Question about # of cranks for compression test

[El Cuervo]

As title says. It is suppossed to be 5 cranks per cylinder when doing a copression test. My question is: why only 5 cranks. Whats so special about 5 cranks instead of say, 6, 8 or 10?
On the universal compression kit that I bought, it says to crank the car (motor) until the needle on the gauge stops moving and that would be my compression. Is this true? This could be more than the said 5 cranks. It could be 8, 10 or 15 cranks. Again, I would like to know if you crank till the needle on the gauge stops moving or only crank an specific number per cylinder and why.nI really need to know why. This way I can make sense out of it and have a more accurate test. It helps for learning too.

thanks guys

jorge.

 

[Calan]

It's just a number that seems to be about right...it's usually enough to get a stable reading without excessive drain on the battery. It doesn't really matter as long as you get a stable measurement (when the pressure stops going up)...within reason of course.

http://www.dsmtuners.com/forums/articles-engine-fuel/338152-compression-test-vs-leak-down-test.html

 

[#40Fan]

Until the needle stops moving is what I always do.

 

[El Cuervo]

Oh ok. Thanks guys. Calan, I also read that thread that you posted. Its very useful. Thanks.
So I guess it doesn't matter how many times yoiu crank the motor, as long as the needle stops moving. So what if two cylinders measure the same PSI but one of them took 9 cranks for the needle to stop moving and the other cylinder took 6 cranks for the needle to stop moving but yet they both measured 155psi. Would the test be wrong because it took more cranks from 1 cylinder to completely stop the needle in the gauge, as oppose to the next cylinder that took less cranks to stop the needle and have the same PSI?

 

[Calan]

Probably not. It could just be that the tester seals better on one cylinder than the other...which is why you want to test each cylinder more than once and average the results. And you have to remember that a running motor is cycling WAY faster than it is during the test, so a few extra revolutions across cylinders during the test isn't really an issue.

But...

Remember in the article I mentioned that a compression test only tells you how much pressure you can build; not if it's leaking. Every time the piston rises on the compression stroke, the pressure is building in the cylinder, and most of it then bleeds off on the exhaust stroke. But if there is a small leak in the cylinder (rings for example), less pressure will be made on each compression stroke, so it takes longer for the pressure to reach it's maximum on the gauge. If the leak is large enough, the final pressure number will be lower.

Think of it like this: You're blowing up a bag with a small hole in it. With each breath, you fill the bag more. In between breaths, some leaks out through the small hole...but the next breath replaces the amount that was lost plus more. Eventually the bag can't be blown up anymore, and you are at maximum pressure. But if the hole is large enough to bleed off almost your whole breath, you're gonna be huffing and puffing a long time to get it blown up...maybe never. You're bag has a problem.

The leak down test will tell you exactly how much air is being lost in an otherwise "sealed" cylinder, and where it is going.

If you are seeing 155 on both of the cylinders in question, I wouldn't worry much (any) about it taking 3 extra cranks to get there. But a leak-down test is always a good idea, if for no other reason than you'll learn a little bit more about your motor's health.

 

[greyspyder117]

Am going to school during the day for automotive and my book says 4 needle jumps and then record what you have. If there is more then a 15% difference between any cylinder service is going to be needed in a fair amount if time. Now this is from a book i just thought that you guys could use another input. By the way if you do it till the needle stops moving what kind of numbers are you getting isn't it going to be almost perfect al the time? Unless you have a large leak in one of the cylinders it's going to built up pressure no matter what you know?

 

[Calan]

Am going to school during the day for automotive and my book says 4 needle jumps and then record what you have.

No disrespect intended towards you or the book, but automotive repair manuals and text books are full of guidelines like this one. There are just too many variables involved to say that 4 cranks is sufficient on any car. One particular engine...maybe. But depending on the tester, engine CR, ring tolerances, starter condition, ambient weather, planet alignment, etc...it may take 3 or it may take 10. My personal rule of thumb is anything over 10 cranks or 5 seconds and I start checking my tester connection...followed immediately by a confused frown if the readings don't change. . The 15% difference in final numbers is valid though.

Unless you have a large leak in one of the cylinders it's going to built up pressure no matter what you know?

Ummm... that's the point of the test.

If there is a big enough leak (and it doesn't take much), the max pressure that can be built will be lower. Your looking for maximum pressure here; not if it bleeds off (since the comp tester has a check valve, you can remove it it from the car and toss it on the bench..and you'll still read pressure).

That's why I always suggest doing both a comp test and leak-down test. They each show a different side of what's going on in your cylinders.

 

[greyspyder117]

nah dont agree if i blow a bag up full of air i can blow slow and it will built up slow or i can blow fast and it will be filled up fast. You want to see in a certain amount of time how much compression you can build. What good is it if you have to do 10 compression strokes to built up lets say 180 hg if my car does it in 5 so i think you should rethink your theory Your looking at it all wrong.. The science of it doesn't match up with what you are saying... NO DISRESPECT

 

[Calan]

nah dont agree if i blow a bag up full of air i can blow slow and it will built up slow or i can blow fast and it will be filled up fast. You want to see in a certain amount of time how much compression you can build. What good is it if you have to do 10 compression strokes to built up lets say 180 hg if my car does it in 5 so i think you should rethink your theory Your looking at it all wrong.. The science of it doesn't match up with what you are saying... NO DISRESPECT

ok

Guess I better yank that new stroker I just built << back out of the car and rebuild it then... cause even though it's making somewhere around 375-400 hp right now with 5000 miles on it, all of the cylinders take between 6 to 10 cranks to max the comp gauge.

BTW - They read 190 190 190 181

EDIT:

What good is it if you have to do 10 compression strokes to built up lets say 180 hg? if my car does it in 5 so i think you should rethink your theory

The more I read that and think about it...

You can't possibly be saying that it should take exactly the same number of cranks for a stable compression reading in your car as it does in mine... right? I can think of about 50 reasons off the top of my head why they could (and probably will) be different...but I'll use just one example that would apply to even the SAME engine in the SAME car under different conditions:

Based on what you're saying (I think), what would happen if you replaced your starter with one that has a lower torque rating...or let's say the temperature drops and your battery isn't cranking quite as well as it would if it was hot out. Your motor is turning over slower... so are you going to crank the car over those exact 4 times, and then conclude that you have bad compression because the numbers are lower than the last time you tested it with 4 cranks?

By that same logic, should I conclude that my motor has a compression problem, since it sometimes takes me about 10 strokes to reach full compression values, when yours takes 5 (and a book says 4)...even though I get about 190 psi across the board on average?

Maybe I'm totally missing your point...I kind of hope so.

 

[snowborder714]

Well, I'm too lazy to read what all Craig posted, but here's how I do my compression tests. I follow this link, specifically this line:

"It may take more than 4 revolutions to get a stable reading; crank for up to 10 seconds or until the needle stabilizes, whichever comes first."

http://web.archive.org/web/20061106180816/http://www.dsmgrrrl.com/FAQs/compression.htm

 

[Calan]

Well, I'm too lazy to read what all Craig posted...

you lazy bum...

"It may take more than 4 revolutions to get a stable reading; crank for up to 10 seconds or until the needle stabilizes, whichever comes first."

That's exactly what I said, except I did it in 10,000 words or more. (Ended up making a freakin' college essay out of it).

 

[greyspyder117]

nice.... never thought about the starter thing guess i still have alot of different ways to think about it also thanks for the insight