Compression test

[Jackson1320]

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 relatively useless for determining an exact reading but still 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 effect on the reading you get.

I believe the cheap harbor freight kit has the check valve in the gauge.
Having all the spark plugs out makes a big difference because it changes the cranking speed. just like having a good battery makes a big difference. But throttle open vs closed just takes more rotations cranking but you’ll get about the same number in the end.
but the problem is not low compression. The problem is people Claiming that their compression test showed 240 on the engine that should make 160 how are they doing this

Sponsored

 

[Jackson1320]

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.

ok I know how to do a compression test I know they should be balance within 10% I’m talking about how do people get readings at 240 250 260 on a engine that should be at 160

 

[IamCDNJosh]

Too many factors here, bad seal, leaky tester etc can affect the overall number. Again you need to be focused on the consistency of the numbers you're getting, not the overall number.

 

[thelostotter]

ok I know how to do a compression test I know they should be balance within 10% I’m talking about how do people get readings at 240 250 260 on a engine that should be at 160

You can't just assume the compression tester is going to read atmospheric pressure multiplied by compression ratio, or 14.7 psi x 11 = 162 psi.

The air heats up as you compress it, and that increases its pressure further.

No 5.0 Coyote in decent condition should read 160 psi on a tester.

 

[IamCDNJosh]

Unless its a harbor freight tester then all bets are off......

My rules are with harbor freight are as follows:

1. No measurement tools (torque wrenches, compression testers etc).
2. If its failure can either hurt or kill you, don't buy it from harbor freight (Jackstands come to mind).

 

[Jackson1320]

Too many factors here, bad seal, leaky tester etc can affect the overall number. Again you need to be focused on the consistency of the numbers you're getting, not the overall number.

Again I’m not worried about consistency what I would like to know is what should a coyote test at

 

[Jackson1320]

Unless its a harbor freight tester then all bets are off......

My rules are with harbor freight are as follows:

1. No measurement tools (torque wrenches, compression testers etc).
2. If its failure can either hurt or kill you, don't buy it from harbor freight (Jackstands come to mind).

You seam to have a opinion on everything except the topic of this thread

 

[IamCDNJosh]

I gave you an answer as to why an exact value can't be given for a compression test. Yes I have opinions and they're all right.

 

[engineermike]

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

Heres another thought...
If you trust the online dynamic compression ratio calculator, then the DCR of a gen3 with the cams in the parked position is 9.95/1. If you adiabatically compress air at a 9.95/1 volume ratio, the resulting pressure and temperature is around 360 psi and 660 deg F. Now, the air actually loses heat rather rapidly to the cylinder walls, which is why you don’t see 360 psi on your gauge. My thermo is a little rusty, but it looks like if the air cools during compression from 660 to 300 deg F, then the gauge should read 230 psi. If it cools to 200, the result is 200 psi. And if it cools to 100 deg F then the result should be 170 psi. This brings me to the next possibility. If the cylinder wall and head is 200 deg when doing a compression test, the the air won’t cool nearly as much. If the cylinder wall and head are 50 deg, the air will cool much more.

Could block and head temperature make the difference we are seeing?

 

[ice445]

Heres another thought...
If you trust the online dynamic compression ratio calculator, then the DCR of a gen3 with the cams in the parked position is 9.95/1. If you adiabatically compress air at a 9.95/1 volume ratio, the resulting pressure and temperature is around 360 psi and 660 deg F. Now, the air actually loses heat rather rapidly to the cylinder walls, which is why you don’t see 360 psi on your gauge. My thermo is a little rusty, but it looks like if the air cools during compression from 660 to 300 deg F, then the gauge should read 230 psi. If it cools to 200, the result is 200 psi. And if it cools to 100 deg F then the result should be 170 psi. This brings me to the next possibility. If the cylinder wall and head is 200 deg when doing a compression test, the the air won’t cool nearly as much. If the cylinder wall and head are 50 deg, the air will cool much more.

Could block and head temperature make the difference we are seeing?

