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I wonder if this is an aortic stenosis type thing? In the heart when the aorta gets too fat the blood flow changes from laminar to turbulent, and significantly reduces the flow. In your close-coupled empty cat the increased diameter is probably enough to cause the trip from laminar to turbulent, but when the matrix is in, the flow is laminarized. I would expect if this is indeed the case then fitting a thinner exhaust pipe as used in the CF2 will increase the exhaust flow by keeping the gas reynolds number the correct side of the transition. Reynolds depends on diameter, speed, viscosity, and density, so you could be going back and forth over the transition at different loads and engine speeds, which seems to be what you are describing.
 

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Discussion Starter #22
Interesting analogy, there might be something in it, though I'm not convinced that 'flow' is really the issue. I suspect it more likely to be related to some affect on harmonic waves in the exhaust system, maybe....

I think the commonly accepted major effect of a sudden increase in pipe ID (such as a pipe / tube exiting into an enclosed void, such as a precat) is that the void generates a reflection of pressure waves in the moving gas column, back toward the open exhaust valve. It might be that emptying the precats (and primary cat) could stengthen this affect (i.e. the strength of innate reflected pressure waves) .

So, if the primary and secondary manifold pipe lengths are not as good as they might be, and consequently positive pressure waves arrive at the open exhaust valve with inopportune timing such that breathing is adversely affected (i.e. precats and cat are located in the system with more of a view to packaging and heating quickly than with a view to maximising exhaust pressure wave timing), then anything which might increase the strength of those pressure fluctuations might cause some loss of power...??

Perhaps emptying the various catalyst canisters might increase the strength of reflected pressure waves, compared to canisters still stuffed with the catalytic cores? If so, then the adverse affect of a reflected positive pressure wave arriving at an open exhaust valve would be greater if that wave were reflected from a gutted precat (or primary cat) than if reflected from an intact precat / primary cat...??

If so, then a gutted precat and / or 'primcat' could adversely affect performance (compared to non gutted), if the manifold primary and secondary pipe lengths were not ideal (or in fact counter productive) for enhancing engine breathing. On the other hand, if the pipe lengths are conducive to negative pressure waves arriving at the open exhaust valves at opportune moments (for enhanced breathing), then the affect of gutting precats / primcats might have the opposite affect, i.e. improve breathing, and therfore improve performance...??

This might explain why some engines appear to gain power from gutting the various catalytic convertors (according to report / rumour / myth...?), and others seem to lose power (again, according to report etc...). If the hypothesis is correct, then whether or not gutting a given catalytic convertor helps to create more power or loses power might depend strongly on the length (and possibly ID) of the pipe between the exhaust valve and the 'void' that the exhaust exits into.

Just thinking out loud...

Regards,
John.
 

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Discussion Starter #23
Applying this thinking to my recent experience with gutted vs non gutted cats;

The CF3 exhaust manifold ('manivertor') appears to be primarily designed to place the precats close to the engine more than it is to have 'ideal' primary pipe lengths. Compared to the CF2 manifold the CF3 primary pipes are quite short. Shorter primary pipes tend to not be ideal for lower / mid range rpm power, so are lilkely to make somewhat less power at lower rpms compared to the CF2 manifold (all else being equal). Engines fitted with the CF2 manifold are known to make somewhat more power than engines fitted with the CF3 manifold, which may be in large part (or perhaps in whole?) attributable to the difference in exhaust manifolds?

The likelihood is that within the usable rev range the relatively short CF3 primary pipes cause positive pressure waves to arrive at the open exhaust valves, inhibiting engine breathing to some degree (as oppsed to the longer pipes of the CF2 manifold encouraging negative pressure waves to arrive at open exhaust valves). The short primaries may work at very high rpm, but concievably this could be so high as to be outside the usable rev range (??).

So, if the cats are gutted the reflected positive pressure waves (as they arrive at the open valves) could be stronger than they might be if the cats were intact (not gutted). When the positive pressure wave arrives at the open exhaust valve, the stronger the wave the more inhibiting it is for exhaust gas exiting the cylinder, and therefore the worse it is for inlet charge entering the cylinder (i.e. breathing is adversely affected). The weaker the positive pressure wave is the less inhibiting it will be for engine breathing, so a weaker pressure reflection from an ungutted cat would be less damaging for engine breathing and so more power would be made (or rather, less power would be lost due to the shorter than ideal primary pipe lengths...).

