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Discussion Starter #1
Hi there,
i am just building a 3.2 Engine for my "fun-GTA" and thought why not buy some maXpeedingrods as there seem to be people who have used them successfully including jimk.

Despite not having the offset the Original rods have i have found something even more strange,
the bearings are offset axial if mounted in the big end...

I wanted to send the crank for balancing because i lightened the flywheel and crank pulley but if i cant come to a conclusion which rods to use (original or the maxpeeding ones) i can not send it off because the rods are 100g lighter and balance would be way out.

Has anybody seen that before ?

 

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Judging from the scuff marks on one of the bearing shells, it looks like the rod has been trial assembled onto the crank?

In the second photo, one of the bearing shells has obvious scuff marks on it, but the other shell doesn't. This suggests that the rod cap is radially offset from the rod big end bore (as well as axially). If so then my suspicion is that the cap is incorrectly installed, or the wrong cap is fitted to the wrong rod (if so then all rod and cap pairings would need to be checked).

Where the rod is split and the ends of the bearing shells abut each other, there should not be any 'edge' that can be felt with a fingernail. If there is a feelable 'edge' then the cap is offset, possibly caused by being fitted to the wrong rod, fitted to the correct rod but the wrong way around, or something is damaged.

Are the standard V6 rods a known weak point? If not then why change them?

The rods are not involved with rotational balance, only the crank, flywheel, clutch, harmonic damper and pulleys. It is important that the rods are all reasonably 'weight' matched with each other, i.e. the mass of each 'big end' matches the masses of each of the the other big ends, and that each 'little end' matches the other little ends (and pistons should also be weight matched with each other). That one set of rods is heavier than another set of rods is not relevant to the rotational balance.

Regards,
John.
 

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Discussion Starter #4
Hi john,
these are the old shells from the original rods, the rods where never in an engine i just test fitted them as i wanted to send one piston with rod and shells along with the crank for balancing.
I want to change the rods because the maxpeeding ones are lighter by almost 100g which is almost 600g less in rotating mass, flywheel (-2.5kg) and pulley (-500g) are lightened already.

The notches are on the same side where they should be and the cap is installed correctly.
IF i would turn the cap around (which i am not doing ofcourse) the bearings would line up actually, but would be either to the left or right depending on how you rotate the rod.

Are you 100% sure that replacing the rods with 100 gram lighter ones does NOT change the balance ?
 

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Are you 100% sure that replacing the rods with 100 gram lighter ones does NOT change the balance ?
Lighter rods will not change the balancing of the engine. As John states, the rods should ideally be weight matched to each other, and so should the pistons. If you'd like, I can email you a report on engine rebuilding, that I wrote. It should provide you with the most basic knowledge you need.
 

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Discussion Starter #6
As it is no inline 4 or 6 i believe that you still need to factor in the weight.
I have been told that they need to make bobweights to balance my crank and need the piston, rod, pistong rings and rodbearings to weight them and calculate the weight of the bobweights.
 

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Stephan,
Brinker and I are in agreement. The reciprocating masses of the rods and pistons have no affect on the rotational balance, so long as all sets of reciprocating masses are matched. Only if the mass of at least one rod and / or piston is significantly 'lighter' or 'heavier' than any of the others is rotational balance affected. This is the same for any engine regardless of the number of cylinders (trying to anticipate your next question, perhaps).

Less rod mass does equate to a reduction in reciprocating and rotational mass (a connecting rod being both reciprocating and rotational mass, though not in equal degree). Reduced rod mass will have a similar affect as taking some mass of the flywheel, but it strikes me as an expensive way to do it, compared to just lightening the flywheel a tiny bit more...

I could be wrong, but, my expectation is that if you had two engines that were identical in all ways other than one had 'light' rods and one had 'heavy' rods, I'll bet that in a back to back comparison very few people could pick which was which.

Regards,
John.
 

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Discussion Starter #8
but it strikes me as an expensive way to do it, compared to just lightening the flywheel a tiny bit more...
There is not much i can take of the flywheel anymore and i thought it might be a nice addition as the rods are not that pricey at all.

