Compression #'s

[AeR0]

anyone know what kind of compression i should be getting on a 99 grand am.

[red98grandamse]

http://www.gaownersclub.com/forum/showthread.php?s=&threadid=1978 here ya go

[AeR0]

Compression Ratio: 9.7:1



so if i do a compression test on my car and it comes back 160? how do i figure this out?

[AeR0]

someone over on the quad 4 forums told me that the compression on the new TC is 9.7:1


so did they change or is he wrong?

[rixGAphx]

Originally posted by AerosGA
Compression Ratio: 9.7:1



So if I do a compression test on my car and it comes back 160? How do I figure this out?
The link from red98 regarding compression ratio is not what you need.

Go to your Haynes/Chilton's, and see what they say the reading should be.

When doing a compression check there are TWO things with which to be concerned:
1. That ALL the readings are within 10% of recommended reading; AND,
2. That one of the readings isn't more than 5% different from the others.

Examples (generic, theoretical engine):
Recommended reading: 170 psi.
Therefore, 10% = 17 psi; therefore, all cylinders must be 153 psi or more.
If the readings (for 4 cyls) are 160 - 155 - 158 - 162; then all's well.
If the readings are 155-154-153-155, then all's well.
If the readings are 154-153-150-155, then it's no good (because one was less than the minimum 153).
If the readings are 163-164-163-154, then it's no good (because the 154 was more than 5% lower than the others; this indicates a broken ring, burnedvalve, or other internal problem).

This isn''t rocket science, but with your level of experience you're REALLY over your head in trying to e-analyze this engine.

Good luck.
-Rick

edit - PS: Is this relative to your existing engine, or one of the used engines you were looking at?

[AeR0]

Originally posted by rixGAphx
The link from red98 regarding compression ratio is not what you need.

Go to your Haynes/Chilton's, and see what they say the reading should be.

When doing a compression check there are TWO things with which to be concerned:
1. That ALL the readings are within 10% of recommended reading; AND,
2. That one of the readings isn't more than 5% different from the others.

Examples (generic, theoretical engine):
Recommended reading: 170 psi.
Therefore, 10% = 17 psi; therefore, all cylinders must be 153 psi or more.
If the readings (for 4 cyls) are 160 - 155 - 158 - 162; then all's well.
If the readings are 155-154-153-155, then all's well.
If the readings are 154-153-150-155, then it's no good (because one was less than the minimum 153).
If the readings are 163-164-163-154, then it's no good (because the 154 was more than 5% lower than the others; this indicates a broken ring, burnedvalve, or other internal problem).

This isn''t rocket science, but with your level of experience you're REALLY over your head in trying to e-analyze this engine.

Good luck.
-Rick

edit - PS: Is this relative to your existing engine, or one of the used engines you were looking at?


lol what do u mean its over my head. i understand how compression tests work.
i just wanted to know why on some of the engines the Compression Ratio: 9.7:1 and on the other ones the Compression Ratio is 9.5:1

[rixGAphx]

Originally posted by AerosGA
lol what do u mean its over my head.
1. We've been reading your posts about this engine for a couple weeks.
You're still on a learning curve, and that's fine. I was there once, too.
IMO, you're still too green.
That's not meant as a criticism, just as a caution.

2. Your most recent statement:
"So if i do a compression test on my car and it comes back 160? how do i figure this out?"

This indicates you're trying to compare a reading of 160 psi to a published ratio of 9.7:1.
That just IS'NT how it's done, and this indicated to me that you're not ready for the project.

3. You have the car, and you're having problems with it.
Your first step should be to purchase a manual for it.
The fact that you haven't indicates to me a lack of experience.
Again, not criticizing, just stating my impression.


I congratulate you for asking the questions.
It shows a high level of maturity and a willingness to learn instead of make mistakes by guessing.

It's true that by asking questions you can get through about any situation, but the trick is to know when to ask questions and of whom.

GA's are full of their own problems, and we come together here to try and help each other thru them.
I'm thankful of the advice I've received here, and in a coupple cases that advice has steered me away from tackling a GA problem myself.
I wish to pass on similar advice, and I apologize if I seem like I'm attacking.

i just wanted to know why on some of the engines the Compression Ratio: 9.7:1 and on the other ones the Compression Ratio is 9.5:1
Your questions BOTH contain the number 9.7:1 (nothing about 9.5:1), so I didn't know what you were asking.

Peace.
-Rick

[AeR0]

yeah the answer to that is the 99 and up motors are 9.7:1

and the rest are 9.5:1

[HeyDace]

Keep plugging away and don't clutter up your head with useless info like compression ratios or cam lift and duration unless your building an engine. I've been a mechanic for 27 years and couldn't tell you what the hell the ratio is on any particular engine. I might have known at one point in my life but it seems to have escaped me now.

:lol:

[AeR0]

right on thanks for the info man!


Originally posted by HeyDace
Keep plugging away and don't clutter up your head with useless info like compression ratios or cam lift and duration unless your building an engine. I've been a mechanic for 27 years and couldn't tell you what the hell the ratio is on any particular engine. I might have known at one point in my life but it seems to have escaped me now.

:lol:

[Lash]

The pre 99 motors have domed pistons making the CR 9.5....the 99+'s have flat tops making it 9.7.


You should really sign up on j-body.org. Alot of this stuff has been discussed years ago and is easliy found in seconds with a quick search.

Also...the compression test #'s for your 2.4 should be around 160 or a little less. If you have one thats alot lower....like more than 2 points lower than the others...you have a problem.

If it looks like this

158-159-158-157....you should be good to go. but this...

158-159-153-158
is a bad thing.

[hvychy]

So I guess my 215,213,215,200 is too high? LOL