Compression test values too high - 1991 Toyota Tercel

These values are absolutely OK for motor in good condition. It is caused by velocity of air which is a little compressed even in bottom dead center. My english is a little poor so somebody may explain it better.

J. Lejsek

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Pressures sound about right - correct range is probably around 80 to 215 PSI. More important is the difference between cylinders, which should not exceed 15 PSI.

You mean 180? I think also the difference is not so much PSI, but more like

I'm not sure what this engine specs are, it does have a specific compression rating, you will have to look this up. If it is high than you most likely have some carbon build up. You can remove carbon with water injection or a product called Seafoam which works really well. Being that the car is 15 years old that's probably the cause.

If you use the Seafoam just feed it into the throttle body with a spray bottle, or let it suck it out of the bottom half of a styrofoam cup using a vacuum line to the TB. This will produce a LOT of white smoke, so beware. Some people also feed the Seafoam into the cylinder, install the plugs and crank the engine a few times, let it sit for 30 minutes and start the car.

I just rebuilt the engine and removed the carbon buildup in the process. The compression ratio is supposed to be 9:1 and the Chiltons manual says a brand new engine should get about 140 -150psi.. so yeah, it's too high.

Interesting theory, that hadn't occrured to me. Though I don't think that the air could be compressed that much in bottom dead center, but your theory sounds the best so far... Except that I've done compression tests on other cars and they always come out normal (100 -

What exactly did you 'rebuild'? As far as somebody suggesting velocity of the air, I highly doubt that has anything to do with it.

I didn't rebuild the whole engine, but I did completely tear apart the head and replaced all gaskets and seals (head gasket, valve stem oil seals, intake gasket, exhaust gasket, ...) While I had the head off I removed the carbon on the head and on top of each piston.

Are you sure your calculations for the ratio are correct? Does it seem to have a lot more power than before the rebuild? What octane gas are you running? If you use the lowest available does it ping a lot?

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I don't see why they wouldn't be correct, unless "compression ratio" doesn't really mean what I think it means. It runs a tad better, but not enough to justify the compression I'm seeing, 13:1 is akin to putting a turbo on. I'm using 87 regular. No, it sounds/runs beautifully.

Oops, typo - I meant to say 180. The problem with using a percentage difference between the highest and lowest rating is that the allowable difference varies with compression. 10% is a fair rule of thumb, 5 to 10 PSI difference is acceptable, 15 PSI difference is at the high end of aceptable, whether it is a high compresison engine or not.

Since the OP replaced the head gasket, I would hope that whoever did the work would not put it back together with a heavy crbon buildup on the pistons or valves.

I only asked because I am ignorant of how to get the compression ratio from the PSI produced in a cylinder.

Or using the wrong gasket (Can you actually get a change of 4 or 5 to 1 from a gasket??!)

Then as someone else suggested: Just drive it!

Seriously, if you had 13:1 ratio it should ping like hell on 87 octane (Does a 91 Tercel have a knock sensor?).

In any event, good luck with the rebuilt engine.

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You are doing something wrong, but the good news is you are doing the same thing worng repeatedly and the test results are that all cylinders are within as narrow range, and this is good.

There is just no way you are running 13:1 I think your calculations are off. At 13:1 even on the highest octane on the stock ECU you would be in trouble.

Solved.

Hum, I don't know why I didn't think of this before... I guess it's been a while since I took a thermodynamics or physics class. The gas-compression in a cylinder causes the gas to heat up and thus increases it's pressure beyond the 8.9:1 volume ratio. infact, if the process is totally adiabatic (no heat in the air is lost to the cylinder walls), the pressure ratio should be about 8.9 ^ 1.4 = 21.3 times atmospheric pressure which gives about 313 psi. Under cranking speeds, the process is pretty far from adiabatic, which is why the pressure is lower...

I guess that solves the compression problem 200psi is fine, just me over simplifying things.

Thanks everybody!