Here is what my boss says about this....

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PV=nRT : Ideal Gas Law

I'm using volume for the discussion, even though it is radius that is changing. The volume is proportional to the square of the radius.

(1)Assuming your engine stuffs moles of fuel into the combunstion chamber until a certain pressure is reached then the Pressure(P)
before your stroke is constant. (2)Also, say you increase the Volume(V) of your combustion chamber by 10%, and therefore the moles of air/fuel mix are also increased by 10%.(n)
(3)R is a constant, and doesn't change.
(4)T is temperature, and lets assume this doesn't change.(Reasonable)

So we have before the stroke:

Unmodified chamber: P(V) = (n)RT
Modified chamber: P(V*1.1) = (n*1.1)RT

Lets say your stroke compresses the chamber by a 2:1 ratio. So after the stroke we have:

Unmodified chamber: P(V*0.5) = nRT -> P = (nRT)/(V/0.5) -> P = 2 * [ (nRT) / (V) ] (Basically 2x the pressure before the stroke... Big surprise, half the volume double the pressure...)

Modified chamber: P(V*1.1*0.5) = (n*1.1)RT
P = (1.1*n*RT) / (0.55 * PV)
= 2 * [ (nRT)/(V) ]
(This is the same as the unmodified chamber.)

So, if you proportionally increase BOTH the *VOLUME* and the amount of fuel air mixture then the pressure
should remain the same. I know very little about cars, but if their fuel system only puts out a certain amount of fuel
and won't put out 10% more fuel & air if you increase the available volume by 10% then something different will happen.
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