I could be wrong, but, my expectation is that if you had two engines that were identical in all ways other than one had 'light' rods and one had 'heavy' rods, I'll bet that in a back to back comparison very few people could pick which was which.
I bet you are right, but i also believe that even little mods will add up when building a engine.
So far in the mix are the lightened flywheel, crank pulley, conrods, a bit of porting on the heads, regrinding of the valves, skimming for a 11:0 CR, reground cams and vernier pulleys (which are lighter too).

If i was only for hp and torque i would slap on a Rotrex like i did on my GTV,
so its kind of a "the route is the goal" project for the fun and learning curve of doing it.

Thanks for your input,
Stephan
 

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I disagree with the above.. in JimK book he says that for what ever reason the only correct way to dynamicallly balance this crank is with bob weights fitted.. i have discussed this at length with him and i have thought of wrapping the correct weigght in lead around the bearing journal as long as its secured as an alternative to making bob weights for every piston rod combo.. i will have to reread his book but cranks need to be balanced with crank pulley flywheel and bob weights according to JimK on this engine.. just my two pence worth..
 

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Less rod mass does equate to a reduction in reciprocating and rotational mass (a connecting rod being both reciprocating and rotational mass, though not in equal degree). Reduced rod mass will have a similar affect as taking some mass of the flywheel, but it strikes me as an expensive way to do it, compared to just lightening the flywheel a tiny bit more...

I could be wrong, but, my expectation is that if you had two engines that were identical in all ways other than one had 'light' rods and one had 'heavy' rods, I'll bet that in a back to back comparison very few people could pick which was which.

Regards,
John.
its not about feel its about reducing the stress on the crank and bearings. a lighter piston rods combination will achieve higher rpm safely than one with heavy rods pistons.. not a massive gain on the v6 as rev limit is limited by the hydraulic tappets on this engine and then valve train really before rod strength. dont know many v6 busso that die from rod failure alone.. but lighter stronger rods with arp bolts wont do any harm
 

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MW is correct. So to backtrack a bit;

The penny dropped that we are discussing a V6 engine (I knew that...) and then I remembered that V6 engines have weird crankshafts, i.e. not a 'flat plane' crank. I have never owned a V6 (just many straight 4 and 6 engines, and one flat four) and it is almost automatic for me to assume a flat plane crankshaft, which is careless thinking. What I said is correct for flat plane crankshafts, but not for multi plane cranks as are found in V6 engines.

The simplest flat plane cranks are found in twin cylinder engines. Each crank 'throw' is 'balanced' by another 'throw' acting exactly on the opposite side of the crank axis, which in theory should perfectly balance the mass of the first throw. A straight or flat four cylinder engine is in effect two twin cylinder engines nose to tail, and a straight six is in effect three twin cylinder engines nose to tail. With a flat plane crank, the mass of each conrod is rotationally in perfect balance with the mass of the conrod that is on the opposite crank throw, so the specific mass of the rod is not important to the rotational balance, so long as the mass of each rod is matched with the mass of the other rods (or at least with the rod with which it is paired on the other side of the crank axis).

'V' engines can be the same when fitted with flat plane cranks, but not all are. No V6s have flat plane cranks.

I'm not sure of the Busso configuration, but as I understand it V6 crankshafts have either six throws (one piston per crank throw) or three throws (i.e. two pistons per throw, in which case the engine is in effect two 'straight three' engines sharing a crankshaft). Either way the crank throws are not inherently 'balanced' by other crank throws acting exactly on the other side of the crank axis, so the throws are not rotationally balanced, and so the engine does not inherently have perfect primary rotational balance.

So, a V6 crank needs its' primary balance to be 'corrected' by the inclusion of additional balancing masses (that are not needed with flat plane cranks). The masses are cast or forged as part of the crank, and balance the masses of the crank throw and conrods that are on the other side of the crank axis (the not quite opposite side...). The mass of each rod is not 'automatically' in perfect balance with the mass of another equally 'heavy' rod, because the rods are not equally located on exactly opposite sides of the crank axis (i.e. not displaced 180°, but at a different 'non balanced' angle).

