Another compression question

[250X_project]

I don't understand why the 300EX has a lower compression than the 250X when they are the same jug, piston and head....The only difference is the 300 has a longer stroke. I do know the cams were different, I am pretty sure the 300 had a more mild cam. yet I don't see how even the most mild cam can more than offset the 8.2mm stroke difference.

250X-9.4:1
300EX-8.9:1

[melloyello]

The bore of the cylinder on the 2 engines is the same yes, but is the height of the piston or cylinder the same between the 2? If the 250x has a shorter stroke then either the cylinder is about 4mm shorter or the piston is that much taller. I don't know much about the similarities between the 2 in question but as far as any engine goes, the ways to increase compression are longer stroke, taller or domed piston, thinner head or base gasket and shortened cylinder.
Maybe someone could measure the height of each piston for you. From top of wrist pin to highest point on the piston. If they used the same height piston on both of the engines then they had to have used a shorter cylinder on the 250x. If not, it wouldn't be a very efficient engine b/c the top of the piston would probably be a couple of mm below the top of the cylinder.

[250X_project]

I am almost positive that BOTH the cyl. and piston are identical....If I have calculated correctly..(used stock compression and displacement to solve for X)....than the 300 does in fact have a bigger combustion chamber volume....the difference appears to be 6.28cc's. I very well could be wrong though.. yet this is what all my research has shown so far.

I know timing and cam grind plays a part as well...300 has a bit more mild cam, and +7*BTDC difference in time. Not enough to say so? Thanks!

-Adam

[dork]

if the piston, deck height and headgasket thicknesses were the same, the 300ex would have higher compression because it has a longer stroke. if you increase an engine's displacement but keep everything else the same, the compression will increase. more swept volume to the same compressed volume. honda reduced the compression on the 300ex for reliability, i believe the piston has a larger dish.

[melloyello]

I agree. Something has to be different in there. The more cc's the more air that is pulled in.

[250X_project]

I was told the pistons are the same, but that would make since, my solving for X tells me there is more cc volume in the 300. Could the piston come just a bit short at TDC?

[exman]

the piston on the 250x is slightly taller

[250X_project]

exman- You sure about this? Have you ever seen it measured? That would make sense. Thank you.