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Old 08-23-2008, 01:29 PM
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doing a compression test

Hey yall

When I go home next weekend I'm going to be putting new spark plugs and wires on my truck and figured it would be a convenient time to do a compression test. What kind of number range should i be seeing for an engine that has 112k miles on it? I've never done this before is there any tips yall could suggest?

Engine is a 4.9L (300-I6) completely stock.
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Old 08-23-2008, 01:48 PM
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It's a fairly low compression motor, at about 8.8 to 1
dont know the specific data for yours... just one tip: watch for 1 cyl starkly lower than the others, two cyl w/ low reading side by side, etc... watch for PUMP UP...meaning a cylinder reaches good compression only after a few strokes, and dont do it wth the valve covers off
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Old 08-23-2008, 03:36 PM
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My repair manual shows 8.8:1 for a 93 F150 with a 300, as Tom said.
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Old 08-23-2008, 03:45 PM
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More important than high numbers, consistent numbers (within about 10%) show a healthy engine.
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Old 08-23-2008, 06:53 PM
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cool. Thanks yall, i'll probably post up some numbers when I do it.
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Old 08-23-2008, 07:22 PM
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sicktight722,

These motors, when in tip-top shape, should be in the 150psi range.

A motor with a little bit of wear would be down in the 130psi range.

That at least gives you an idea of what to expect.

Just make sure that the motor is hot, all spark plugs are out and the coil is disconnected. This test on a cold engine will not be 100% accurate.

Quote:
Originally Posted by mrfixit View Post
It's a fairly low compression motor, at about 8.8 to 1
dont know the specific data for yours... just one tip: watch for 1 cyl starkly lower than the others, two cyl w/ low reading side by side, etc... watch for PUMP UP...meaning a cylinder reaches good compression only after a few strokes, and dont do it wth the valve covers off
8.8:1 is the static compression ratio, and really means next to nothing in the context of a compression test. Things like volumetric efficiency and the dynamic compression ratio are much more meaningfull - unless we are talking about a motor that is 100% volumetrically efficient (not possible without forced induction, and even then the VE is inconsistant), a motor that has each cylinder completely sealed at the bottom of the stroke and that the volume compressed is the actual volume taken in on the intake stroke. This is not possible either because intake valve closure (the cylinder sealing itself) always takes place after bottom dead center, which causes some of the air/fuel to be forced back into the intake manifold by the rising piston.
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Old 08-23-2008, 07:47 PM
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Quote:
Originally Posted by eco View Post
sicktight722,

These motors, when in tip-top shape, should be in the 150psi range.

A motor with a little bit of wear would be down in the 130psi range.

That at least gives you an idea of what to expect.

Just make sure that the motor is hot, all spark plugs are out and the coil is disconnected. This test on a cold engine will not be 100% accurate.



8.8:1 is the static compression ratio, and really means next to nothing in the context of a compression test. Things like volumetric efficiency and the dynamic compression ratio are much more meaningfull - unless we are talking about a motor that is 100% volumetrically efficient (not possible without forced induction, and even then the VE is inconsistant), a motor that has each cylinder completely sealed at the bottom of the stroke and that the volume compressed is the actual volume taken in on the intake stroke. This is not possible either because intake valve closure (the cylinder sealing itself) always takes place after bottom dead center, which causes some of the air/fuel to be forced back into the intake manifold by the rising piston.

Thanks man. I was also told to pull the fuse to the injectors so they don't squirt gas while turning over the engine.
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Old 07-23-2009, 04:46 PM
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Compression test

PSI should be no more than 15 difference between cyclinders. I've seen no more than 10 psi difference on healthy engines.
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