Piston velocity

Competition engines and ancillaries - general discussion
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trickymex
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Piston velocity

Post by trickymex »

I just need some opinions please

What is regarded as a safe maximum piston velocity on a set of modern aftermarket forged pistons and rods

I know its dependant on many variables but i just need a safe ball park figure.

Thanks Ricky
1NRO
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Re: Piston velocity

Post by 1NRO »

It's not exactly relevent but in 2005 Ferrari was running a 1.6" stroke and turned 18K revs during the race. This translates into 4800 ft. per minute piston speed. Tested in development at 20k which is something like 5300 ft,per minute. Testing pushed past 22k and ended in failure but apparently was due to crankshaft harmonics not piston failure. I've seen engine combinations with figures in excess of this quite often, over 6000ft,per minute, they coped it seems. As far as I've understood the subject the threshhold is when ring flutter is experienced and directly related to the quality of the piston/ring package. What's considered appropriate for choice is not something I'm qualified to preach about. A wild guess would put 5000ft.per minute in my betting slip. Guy might have time to post, I'm sure he has a better opinion than mine.
Sorry about the lack of metric, my sum and source is imperial.

Nik
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trickymex
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Re: Piston velocity

Post by trickymex »

Thank you Nik,

The imperial measurments are fine, i can work in either without to much of a problem

The setup i am investigating would be pushing just past 5100 ft. per min, or for the benifit of the metric users out there, just over 156 meters per min


Looking form the outside in it looks as though alot of a race engine designers base there designs around a short stroke setup for a few technical reasons, here is a short list

1. Lower piston velocity

2. The ability to run longer rods, and this combined with a short stroke setup means the ability to have better rod to stroke ratio's

For the above it is my understanding that a good rod to stroke ratio means less rod articulation and less force exhurted pushing the pistons against the thrust sides of the bore and so reduces the lost energy in the rotating assembly by reducing these factors

Plus with a longer stroke it means that the piston is being subjected to higher piston velocity and it must take more energy to accelerate the piston to these higher speeds, again another reason why a short stroke engine seems to me a good idea and should make the whole setup more energy effiecient and thus more powerfull


Not to mention that if you have a short stroke setup for the same capacity you will have a larger bore and that equals more room for larger valves

I cannot think of many negative points to a short stroke setup

Regards

Ricky
Guy Croft
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Re: Piston velocity

Post by Guy Croft »

This question is too important for guesswork though I will say that with race rods and pistons your rev limit in practice is going to be set by spring and cam behaviour.



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trickymex
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Re: Piston velocity

Post by trickymex »

Thaank you Guy,

Maybe i should give you more info on what i am trying to work out

You have seen my other thread with regard to a car for the DMN series, and so i am looking into an BMW E30 318is this car is fitted with an M42 engine

Brief spec

1796cc 16v 4 cylinder engine, bore 84mm, stroke 81mm, Std rod length is 140mm

For this class i am able to run upto 2000cc in a 16v format so what i am trying to understand is what are the negative points when enlarging this engines capacity

Please bare in mind that i want to get as much power as possible from this engine and i will need at least 250-280bhp to be on par with other cars in this class, i know its not going to be easy nor cheap

My investigaion has brought to light another BMW O/E crank that fits in this block that is 87mm stroke, this combined with a new bore of 85.5mm gives me a capacity of 1998cc

The O/E Rod ratio is just about 1.73:1, but with the 87mm crank and the shorter rods that will be needed you end up with a rod ratio of 1.57:1 not terrible i guess but not as good as the O/E setup

On top of this, with the longer stroke the piston velocity will go from 145.8 meters per min at 9000rpm to 156.5 at the same 9000rpm

As you have pointed out Guy, the RPM limiting factor with the standard engine is the valve train(hydraluic lifters), fortunatly the S50 M3 engine has solid lifters that are interchangable and in road form that engine produces its peak BHP at over 8300rpm, using a combination of S50 parts and after market cams, valves and spring i would hope to be able to push the M42 past 8500RPM, this engine to be competative and produce the desired power will no doubt have to rev this high

It all seems good in theory but i have some reservations about the changes needed to get the capacity up to the 2000cc, mainly the piston velocity and the rod ratio but also the O/E 87mm BMW crank needed to bring the capacity up, will this be capable of such high RPM reliably, im not so sure its worth taking the risk

I will also be moving a heavier piston(larger bore) and that combined with the higher piston velocity worries me somewhat

I guess i am trying to pre-empt parts failures by working out the loading on the rod and piston with maths but im not sure how, the more i look into it the more i keep asking myself is it worth it with this engine? the only reason i investigated this engine was that its the only other 16v 4 cylinder engine fitted to the E30 and i was hoping that it may work out cheaper than the more established S14 (E30 M3 Engine) but when you take into consideration that the S14 engine has 38mm inlet valve's rather than the M42's 33mm valve's and that the S14 has solid lifters as standard it seems that the costs are going to be smaller than i thought if anything at all

Also with the S14 i will be bringing the capacity down rather than up to be eligable for the 2000cc class and this gives me the added benefit of a much better rod ratio and slower piston velocity


Its seems as though i have enough knowledge to work out potential problems but not enough to devise a solution to them, so i need some guidence

Regards

Ricky
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