L48 compression test questions
Thread Starter
Advanced
Joined: Mar 2012
Posts: 81
Likes: 0
From: Dallas
L48 compression test questions
I bought a new2me 1980 corvette on Thursday. Today I set off to run a compression test.
I measured on a warm (still hot enough to burn skin) engine. It wasn't at full temp because it took me too long to remove the spark plugs from those terribly placed exhaust headers!
All cylinders were 180psi +\- 4psi.
So the question is... Is this good? What is the expected range? I don't yet have a 1980 shop manual.
It is obvious the engine has been modified; but the block is original. I can't yet tell if the crank, cam, or head are stock.
I measured on a warm (still hot enough to burn skin) engine. It wasn't at full temp because it took me too long to remove the spark plugs from those terribly placed exhaust headers!
All cylinders were 180psi +\- 4psi.
So the question is... Is this good? What is the expected range? I don't yet have a 1980 shop manual.
It is obvious the engine has been modified; but the block is original. I can't yet tell if the crank, cam, or head are stock.
Race Director
Joined: May 2005
Posts: 10,674
Likes: 122
From: Millington Illinois
St. Jude Donor '08-'09
I bought a new2me 1980 corvette on Thursday. Today I set off to run a compression test.
I measured on a warm (still hot enough to burn skin) engine. It wasn't at full temp because it took me too long to remove the spark plugs from those terribly placed exhaust headers!
All cylinders were 180psi +\- 4psi.
So the question is... Is this good? What is the expected range? I don't yet have a 1980 shop manual.
It is obvious the engine has been modified; but the block is original. I can't yet tell if the crank, cam, or head are stock.
I measured on a warm (still hot enough to burn skin) engine. It wasn't at full temp because it took me too long to remove the spark plugs from those terribly placed exhaust headers!
All cylinders were 180psi +\- 4psi.
So the question is... Is this good? What is the expected range? I don't yet have a 1980 shop manual.
It is obvious the engine has been modified; but the block is original. I can't yet tell if the crank, cam, or head are stock.
Stock is about 150. Somebody juiced it up a little and sounds like they did a good job.
Thread Starter
Advanced
Joined: Mar 2012
Posts: 81
Likes: 0
From: Dallas
That would be nice! I read somewhere that carbon build-up bumps the PSI... I'd rather not pull the heads, so is there a way to figure out where it would it be coming from?
It's pretty worn, but the block ID seems to be the original block for the car. I don't know how to prove original heads, but the casting number seems to be correct.
Perhaps it's stroked? Would a vacuum tester tell me that?
A perf cam wouldn't change compression would it?
It's pretty worn, but the block ID seems to be the original block for the car. I don't know how to prove original heads, but the casting number seems to be correct.
Perhaps it's stroked? Would a vacuum tester tell me that?
A perf cam wouldn't change compression would it?
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
That's pretty decent compression, man. I bet it goes pretty good, huh? Are you able to run regular gas in it?
I've been thinking about it, and without actually trying it, I don't know if it's possible to do, but if you could stick a very thin, stiff rod, like maybe one of those little wooden skewers that beef teriyaki comes on, down in the spark plug hole, you might be able to tell if it has a 3.75" or 3.48" stroke. I can't really visualize all the angles in my head, so I don't know for sure if it's even possible.
Cam timing definitely has an effect on compression, because it controls when the valves are open and closed during the cycle. Read this article:
http://www.badasscars.com/index.cfm/...d=68/prd68.htm
Keep the shiny side up!
Scott
I've been thinking about it, and without actually trying it, I don't know if it's possible to do, but if you could stick a very thin, stiff rod, like maybe one of those little wooden skewers that beef teriyaki comes on, down in the spark plug hole, you might be able to tell if it has a 3.75" or 3.48" stroke. I can't really visualize all the angles in my head, so I don't know for sure if it's even possible.
Cam timing definitely has an effect on compression, because it controls when the valves are open and closed during the cycle. Read this article:
http://www.badasscars.com/index.cfm/...d=68/prd68.htm
Keep the shiny side up!
Scott
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
Have you started a thread on the transmission, or is it something you have a handle on, and don't need any help with? If you are looking for a manual, I have a Chilton's and a Haynes, I would recommend both, because each one covers some things more in depth than the other, between the two of them, I'm in pretty good shape. But the most important 'vette book I own is the Assembly Instruction Manual, known around here as the AIM, it's a lifesaver! It's basically the manual that the guys on the assembly line used to build your car, and it show how everything goes together, and I mean everything! Right down to how they installed the emblems.
Keep the shiny side up!
Scott
Keep the shiny side up!
Scott
Thread Starter
Advanced
Joined: Mar 2012
Posts: 81
Likes: 0
From: Dallas
No I've not started on the tranny yet. Need to figure out my lift setup first. I could/will eventually buy a real shop lift but not for at least a few months. But then I'd need a tranny jack too. Seems more realistic to borrow an engine hoist for $17/day & pull it all.
Drifting
Joined: Mar 2011
Posts: 1,886
Likes: 9
I bought a new2me 1980 corvette on Thursday. Today I set off to run a compression test.
I measured on a warm (still hot enough to burn skin) engine. It wasn't at full temp because it took me too long to remove the spark plugs from those terribly placed exhaust headers!
All cylinders were 180psi +\- 4psi.
So the question is... Is this good? What is the expected range? I don't yet have a 1980 shop manual.
It is obvious the engine has been modified; but the block is original. I can't yet tell if the crank, cam, or head are stock.
I measured on a warm (still hot enough to burn skin) engine. It wasn't at full temp because it took me too long to remove the spark plugs from those terribly placed exhaust headers!
All cylinders were 180psi +\- 4psi.
So the question is... Is this good? What is the expected range? I don't yet have a 1980 shop manual.
It is obvious the engine has been modified; but the block is original. I can't yet tell if the crank, cam, or head are stock.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Race Director
Joined: May 2005
Posts: 10,674
Likes: 122
From: Millington Illinois
St. Jude Donor '08-'09
That is where I was coming from. +/- 10% is typical good readings. To be +/- 2% and 30 PSI over what you would see on a stock engine is great. Well built and beefed up is a good start. If tuning is right it should be a fun engine. Doubt carbon buildup is the reason for higher compresion readings
Thread Starter
Advanced
Joined: Mar 2012
Posts: 81
Likes: 0
From: Dallas
Well thanks for the good news! I have decided not to pull the heads because I believe everything is in good shape.
The one last internal engine issue: most (not all) of my spark plugs are oil fouled! I'm assuming that since my compression is good the most likely failure point is leaky valve seals. I bought the complete set yesterday and plan to replace them all today.
Assuming I have time for a good carb tune and replacing the coolant temp sender, I should have a finished happy engine by the end of the day!!
The one last internal engine issue: most (not all) of my spark plugs are oil fouled! I'm assuming that since my compression is good the most likely failure point is leaky valve seals. I bought the complete set yesterday and plan to replace them all today.
Assuming I have time for a good carb tune and replacing the coolant temp sender, I should have a finished happy engine by the end of the day!!