Compression test questions
Thread Starter
Racer
Joined: Sep 2011
Posts: 352
Likes: 6
From: Centreville Maryland
Compression test questions
Do you do differential compression (known as leak down) tests on these engines?
Won't they tell you more than a steight compression test?
Background:
Just bought a 91 (L98) and very new to Corvettes. Former Porsche owner and aircraft mechanic. Don't know my way around the engine yet and don't own a code reader (yet). Maint manuals on order.
Won't they tell you more than a steight compression test?
Background:
Just bought a 91 (L98) and very new to Corvettes. Former Porsche owner and aircraft mechanic. Don't know my way around the engine yet and don't own a code reader (yet). Maint manuals on order.
Drifting
Joined: Dec 2007
Posts: 1,647
Likes: 4
From: Argillite KY
As you know a leakdown test can more easily pinpoint problems with an engine. It's easier to hear burnt exhaust or intake valves and the old blow by issue and of course cylinder balance. Do you have a problem or expect to find a problem by doing this. If it's running well why go looking for problems? Since it's your first Vette just go out and drive and enjoy it. Unlike a Lycoming or Continental engine, if your's quits you're not going to corkscrew into a cornfield in Iowa at 150 MPH.
Melting Slicks
Joined: Nov 2008
Posts: 2,765
Likes: 5
As you know a leakdown test can more easily pinpoint problems with an engine. It's easier to hear burnt exhaust or intake valves and the old blow by issue and of course cylinder balance. Do you have a problem or expect to find a problem by doing this. If it's running well why go looking for problems? Since it's your first Vette just go out and drive and enjoy it. Unlike a Lycoming or Continental engine, if your's quits you're not going to corkscrew into a cornfield in Iowa at 150 MPH.
You & I are from the same school of Hotrod Insanity.
LOL
Corkscrew & crash & burn..........LOL
I have to drive down to Kentucky this fall & hang out.
You can smoke by me very fast forward in your Vette against my 87 vert.
No 410ci in yet.
Be a good time
Brian
Melting Slicks
Joined: Nov 2008
Posts: 2,765
Likes: 5
Do you do differential compression (known as leak down) tests on these engines?
Won't they tell you more than a steight compression test?
Background:
Just bought a 91 (L98) and very new to Corvettes. Former Porsche owner and aircraft mechanic. Don't know my way around the engine yet and don't own a code reader (yet). Maint manuals on order.
Won't they tell you more than a steight compression test?
Background:
Just bought a 91 (L98) and very new to Corvettes. Former Porsche owner and aircraft mechanic. Don't know my way around the engine yet and don't own a code reader (yet). Maint manuals on order.
Have a MAC leakdown tester too.
I hate the MAC.
Lies easily.
Thread Starter
Racer
Joined: Sep 2011
Posts: 352
Likes: 6
From: Centreville Maryland
What model code reader should I buy?
Corkscrew??? Aircraft don't quit flying until they run out of air over the wings, then... bump!
Ask a pilot what he does if his engine quits at NIGHT!!
Answer: You turn on your landing light, and if you don't like what you see, you turn it off!
Drifting
Joined: Dec 2007
Posts: 1,647
Likes: 4
From: Argillite KY
Corkscrew??? Aircraft don't quit flying until they run out of air over the wings, then... bump!
Quick...hard left rudder, hard left aileron and drop the nose. Been there....done that
Answer: You turn on your landing light, and if you don't like what you see, you turn it off!
Drifting
Joined: Dec 2007
Posts: 1,647
Likes: 4
From: Argillite KY
I have to drive down to Kentucky this fall & hang out.
You & I are from the same school of Hotrod Insanity
Be a good time
Safety Car
Joined: Jan 2004
Posts: 3,759
Likes: 8
When you run a standard compression test. Are you doing it on a hot or cold engine? The FSM doesn't mention anything about the temp of the engine when doing this test.
Some say do it on a warm engine, others say do it on a cold engine. I have heard the numbers can get scewed when doing it on a warm engine as by the time you get to the last few cylinders, the engine has cooled down and lowered the compression results.
Others also say not to remove the spark plugs on a warm engine as it could gall the threads.
Some say do it on a warm engine, others say do it on a cold engine. I have heard the numbers can get scewed when doing it on a warm engine as by the time you get to the last few cylinders, the engine has cooled down and lowered the compression results.
Others also say not to remove the spark plugs on a warm engine as it could gall the threads.
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Team Owner
Joined: Oct 2004
Posts: 81,241
Likes: 3,062
From: altered state
St. Jude Donor '05
You can get a code reader from Oreillys
One is 59 which reads them, one is 79 or 99 which reads, resets and does some other functions (obd and 2). Easier than screwing with a paper clip under the dash.
on the to buy list.
One is 59 which reads them, one is 79 or 99 which reads, resets and does some other functions (obd and 2). Easier than screwing with a paper clip under the dash.
on the to buy list.
