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Slow learner

rwarre

Well Known Member
So I am getting new cylinders going from 7:1 to 8.5:1. My understanding is that the volume of gas in the cylinder as the pistons move changes with more force with the 8.5 to 1 because of the stroke. Like I said, slow learner not an engineer.
 
That's essentially correct. The compression ratio is the ratio of the volume of air in the cylinder from bottom dead center to top dead center in the cylinder. You say you have an 8.5:1 cylinder but what's really changed is the stroke of the piston, not the volume in the cylinder.

With a longer stroke on the piston, you have the same volume of space at bottom dead center, but at top dead center you have reduced volume, giving a higher compression ratio.
 
Yes. You're starting with 80 cubic inches of air in each cylinder at atmospheric pressure. On your 7:1 pistons, that gets squeezed down to 80/7 = ~11.5 cubic inches.

On your new 8.5:1 pistons, it is getting squeezed down to 80/8.5 = ~9.4 cubic inches. It takes more force to compress the same amount of air more. But you get it all back and then some on the combustion stroke.

The new pistons will stress everything in the engine slightly more.
 
Not really

Can-o-worms here, friend. Could go into a lot but you stated your background. The taller pistons are still tall at the bottom of the stroke; so, stroke (and volume of your air/fuel charge) does not change.

Start by checking out this active thread

https://vansairforce.com/community/showthread.php?t=185266

The responders quickly deviated from the OPs question but your answer is probably in there. The taller pistons will give you more power and a bit more efficiency (generally). Component life will be affected. Be happy with the result and don’t sweat the details (Unless you get into detonation margin; this/octane requirements/nother can-o-worms). Best of luck.
 
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