Does any of this make sense?

Regards,
John.
 

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Yep...basically it's as I said above...you need a little bit of back pressure at low revs if you want good torque at low revs. Otherwise you lose mixture straight out of the exhaust valve on the valve overlap. Unless you have modern variable valve tuning this cannot really be avoided. Straight through systems are great on race it drag cars that are mostly at maximum revs and low down torque and flexibility stands for nothing...but on a road car a torquey flexible engine is nicer to drive than a rev hungry screamer...which is fun for about 5 minutes...and why I'm not a fan on the 2.2 JTS...not much torque and what there is, is high up the rev range.
 

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All the twinsparks have variable inlet cam timing. Generally this operates between 2800 to 4800 rpm. , so maybe the reflected waves in the primary are a smaller issue, since it would be way above that range. Then, to add another idea to the party - secondary combustion would be happening in the primary, if not catalysed it might make other gas species that impede gas flow, although my gut feeling is this is also a tiny effect.
 

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Discussion Starter #26 (Edited)
I don't think it is as simple as back pressure, and I'm not at all convinced that back presure is ever a good thing (though it is more or less unavoidable in whatever degree). The higher the back pressure the harder the engine is working to get the spent gasses out, which I think means it's harder to get the fresh air and fuel in, at any rpm. Back pressure as I understand the concept relates to the average pressure in the exhaust system (the basic resistance to gas flow). There could be low back pressure with high or low pressure fluctuations, or high back pressure with low or high pressure fluctuations.

What I'm talking about is the harmonic pressure fluctuation that occurs in the gas column within the exhaust system. The gasses in the exhaust system are highly energetic and dynamic. Significantly strong positive and negative pressure waves move up and down the exhaust pipes at the speed of sound (a lot faster than the gasses actually move through the system). When a negative pressure wave arrives back at the beginning of the exhaust system (i.e. at the exhaust valve), if the valve is open at that instant then this momentary low pressure makes it easier for exhaust gasses to exit the cylinder (in turn assists cylinder filling with fresh air / fuel, which makes more power).

On the other hand, if a positive pressure wave arrives at the open exhaust valve, then this momentary high pressure makes it harder for the exhaust gasses to exit the cylinder. 'High' and 'low' pressure are relative in that the reflected harmonic pressure waves within the exhaust system are always lesser than the pressure escaping from the open exhaust valve, but it is always helpful if the pressure in the exhaust port is as low as possible when the exhaust valve opens. Higher pressure moves toward low pressure, and the greater the pressure differential the more strongly the higher pressuer gas will move, so the greater any pressure differential the better, so the lower the momentary pressure in the exhaust port the better.

The length of the pipes in the exhaust system has a huge affect on the time it takes for a pressure wave to move along the exhaust system (down, then reflect back up), and within what range of rpm a negative or positive wave arrives at the open exhaust valve. The pressure wave moves from the exhaust valve to the first point at which the ID of the system suddenly increases (and any subsequent points where the ID increases), then the wave reverses direction ('reflects') back toward the valve. This is a lot more complex than I am presenting it to be...

My hypothesis is that the CF3 manifold is not a well designed 'header' manifold. It's primary pipes are most probably significantly shorter than ideal for generating negative pressure waves which arrive at the exhaust valves at the right time to enhance power (within a useable range of rpm). Comparing the CF3 manifold to the earlier CF2 manifold, the CF2 has much longer primary pipes (and no precats). The CF2 manifold is a 'cleaner' design than the CF3, as the CF2 does not have to make provision for accomodation of the two precat convertors, and so was able to be designed purely to enhance exhaust performance (at least with a lot less compromise than the CF3). Because the CF3 manifold has to include the precats, and to get the precats as close to the cylinder head as possible (for more rapid heating), the designers were forced to substantially shorten the primary pipes, to a quite significantly less than ideal length (IMO).

Because the CF3 primary pipes are quite short, the rpm at which negative pressure wave arrives at the open exhaust valves will be quite high (i.e. the shorter the primary pipes the higher the rpm at which this occurs, dictated by the speed of sound, i.e. the speed at which the pressure waves move within the exhaust system). This is the same deal for positive pressure waves, i.e. they move at the speed of sound along the pipes, alternating with negative pressure waves.

