LSJ Compression Test
You don't "have" to, but it's suggested. The idea is that you don't want any restriction to the airflow into the cyl. Although, as long as it's the same on all 4, you don't open the throttle on 3 of them only or something, it won't matter.
The actual number you get is pretty irrelevant, as long as they are reasonably close between all 4, no more than 10-15% variation from highest to lowest is considered normal. If you see one of them down 30% or so, you have a problem.
The actual number you get is pretty irrelevant, as long as they are reasonably close between all 4, no more than 10-15% variation from highest to lowest is considered normal. If you see one of them down 30% or so, you have a problem.
OP just run the straight compression check, no throttle, then post the numbers.
also I thought the cam(s) open the valves? how would opening the throttle work anyway?
Ideally you should have the throttle wide open. But for 6000ft and 60k miles those numbers arent too bad....if you were at sea level youd be running High 160s, low 180s.
there shouldnt be any "trying" to hold the throttle down....just prop the pedal WFO. done.
Its harder to suck air through a closed throttle blade than it is an open one, thats why you open the throttle.
Cams open the valves, but air comes through the throttle body.
there shouldnt be any "trying" to hold the throttle down....just prop the pedal WFO. done.
Its harder to suck air through a closed throttle blade than it is an open one, thats why you open the throttle.
Cams open the valves, but air comes through the throttle body.
Last edited by Maven; Aug 1, 2010 at 03:01 AM. Reason: Automerged Doublepost
Senior Member
Joined: 09-02-09
Posts: 4,210
Likes: 2
From: Wisconsin
I did a Dry Compression Test last night.
2006 LSJ
63,800 Miles
2.8 Pulley
Last Dynoed at only 230whp (2006 Dyno)
The car was at operating temps.
Results:
Cylinder 1=155
Cylinder 2=140
Cylinder 3=155
Cylinder 4=145
Note: I did forget to hold open the throttle body somehow and/or hold the throttle down when cranking the motor. Is there an easy way to do this with the Fly By Wire Throttle and how badly can this effect my compression numbers?
I was a little disappointed with the results, especially Cylinder 2.
Any thoughts on how these look? They all seem to be relatively even but also pretty low.
I was expecting 180-200.
2006 LSJ
63,800 Miles
2.8 Pulley
Last Dynoed at only 230whp (2006 Dyno)
The car was at operating temps.
Results:
Cylinder 1=155
Cylinder 2=140
Cylinder 3=155
Cylinder 4=145
Note: I did forget to hold open the throttle body somehow and/or hold the throttle down when cranking the motor. Is there an easy way to do this with the Fly By Wire Throttle and how badly can this effect my compression numbers?
I was a little disappointed with the results, especially Cylinder 2.
Any thoughts on how these look? They all seem to be relatively even but also pretty low.
I was expecting 180-200.
Thread Starter
Senior Member
Joined: 02-22-06
Posts: 1,730
Likes: 0
From: Colorado
Thanks everyone for posting! I appreciate it!
The only reason I would hold the throttle down is because thats what I was told to do form two sources when I looked into doing a compression test. I just wasn't sure if that worked for us with our fly-by-sire throttle system? Any ideas?
It does make sense that the air being compressed would have to come from somewhere but it also makes sense that the throttle body doesn't seal 100% at 0% throttle.
Anyway, the numbers I posted were with no throttle (I was not pushing the throttle down at all as I forgot too. lol)
I will re-try soon and post those numbers aswell.
As far as the process I used:
1. Drive car around for about 15 minutes (Went to buy tester)
2. Pull fuel pump relay and let the car die (No chance of flooding motor during test)
3. Let car cool down for about 5 minutes (Easier to work with cooler spark plugs and less chance of damaging the head)
3. Pull all 4 spark plugs
4. Disconnect coil-on-plug deals from wiring harness to avoid any chance of grounding or shock
5. Connect tester and crank 5 times per. cylinder
Also, I have always used and heard that 3% per. 1000 feet above sea level. This includes power loss compared to the same car at sea level and is also a standard that is used in the HVAC business (My Day Job).
So at between 5000-6000 feet above sea level, I should be seeing around 15-18% losses.
At 180-200 psi average that others are seeing on compression tests of LSJ's, minus 15% = 153-170 psi or minus 18% = 147-164 psi
Maybe this explains the numbers?
Either way, I will re-do the test soon and post again.
I just found this online and it might help explain the numbers I am getting?
The only reason I would hold the throttle down is because thats what I was told to do form two sources when I looked into doing a compression test. I just wasn't sure if that worked for us with our fly-by-sire throttle system? Any ideas?
It does make sense that the air being compressed would have to come from somewhere but it also makes sense that the throttle body doesn't seal 100% at 0% throttle.
Anyway, the numbers I posted were with no throttle (I was not pushing the throttle down at all as I forgot too. lol)
I will re-try soon and post those numbers aswell.
As far as the process I used:
1. Drive car around for about 15 minutes (Went to buy tester)
2. Pull fuel pump relay and let the car die (No chance of flooding motor during test)
3. Let car cool down for about 5 minutes (Easier to work with cooler spark plugs and less chance of damaging the head)
3. Pull all 4 spark plugs
4. Disconnect coil-on-plug deals from wiring harness to avoid any chance of grounding or shock
5. Connect tester and crank 5 times per. cylinder
Also, I have always used and heard that 3% per. 1000 feet above sea level. This includes power loss compared to the same car at sea level and is also a standard that is used in the HVAC business (My Day Job).
So at between 5000-6000 feet above sea level, I should be seeing around 15-18% losses.
