Compression test

[Jackson1320]

Hello I m trying to get a accurate idea of what kind of numbers a compression test should show on a coyote. I hear numbers of all kind so Anyone that has done a compression test I would like to hear your input

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[thelostotter]

Here's what I got on my engine when checking compression with all the plugs out using the starter. I let it spin through 8 compression strokes for each.

 

[engineermike]

@thelostotter yours is NA? If so that rules out the theory about blowers increasing compression numbers.

Any idea why yours broke?

 

[thelostotter]

@thelostotter yours is NA? If so that rules out the theory about blowers increasing compression numbers.

Any idea why yours broke?

Yes, NA. #8 piston ring land broke. #7 and #6 were cracked but hadn't completely failed.

Not sure exactly why it happened. I do track the car and was using 91 octane which is the highest pump gas available here. So there may have been some detonation going on although I never heard it. The car never really got hot on track either according to the gauges. I did have the FR Power Pack 2 for a bit which definitely DID ping sometimes, so maybe that was it. Or this motor was built with ring end gaps on the tight end. They were all at 0.015" when I tore it down, even #8 which was totally worn out so it likely started life a bit tighter.

Had the block honed with a torque plate, mains align honed, and rebuilt with Manley rods and Wiseco pistons. Put on an Earl's 50 row extra-wide oil cooler and a real oil temp sensor in the pan. Race fuel only on the track now. Taking no chances

 

[engineermike]

Top ring gap spec from Ford is .008-.012 FWIW.

 

[Jackson1320]

@thelostotter yours is NA? If so that rules out the theory about blowers increasing compression numbers.

I’m wondering if different brands of compression testers are contributing to the inconsistency. I’m sure that some of the inconsistency’s are caused by each person doing the test slightly different. But it still doesn’t answer how people are getting test results 60-70Lbs or more over what the dealer says it should be

 

[CorvZ061]

I’m wondering if different brands of compression testers are contributing to the inconsistency. I’m sure that some of the inconsistency’s are caused by each person doing the test slightly different. But it still doesn’t answer how people are getting test results 60-70Lbs or more over what the dealer says it should be

throttle open or closed makes a big difference, both are done very often. Compression ratio also makes a big difference, a gen 3 Coyote should always have more pressure than a gen 1 or 2.

if you have cams or a tune the cam phase of intake or exhaust cams alters ford’s valve timing which could either build more compression or bleed off compression. There are a bunch of factors, even as simple as air temp or altitude density.

 

[Jackson1320]

throttle open or closed makes a big difference, both are done very often. Compression ratio also makes a big difference, a gen 3 Coyote should always have more pressure than a gen 1 or 2.

if you have cams or a tune the cam phase of intake or exhaust cams alters ford’s valve timing which could either build more compression or bleed off compression. There are a bunch of factors, even as simple as air temp or altitude density.

Cam phasing does not happen while cranking the engine over or at idle. And none of the factors should be able to have a 160psi engine show 230-250. Your talking about something that can alter the test by a few psi. A compression test is not done with the throttle closed and even if you did it doesn’t make as much difference as you think. Cylinder pressure will still max out the same it just takes longer to get there

 

[CorvZ061]

Cam phasing does not happen while cranking the engine over or at idle. And none of the factors should be able to have a 160psi engine show 230-250. Your talking about something that can alter the test by a few psi. A compression test is not done with the throttle closed and even if you did it doesn’t make as much difference as you think. Cylinder pressure will still max out the same it just takes longer to get there

Open vs closed throttle can make a 20 psi difference in some cases, I’ve tested that myself. Compression changing from 11:1 to 12:1 will certainly increase cylinder pressure as well.

Cam phasing doesn’t happen at start and idle with factory ford tune, do no tuners play with those settings on a tuned car? I’d bet they do.

As an example my Chevelle use to have 12.5:1 compression, with a compression tester screwed in and all plugs removed it would read 245psi. The same displacement engine with different pistons dropping compression to 10.5:1 to live on pump gas made 165psi doing the same test.

 

[ice445]

I've always gone by the rule that the numbers don't matter at all, consistency across cylinders is what matters. Yeah, it may seem like your engine is broken if you're 60PSI down compared to some guy on the internet, but if all your cylinders are within 2% of each other, your engine is either miraculously broken in a perfectly consistent way, or it's perfectly fine.

Most testers are shitty anyway, and there's no way to really test their accuracy.

 

[engineermike]

You can definitely check the accuracy of the pressure gauge.

I wonder if the hose length/volume added when using a gauge is causing some of the difference. That would inadvertently change the compression ratio.

 

[Jackson1320]

Open vs closed throttle can make a 20 psi difference in some cases, I’ve tested that myself. Compression changing from 11:1 to 12:1 will certainly increase cylinder pressure as well.

Cam phasing doesn’t happen at start and idle with factory ford tune, do no tuners play with those settings on a tuned car? I’d bet they do.

As an example my Chevelle use to have 12.5:1 compression, with a compression tester screwed in and all plugs removed it would read 245psi. The same displacement engine with different pistons dropping compression to 10.5:1 to live on pump gas made 165psi doing the same test.

I agree compression makes a big difference. I’m not talking about apples to oranges. Gen2 to gen2 stock to stock. No game no b.s just a honest test to test. How are they 70-80 psi different
cam phasing can not be modified at crank and the only time it is modified at idle is in a cam lope

 

[Jackson1320]

You can definitely check the accuracy of the pressure gauge.

I wonder if the hose length/volume added when using a gauge is causing some of the difference. That would inadvertently change the compression ratio.

That is the kind of information I’m looking for. Anything that might help explain why one gen2 coyote tests at 160 and the next at 240

 

[thelostotter]

Most compression gauges have a check valve in the fitting that threads into the head, so in practice they are not lowering the compression ratio a measurable amount. If they don't have a check valve then yes they are lowering the CR, and would be useless for determining an exact reading but still relatively useful for comparing each cylinder relative to the 7 others.

I think variables like throttle body open or closed, all plugs removed versus just the one being tested, and the number of compression strokes you spin the engine through will have a pretty big affect on the reading you get.

 

[IamCDNJosh]

Since pressure gauge readings can vary depending on the manufacturer, you're going to be more concerned with the consistency of the measurement between all cylinders.

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