How to equalize volume between different piston?

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Biancorally
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How to equalize volume between different piston?

Post by Biancorally »

Hi Guy,

is it possible to equalize piston volume difference?

Here the piston dome volume difference:
Cyl.1 = -12,2cc
Cyl.2 = -12,4cc
Cyl.3 = -12,0cc
Cyl.4 = -11,8cc

Is it possible to resume these differences from the combustion chamber in the cylinder head?

Many thanks

Regards

Daniel
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measurement of piston intruder dome
measurement of piston intruder dome
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IMG_0427.jpg (115.82 KiB) Viewed 5701 times
measurement dummy cylinder
measurement dummy cylinder
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Guy Croft
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Re: How to equalize volume between different piston?

Post by Guy Croft »

no doubt it is interesting but it is pointless to chase cc differences on that small scale.

Whilst in theory (?) the CR should be the same from cylinder to cylinder there are significant differences in the cylinder filling & purging due to pressure wave effects in the header, inlet tract and across the cylinder (at overlap) that essentially mean you'd be pretty well wasting your time.

If you are determined to equalise take material out of the cylinder head.

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Biancorally
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Re: How to equalize volume between different piston?

Post by Biancorally »

Hi Guy,
new mystery solved, thank you very much for explanation.


Regards

Daniel
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WhizzMan
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Re: How to equalize volume between different piston?

Post by WhizzMan »

I wouldn't try to even these out without determining what is the cause of the differences. Cleaning piston domes and the amount of play/lubrication in the bearings could very well make those figures differ by 0.6 cc. It could be deck height for the piston bottom or length of the con rod for all we know as well. Maybe by doing a mix and match of pistons, rods and what cylinder you put them in, you could even out the differences as well?

If you're going to be meticulous about this, I'd advice you to tackle the problem at it's source, not mitigate with another non-equal measurement somewhere else. Taking out material in the combustion chamber will (in theory) change flow and burn characteristics between individual cylinders. That would give you a complete new thing to compensate for and how would you want to measure that, compensate for it? The whole idea of this is to get individual parts as closely matching to the "ideal" configuration as possible and then end up with four "as equal as possible" cylinders. once you start making them more unequal at one place, you will end up with having to compensate with more unequal things in other places to compensate and so on.

In modern cars with the latest generations in injection systems, they measure the acceleration of the crank shaft per cylinder and have a wide-band lambda probe before the first catalytic converter. By doing smart tricks during running like leaning out one cylinder at the time for a very short period while the car is being driven, or experimenting with the ignition timing of just one cylinder, they compensate for individual cylinder differences by giving each injector, each cylinder it's own injection map and each cylinder it's own ignition map as well. These are all dynamic and change constantly with engine wear or if you put in different fuel. Engine factories have high precision manufacturing techniques these days. They don't have a lot of difference in tolerances and weight of the individual parts due to the manufacturing process itself. They build the complete engine within tolerances and use final tuning to compensate for any differences that occur on the completed product. It's much less complicated and gives very good results, not just for environmental factors, but also power output. These people spend literally billions on developing engines and effective manufacturing of those. They're not compensating elsewhere, but go for stricter tolerances on the individual components. They no doubt have tried if it's better/cheaper to compensate with offsets elsewhere and found it not working in general. To me that is an indication that you'd have to have a very specific case where you can actually prove that it will help before you should do it.
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TomLouwrier
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Re: How to equalize volume between different piston?

Post by TomLouwrier »

Very true. Modern engines straight off the lines are about as 'blueprinted' as you would get by hand 20 years ago. It's the result of producing in large numbers, using highly automated machines and processes and of course a fundamental improvement of the products on the drawing board; "Design For Quality".
What's left in terms of inequality between cylinders -being individual engines in themselves- is solved with very subtle and powerful computing.

In this case the measured volume is related to too many sizes and tolerances to be able to point out 'the one culprit'. Deck height and rod length are already mentioned, but where exactly is the pin hole in the piston and how tall is the actual throw per crank? They differ.
To take those out you should stick that clear plastic straight on the deck and make the piston just touch it. That way you measure the volume against one and the same reference: the uppermost point of the piston. Then again, that does not tell you much about the effective difference between cylinders -you already know that- or where that difference comes from.
I can imagine you take some metal off of the domes in order to equalize the volumes. That way you could equal out a combined lot of differences, as long as you keep the crank, rods, pistons together as a set. Just like balancing a crank and flywheel together as one assembly.
It will only work if there is room to shave the domes (weight, wall thickness). If there is not, you must compensate again elsewhere like Whizzman says, giving you all kinds of compensations for compensation...

Interesting, but I'm going with Guy here. The rest of the system has so many factors that contribute to the cylinders all running just that bit different, that chasing this particular issue wouldn't show up in the overall performance of the engine.

Of course that raises the question on accurate buretting of chambers. How good is good enough?

regards
Tom

(small edit to make it more readable)
Last edited by TomLouwrier on June 7th, 2012, 11:45 am, edited 2 times in total.
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Biancorally
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Re: How to equalize volume between different piston?

Post by Biancorally »

Hi,
many thanks Tom and WhizzMan for your explanation.

FWIW, i would't undertake any major changes.

Thanks

Regards

Daniel
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