partially right...) that the damper ring mass, moving on the rubber insert, more or less acts as if it were a mass of much greater mass than it actually is (due to its' reactive oscillation). In this way it mimics the effect of damping crankshaft oscillation that you would get if there were to be another flywheel of equivalent mass (and mass distribution around the crank axis) fitted to the front of the crank. This would be an additional flywheel to that already fitted at the gearbox end of the crank.

This additional flywheel would 'balance' the existing flywheel, and this arrangement would also dampen crank oscillation, but is of course an impractical solution due to there usually being no room for an additional frontally mounted flywheel, and, this flywheel would have to be much heavier than the stock damper, so response would suffer, as would acceleration (would you want a flywheel double the mass of the one already fitted?). Having said that, it occurs to me that if a frontally mounted flywheel were additionally fitted, then the mass of both flywheels could probably be reduced...

Regards,
John.
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I don't know what theory said in details, but i strongly believe in practice results....
And my practice is as follows.....
Had 650gr alloy crank pulley on my first 2.0TS - with lightened FW as well.....had almost 350k km personally, changed bearings on 300k just for curiosity......still drive it.....
Almost 150k on my second TS with 750gr alloy pulley - not even changed bearings still......
Fiat Coupe 20VTurbo, about 320 - 330bhp, 600gr pulley, did 12-15k road use, and almost 9 hours in our local Endurance championship......dropped the sump afterwards to inspect the bearings - did not worth changing.......
I don't know what theory said, but my practice show me different.....and i guess that it's not just luck.....
Maybe worth to note that i even grind some metal from my trackday V6 pulley no matter that it's different from TS and have ballast on it, because as all we know V6 use this part with CS and FW to balance the engine.....however i removed as much as i can from outer diameter of the pulley, not touching the balance......running that pulley since 2011, many trackdays every year, redlined every run.....no problems......
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My TS has a lightened flywheel. There is an improvement in acceleration in the lower gears. There is not really an improvement in the higher gears because the acceleration of the crankshaft is minimal. The flywheel has a much greater mass and diameter than the crankshaft pulley. From the point of mass, I cannot see the pulley making much difference.

These pulleys are predominantly for racing use. Every slight benefit is used for racing. In any case, engine speeds are usually higher than 4000rpm so a larger crankshaft pulley is not needed. The ancilliaries will work with a smaller pulley due to increased engine speed.

For road use, any benefit will be so slight that it probably makes no odds. For that reason, I don't see that increasing the risk of major engine damage, or more likely, putting increased load on the alternator due to it turning slower is worth it.

In short, any modifications I do are intended not to adversely affect reliability so for me, the smaller lightened pulley is one I won't bother with.

Alfa probably fit these to reduce the possibility of crankshaft damage which is possibly only 1 or 2 engines in every 100. They could have saved money with solid pulleys but over the life of the 156, that may have been 6000 cars which is enough to give a reputation for unexplained crankshaft failure due to 'poor engineering' or 'underdevelopment'.
For production and reputation reasons, it is not a risk worth taking for an imperceptible improvement in performance.
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Quote:
Originally Posted by christiana View Post
Are you on about the auxiliary pulley that sits on top of the crank pulley? Changing that pulley for a lighter weight alloy one in a smaller diameter is one of the best bang for buck mods you can do on the Twin sparks.
It may only give a few bhp extra but In real on the road terms the car feels like you’ve left a passenger behind.
There is a kit on eBay at the moment with shorter belt included for 175 pound I think.
Explain to me how replacing a lighter pulley with a smaller diameter will magically increase bhp to the engine?
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Explain to me how replacing a lighter pulley with a smaller diameter will magically increase bhp to the engine?
The short answer? See what I can do, probably fail...

Strictly speaking, neither a less massive damper nor a smaller diameter pulley can or do increase the actual power manufactured by the engine (i.e. they don't). It's not remotely similar to say fitting different cams or a supercharger (which will increase actual power). This begs the question, so what is the rationale behind reducing rotating mass or wanting to reduce pulley diameter?

It's not magic, but because doing so in pratice and / or in theory reduces the amount of a given power output that is 'parasitically lost', and therefore is unavaliable as power to accelerate the car forward, and also improves throttle response for much the same reason.

Reducing rotating mass:
Whether by fitting a lighter pulley / damper or a lighter flywhel etc. this doesn't increase power, sort of depending on how that power is measured.