This is 200IQ thinking.

 

[Jackson1320]

I gave you an answer as to why an exact value can't be given for a compression test. Yes I have opinions and they're all right.

Your opinion is like your ass, It stinks!
You can’t even understand the purpose of the thread.
You think you are right and that is the worst part. You are to narrow minded to comprehend that your way is not always the right way and almost never the only way.
Im not asking for a exact number Im asking for what kind of numbers people are getting. If you have not or cannot do a compression test then move on

 

[CorvZ061]

Your opinion is like your ass, It stinks!
You can’t even understand the purpose of the thread.
You think you are right and that is the worst part. You are to narrow minded to comprehend that your way is not always the right way and almost never the only way.
Im not asking for a exact number Im asking for what kind of numbers people are getting. If you have not or cannot do a compression test then move on

Same can be said for you, you’ve been given several answers on why there could be so many variables but you immediately shoot them all down because you don’t think it’s correct. “Nobody does a compression test with throttle closed” being a perfect example. It happens all the time. You’ve been told that the outright number doesn’t matter nearly as much as consistency, but you’re still stuck on a outright number. There is no answer to your question since all answers are wrong

 

[IamCDNJosh]

Your opinion is like your ass, It stinks!

My ass does stink, no argument there.

You can’t even understand the purpose of the thread.

I can assure you I do.

You think you are right and that is the worst part. You are to narrow minded to comprehend that your way is not always the right way and almost never the only way.

I come from a highly technical background, also have been wrenching on cars since I was a teenager.

Im not asking for a exact number Im asking for what kind of numbers people are getting. If you have not or cannot do a compression test then move on

Again, it varies. Did my first compression test as well as a leakdown test on an engine in the mid 90's when I was 17. This was in an Acura Integra and was done just before we swapped out the engine. We wanted to be sure we could sell the engine as one that ran, rather than a rebuild.


I'm sorry you didn't like the answer I gave, but what I'd said was based on experience. There are a number of posters here (like engineermike) who really know there stuff here but for every one that does, there are a lot who don't.

 

[Jackson1320]

Same can be said for you, you’ve been given several answers on why there could be so many variables but you immediately shoot them all down because you don’t think it’s correct. “Nobody does a compression test with throttle closed” being a perfect example. It happens all the time. You’ve been told that the outright number doesn’t matter nearly as much as consistency, but you’re still stuck on a outright number. There is no answer to your question since all answers are wrong

Did you enjoy riding the little bus to school. I never said that you should do a compression test with the throttle shut. I said it would not change the results much if any. It doesn’t matter what you think about variables because I didn’t ask for variables. I’m asking for numbers that people are getting from a compression test. None of your business what I use the information for. If you don’t want to provide the numbers you have experienced fine. mind your business, shut your mouth and Move on already. Some of the variables mentioned don’t do what you think. I have probably built more coyotes than you have driven. I doubt you could give me one piece for information I don’t already know on the coyote. So hop back on the little bus and be gone

 

[Jackson1320]

My ass does stink, no argument there.



I can assure you I do.



I come from a highly technical background, also have been wrenching on cars since I was a teenager.



Again, it varies. Did my first compression test as well as a leakdown test on an engine in the mid 90's when I was 17. This was in an Acura Integra and was done just before we swapped out the engine. We wanted to be sure we could sell the engine as one that ran, rather than a rebuild.


I'm sorry you didn't like the answer I gave, but what I'd said was based on experience. There are a number of posters here (like engineermike) who really know there stuff here but for every one that does, there are a lot who don't.

Apparently you don’t know what the purpose of the thread is. I don’t need you to try and share the little bit of knowledge you think you have. I’m good. I’m interested in what numbers people are getting. Not in this area or tune or cams just what numbers people are getting. If you don’t want to share fine. But telling me things that I already know is not helping. Should I be impressed by your vast knowledge? I have more. Your technical background means nothing to me. How many coyotes have you built? Not as many as me I’m sure. So help out with numbers or move on down the road

Sponsored