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Compression Test Numbers Always Low

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Compression Test Numbers Always Low

 
Old 06-04-2017, 07:09 PM
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Default Compression Test Numbers Always Low

Every year I test my compression in the car and every year it is about the same. The numbers are way off from others but I have never figured it out. I have (2) compression testers and I always get 145 psi plus or minus a few in each cylinder. Running a cylinder leakage test all are around 4-5% every year. I check the valves a few times a season..... Never any issues and the car runs has the same numbers down the straights as the other S2K's. Anyone have a guess as to why they would be so low?

Details:
2001 w/ 55K Miles
Test completed with the car hot
Innova and OTC Compression Tester

​​​​​​​The only thing I can think of is that I use a motor cycle battery that can't turn the piston fast enough but I am not sure.
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Old 06-04-2017, 07:19 PM
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You mean your car doesn't blow a perfect 240 PSI across all cylinders, with the mechanic/tester blind-folded, piss drunk, and as high as a kite while conducting the test?

-Pull all plugs out
-Fuel pump fuse removed
-WOT
-Hot engine

No really. Consistency across cylinders is key. You're overthinking it. How far are you off sea-level? Your car is fine. Why are you testing it ever year?

If it makes you feel better, mine consistently blows 180s. Next year, buy a different tester.
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Old 06-04-2017, 08:24 PM
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After looking at your signature I have to ask if your car is stock? I see some FI links in there. One reason the compression would not be as high as what most people get on here is because the compression ratio might be lower if the engine was rebuilt (ie. for boost).
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Old 06-05-2017, 06:38 AM
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Leakdown numbers are far more important than comp numbers. Not holding the throttle wide open, weak battery, or a faulty comp gauge can lead to low comp readings.
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Old 06-05-2017, 09:24 AM
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Or anything that would cause it not to crank at full speed. Which are tje things mentioned already. Plugs all out. WOT. Engine hot. Fuel pressure disabled. Etc.

But also super important is fully charged and full capacity battery. If you are asking if your motorcycle battery might be the issue here, yes it could be.

Got another battery you could temporarily swap in? Doesn't have to fit well, just needs a good electrical connection to cables.

If not, even just good, heavy duty jumper cables with a good, large capacity battery to help out your cranking would be good to try.
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Old 06-05-2017, 03:57 PM
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Originally Posted by Soviet View Post
After looking at your signature I have to ask if your car is stock? I see some FI links in there. One reason the compression would not be as high as what most people get on here is because the compression ratio might be lower if the engine was rebuilt (ie. for boost).
I have had (5) S2K's over the years, this one in particular is N/A.


Thank you all for the input, I did not have much concern but it was always strange to me but the car runs well.
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Old 02-19-2019, 08:07 PM
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Well .... Alex- Awhit has a pretty nice built head from Endyne/Urge he’d ship you for 2,300.... I’m also sure someone would pick up the ptuning kit off of ya. I’m sure neither of these are the answer you really wanted though.


good luck!
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Old 02-20-2019, 02:47 AM
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Do not pull the full pump fuse for a compression test. Fuel will still spray into the cylinders and skew the readings. It is also unhealthy for the engine.

Disconnect the injectors instead.
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Old 02-20-2019, 03:09 AM
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Originally Posted by Billman250 View Post
Do not pull the full pump fuse for a compression test. Fuel will still spray into the cylinders and skew the readings. It is also unhealthy for the engine.

Disconnect the injectors instead.
Fuel washing the oil?
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Old 02-20-2019, 03:31 AM
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Correct.
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