Compression Figures

[SAM]

Hi All

What should the compression figures be on a stock standard engine. I have just bought a cheapie Chinese compression tester and I am getting identical figures of 105psi on both cylinders. I only screwed the thing in hand tight as I was expecting one to be well down but I am really interested to see what you guys think?

Sam

[AustHaflinger]

I have numbers for my engine but it is no longer standard so not all that relevant to you (before engine failure no 1 - 118psi, after first rebuild 165psi, after second rebuild 150psi).

As your numbers are the same on both cylinders the odds are that your engine is good and given my first modified engine was at 118psi I think your numbers are probably OK.

Jim LaGuardia (Goatwerks) will know and hopefully he will be along and give you a definite answer.

Cheers

Garry

[heinkeljb]

As a quick test. you could remove both spark plugs.

Make a suitable tool to block a spark plug hole. (Wooden dowel, rolled up cloth etc.)
Get some one to turn the engine over whilst you hold the tool covering a spark plug hole.

You will be able to tell if the force of the air that is pushed out is similar on both cylinders - that will help you believe that your compression tester is actually working.

I have 3 of them and two produce wildly differing results if they register anything at all! The joints inside are obviously not what they should be, or the diaphragm does not move at all.

The standard engine does not have very high compression, but I think your figures are a bit low - did you try again with a teaspoon of oil down the bores?
The fact they are both the same (if your kit is working correctly) would be what I was looking for. Unless you actually think you have an engine issue, I would leave well alone.

John

[SAM]

Thanks Guys

I thought one was going to be low but the fact that they are the same sounds good and I am inclined to leave it undisturbed!

Sam

[jhon]

At 105psi on both cylinders I would be inclined to leave alone, particularly if the vehicle runs reasonably well and doesn't smoke excessively. I think mine read about 115psi after I did the rings (no idea how accurate the gauge is).

When you're checking make sure that the throttle is wide open to ensure that air gets in to the cylinders, it makes a big difference to the readings!

[Goatwerks]

65-115 PSI, depending on year and compression ratio.
Available stock ratio's were, 6.7:1, 7.2:1, and 7.8:1.

[SAM]

Thanks all

Those figures are surprising and quite a range. This vehicle is a 74 and my figures are pretty close to the 115 so it sounds like it is in good shape.

I thought it sounded a bit low. I read Garry's post and he had 165 (or something like that) so I was a bit concerned.

Sam

[AustHaflinger]

I thought it sounded a bit low. I read Garry's post and he had 165 (or something like that) so I was a bit concerned.

Sam

Yes but as I said my engine was heavily modified so not all that relevant to you - 762cc, big valves with different pistons with higher crowns giving a much higher compression - piston cracked soon after. Current engine is 150psi again with all the above but a bit less compression.

Your standard 74 engine with 115psi sounds about right.

Garry