If we were to measure power on a 'brake' dynomometer (with which engine power is determined with the engine held at a given steady rpm against a 'braking' force that is 'trying' to stop the crank from rotating, i.e. brake horsepower, or brake KW if you like), then at any given steady rpm the as measured power would not increase with a lower rotating mass. This is because at a steady crank rpm we are not accelerating the the speed of the rotating masses (which takes some power to do, 'robbed' from the engine).

However, if we were to measure power with an 'inertial' dynomometer, then with less rotating engine mass there will be an apparent increase in 'measured' power. Note that with an inertial dynomometer the power is not directly measured at a given steady rpm (as with a 'brake' dyno), but rather is 'inferred' by the time it takes for the engine power (at continuously increasing rpm with a WOT) to increase the rotational speed of a heavy mass of known proportion (part of the dyno typically being a heavy cylinder of known weight and diameter, hooked up to sensors feeding data to a computer). Engine power hasn't really increased as a result of the reduction in rotating mass, but the inertial dyno will 'say' that power is increased. This is because the inertial dyno is 'seeing' the reduction in the time that it takes to increase the rpm of the rotating known mass, which is less time because of the decrease in parasitically 'lost' power (due to the reduction in rotating engine mass). The dyno interprets this reduction in time (needed to accelerate the known mass) as an increase in power.

Inertial dynos can't determine steady rpm power because at steady rpm the known mass is not changing its' rate of rotation. They give a 'measure' of power at any given rpm based on the time it takes to rotationally accelerate the known mass. The known mass and the rotating engine mass both affect the time it takes to increase the rpm of the known mass (the attached computer can't tell where the difference in reduced mass is coming from, all it 'sees' is the time it takes for the known mass to change rotational speed). My understanding is that an inertial dyno measures this change in rotational acceraration over very short time increments, which it computationally 'joins together' to create a 'graph' of inferred power over the engines' rpm range (or can detect the rate of change in acceleration of the known mass). Inertial dynos don't directly measure torque, but infer it mathematically from the measured power and the engine rpm at any given moment.

A 'brake' dyno is different, it directly 'measures' measures torque, and only infers power via a mathematical calculation. It quantifies torque by applying a mechanical resistance to engine rotation (i.e. a 'braking' force), and in some manner measures the force required to hold the engine at a given rpm. This can either be a resistance directly applied to the crank (engine dyno), or less directly at the wheels (chassis dyno, somewhat less accurate). With a brake dynomometer, time doesn't come into it, it is an 'instantaneous' measure of torque that is not time dependant (though time is an element used in the calcuation used to infer power, because crank rpm is used in the calculation, and the 'm' in 'rpm' represents 'per minute', i.e. time). A reduction in rotating engine mass isn't directly 'seen' by a brake dynomometer, because it doesn't use time to measure torque.

What we learn from understanding this (I hope), is that a reduction in rotating mass improves acceleration (to whatever degree) because of reduced inertial loss. But, it won't affect top speed when the forces acting to limit top speed hold the engine at a power limited maximum steady rpm, because the reduction in rotating mass doesn't make any more steady rpm power (rotational inertia is not a factor at steady rpm, only the aerodynamic and rolling resistances).

So, it takes power and time to increase the rotational speed of the engines' rotating masses. When rotational mass is reduced, this doesn't affect the power that the engine is actually making, but does decrease the time it takes for a given amount of power to increase the rotational speed of the rotating masses, and so acceleration improves.

This explains why a reduction in rotating mass has a more evident affect on acceleration in the lower gears, i.e. in a lower gear the crank rpm increase more rapidly, and any improvement is more subjectively evident than in a higher gear when the rpm increase more slowly, and, rotational inertia is less relative to the increasing aerodynamic and rolling resistances at higher speeds in taller gears.

Reducing rotating mass is most commonly achieved by lightening the flywheel, or fitting a flywheel that is lighter to start with. Reducing the mass of the crank pulley has a similar if much smaller affect on rotating mass.

Reducing the diameter of the pulley:
Lessens the rpm of the pulleys attached to the auxilliary components, at any given crank rpm. Supposedly this reduces the power required to drive the auxilliary components, which it might do in certain circumstances. I explained my doubts about how true this might be, or how much of an effect it might actually have, in an earlier post.

Regards,
John.

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Thank you John.

