Quote Originally Posted by fs5 View Post
thanks mate, what about a low rever like mine ? it likes to sit around 5300/5800 tops.
i run longer plugs , with a couple extra washers so the plugs sit further in the dome giving me a bit more comp. this works for me giving us around 3kph better topend.
would oversize boring increase compression ?
thanks
That is good that you find the longer plugs work and not a problem since you pull them often before carbon and heat affect the exposed threads. But note that the sharp edges of the exposed threads is a potential hot spot that could promote pre-ignition.

Since you ask about compression ratio only way to get specific is by math, so here is an example.

CR = Compression ratio = (CV + HCV)/ HCV

CV = cylinder volume TDC to BDC, but the real volume for a 2 cycle is TDC to top of exhaust port which is less.
HCV = head combustion chamber volume

all units the same like cubic inches.

So if you increase the CV with bore job by 0.030 and leave the HCV the same there might be a slight increase as example will show.

Example: Say the bore is D = 3", stroke is 3" and HCV = 2.5 cu. in

CV = stroke x pi x D^2/4 = 3 x (3.14 x 3 x 3)/4 = 21.21 cu. in

CR = (21.21 + 2.5)/2.5 = 9.48 :1

If you bore 0.030 over then D = 3.030"

CV = 3 x (pi x 3.030 x 3.030)/4 = 21.63 cu. in

If HCV is the same at 2.5 cu. in

Then CR = (21.63 + 2.5)/2.5 = 9.65:1 and increase of 1.8%

Weigh that against the increased weight of the oversize piston. A low revving engine will be better
tolerate the heavier piston assembly that a high revving engine. More rotating mass affects acceleration.

Run specific numbers with your engine.

Do you have a blind bore engine with cast on cylinder head or is it a removable cylinder head? Is it legal to machine the head to reduce the HCV? That would be a better way to increase compression ratio.