So, because the CF3 primary pipes are too short (for useable engine rpm), rather than negative pressure waves arriving at the open exhaust valves, positive pressure waves arrive at the valves (a bad thing). Positive pressure waves somewhat inhibit the exhaust gasses as they exit the cylinder, and the higher the pressure of the reflected positive pressure waves the worse this will be. The pressure waves are reflected back toward the valves from the points at which the primary pipes are welded to the first other pipe they encounter, and from the point where the secondary pipes exit into the precat canister (a very abrupt and very susbstantial increase in effective pipe ID, probably promoting a strong wave reflection).

This next part is the crux of my hypothesis. If the precat matrix has some dampening effect on the pressure wave that is reflected from the precat (which I suspect it would, but might not...), then removing that matrix (gutting the precat) may well increase the strength of the reflected positive pressure wave. This would mean that with a gutted precat the unwanted positive pressure waves that arrive at the open exhaust valves would be stronger than would be the case if those pressure waves were dampened by an intact precat matrix...

Do you see what I'm clumsily trying to get at? I'm hypothesising that the CF3 manifold has an inherent pressure wave problem created by excessively short primary pipes causing positive pressure waves to arrive at the open exhaust valves (rather than negative waves), but that this problem is quite possibly somewhat ameliorated by the existance of the precat matrix acting to dampen the strength of these disadvantageous reflected positive pressure waves. If this is correct assumptioning, then if the matrix is removed its' dampening affect is also removed, and the strength of the reflected waves increases, impeding breathing somewhat more and causing a greater loss of power than would be the case (if the precat matrix were intact).

I am just speculating here, but I think it does at least explain the power increase I have experienced when I replaced the gutted precats with intact precats. I think it also explains why it appears to be (anecdotal reports) that with some cars gutting the precats creates an increase in power, but with others causes some loss. I suspect that if the CF3 primary pipes were significantly longer (i.e. 'proper' length, if that were practically possible) then it is less likely that there would be a power loss as a result of gutting the precats, rather I suspect there would be some power gain as a result of enhanced negative pressure wave strength (as well as from some reduction in back pressure). Some other cars may well have primary pipes long enough that this is what would happen if the precats were gutted...?

I could speculate all day, but its not easy to prove...

Since fitting the intact precats, the more I drive the car the more convinced I become that the power increase is quite real and not a figment of my imagination.

Regards,
John.
 

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Discussion Starter #27
Alfahaze said;
"All the twinsparks have variable inlet cam timing. Generally this operates between 2800 to 4800 rpm. , so maybe the reflected waves in the primary are a smaller issue, since it would be way above that range."


I'm only speculating on the possible affects of changes in the exhaust, which I think is somewhat divorced from the inlet side.

I think it should be noted just how short the primary pipes are on the CF3 exhaust manifold ('header'). Compare the CF3 unit to just about any other 'header' style manifold that has been properly designed for 'high performance' without space contraints or the need to accomodate precats, and they are quite substantially longer in just about every case I've ever seen. There are exceptions, but I would suggest they are either not well designed, or have to be fitted into tight spaces (or just intended to look nice...). The implication of such short primary pipes on the CF3 manifold is that they will 'work' at very high rpm, more than likely well above what a TS Alfa engine will ever actually see (even at 7000 redline...).

It is my understanding that a positive pressure wave moves from the exhaust valve to the point at which the ID of the pipe significantly increases, then a negative pressure wave is 'reflected' back along the pipe until it reaches the exhaust valve (hopefully in time to find that valve is opening again, but often not, depending on rpm and pipe length). The pressure wave in the pipe moves at the speed of sound, and though the speed of sound can be somewhat variable at differing gas temperatures and pressures, even with different engines will take a very similar time to travel a particular distance along a similar length of exhaust pipe.

So, pipe length X can have a similar effect in various engines at similar rpms. If with a variety of engines we commonly see most serious 'header' pipes ranging between X length and Y length, then I think it is not unreasonable to assume that lengths longer or shorter than X and Y are not likely to be particularly effective for most fully employing harmonic pressure waves to help scavenge the exhaust gasses from the cylinders. The CF3 'header' pipes do I think fall on the short side of 'useful'...

"Then, to add another idea to the party - secondary combustion would be happening in the primary, if not catalysed it might make other gas species that impede gas flow, although my gut feeling is this is also a tiny effect."

I'm not following...

Regards,
John.
 