Would, therefore, your consideration be that possibly
a lightened flywheel would be at disadvantage compared to a heavier flywheel to pull the car up steep gradients and hills and final acceleration towards top speed?

Regards,

Dino

Quote:
Originally Posted by johnlear View Post
The short answer? See what I can do, probably fail...

Strictly speaking, neither a less massive damper nor a smaller diameter pulley can or do increase the actual power manufactured by the engine (i.e. they don't). It's not remotely similar to say fitting different cams or a supercharger (which will increase actual power). This begs the question, so what is the rationale behind reducing rotating mass or wanting to reduce pulley diameter?

It's not magic, but because doing so in pratice and / or in theory reduces the amount of a given power output that is 'parasitically lost', and therefore is unavaliable as power to accelerate the car forward, and also improves throttle response for much the same reason.

Reducing rotating mass:
Whether by fitting a lighter pulley / damper or a lighter flywhel etc. this doesn't increase power, sort of depending on how that power is measured.

If we were to measure power on a 'brake' dynomometer (with which engine power is determined with the engine held at a given steady rpm against a 'braking' force that is 'trying' to stop the crank from rotating, i.e. brake horsepower, or brake KW if you like), then at any given steady rpm the as measured power would not increase with a lower rotating mass. This is because at a steady crank rpm we are not accelerating the the speed of the rotating masses (which takes some power to do, 'robbed' from the engine).

However, if we were to measure power with an 'inertial' dynomometer, then with less rotating engine mass there will be an apparent increase in 'measured' power. Note that with an inertial dynomometer the power is not directly measured at a given steady rpm (as with a 'brake' dyno), but rather is 'inferred' by the time it takes for the engine power (at continuously increasing rpm with a WOT) to increase the rotational speed of a heavy mass of known proportion (part of the dyno typically being a heavy cylinder of known weight and diameter, hooked up to sensors feeding data to a computer). Engine power hasn't really increased as a result of the reduction in rotating mass, but the inertial dyno will 'say' that power is increased. This is because the inertial dyno is 'seeing' the reduction in the time that it takes to increase the rpm of the rotating known mass, which is less time because of the decrease in parasitically 'lost' power (due to the reduction in rotating engine mass). The dyno interprets this reduction in time (needed to accelerate the known mass) as an increase in power.

Inertial dynos can't determine steady rpm power because at steady rpm the known mass is not changing its' rate of rotation. They give a 'measure' of power at any given rpm based on the time it takes to rotationally accelerate the known mass. The known mass and the rotating engine mass both affect the time it takes to increase the rpm of the known mass (the attached computer can't tell where the difference in reduced mass is coming from, all it 'sees' is the time it takes for the known mass to change rotational speed). My understanding is that an inertial dyno measures this change in rotational acceraration over very short time increments, which it computationally 'joins together' to create a 'graph' of inferred power over the engines' rpm range (or can detect the rate of change in acceleration of the known mass). Inertial dynos don't directly measure torque, but infer it mathematically from the measured power and the engine rpm at any given moment.

A 'brake' dyno is different, it directly 'measures' measures torque, and only infers power via a mathematical calculation. It quantifies torque by applying a mechanical resistance to engine rotation (i.e. a 'braking' force), and in some manner measures the force required to hold the engine at a given rpm. This can either be a resistance directly applied to the crank (engine dyno), or less directly at the wheels (chassis dyno, somewhat less accurate). With a brake dynomometer, time doesn't come into it, it is an 'instantaneous' measure of torque that is not time dependant (though time is an element used in the calcuation used to infer power, because crank rpm is used in the calculation, and the 'm' in 'rpm' represents 'per minute', i.e. time). A reduction in rotating engine mass isn't directly 'seen' by a brake dynomometer, because it doesn't use time to measure torque.

What we learn from understanding this (I hope), is that a reduction in rotating mass improves acceleration (to whatever degree) because of reduced inertial loss. But, it won't affect top speed when the forces acting to limit top speed hold the engine at a power limited maximum steady rpm, because the reduction in rotating mass doesn't make any more steady rpm power (rotational inertia is not a factor at steady rpm, only the aerodynamic and rolling resistances).

So, it takes power and time to increase the rotational speed of the engines' rotating masses. When rotational mass is reduced, this doesn't affect the power that the engine is actually making, but does decrease the time it takes for a given amount of power to increase the rotational speed of the rotating masses, and so acceleration improves.

