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

[400eXr1d3rZ]

Gaskets may be thinner?

[250X_project]

Thinner gaskets would rise compression I think, but I am pretty sure they are the same anyway. Thanks though

[NTPRacing#19]

mild cams keep valves open longer correct? if so then the time to build compression is shorter so creating less compression.

its a trade off i believe, mild cam lets more exhuast to be pushed out and and more fuel/air sucked in for say more power but less compression results due to valves being open longer :confused:

[250X_project]

That would make sense, but doesn't the mild cam have less duration and close the valves sooner? Can anyone clear this up for sure? Thanks though, NTPRacing#19, I appreciate it!

[GPracer2500]

If the head and piston are identical then the only thing that leaves is differences in the crank and rod. The 300EX version of that engine must not push the piston quite as far up into the bore leaving a different deck height measurement than the 250X. The shallower the piston sits in the bore the less compression (all else being equal).

Cam specs come into play for dynamic compression calculations--not the static compression figures you're talking about.

[250X_project]

I think that must be it, it must not come flush with the deck Thanks

[wilkin250r]

I don't even think it's that complex.

I don't think the 250X and the 300EX use the exact same piston. Just because the aftermarket pistons are the same, doesn't mean the stock ones are. I believe the stock piston for the 300EX is lower compression intentionally.

[250X_project]

Okay, Honda must have given the 300 a lower compression piston than the 250. Everything I have checked was aftermarket...though Wiseco lists the two separately the 300 being lower compression. Thank you!