Compression test

[Jackson1320]

Liquid is essentially incompressible and artificially increases the compression ratio.

True but the fuel injected would not be enough to make a noticeable difference. Oil in the cylinder is used to determine what part of the engine is damaged. It will help seal the rings even if there is damage to the cylinder or rings. But on a healthy engine it really shouldn’t do much if anything

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[Jackson1320]

According to Ford technicians gen1 and gen2 coyote compression should be 160 give or take 10%

 

[96gt4.6]

This has bothered me since people started posting numbers, being that mine are in the 200+ range.

However, it also got me thinking.

For my readings, it's obviously either the equipment being incorrect, or procedure being incorrect. However, my readings were 100% dry, and done as per the factory manual (running starter manually from under hood, throttle held open manually).

One thing to consider that I didn't think of, with a PD blower on the motor, during this test the supercharger bypass valve is closed (no manifold vacuum), so the blower is effectively helping pump air into the cylinders during cranking, wonder if that's why some of our results are higher than others?

I never did check the readings before I did the blower. Just once before the start of every race season for good measure, and I never have documented actual compression readings of a GEN2 at work, but those numbers sound about right on.

 

[Jackson1320]

This has bothered me since people started posting numbers, being that mine are in the 200+ range.

However, it also got me thinking.

For my readings, it's obviously either the equipment being incorrect, or procedure being incorrect. However, my readings were 100% dry, and done as per the factory manual (running starter manually from under hood, throttle held open manually).

One thing to consider that I didn't think of, with a PD blower on the motor, during this test the supercharger bypass valve is closed (no manifold vacuum), so the blower is effectively helping pump air into the cylinders during cranking, wonder if that's why some of our results are higher than others?

I never did check the readings before I did the blower. Just once before the start of every race season for good measure, and I never have documented actual compression readings of a GEN2 at work, but those numbers sound about right on.

That would absolutely answer some questions

 

[sloride]

In the end for the most part it should be a percent differential comparison from cylinder to cylinder

 

[DougS550]

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

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

A compression test is a quick way to check if you have an issues, but the most accurate way to troubleshoot a problem is a perform a cylinder leak down test. You will be able to check your piston rings, valve seats, head gaskets for excessive leakage. Good luck.

 

[Jackson1320]

A compression test is a quick way to check if you have an issues, but the most accurate way to troubleshoot a problem is a perform a cylinder leak down test. You will be able to check your piston rings, valve seats, head gaskets for excessive leakage. Good luck.

I understand the benefits of a leak down test. But a leak down is normally only done if the compression test showed a problem. We are talking about a healthy engine. I Would like to find some accurate numbers to compare to

 

[Jackson1320]

In the end for the most part it should be a percent differential comparison from cylinder to cylinder

Ok but if all cylinders test at 125 on a engine that should test at 180 it doesn’t really matter if they are all the same. So if you you don’t know that they should have been at 180 you might think you have a healthy engine when you don’t

 

[DougS550]

I understand the benefits of a leak down test. But a leak down is normally only done if the compression test showed a problem. We are not talking about a healthy engine. I Would like to find some accurate numbers to compare to

Well I will check my service/Repair manual, it should be their.

 

[sloride]

Ok but if all cylinders test at 125 on a engine that should test at 180 it doesn’t really matter if they are all the same. So if you you don’t know that they should have been at 180 you might think you have a healthy engine when you don’t

Then insert leak down test as mentioned above

 

[engineermike]

I don’t think @Jackson1320 is in need of instructions on diagnosing an engine.

 

[Jackson1320]

Then insert leak down test as mentioned above

No. Because if all cylinders test the same 99% of the time that will be it. They will assume everything is ok and go no further. That is way it is good to know what the cylinders should test at. I would just like to have a accurate number to compare to. I agree a leak down test is beneficial but it still doesn’t answer the question of what the coyote should test at

 

[DougS550]

I understand the benefits of a leak down test. But a leak down is normally only done if the compression test showed a problem. We are talking about a healthy engine. I Would like to find some accurate numbers to compare to

A compression test was only used when I was at the track to check if I had a dead cylinder. The actual compression number wasn't that important. Finding one cylinder which was like 90 and the others were 160, that's was a problem to send me home. To troubleshoot a dead cylinder, I would perform a leak down test which was the only test that mattered. The actual compression test psi numbers really didn't matter. I mean those numbers can be drastically changed by holding throttle closed, partly open or fully open. Spraying a few squirts of wd-40 would also make those numbers differ. Good Luck

 

[sloride]

I would say from readings/internet historically anything mid-100’s and higher is very good compression. With many variables skewing those numbers. But I disagree and believe that a leak down is a better indicator of engine health. If you’re leaking somewhere compression numbers if even registering don’t mean anything.

 

[Jackson1320]

I would say from readings/internet historically anything mid-100’s and higher is very good compression. With many variables skewing those numbers. But I disagree and believe that a leak down is a better indicator of engine health. If you’re leaking somewhere compression numbers if even registering don’t mean anything.

That’s exactly what I’m saying. Just because the numbers are even that doesn’t mean that the engine is healthy. So if someone does a compression test and the numbers are all even they might think that the engine is healthy when it is not. So it is a good idea to know what the Engine should test at

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