22 replies to this topic

[Sprocket]

The PSI reading is only useful to compare cylinders with each other, ie balancing or problem determination.

You can work out the actual compression ratio from the psi reading, but you also need temperature, altitude and also barometric pressure and some long winded maths...

but from your readings you're going to be taking the head off at least so it'll then be a good time to measure volumes etc...

You can only work backwards if you know what the cylinder volume is when the valves are closed as well as the pressure at this exact point. Any other way will have errors in the calculation and not worth the bother

i dont want to know the compression ratio. all i want to know is what psi or bar to expect from a good 1275 on compression test and on a worn one so i can compare to mine. i know you shouldnt have no more than about 2 bar difference between cylinders but nobody seems to know what is an acceptable pressure on a good engine.

Prssures will vary depending on the engine. For instance, a fairly standard production engine may have pressures of 140psi, where as a high performance engine could give pressures of 240psi. I would be happy with 175psi and confident that its a good cylinder seal. That 110psi is p!ss poor. I recon its a bent inlet valve

[jack_marshall]

Any time Ive ran a compression test on an engine with bent inlet valves I get 0 compression, even when the bent valve isnt noticable to look at?

Anyway, I get 175psi across all 4 cylinders in my 1275, and its healthy engine.

[Sprocket]

Any time Ive ran a compression test on an engine with bent inlet valves I get 0 compression, even when the bent valve isnt noticable to look at?

Anyway, I get 175psi across all 4 cylinders in my 1275, and its healthy engine.

I have experienced bent valves, two, on the same head, at the same time. Compression was down on these two cylinders by 120psi from the other two at 240psi. I had the valves replaced and all the seats recut and now the compression is 260psi on all cylinders even with two cracked valve seats.

So, worn and pitted valve seats are worth less than 1% of the overall compression pressure, but bent valves are worth 50%

[Cooper16]

Just done mine, 1991 rover cooper 1275cc, standard engine. 1-144 2-157 3-152 4-142. Maybe a little low for mine. Hope this helps

[racingenglishcars]

The closing angle of the inlet valve must also be considered to convert cylinder pressure at cranking speed to compression ratio

[Mini Manannán]

The closing angle of the inlet valve must also be considered to convert cylinder pressure at cranking speed to compression ratio

 

In other words 'Dynamic Compression'?  I still haven't got my head round that!

[Spider]

 

The closing angle of the inlet valve must also be considered to convert cylinder pressure at cranking speed to compression ratio

 

In other words 'Dynamic Compression'?  I still haven't got my head round that!

 

 

Yeap.

 

It's possibly a bit back to front how most people would think of it, but the Static CR will only have a small effect of the pressures seen from a compression test and the more 'sporty' the cam, the lower the pressures will be on such a test.

 

Don't get too hung up by the actual numbers from these tests, really all you're looking for is consistency across all cylinders, +/- 10 psi and even then, unless there's a huge difference, I wouldn't stress too much, particularly if the engine's done a lot of miles.

 

Also, because these tests are not done under actual running conditions, but just on the starter motor, the rings are not going to behave as they do when running, so this can and often does, give results which may appear alarming, when in fact there's nothing to be overly concerned about. A Wet test is more reliable and a Dry one IMO is a waste of time.

 

I've got a compression tester, however, I think it was about 6 or 7 years ago I had it out. They can be most helpful for diagnosing a possible blown head gasket but not a reliable indicator of engine condition.

Edited by Moke Spider, 02 February 2017 - 06:38 PM.

[Chris.Williams]

I've got a compression tester, however, I think it was about 6 or 7 years ago I had it out. They can be most helpful for diagnosing a possible blown head gasket but not a reliable indicator of engine condition.[/quote]

Very true, my last compression test was 175 across all 4 cylinders, but when I pulled it down, my rings where knackered.