390SE compressions

[ed_crouch]

Did my compressions a while back, and got 190PSI +/- about 5 across all 8. Done with the throttle butterfly closed.

NCK motor.

How does this compare with other peoples? I thought it seemed pretty good.

Ed.

[rev-erend]

Ed - I'm pretty sure mine were around 185~195 at WOT.

Although it did not seem to make much difference with throttle closed.

I would say yours are spot on.

[ed_crouch]

Geezer.

Thought they sounded pretty good.

Ed.

[GRIFFONMARK]

Mine (350i) were 152,165,152,180,152,162,152,172 going from no.1 to no.8. The highest ones were the RH bank where the headgasket had clearly leaked on all cylinders. I think the engine sucks in a small amount of oil which increases compression pressure. So you've probably got badly blown head gaskets (TIC)

Seriously though, how does this compression thing work? Isn't air at 14.5psi, so if you increase 10 fold (10:1 CR) you get 145, plus ambient a total of 159.5psi? How do you get 190psi then?

[GreenV8S]

GRIFFONMARK said:

Seriously though, how does this compression thing work? Isn't air at 14.5psi, so if you increase 10 fold (10:1 CR) you get 145, plus ambient a total of 159.5psi? How do you get 190psi then?

When you compress air, it heats up which causes an additional pressure increase.

[leorest]

GreenV8S said:

GRIFFONMARK said:

Seriously though, how does this compression thing work? Isn't air at 14.5psi, so if you increase 10 fold (10:1 CR) you get 145, plus ambient a total of 159.5psi? How do you get 190psi then?

When you compress air, it heats up which causes an additional pressure increase.

I tried to work this out a while ago but gave up
IIRC mine (350i) were iro 160psi. Probably searchable on here if you're interested

[ed_crouch]

Search google for adiabatic heating.

Its not QUITE fully adiabatic as you lose a tiny smidge of temperature to the cylinder walls, but for all but the slowest compression stroke, its a good enough model.

I reckoned that the air in my cylinders gains 60ish celsius upon compression.

Ed.