Drifting
Joined: Dec 2007
Posts: 1,647
Likes: 4
From: Argillite KY
Obviously a hot (operating temp) engine would be ideal but primarily we're looking at cylinder balance. You couldn't be sure that the temps of the pistons and rings will be the same from when you start the test until you finish and results could be skewed. Also remember that even if it is hot, when you start injecting cold air into it they cool down quickly. That would be very quickly in the case of the compression ring with airflow all around it.
For these reasons I always do them cold. I've been removing spark plugs for a few years now and I've done them hot, warm and cold. I've never galled a thread in my life. Based on my experience I would think it's not too important. I'm sure there's a tech article that will disprove what I said but if it doesn't make a difference then.......poopy on it.
Well horse snot. I just re-read your question and you said a standard compression test. Same thing, cold is just fine. Make sure your throttle stays in the same position for each cylinder. Stick a tool of some sort in the TB opening and leave it there.
For these reasons I always do them cold. I've been removing spark plugs for a few years now and I've done them hot, warm and cold. I've never galled a thread in my life. Based on my experience I would think it's not too important. I'm sure there's a tech article that will disprove what I said but if it doesn't make a difference then.......poopy on it.
Well horse snot. I just re-read your question and you said a standard compression test. Same thing, cold is just fine. Make sure your throttle stays in the same position for each cylinder. Stick a tool of some sort in the TB opening and leave it there.
Last edited by 1963SS; Sep 10, 2011 at 04:00 PM.
Race Director
Joined: Sep 2003
Posts: 16,834
Likes: 337
From: Database Error Indiana
Do you do differential compression (known as leak down) tests on these engines?
Won't they tell you more than a steight compression test?
Background:
Just bought a 91 (L98) and very new to Corvettes. Former Porsche owner and aircraft mechanic. Don't know my way around the engine yet and don't own a code reader (yet). Maint manuals on order.
Won't they tell you more than a steight compression test?
Background:
Just bought a 91 (L98) and very new to Corvettes. Former Porsche owner and aircraft mechanic. Don't know my way around the engine yet and don't own a code reader (yet). Maint manuals on order.
Safety Car
Joined: Jan 2004
Posts: 3,759
Likes: 8
Obviously a hot (operating temp) engine would be ideal but primarily we're looking at cylinder balance. You couldn't be sure that the temps of the pistons and rings will be the same from when you start the test until you finish and results could be skewed. Also remember that even if it is hot, when you start injecting cold air into it they cool down quickly. That would be very quickly in the case of the compression ring with airflow all around it.
For these reasons I always do them cold. I've been removing spark plugs for a few years now and I've done them hot, warm and cold. I've never galled a thread in my life. Based on my experience I would think it's not too important. I'm sure there's a tech article that will disprove what I said but if it doesn't make a difference then.......poopy on it.
Well horse snot. I just re-read your question and you said a standard compression test. Same thing, cold is just fine. Make sure your throttle stays in the same position for each cylinder. Stick a tool of some sort in the TB opening and leave it there.
For these reasons I always do them cold. I've been removing spark plugs for a few years now and I've done them hot, warm and cold. I've never galled a thread in my life. Based on my experience I would think it's not too important. I'm sure there's a tech article that will disprove what I said but if it doesn't make a difference then.......poopy on it.
Well horse snot. I just re-read your question and you said a standard compression test. Same thing, cold is just fine. Make sure your throttle stays in the same position for each cylinder. Stick a tool of some sort in the TB opening and leave it there.
I've got an extra 75 battery. So, I'll do four cylinders on one battery. And then swap in the other battery for the other four cylinders since you're going to be cranking it over 4 times a cylinder.
Drifting
Joined: Dec 2007
Posts: 1,647
Likes: 4
From: Argillite KY
One battery should be good if you're just cranking a short while. With the compression tester in place you'll see when to quit. It will stop increasing. If you pull the plugs (8) first it'll crank easier and give you a few more RPM's also. Good luck. If you don't mind, post the pressures. It's been awhile since I've seen something near stock and my memory needs boosted.
Melting Slicks
Joined: Nov 2008
Posts: 2,765
Likes: 5
Run them.
Smoke me when you shift at 7,500 RPM's.
Take me for a ***** out ride in your Race Vette.
Then I take you for a ***** out ride in my car.
Then beer & hamburgers & bench racing till we both drop.
Thread Starter
Racer
Joined: Sep 2011
Posts: 352
Likes: 6
From: Centreville Maryland
One battery should be good if you're just cranking a short while. With the compression tester in place you'll see when to quit. It will stop increasing. If you pull the plugs (8) first it'll crank easier and give you a few more RPM's also. Good luck. If you don't mind, post the pressures. It's been awhile since I've seen something near stock and my memory needs boosted.
Drifting
Joined: Dec 2007
Posts: 1,647
Likes: 4
From: Argillite KY
Do you pull a fuel relay somewhere?