At 180-200 psi average that others are seeing on compression tests of LSJ's, minus 15% = 153-170 psi or minus 18% = 147-164 psi
Maybe this explains the numbers?
Either way, I will re-do the test soon and post again.
There are some variables that affect the readings obtained from compression testing.
They are cranking speed, altitude, temperature, worn camshaft lobes and high-performance, long-duration profile camshafts.
The cranking speed needs to be maintained the same for each cylinder. This may mean jumping your battery to maintain the speed.
There are factors to compensate for the different altitudes and the corresponding temperature differences.
These are as follows:
1,000 feet = .9711
2,000 feet = .9428
3,000 feet = .9151
4,000 feet = .8881
5,000 feet = .8617
6,000 feet = .8359
7,000 feet = .8106
8,000 feet = .7860.
The equivalent compression reading for a cylinder that should be 180 psi at Sea Level by the data at 5,000 feet would be 180 x .8617 = 155.106
They are cranking speed, altitude, temperature, worn camshaft lobes and high-performance, long-duration profile camshafts.
The cranking speed needs to be maintained the same for each cylinder. This may mean jumping your battery to maintain the speed.
There are factors to compensate for the different altitudes and the corresponding temperature differences.
These are as follows:
1,000 feet = .9711
2,000 feet = .9428
3,000 feet = .9151
4,000 feet = .8881
5,000 feet = .8617
6,000 feet = .8359
7,000 feet = .8106
8,000 feet = .7860.
The equivalent compression reading for a cylinder that should be 180 psi at Sea Level by the data at 5,000 feet would be 180 x .8617 = 155.106
Last edited by leviticus88; Aug 1, 2010 at 09:20 PM. Reason: Automerged Doublepost
Senior Member
Joined: 06-27-05
Posts: 2,057
Likes: 0
From: southeast florida
Figured I would post my results from last week. Decided to do it cause I was having some terrible running issues and I took out the plugs and one of them was missing the tip.
Stock pulley for 35k, stg 2 pulley for 10k, 15k full stage 2, 2.8 7k, and the last 3k I took the 2.8 off for highway driving and havent put it back on.
So 70k total hard miles.
I got 190 across the board using 2 different guages.
Stock pulley for 35k, stg 2 pulley for 10k, 15k full stage 2, 2.8 7k, and the last 3k I took the 2.8 off for highway driving and havent put it back on.
So 70k total hard miles.
I got 190 across the board using 2 different guages.
205 across the board here.
Many things can come into play. make sure you have a good seal when you thread the fitting into the head. since our plugs are recessed far into the valve cover it's often hard to thread them tightly, and dirt/grit can sometimes get in the way of the O-ring sealing.
As for the process, sounds like you did it right. the numbers are close enough across all cyl so i wouldnt worry about it given your elevation
Also, you may want to hook up a battery charger while you are "dry cranking" during the test. slow cranking speed will cost you about 20 psi
Many things can come into play. make sure you have a good seal when you thread the fitting into the head. since our plugs are recessed far into the valve cover it's often hard to thread them tightly, and dirt/grit can sometimes get in the way of the O-ring sealing.
As for the process, sounds like you did it right. the numbers are close enough across all cyl so i wouldnt worry about it given your elevation
Also, you may want to hook up a battery charger while you are "dry cranking" during the test. slow cranking speed will cost you about 20 psi
Last edited by Bika; Dec 27, 2010 at 01:20 PM.
You don't "have" to, but it's suggested. The idea is that you don't want any restriction to the airflow into the cyl. Although, as long as it's the same on all 4, you don't open the throttle on 3 of them only or something, it won't matter.
The actual number you get is pretty irrelevant, as long as they are reasonably close between all 4, no more than 10-15% variation from highest to lowest is considered normal. If you see one of them down 30% or so, you have a problem.
The actual number you get is pretty irrelevant, as long as they are reasonably close between all 4, no more than 10-15% variation from highest to lowest is considered normal. If you see one of them down 30% or so, you have a problem.
Senior Member
Joined: 09-02-09
Posts: 4,210
Likes: 2
From: Wisconsin
Confused
Ok I'm finding a catch 22 here....
Some one please correct where I may be in error.
To run a compression test, you must remove the spark plug for the cylinder under test, and install the test gauge to the sparkplug hole.
To Remove the spark plug, you must wait till the engine has cooled down, or damage to the head may occur.
With that in mind, how does one do a compression test at operating tempature?
I'm guessing that aperently it *IS* safe to remove the plugs when the engine is (obviously off) and at the normal operating tempature of 180 degrees?
Some one please correct where I may be in error.
To run a compression test, you must remove the spark plug for the cylinder under test, and install the test gauge to the sparkplug hole.
To Remove the spark plug, you must wait till the engine has cooled down, or damage to the head may occur.
With that in mind, how does one do a compression test at operating tempature?
I'm guessing that aperently it *IS* safe to remove the plugs when the engine is (obviously off) and at the normal operating tempature of 180 degrees?
Yes, you can remove plugs with the engine warm. Put a small amount of anti-sieze on the threads when you install plugs, and they will come out easy every time with no risk of damage to the head.
New Member
Joined: 06-21-16
Posts: 16
Likes: 1
From: bailey colorado
so while we are on the topic of compression, what is the proper compression rating for an 06 cobalt ss 2.0 supercharged? i am trying to buy an engine off of ebay and asked what the compression ratings were for it, he said 220 psi. seemed a little high, is that normal?
Thread
Thread Starter
Forum
Replies
Last Post