AJP8 Typical Compression PSI

[ukkid35]

Whenever I've done a compression test in the past I've always got 220psi or more, but this time I'm getting 270psi

This is the method I have used

• Warm engine
• All plugs out
• Battery on charge
• Throttle fully open
• Ignition & Fuel pump disabled
• Spin engine for X revolutions

When X is 4 - 210psi
When X is 5 - 240psi
When X is 6 - 270psi
When X is 10 or more 272psi

Engine is running fine, just slight hesitation on accelerating, so I thought I pull the plugs to check

While I was at it I did a compression test

Figures are very consistent across all cylinders, heads have fairly new valves, springs, seats, but piston rings are original

I am running the cam timing with level lifters on TDC

I have had reputable engine builders on PH tell me that I'm smoking crack to report figures as high as 240psi in the past

However the gauge I'm using reports figures that are exactly as expected on other cars I've used it with

What compression figures are typical?

[spitfire4v8]

I usually see around 220 on a std engine, 240 on a red rose ..

[ukkid35]

spitfire4v8 said:

I usually see around 220 on a std engine, 240 on a red rose ..

My heads are missing 0.5 - 0.7mm, so perhaps not too dissimilar to a red rose

[Imran999]

Here’s what I got:
(Testing procedure as per your list, except that the engine was cold)
1 - 245
2 - 250
3 - 250
4 - 255
5 - 265
6 - 243
7 - 270
8 - 240

Of-course, my gauge was a non-calibrated, cheap product, but the results were consistent when I went back and re-measured.
Engine had about 4500 miles on it and had recently had a full 12k service (tappets).

[ukkid35]

Thanks both

Should I be spinning the engine until it maxes out, or is there a recommended number of revs?

[notaping]

This is really interesting. My 4.2 block is currently away getting upgraded to 4.5, but typically the figures I was getting before were in the range 175 - 185 psi. On rebuild should I be looking for something in the 220 range.

[Jhonno]

ukkid35 said:

Thanks both

Should I be spinning the engine until it maxes out, or is there a recommended number of revs?

Mine went off a 300psi gauge..

I turned it over about 8 rotations.

[Imran999]

I did mine back in 2016, but if I recall correctly, I ended up cranking the engine for about 5-8 seconds.
I had the gauge mounted such that I could see from the drivers seat.
While cranking, the needle jumps up and continues to climb with each rotation, until you can see that it isn’t really climbing much more - I was told that anything over 200psi meant there were no issues, so I basically stopped cranking when I saw the needle pass that point.

[mrniceguy351]

I got 225psi across the board using the same method. Over 100000miles on the engine!

• Edit* double checked and it was 225 each.

Edited by mrniceguy351 on Saturday 17th April 17:17

[ukkid35]

mrniceguy351 said:

I got 250psi across the board using the same method. Over 100000miles on the engine!

That is impressive

[LLantrisant]

compression test is quite useless in terms of max. values.

depends on battery-charge, type of starter motor, throttle full open or not (usually it should be open) and so on....

important is that all cylinders are equal. and thats the ONLY thing you can use as reference.....not the max. values.