This explains why a reduction in rotating mass has a more evident affect on acceleration in the lower gears, i.e. in a lower gear the crank rpm increase more rapidly, and any improvement is more subjectively evident than in a higher gear when the rpm increase more slowly, and, rotational inertia is less relative to the increasing aerodynamic and rolling resistances at higher speeds in taller gears.

Reducing rotating mass is most commonly achieved by lightening the flywheel, or fitting a flywheel that is lighter to start with. Reducing the mass of the crank pulley has a similar if much smaller affect on rotating mass.

Reducing the diameter of the pulley:
Lessens the rpm of the pulleys attached to the auxilliary components, at any given crank rpm. Supposedly this reduces the power required to drive the auxilliary components, which it might do in certain circumstances. I explained my doubts about how true this might be, or how much of an effect it might actually have, in an earlier post.

Regards,
John.
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Firstly, can we stop quoting complete dissertations? There is no need to quote or quote everything all the time. Some posts are long.

Lightened flywheels are known to have a negative effect for going uphill, or at least such a steep hill that engine speed will reduce.

A flywheel is an energy store. It stores kinetic energy. A smaller store (lightened flywheel) store less energy so will decelerate more readily going steeply uphill. Therefore, the car will decelerate more readily.
However, it will still accelerate slightly better going uphill.

A side effect of a lightened flywheel is less large-diameter gyroscopic mass. I'm not going to try to explain about that so just accept that reduced gyroscopic mass helps the car turn easier when travelling at speed.

I'm sure I read that removing around 5kg of flywheel mass is equivalent to removing 100kg of chassis mass. On a TS a lightened flywheel feels like less than a 50kg weight loss to me.
The improvement in engine response is probably the main gain.
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I replaced the stock 1.4kg damper-pulley on my 2.0 TS for a 380gram billet alloy pulley with 15% reduced diameter, to reduce drive to the ancillaries. I didn't notice any drop in steering assistance when at idle which was nice, nor did the battery light come on even at idle with all electrics turned on. So maybe you could go more than 15% reduced diameter, I'm not sure. For me it worked very well, the car felt noticeably quicker off the mark, 1st & 2nd gear felt way better. I also took the balance belt off at the same time though, so not exactly scientific. Both mods together made the car a nice bit quicker, so for me it was worth it. Had no issues for years afterwards. That's not to say I disagree with the theory posted above.
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About the smaller diameter pulley, I remember years ago there was often confusion with the Alfa Nord 1.8 engine output with regard to SAE or DIN measurements.

The DIN rating was 122bhp but the SAE rating was 135bhp.
AFAIK, DIN was as installed in the car and SAE was the bare engine in a test bed which had no auxilliary belts on it.
I also guess that the DIN figure is not at maximum electrical load so in real terms, the DIN figure could be lower.

I can see how a smaller diameter pulley could 'free-up' lost power. Being lighter is of no benefit but smaller diameter for high RPM makes perfect sense.
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Alfa TS Underdrive Engine Pulley | TTV Racing Component Manufacturers

I think the description of this pulley is more informative as to its intended purpose.
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I don't know what theory said in details, but i strongly believe in practice results......

......I don't know what theory said, but my practice show me different.....and i guess that it's not just luck.....
I think it will be a bit of a lucky dip. A lot would depend on the specific engine and its' crank harmonic characteristics, and how it's used. Perhaps the TS crank (and other cranks) is not particularly fragile in this respect(?), but others may well be. In the field, there might be only say one in fifty crank failures (to pick a random number for sake of illustrating a point) as a result of deleting the damper mass, against say one in a thousand when using the stock damper. No data, no proof, but still...

I recall reading somewhere that the original Datsun 240Z engines came with only a light pressed steel pulley, and Datsun (Nissan) soon found themselves with a significant spate of warranty claims related to crank breakage. A 'proper' damper was then fitted on subsequent engines, and the problem went away. I'll bet this isn't the only such case...

I just can't see it being worth the risk, especially when the reduction in rotational mass is minimal, and the affect will be even more minimal considering the relatively small diameter of the damper ring. Taking say 1kg off the pulley won't have nearly the same affect as removing 1kg from the near the edge of the flywheel, because of the relative distances from the crank axis (near the edge of flywheel being a LOT farther away from the crank axis than the damper ring is, i.e. 1kg out near the edge of the flywheel has far more rotational inertia than 1kg of damper mass much closer to the crank axis).

