Compression Test Results: A guide for results/testing

Just wondered how healthy everybody’s motor’s are, Mine are as follows

Cylinder 1. 150psi
Cylinder 2. 150psi
Cylinder 3. 150psi
Cylinder 4. 150psi
Cylinder 5. 150psi
Cylinder 6. 149.5psi

When I last did mine they were:

I did these the day I got the car from the docks so I am due another check.
I was obviously happy with the results first time around. :slight_smile:
I wonder what the next time will bring :roll: :lol:

Spot on results Andy.
Don’t get much sweeter than that.

Mine are 1, 150
2, 149
3, 150
4, 150
5, 150
6, 150
7, 150
8, 150
9, 151.128747

Checked mine a few months back and they were all 145psi +/- 1psi


[quote]Just wondered how healthy everybody’s motor’s are, Mine are as follows

Cylinder 1. 150psi
Cylinder 2. 150psi
Cylinder 3. 150psi
Cylinder 4. 150psi
Cylinder 5. 150psi
Cylinder 6. 149.5psi[/quote]

What sort of gauge of you using to be able to clearly see .5 psi.

As i have just bought a Gunson’s HI.GAUGE plus from Halfords and i’m lucky to be able to read 1 PSI either way.

I have just tested the front set and got the following results.

From left to right
149 +/- 1psi
143 +/- 1psi
149 +/- 1psi

When doing these tests please remember to get your engine hot as i done my first test when warm and got 142 Psi all the way across.

Done the second test when very hot and got the above results.


When I test my compression;

  1. I remove all spark plugs,

  2. I disconnect the HT leads,

  3. I disconnect the fuel injector wiring,

  4. I turn the engine on the key for about 3 turns (you can hear each revolution),

  5. I read the gauge to the nearest mark (same gauge as Dave),

  6. I vent the gauge, so it’s reading zero before refitting,

  7. I repeat for all 6 cylinders

Turbo engine, 115 to 155, no more than 15 PSI difference between the best and worst cylinders.

Putting the gauge on your car and turning the engine over and over will give you false results. Get someone to turn the ignition whilst you watch the gauge. You might have a bad cylinder, which could only go to 100psi on the first turn and increase 5-10 psi on each susequent revolution of the engine. After 6 or 7 turns, it could be reading the same as all the other cylinders, which went straight to 145 psi, and didn’t increase on any subsequest revolutions.

Of course, compression testing is only a rough guide to what is happening inside your engine. You can have broken ring-lands and still get good compression!



Number 10 = 10 Bar which is not 150 PSI is more like 145 PSI.

My question was how the hell can you tell that you have 149**.5** PSI.

Dave :wink:

If you check engine compression pressures with the throttle shut, a big butterfly valve blocks up the inlet… the figures will be low …like trying to breathe with a sock stuffed in your mouth…
Any roadgoing petrol engine … around 150 psi is good … it should get there within three strokes … if you need to keep leaning on the starter motor, waiting for 150 to appear, you have a valve or piston problem .
If the battery is good enough, you can, eventually, spin it over fast enough to make up for cylinder leakage …
Why am I telling you this ? … you know it …
Balance ? Within 5% is OK… otherwise recheck …
Where’s it leak ? Two or three squirts of oil down the plug holes to temorarily seal the piston rings and recheck compressions … now OK … it’s a piston/ring problem … no change … it’s a valve/head gasket problem.



You don’t need to hold the throttle open if you’re doing all six, as you’ll have the plenum removed! Just make sure you don’t leave tools or parts lying around the engine bay or you’ll be crying!

How do you do your tests Jim, with all the plugs removed, or with only the one plug removed at a time?


My readings were 150 right across all six when they rebuilt my engine.


1…130 PSI
2…150 PSI
3…130 PSI
4…145 PSI
5…150 PSI
6…150 PSI

I’m going to keep pushing her until she pops though. So the variation between cylinders isn’t bothering me too much at this point. :wink: After all, 2 weeks ago, she produced the ET you see in my sig.

My last compression results:

1: 0 PSI
2: 0 PSI
3: 0 PSI
4: 0 PSI
5: 0 PSI
6: 0 PSI

Nice and even across the board… :cry:

But this is what happens when you bend 12 inlet valves!


Why do people get 150psi when the compression ratio is 8.1:1 ? Surely it should be more like 120psi? 150psi would indicate a compression of 10:1 which is a N/A.

Engine now rebuilt but not run in:

155 PSI accross the board with two turns of the engine.