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I replaced the front cats today in the 1.8TS CF3. I used BM cats as it was only £126 delivered. I had to send it back last weekend as one of the welds had a huge hole in it. I think I got the same one sent back with the hole welded up. I notice on the four bolt flange section which mates to the manifold that the holes are way to big to fit the M8 studs in, so that was a massive faff having to fit M8 bolts and and nuts. Then the rear flange for some reason they have done with massive M12 bolt holes, instead of the M8, so again a massive faff with lots of increasing size washers to fit the thing back in. Then I noticed that one of the O2 sensor flanges is not welded in the right location, and the sender is pressed hard against one of the A/C hoses. Well, I know why it was so cheap. That will teach me for not using IMASAF, which always fit perfectly with no drama.
 

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I remember reading a book about exhaust design a long while ago, by Vizard? So maybe what I remember might be true. Or I might of dreamed it up, you can decide

Yes, I agree with everything you are saying - the CF3 front cats are a terrible design, smells like a last minute design change to get through Euro III.

Theres two things at play, the steady slow-ish stream flow, and the high speed pulse waves. In steady stream when the gas meets a sudden change in diameter it slows and increases in pressure, eddy currents are formed, and it becomes turbulent. At the same time pulses are going through this steady flow situation, and when the pulse meets the change in density it is partially reflected. If the reflections are timed right it can help scavenge the cyclinder during the overlap period - which in the TS is quite large when the CCT is active. But the CF3 front cats have a massive change in section, right up close to the cylinder. This would only possibly be of help if you were running at stupid high RPM like an F1 engine 10-15k or so.

So, has anyone swapped the CF3 front cat for a CF2 pipe and managed to keep the CEL working correctly? My 2.0 CF2 is significantly faster than the 1.8 CF3, seat-of-pants dyno says like 40% more power.
 

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Discussion Starter #30
That author must be David Vizard. Decades ago I built a potent little BMC A Series (Austn Healey Sprite) engine with one of his books on the workbench ("Tuning the A Series Engine", imaginitively...). The engine was only 1L, with Cooper S head, 'rally' cam, Dellorto DHLA, light flywheel etc. It dynoed at about 65 wheel horsepower, and lived in a Nota (handmade sports car, like an Australian Lotus Seven), which only weighed about 500kg, so was quite rapid. This was a fantastic little motor, plenty of power for its' size and weight of the car, and the best throttle response I've ever encountered before or since (revs jumped like a bike engine).

"Yes, I agree with everything you are saying - the CF3 front cats are a terrible design, smells like a last minute design change to get through Euro III."

Smells exactly like that, more than a faint pong...

"Theres two things at play, the steady slow-ish stream flow, and the high speed pulse waves. In steady stream when the gas meets a sudden change in diameter it slows and increases in pressure, eddy currents are formed, and it becomes turbulent. At the same time pulses are going through this steady flow situation, and when the pulse meets the change in density it is partially reflected. If the reflections are timed right it can help scavenge the cyclinder during the overlap period - which in the TS is quite large when the CCT is active. But the CF3 front cats have a massive change in section, right up close to the cylinder. This would only possibly be of help if you were running at stupid high RPM like an F1 engine 10-15k or so."

I agree, the rpm at which such short primary pipes are likely to work is probably very high indeed (two stroke racers use the term "come on the pipe"). With a 7000rpm redline, in all probability not even close...

My very basic understanding is that a big pipe ID tends to create less back pressure (i.e. more free flowing exhaust), but decreases the strength of the harmonic pressure waves. Smaller pipe ID restricts flow so creating higher back pressure, but tends to create stronger pressure waves. So since flow is a good thing, and strong pressures wave pulses can be a good thing (depending...), then the choice of pipe ID is a balancing act, trading off one benefit against another benefit (potential benefit, or hindrance, pressure waves being a two edged sword...).

Of course stronger pressure waves are only a 'good thing' if they are working for us and not against us. So if we have negative pressure waves arriving at our exhaust valves when they are open then this is good and the stronger the wave the better. If we have the opposite, i.e. positive pressure waves arriving at our open exhaust valves, then this is bad and the weaker the waves the better. This is why I think a wave weakening precat core matrix left in place is better than gutting the precat, at least with the CF3 manifold. In this case all that gutting the precats does is (I think) make an existing problem into a worse problem...