Regards,
John.
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Would, therefore, your consideration be that possibly
a lightened flywheel would be at disadvantage compared to a heavier flywheel to pull the car up steep gradients and hills and final acceleration towards top speed?
Dino,
IMO a heavier flywheel only means that the car will roll just a bit further up the hill before the driver needs to start doing something about the car beginning to significantly lose speed. I don't see this as a problem. The kinetic energy embodied in the greater rotating mass will help the car to climb the first part of an incline without as much need to increase throttle opening as much, but it's not a great difference.

A heavier flywheel won't help the car "pull up steep gradients" (after the embodied kinetic energy that was there at the start of the climb has been dissipated). If the speed and rpm remains the same then the throttle position will be the same with a heavy or a light flywheel. Conversely, if the throttle position remains the same then the speed and rpm will remain the same, regardless of flywheel mass. This is because the mass of the flywheel makes no difference if the rpm is not changing.

A heavier flywheel won't be advantageous with "final acceleration towards top speed". A lower rotating mass will always allow the car to accelerate faster, regardless of the gear the car is in or the speed it is going at. This advantage will just be less obvious the higher the gear and the closer the car is getting to it's terminal velocity (which will be the same speed with a light or heavy flywheel).

Regards,
John.
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Some posts are long.
Who you looking at...

Regards,
John.
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Lightened flywheels are known to have a negative effect for going uphill, or at least such a steep hill that engine speed will reduce.
Fruity, I partially disagree. As I said in another post, it's only the first part of an incline in which a heavier flyweel (rotating mass in general) is of some assistance.

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However, it will still accelerate slightly better going uphill.
Yes. If the car were to start the climb from a standstill at the bottom of the hill, with a lesser flywheel effect (lesser rotational inertia from any source) the car would accelerate from rest all the way to the top of the hill with somewhat greater alacrity, than it would with a greater rotational engine mass.

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A side effect of a lightened flywheel is less large-diameter gyroscopic mass. I'm not going to try to explain about that so just accept that reduced gyroscopic mass helps the car turn easier when travelling at speed.
This is an interesting thought. It has crossed my mind before that at least theoretically there must be some degree of gyropscopic action (associated primarily with rotational engine mass, but also gears and wheels etc.) that would assist the car to change direction when steering one way, but hinder it when steering the other way. I can't say that I've ever felt a difference that I could attribute to such an effect. I'm fairly sure such an effect exists, just not at all sure what magnitude it might have. I assume it would be different for engines mounted east/west than it would be for engines mounted north/south.

If anyone doubts the existence of this effect, take a bicycle wheel and hold it in front of you with both hands (by the axle) at arms' length, have some rotate it fast, and then 'steer' it to the right, then 'steer' it to the left. You'll feel the gyroscopic affect, quite different left vs right. The wheel will 'steer' easily one way, and be quite resistant 'steered' the other way (can't remember which way is what, years since I did this experiment). The wheel will also try to rise higher when 'steered' one way, and try to drop lower 'steered' the other way.

Quote:
Originally Posted by Fruity View Post
I'm sure I read that removing around 5kg of flywheel mass is equivalent to removing 100kg of chassis mass. On a TS a lightened flywheel feels like less than a 50kg weight loss to me.
The improvement in engine response is probably the main gain.
Agreed. I don't know what the equivalence ratio is (differs at diferent rpm I'm sure), but it is quite significantly harder to get a rotating mass to move forward than an equivalent mass that is not rotating. Gyroscopes don't 'like' to move, other than rotationally.

Regards,
John.
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Originally Posted by Pud237 View Post
I replaced the stock 1.4kg damper-pulley on my 2.0 TS for a 380gram billet alloy pulley
So you lost a tad over 1kg of rotating mass from the damper / pulley. But, this mass is lost from a position only a few cm from the crank axis (the closer a mass is to the axis of rotation the less rotational effect that mass will have, because the mass moves a lesser distance per degree of rotation if it is closer to the axis). How much would you gain from taking 1kg off the flywheel, and that 1kg from a position a lot farther from the crank axis? I suggest it would not be noticable...

