compression ???'s

[headache]

Hey guys i'm just wondering what is the stock compression PSI for a 400ex?? My quad is supposed to have a 11 to 1 wiseco piston and a stage 2 hotcam. So what kind of compression should my motor have?? I'm going to check it as soon as i find a damn socket thin enough to get the sparkplug out... thanks for the help.

[redexrider]

with regards to the socket, any automotive place should cary an 18mm spark plug socket. It fits in there, a little tight, but it works. Bought mine at Sears, but I have seen them at autozone.

[headache]

thanks. i know my craftsman socket is way too thick to go down in there, and i don't want to grind on it...

[GPracer2500]

IMO, the spark plug wrench in the stock tool kit works best, if you have it.

OEM service manual specifies 100-128psi. That may sound low but it's because the stock cam's auto-decompression mechanism keeps one exhaust valve open a little at cranking speeds. If not for the auto-decomp, it would be around 160psi (stock cam, stock 9.1:1 piston, at ~1000ft elevation, in perfect condition).

Since a HC cam almost certainly does not have an auto-decomp installed on it, that won't come into play for your engine. With a HC Stg 2 and an 11:1 static compression piston, my math gets you almost 190psi at 1000ft elevation. That's just an upper level estimate though--variations in gaskets, ring sealing, valve sealing, elevation, etc. will influence the numbers (mostly downward).

[headache]

thanks man. i was thinking probably 170 or so. My ported and milled blaster engine is 160-170 and i run race fuel in it. I was also guess about 120-130 for stock.

[Daver]

GP2500 gotta be close cause my brother`s stage 2 and 426 10:1 reads 170psi warm WOT , but my 426 stage 2 11:1 with thinner head & base gasket reads 250lbs warm WOT