Thread: Compression Test Woes

Well, it's good to see that 120 psi isn't the end of the world..I was just looking up used '11 Mustang GT's and accompanying turbo kits

I'll track the progress of the compression over the next month. Hope I can make it to the convention as I got a TurboFX Bellmouth housing coming soon I want to use. 900 mile road trip should break some more stuff in along the way.

-G

I ran 20psi daily on pump on my built motor but that was a good flowing large .82 turbine housing T3 60 trim turbo. Never had a problem with running that much boost on 91 octane.

Patm X CR = Max Stat. Cyl. Pressure

14.7psi X 8.5=125psi. I don't see any problem.

Walker

Edit: fyi cams should have no substantial impact on cr in this case someone please explain to me why they would; at cranking speed cylinder filling to atmospheric should have more then ample time. Overlap occurs at intake valve open, but that is of no significance, once the exhaust valve closes under cranking the cyl will be at atmo pressure very quickly, then closes slightly after tdc. Then CR will take affect. Simple math. Cranking is well under 400rpm...I see overlap lowering compression less then %2 which accounts for the max measures cyl pressure around 120psi so this is perfect. I don't know the cam spec. so I do not know the deg. at which the intake valve closes, but I estimate it will not affect things too much. With the head being machined, 120psi sounds right.

you will notice a drop in compression with cams period. The lift/duration/overlap all have an effect. Same reason why your vac is always lower with s4's compared to stock. Stock cams its common to see 20-22 in-hg of vac wheras with s4's you will see anywhere between 16-18 in-hg of vac. There is obviously less of a vac that would get cylinder pressure up.

Definitely remember to hold the throttle Wide Open when doing the test if you wernt already. That will make a big difference. Ive seen 30-40psi difference just doing it with throttle closed vs open.

Wrong.

Lift and duration will have no affect on compression. It is the closing of the intake* valve that will. Think, how can a cylinder fill at 8000rpm when it cannot fill at 400rpm? The only affect on CR will be when the valve closes. Any good cam will close slightly after bdc to utilize inertial affects of the intake velocity which will result in packing of the cyl. At cranking rpms this will effectively lower you CR. This is why you see lower peak cylinder pressure at cranking rpms with an aftermarket cam. At any rate, the pressure levels look fine, no need to fret.

Walker

Pumping loss.

Walker

Numbers are solid for a forged piston 8.5 comp motor and s4 cams. Don't even worry about it.

my stock u13 det only has 120 across the board....well 90 in #3 but thats a HG.....even with the low # in #3 my car pulls like a raped ape.....well before 3rd gear let go.....damn...my car does not like the number 3! LOL

Did a compression test again recently. Engine has about 2000 more miles on it than last time.

The first compression test I really had 130-125-120-120 (even though I said 120 across the board)

This time I get 130-125-115-120

Thoughts? Something going on w/cylinder #3 maybe. Was thinking about re-torquing head studs(even though my coolant level has been rock solid). Not sure what else it could be besides headgasket or rings. Valve seals supposedly don't cause compression issues.

-G