Quote:
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with 15% reduced diameter, to reduce drive to the ancillaries. I didn't notice any drop in steering assistance when at idle which was nice, nor did the battery light come on even at idle with all electrics turned on. So maybe you could go more than 15% reduced diameter, I'm not sure. For me it worked very well, the car felt noticeably quicker off the mark, 1st & 2nd gear felt way better. I also took the balance belt off at the same time though, so not exactly scientific. Both mods together made the car a nice bit quicker, so for me it was worth it. Had no issues for years afterwards. That's not to say I disagree with the theory posted above.
I've removed my balance belt. The difference in acceleration is barely perceptible, so slight I could be imagining that I can actually detect it.

As for pulley diameter reduction, well I'm very skeptical, and suspect a placebo affect. I do agree that the PS will be somewhat less parasitic, but doubt the difference will be significant. I have my doubts about reducing the power lost to driving the alternator with a smaller pulley diameter, as I've already said.

Of course so far this is all theoretical and subjective on all our parts. Engineers have a saying; "in god we trust, all others bring data". Not that I believe in God mind you...

Regards,
John.

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Apparently at 7,000 crank rpm, the balance shafts (which would be spinning at 14,000rpm) consume 7hp. No idea if its correct but I've heard it a lot over the years particularly on this forum, it sounds plausible. Looking into the factory power data, the 1.8 TS makes 82.4hp per litre, the 2.0 with balance shafts makes only 78.7hp per litre. 3.7hp less per litre. 7.4hp less than it "should". Hard to say how noticeable it is to add 7hp to a 150hp engine but to me, the car certainly felt perkier, particularly at low engine revs. I was happy with what the modifications cost to do and would do so again, placebo or not. The interesting test would be to revert back to standard and see if the opposite effect was noticed.

Anyway, there's some good discussion here on Pistonheads which you might find interesting:

https://www.pistonheads.com/gassing/...&t=1220819&i=0
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I think it will be a bit of a lucky dip. A lot would depend on the specific engine and its' crank harmonic characteristics, and how it's used.
I don't know if it's lucky dip, but IIRC, when we make such a pulleys, have them ordered for numerous drift used BMW's, few Civic's, Saxo's, Peugeots, and only god knows how many TSparks.......At least 70% of the cars was driven on track only, and still don't heard about any pulley related fails......
One customer even wants pulley for it's 145's JTD.....he did same endurance races which i did with no problems....except numerous turbo failures......
So as Pud said, i may agree with 80% of the theory above, but practice sometimes different......
For me, damping pulleys, heavy FW are made for comfort, and engine smoothness......
I don't know if its true, but some said to me that bigger FW allows car makers to pass emissions easier......test is conducted with steady RPM, and that make sense for me.......

Quote:
Originally Posted by Pud237 View Post
I also took the balance belt off at the same time though, so not exactly scientific.
Forgot to mention that did that since i got the car.......my other 2.0TS uses regrinded 1.8 block which is lighter and don't have balance shaft......
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So, let's imagine a hypothetical engine that has a critical crank resonance at and very close to, oh I don't know, let's say 5,200rpm. The crank isn't at risk at any other rpm within the usable rev range, but at 5,200 it starts to generate a 'resonant oscillation of death'.

However, the resonant oscillation does not instantly reach a dangerous level the moment 5,200 rpm occurs, it takes at least a few seconds for the torsional oscillation to start getting out of hand. Not unless the rpm are held steady at or very near to 5,200 for a significant number of seconds does the resonance climb toward catastrophy.

But, in hypothetical practice this is something that just never actually (hypothetical actuality) seems to happen with this engine. Rather, the rpm are always rising or falling as they pass through the 5,200 rpm danger zone, so there is not ever time for the 'death resonance' to reach a destructive level.

When the car is cruising the rpm are always well below 5,200. None of the speed limits correlate to 5,200 rpm (nor the speed somewhat in excess that the driver has a habit of trying to get away with...). The car is always accelerating hard when 5,200 arrives and is then quickly exceeded. As rpm fall, say when shifting gear, they tend to drop straight past 5,200.

Were this engine to one day unusually be asked to hold 5,200 rpm for even a short time, then it might not be a happy day for the engine, or it's owner...

Sounds a plausible possible reality to me. It would explain why a car maker would fit a harmonic damper, i.e. because they know that a dangerous crank resonance exists at a certain rpm and of course would want to avoid waranty claims and reputational damage that might come from potential quite foreseeable failures. It would also explain why a certain number of users of this engine can remove the damper and suffer no adverse consequences.

Regards,
John.
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