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I supported Toymods
Location:
Perth
Registered:
May 2002
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 What is a compression test?
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Wed, 06 November 2002 11:11
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Stupid question maybe, but one I don't know the answer to  What exactly is involved in a compression test?
I'll be getting an engine in a little while, but the car won't be ready for it to go straight into. Therefore, going to be doing a compression test ASAP to check it all out while there is still warranty on the thing...
What does a mechanic charge to do one? Is it cool that the engine isn't wired up and stuff? Can anyone tell me any more on it?
Thanks,
Nathan
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Location:
Wollongong
Registered:
May 2002
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Re: What is a compression test?
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Wed, 06 November 2002 11:26
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Compression tests test the compression (whoa!) of each cylinder.
Each spark plug is removed, one at a time, and a compression testing device held in the little hole (kinda like a tyre gauge) and the engine is cranked for a few seconds. ie. the engine needs to be in general working order to do the test.
This will give a reading in PSI/KPa usually.
For a turbo engine, you are looking around the 180 mark I think (can't remember PSI or KPa), but don't quote me on that.
Best way to check if it's decent is if all cylinders come up within a few PSI/KPa of each other.
If one or two (hopefully not more) come up with a lesser value, the rings are probably shot, or valves are stuffed in some way. Leaking air/fuel mix will lead to less compression, and a no-no in a properly tuned engine.
(Prepares flame-proof suit for anything I might have gotten wrong, but it is the general gist of things)
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I supported Toymods
Location:
Perth
Registered:
May 2002
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Re: What is a compression test?
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Wed, 06 November 2002 11:30
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So I cant do a compression test on an engine and gearbox package? Or I can, but I have to connect it to fuel and power first?
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I supported Toymods
Location:
I renounced punctuation
Registered:
May 2002
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Re: What is a compression test?
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Wed, 06 November 2002 11:32
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A comp check is done to check the integrity of the cylinder seal....in other words to look for wear.
Spark plugs are removed and a comp guage is placed/screwed in the spark plug hole. The engine is cranked over usually with WOT and after a prescribed period of time (usually 10 or so revolutions), a plateau value is reached. This should fall within certain values as dictated by the manufacturer. This process is repeated for each cylinder.
The values should all be within a specified range....if one or more is well outside this range, then the engine should be opened and the cause determined, or not bought/returned. Wet compression can also be checked on a dodgy cylinder....basically oil is squirted into the cylinder (it temporarily aids in sealing the suspect cylinder) and if there is a significant improvement, then it suggests ring + bore wear. You can also do what is known as a leak down test. I won't go into it but it can give a better idea of where the motor is losing compression.
This is by no means definitive, and I suggest you read up in greater detail. No idea what a mechanic charges to do this.
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I supported Toymods
Location:
I renounced punctuation
Registered:
May 2002
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I supported Toymods
Location:
Perth
Registered:
May 2002
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Re: What is a compression test?
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Thu, 07 November 2002 01:21
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Thanks dude, that is exactly what I wanted to hear
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