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Discussion Starter #1
I've just had compression and leak-down tests done on my freshly rebuilt motor that now has accumulated approximately 500 miles. The builder purportedly installed 10:1 compression pistons.

While the leak-down indicated the rings were seated well, the compression numbers were 120 across the board.

Are these the results I should expect?

Thanks for any and all input.

Ken
 

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You sure you had the throttle plates wide open? Inaccurate gauge? The 9.5 pistons in my 1750 gave me 190psi readings. You should be able to tell if you have 10.1 pistons by looking in the spark plug hole. Somewhere there's a post that shows the different piston head profiles.
 

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You sure you had the throttle plates wide open? Inaccurate gauge? The 9.5 pistons in my 1750 gave me 190psi readings. You should be able to tell if you have 10.1 pistons by looking in the spark plug hole. Somewhere there's a post that shows the different piston head profiles.
Wow, 190 psi?

Now that we are talking about it, just did a new head gasket on my 87 spid , but did not check compression yet . Assuming all is well, what should I expect it to be?
 

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You sure you had the throttle plates wide open? Inaccurate gauge? The 9.5 pistons in my 1750 gave me 190psi readings. You should be able to tell if you have 10.1 pistons by looking in the spark plug hole. Somewhere there's a post that shows the different piston head profiles.
Right. But I should add that engine should be HOT, say, just after a ride. Remove coil wire, and pedal to the floor as one cranks over the engine about 3-5 turns. A cold engine will give lower readings, I bellieve.
 

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Somewhere there's a post that shows the different piston head profiles.
Look here: IAP's tech page showing Alfa 2.0L pistons .

There are a few things that can affect the readings during a compression test. Health of battery. All spark plugs removed. Throttle propped open. Accuracy of gauge. Valve timing/overlap.

What were the results of the leak down test? If the rings are seating well and the valves are not leaking then the engine should be in good health. And 500 miles may not be well broken in. There is some controversy about the best way to break in an engine.
 

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Discussion Starter #6
the leak down indicated good seating. assuming the test was performed correctly and the gauge was accurate, what would be in the range of expected compression numbers?
 

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Hi -

120 seems pretty low. You may want to do the test again. Make sure the engine is warmed up and you do a wet test by squirting a little WD-40 in each cylinder before you attach the gauge. I can't for sure claim that I have 10.4:1 pistons but the PO (former forum member) claimed that the guy she bought the car from used 10.4:1 pistons when he had the engine rebuilt. What I can tell you is that when I did a compression test before I bought my '79 my numbers were between 210 and 220 wet.

Drew
 

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A VVT engine is not going to show the same numbers as a non-VVT engine during conventional compression tests.

They will be lower because the intake cam is retarded by around 11 degrees (not to mention the cam profile differences themselves) and the only way to get it more or less comperable with a pre-VVT engine would be to have enough oil pressure to activate the VVT (not gonna happen while cranking) or in the case of the centerfuge type VVT's something over 2000-2500rpm while cranking. (again, not gonna happen)

Last I gathered from one source was that anyhting between 118 and 130 was pretty well normal for VVT Bosch engines with numbers up into the the 150-ish range being within the relm of possibility if the planets are in line and all is right with the universe.
 

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Guys, its easy to figure out the MAXIMUM Theoretical compression ratio if every thing is perfect.

Atmospheric pressure is 14.7. All compression gauges read "gauge pressure" - start off at zero. If you compress 10 atmospheres you get 147 psi minus the one the gauge doesn't read so the best you can do is a theoretcal perfect 132 psi. Now take into account that the intake valve is still open many degrees past bottom dead center based on your cam the result is some of the intake air is pushed back out the intake and you get something less than the max number.

At crancking speeds non of the wonderfull tuning effects of packing or exhaust/intake valve overlap pulling additional air in count.

Open throttle, all spark plugs out, warm engine, etc are good ways of getting consistant results. Also buy an accurate or check your gauge.

Chris
 
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