So, if the rod masses are changed (e.g. lighter rods) then the masses of the counterweights would also need to be changed to take this into account. This is why 'bob weights' are fitted to the crank big ends while dynamically balancing the masses of a V6 rotating assembly, i.e. to simulate rod mass. This is also true for other 'V' engines if they don't have flat plane cranks (most V8s have 'cross plane' cranks, not sure about V12s etc.).

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So, if the aftermarket rods are lighter, then I would assume (but could be wrong) that the crank counterweights would need an equal material mass to be removed to account for the lighter rod masses (? it might not be quite that simple...). So, if each aftermarket rod is 100g lighter than a stock rod then there is a direct 600g reduction in rotating mass, and, probably a similar reduction in mass from the crank counterweighs(?).

It still seems a lot of effort to me, for a relatively small gain. And, the bearings are not right....

If I could afford this, I think I'd be having another look at the supercharger...

Regards,
John.
 

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its not about feel its about reducing the stress on the crank and bearings. a lighter piston rods combination will achieve higher rpm safely than one with heavy rods pistons.. not a massive gain on the v6 as rev limit is limited by the hydraulic tappets on this engine and then valve train really before rod strength. dont know many v6 busso that die from rod failure alone.. but lighter stronger rods with arp bolts wont do any harm
Yes, but considering that the useable rpm is otherwise dictated / constrained (e.g. by rev limiter, valve bounce, or just running out of puff) then the other valid reasons for lightening parts of the rotating assembly is for much the same reason as for lightening a flywheel, i.e. some degree of improved throttle response and a slight reduction in inertial parasitic power loss.

I doubt either will be significant improvements. It just seems to me like a less than efficient way to spend money, especially if a substantial rpm increase is not envisaged...

And, do we know what the quality of these aftermarket rods actually is? Stephan says "the rods are not that pricey at all", so what might this imply? They look pretty, and it's easy to know their mass, but how do we know that they actually are stronger than the stock rods? (which don't appear to be a problem in any case). I think my point is, if it isn't broken then don't take a risk trying to fix it...

Regards,
John.
 

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Yes, but considering that the useable rpm is otherwise dictated / constrained (e.g. by rev limiter, valve bounce, or just running out of puff) then the other valid reasons for lightening parts of the rotating assembly is for much the same reason as for lightening a flywheel, i.e. some degree of improved throttle response and a slight reduction in inertial parasitic power loss.

I doubt either will be significant improvements. It just seems to me like a less than efficient way to spend money, especially if a substantial rpm increase is not envisaged...

And, do we know what the quality of these aftermarket rods actually is? Stephan says "the rods are not that pricey at all", so what might this imply? They look pretty, and it's easy to know their mass, but how do we know that they actually are stronger than the stock rods? (which don't appear to be a problem in any case). I think my point is, if it isn't broken then don't take a risk trying to fix it...

Regards,
John.
As i said the gains in a v6 limited by the valve train wont be massive.. but if the current rods need replacing etc.. worth doing.. much easier to turbo or super charge any engine if power is the goal.. if improving the natually aspirated busso is the goal harder to achieve while retaining the N/A characteristic. ive done the above big turbos huge horsepower undriveable street engines.. getting a busso to breath with out forced induction is much harder .. so while lightened rods, pistons and crank may not increase the rev limit it will speed up the way it revs to that limit and increase the engines lifespan when being used in anger but as you say not cheap for small gains.. heads valves and cams usually with solid lifters wil get you around 8.5k though.. thats what im aiming for with itbs and a capacity increase.
usually max speeding rods are a known quanity being a steel h beam rod with good marterials and arp bolts.. if i was building an engine arp bolts in a lightened new standard rod would be a minimum. new rods arent cheap.. few hours spent lightening and balancing them.. arp bolts .. nearly the same money as new better lighter rods ...
 
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