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Engine compression

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Hello all,

 

I've done a compression test on my 95 2.5L and I found these numbers:

 

1 cylinder has 100 psi

5 cyliners have 115 psi

 

I feel that those numbers are low and making numbers, the compression would be 7:1 and 8:1 for my engine.

 

Any of you have at hand the right compression in "psi" for this engine?

 

Thank you

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I don't know what the correct number should be.

 

But - when all the numbers are so close that is a good sign, so I'd think that you are OK.  Do you have any other evidence of low compression?

 

Best Regards,

 

Al Seim

SVT Contour 2000

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I notice very low acceleration and very low milleage, it has new head gaskets, plugs and plug wires.

 

On the other hand, in the engine specifications I found that the engine provides a compression of 9.7:1, if my maths are rigth this equals to 181.3 psi, at least I would belive this will be at sea level, it should be less at 1.4 miles over sea level but I don't think that justifies 65 psi less.

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Normally all cylinders are supposed to be within 10% of eachother. Sounds like you have a burned valve or internal issue. Most cylinders should be 120+ PSI.

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Somehow I misread 100 as 110, so yes there is some difference there.

 

BUT - I'm not convinced that the other numbers are low.  Since the intake valve stays open during the first part of the compression stroke, you will never see as much pressure during slow speed cranking as you'd expect from the compression ratio.  Taking Buckeye's number of 120+ at lowish altitude, 115 at your altitude sounds very reasonable.  Altitude calculator online says that your air density is only 77% of sea level which should cause a 23% reduction in the compression reading vs sea level.

 

If you can find a "leakdown tester" that may give some indication, in addition to numeric values it is helpful to just listen to where the air is going on TDC of the low cylinder vs TDC of a good one.  Crankcase (oil filler cap) = rings, exhaust pipe = exhaust valve, throttle body = intake valve.

 

If the engine has not been run recently, all these things are iffy as a tiny bit of carbon or corrosion can cause a biggish leak.

Quote

 

 

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