compression test

[clive f]

can anyone tell me what readings to expect from a compression test on a 4.5 AJP please?

[ridds]

Don't know specific pressures but as long as they're even and above 175 PSI they should be good. A couple of high 10:1 CR cars I know of run 225 to 250 PSI but this will be dependant on the cam timing, duration and lift.

[clive f]

thanks.

[Boosted LS1]

And they should all be similar readings. Any more then 14 psi difference means there's a problem.

[ro_butler]

I had mine done as part of the inspection prior to purchase, they all came out at 250+ (I am guessing this is psi??).
However my car is allegedly a Red Rose.

[Pasco]

clive f said:

can anyone tell me what readings to expect from a compression test on a 4.5 AJP please?

Have you done this test now mate ? how was it buddy ?

[clive f]

Pasco said:

clive f said:

can anyone tell me what readings to expect from a compression test on a 4.5 AJP please?

Have you done this test now mate ? how was it buddy ?

didn`t get time this weekend, playing with a mutual friends car, will look to get it done next week hopefully.

[Duke Thrust]

Yep, let us know Clive, it was compression that caused the problems originally.

[clive f]

been busy this evening, the engine is sat on some wood on my workshop floor, connected up the oil pipes, topped up the oil and connected a battery to the starter motor, and took some compression readings.
the engine hasn`t been fired up for some 4 weeks or so.

I let the engine turn over for about 6 seconds per test.

cyl 1.....200psi cyl 2.....200psi
cyl 3.....205 cyl 4.....195
cyl 5.....200 cyl 6.....190
cyl 7.....200 cyl 8 ....190

not sure if I should have left it turning over any longer when testing, but didn`t want to flatten the battery out of my pick up, or I wouldn`t be a happy bunny at 6am tommorow morning.

so should I be satisfied with these results?

[ridds]

Which did you start and what was the order?

They look fine tbh.

[clive f]

order was 1,3,5,7. then 2,4,6,8.

Edited by clive f on Wednesday 22 October 21:42

[clive f]

bump: anyone else happy with these, should note that this test is from my engine, not the one in mikes car which is under a different topic, just to save any confusion, or possibly add to it

[rev-erend]

Clive - I'd be happy.

It's not the CR.

Next.

[FarmyardPants]

Numpty question having never done a compression test, but 200 seems quite high, how could you get near 250? Assuming it reads zero when not connected, and a swept area the same as the head volume (CR=2) gives a reading of 1atm (14.7psi), swept area of 2 head volume units (CR=3) gives 2atms, etc, then 200 psi equates to a CR of 12.6, assuming there's no dynamic effect at cranking speed.

Seems quite high, what is the std CR?
Is there any inertia in the measuring equipment as the pressure spikes?

[clive f]

sorry mate, you`ve just gone straight over my head with that lot

very technical for a "weary banker".

[GreenV8S]

FarmyardPants said:

Numpty question having never done a compression test, but 200 seems quite high, how could you get near 250? Assuming it reads zero when not connected, and a swept area the same as the head volume (CR=2) gives a reading of 1atm (14.7psi), swept area of 2 head volume units (CR=3) gives 2atms, etc, then 200 psi equates to a CR of 12.6, assuming there's no dynamic effect at cranking speed.

Seems quite high, what is the std CR?
Is there any inertia in the measuring equipment as the pressure spikes?

What you're describing is correct for isothermal compression where the gases are held at constant temperature. In a compression test what you're seeing is closer to adiabatic compression (no heat energy in or out) so the gas will heat up as it is compressed. This means the pressure ratio is more than the volume ratio.

[TJW]

GreenV8S said:

FarmyardPants said:

Numpty question having never done a compression test, but 200 seems quite high, how could you get near 250? Assuming it reads zero when not connected, and a swept area the same as the head volume (CR=2) gives a reading of 1atm (14.7psi), swept area of 2 head volume units (CR=3) gives 2atms, etc, then 200 psi equates to a CR of 12.6, assuming there's no dynamic effect at cranking speed.

Seems quite high, what is the std CR?
Is there any inertia in the measuring equipment as the pressure spikes?

What you're describing is correct for isothermal compression where the gases are held at constant temperature. In a compression test what you're seeing is closer to adiabatic compression (no heat energy in or out) so the gas will heat up as it is compressed. This means the pressure ratio is more than the volume ratio.

Don't know what a standard cerb is like for compression to be honest, but I seem to recall mine being around/over 250, but its not standard.

[HarryW]

Whoops last post was mine not Tim's

[The AJP Griff]

clive f said:

bump: anyone else happy with these............

I'd be really by happy,just right now!

[FarmyardPants]

GreenV8S said:

FarmyardPants said:

Numpty question having never done a compression test, but 200 seems quite high, how could you get near 250? Assuming it reads zero when not connected, and a swept area the same as the head volume (CR=2) gives a reading of 1atm (14.7psi), swept area of 2 head volume units (CR=3) gives 2atms, etc, then 200 psi equates to a CR of 12.6, assuming there's no dynamic effect at cranking speed.

Seems quite high, what is the std CR?
Is there any inertia in the measuring equipment as the pressure spikes?

What you're describing is correct for isothermal compression where the gases are held at constant temperature. In a compression test what you're seeing is closer to adiabatic compression (no heat energy in or out) so the gas will heat up as it is compressed. This means the pressure ratio is more than the volume ratio.

Ah yes, good point. That explains it.