This is the crux of my theory as to why gutting the CF3 (specifically) precats results in a power losss and not a gain (when with other engines the effect of the same modification may be a power gain, or maybe not, depending on manifold primary pipe lengths). If we gut the precats then in all cases (probably) we increase the strength of the pressure waves (I think likely but not proven...). However, with the short pipes on the CF3 manifold in particular the pressure waves are probably working against us rather than for us, being because we are very likely to have positive waves arriving at our open exhaust valves rather than negative waves. The positive pressure wave at the open exhaust valve inhibits breathing, so strengthening the positve waves (by decoring the precats) causes a loss of power. My seat of the pants says it is a significant loss from just above idle to max rpm...

"So, has anyone swapped the CF3 front cat for a CF2 pipe and managed to keep the CEL working correctly? My 2.0 CF2 is significantly faster than the 1.8 CF3, seat-of-pants dyno says like 40% more power."

Not myself, though I have read of it heard it being done with good result (just anecdotal report, no dyno numbers or acceleration times, so unsubstantiated). I suspect that "40%" is a suspiciously high difference between your cars. The suspicion I have is that the less powerful CF3 engine may have something wrong with it, even given the 200cc lesser capacity and suspect CF3 exhaust design (I could be wrong, I've never driven CF2 TS Alfa, whether 1.8 or 2.0...).

So, just thinking out loud;
The CF2 manifold has reasonably long primary pipes, quite a lot longer than the CF3 manifold not so much because the CF2 primary pipes are particularly long but that the CF3 primaries are so very short, (not unlike most cast iron manifolds). So, with the CF2 manifold the pressure waves are probably working with us and not against us, i.e. the pressure waves arriving at the open exhaust valves are probably negative rather than positive (and of course in this context 'negative' is a positive thing, and 'positive' is a negative thing...).

So, according to my working theory, with the longer CF2 primary pipes (and to some lesser degree the longer secondary pipes) the negative pressure waves should arrive at open exhaust valves (good for power), so the stronger the wave strength the better (even gooder). Replacing the CF3 with CF2 manifold will also mean deleting the precats, so if it is true (as I postulate) that the precat matrix has a damping affect on the strength of the harminic pressure waves, then the deletion of the precats would also be a good thing, probably...

I have read somewhere or other (I vaguely recall) that the CF2 manifold is worth 5 or 6 extra maximum horsepower over the CF3 manifold. So if this is true, and it is the only benefit, then I personally don't think it would be worth the bother or expense to swap the CF3 for a CF2 manifold. However, there may be benefits beyond an extra 5 or 6 max horsepower, there may be a more significant increase in power 'below the curve', i.e. we may pick up significant power at rpm lower than the rpm at which maximium power is made...? If so then this may well be quite noticable and worthwhile, even if the max power increase were only relatively modest...

Put it this way, if I had easy access to a cheap CF2 manifold I'd be very tempted to fit it. I am quite confident this would not cause a CEL (given that my ECU ran for so long with internally disintegrated primary cat and precat, with only one functional precat, and never lit a CEL). I would expect to experience a significant increase in power, maximum power as well as at lower rpm (though I'd be prepared for disappointment...).

Of course deleting the precats is not a legal modification, so there is a legal consideration as well as a moral one. Legally the precat deletion would first have to be detected, where I live I estimate the chances of this to be about the same as winning the lottery.

Morally there is a question about pollution of the air that other people breathe. My understanding is that the main cat does all the real work the vast majority of the time, with the precats only there to help in the first minute or so (before the main cat reaches operating temperature). I live in rural isolation. I just don't think the impact is significant in the scheme of things. Besides, I have some moral brownie points for replacing the completely dead cats that were in my car up until last week...

Regards,
John.
 

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I'd say you are both right and have a combined good understanding of what goes on with the tuned length and possibly turbulent flow within a larger diameter chamber which has no honeycomb monolith to ensure it is laminar.

As the JTS is similar to the CF3 TS, I have looked at this and suspect a good exhaust manifold will help quite a lot. That said, I believe no one has discovered how to deburr the valve throats ideally but changing to a CF2 system is a worthwhile cheap upgrade. However, the precats probably do all the work. GM & other European engines moved to a single precat or maniverter for Euro 4 engines so don't be to concerned about the efficiency of a single cat. Using heat wrap on the pipes would help a CF2 cat efficiency and even reduce the time the rear O2 sensor has its heater coil energised.

Yes, B&M cats are absolute rubbish compared to IMASAF. It is a shame Regal branded stuff does not appear to be readily available anymore as that was actually well made and fitted really well.
 
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