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400exBRAAP
06-20-2008, 03:16 PM
what psi should i have on my 400? its gotta 10:1 piston in it... its got good run time on the piston so it aint BRAND NEW. i checked it, but was dumbfounded by the number, so lets hear what everyone thinks i should have.
drew416ex
06-20-2008, 03:40 PM
Well my 11:1 had around 180-190 psi if I remember right. And stock is between 100-120 so im guessing somewhere around 150psi. I dunno.
400exBRAAP
06-20-2008, 03:49 PM
200
drew416ex
06-20-2008, 04:01 PM
Thats kinda crazy. I had a 12.5:1 that had 210-220.
drew416ex
06-20-2008, 04:01 PM
Well I just noticed you have a stage 1 cam. I had a stage 2 so that will lower my compression a little.
400exBRAAP
06-20-2008, 04:06 PM
will my cam actually make a difference? i wasnt sure about that, i know the fact that it aint gotta decompression system makes a differnce, but i wasnt sure that a cam would.

why would it be higher with a stg. 1 than with a stg. 2? cuz the valves are stayin open longer on a stg. 2, lettin more of the air out?
drew416ex
06-20-2008, 04:58 PM
THe only thing the decompression mech. did on it was to help it start easier. But yeah, a cam with a longer duration has less cylinder pressure. Thats why whenever someone on here wants a bigger cam, I always recommend getting a higher compression piston.
GPracer2500
06-20-2008, 05:34 PM
Not sure if this helps, but......I had a 416 10.8:1 JE with a thin(ish) base gasket (didn't actually measure it) and a HC stg 2 (old version). I pumped ~175 at 1300ft.

So if my numbers and drew's numbers are any kind of yardstick, your 200psi seems a little high for a 10:1 piston. A stg 1 vs. stg 2 should translate into the equivalent of around 0.3 points of static compression on a 400EX. So I doubt that factor is completely bridging the gap.

Assuming your gauge is accurate, there's probably a handful of factors that are conspiring to produce the measured cranking compression. There are plenty of things that influence the measurement that can't be chalked up to the piston itself.
400exBRAAP
06-20-2008, 06:08 PM
idk, but my gauge is accurate, cuz we've compared the numbers on a snowmobile compression to the tester at our powersports shop acrossed the way, and they were both the same number.

but im suprised at how high the number was. i did it a couple times too. WOT too. also, it has a regular thickness base gasket, nothing special, nad a cometic head gasket. not sure if that matters but...
and the elevation around these parts is 1600ft.

and, i just ran a top speed run, and the gps said 72, so im perfectly happy with it