Compression Ratios and Bore Sizes - The Truth

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jerrygomez2002 · #1 Post by **jerrygomez2002** » Fri Jun 11, 2004 5:43 am

According to some reading and using this utility using 40cc as the "Head Chamber Volume" and FLAT piston:

http://home.quixnet.net/~cosmic/EngineCalc.htm

I came to the following conclusions:

OEM
95mm bore x 61mm stroke = 432cc @ 11.8 compression ratio

450 Kit - "so called"
96mm bore x 61mm stroke = 442cc @ 12.05 compression ratio

450 kit, not legal
97mm bore x 61mm stroke = 451cc @ 12.28 compression ratio

460 kit monster
98mm bore x 61mm stroke = 460cc @ true 12.5 compression ratio

This makes sense to you guys?

cannondale27 · #2 Post by **cannondale27** » Fri Jun 11, 2004 11:57 am

Makes sense but the only flat top piston came in the FX400(stockWiseco)all the 450 pistons I have seen are dished like the stock italian piston.So ratios are actually lower.

Happyboy · #3 Post by **Happyboy** » Fri Jun 11, 2004 6:45 pm

Is the 450 kit really illegal? I thought it brought the dale up to 449cc....

CdaleRacer0128 · #4 Post by **CdaleRacer0128** » Fri Jun 11, 2004 6:51 pm

wade's kits HAD more compression than that, maybe another issue on why they didn't run:(

440EX4ME · #5 Post by **440EX4ME** » Fri Jun 11, 2004 7:40 pm

Dont know the actual particulars for each of those examples but I do know a little change in any of the info punched into a comp calculator will make a noticable difference in the resulting comp ratio.

If its just a little off in the deck height, piston dome, head volume or the gaskets are a couple thou thinner when compressed it can throw things off.

One thing is that you guys should be happy since it doesnt seem wiseco screwed up with the c-dale pistons like they did on the EX cause some of the big bore 10.8:1 kits were under 9:1 actual.

If you cant or dont want to actually cc your engine to get the actua comp those calculators are great!!