Compression test

[Jackson1320]

Ok I keep having people tell me that there coyote tests at 230-250psi. I have never seen one test that high. So what are you guys getting when you do a compression test on your coyote. Gen1 gen2 or gen3

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[Adamone92]

My gen 3 had all 8 cylinders between 240 and 250psi

 

[Jackson1320]

My gen 3 had all 8 cylinders between 240 and 250psi

I have had gen3 test in the high 220s but never 240 or 250.
That’s a healthy engine

 

[Adamone92]

I have had gen3 test in the high 220s but never 240 or 250.
That’s a healthy engine

To be fair..it has around 5k miles on it.

 

[Jackson1320]

To be fair..it has around 5k miles on it.

Built gen1 or gen2 coyotes don’t test that high. Take care of it

 

[sloride]

Gen 2

 

[cbrtrx]

I wouldnt worry much at the number but at the difference between cylinders. I've checked many gen 1s and gen 2s, all had low compression compared to these 200 plus numbers posted here. All of them performed flawlessly and made good power. I just suspect that not all the gauges are extremely accurate. Also testing procedures can have many variations.

 

[96gt4.6]

GEN 2 here.

230-235 all cylinders at the start of last racing season.

Will vary based on altitude, method of doing the test, and engine cranking speed.

 

[Jackson1320]

Most gen1-2 test between 170-200 at sea level at operating temperature. So at what point is it considered low. Not just one cylinder but all cylinders equal. I have seen 130 and car ran good. So WTF?

 

[markmurfie]

Dry, no fuel or oil getting in warm cylinder, it should be near ~160. ~175 for Gen 3's

If you are seeing 200+ thats a wet test, usually only done when you are trying to determine if the loss is caused by rings or valves(valves it will continue to test low). unless you get a known quantity of liquid in each cylinder this can cause a variance between cylinders, if you are comparing cylinder to cylinder you want them dry.

Many many shops, even reputable ones do compression checks wrong, and interpret the results wrong. Draining all fuel pressure from the rails like you are going to work on the injectors is a critical step, not just pressing the pedal all the way to the floor.

 

[Jackson1320]

Dry, no fuel or oil getting in warm cylinder, it should be near ~160. ~175 for Gen 3's

If you are seeing 200+ thats a wet test, usually only done when you are trying to determine if the loss is caused by rings or valves(valves it will continue to test low). unless you get a known quantity of liquid in each cylinder this can cause a variance between cylinders, if you are comparing cylinder to cylinder you want them dry.

Many many shops, even reputable ones do compression checks wrong, and interpret the results wrong. Draining all fuel pressure from the rails like you are going to work on the injectors is a critical step, not just pressing the pedal all the way to the floor.

without any added oil, injectors unplugged, throttle body wide open, engine at operating temperature, with a battery charger maintaining battery. 170-200 is what I have seen. Oil may be the reason for some high numbers. But how do people see 240+ on new engines. There should not be any oil in the combustion chamber. Maybe people are adding oil. I’m going to check and see if I get any real increase with oil on my car. I know adding oil will help determine where a engine is failing but on a healthy engine I’m not sure oil would do anything

 

[markmurfie]

Liquid is essentially incompressible and artificially increases the compression ratio.

 

[ice445]

Most people do compression testing with the throttle closed which jacks up the numbers. Easy mistake to make.

 

[Jackson1320]

Most people do compression testing with the throttle closed which jacks up the numbers. Easy mistake to make.

What do you mean jacks up the numbers. If you mean causes higher compression numbers. It does the opposite by having the throttle closed

 

[ice445]

What do you mean jacks up the numbers. If you mean causes higher compression numbers. It does the opposite by having the throttle closed

I meant it gives an inaccurate result. I guess on some engines it doesn't matter though

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