Compression Test, DYNO & expected results
Thread Starter
Racer
Joined: Jul 2005
Posts: 420
Likes: 0
From: Yerington NV
Compression Test, DYNO & expected results
I have a friend that has just about every imaginable tool, test equpment and lift, that I am going to see next month in the 80.
He has already said that we are putting her on DYNO to see where we stand after all my mods (see profile) .....
We are also going to weigh her, and do a compression test, just so that I know where I stand.
I am very interested, as I have worked on this car almost daily for 3 years now, and from all I have read, I am probably in for a disappointment, so with that in mind, I am prepared.
ON TO compression testing, in laymans terms, what is it all testing, and what kind of results should I expect with my setup? I am basing everything against "STOCK" as I never did any testing before tearing into motor with new AFR 180cc/64cc heads, Comp camshaft 274H, Edelbrock 2701 EPS intake, Edelbrock EPS 1406 carb, removing all the smog and installing 2.5" true duals with some cheapie Summit headers.
Stock on the 80 was advertised something like: SBC350 190HP 280 ft/lb torque, Comp: 8.2:1
Anyone want to venture a guess to where I stand now?
He has already said that we are putting her on DYNO to see where we stand after all my mods (see profile) .....
We are also going to weigh her, and do a compression test, just so that I know where I stand.
I am very interested, as I have worked on this car almost daily for 3 years now, and from all I have read, I am probably in for a disappointment, so with that in mind, I am prepared.
ON TO compression testing, in laymans terms, what is it all testing, and what kind of results should I expect with my setup? I am basing everything against "STOCK" as I never did any testing before tearing into motor with new AFR 180cc/64cc heads, Comp camshaft 274H, Edelbrock 2701 EPS intake, Edelbrock EPS 1406 carb, removing all the smog and installing 2.5" true duals with some cheapie Summit headers.
Stock on the 80 was advertised something like: SBC350 190HP 280 ft/lb torque, Comp: 8.2:1
Anyone want to venture a guess to where I stand now?
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
A compression test is used as a diagnostic tool most of the time. You can kind of gauge the health of the engine with a little experience, or pinpoint a problem..
Thread Starter
Racer
Joined: Jul 2005
Posts: 420
Likes: 0
From: Yerington NV
Now to the HOW
Now tell me if I am barking up the right tree (and ignore my ignorance, as I have never done/seen one done before): The compression test is is a test that is done cylinder by cylinder, hooking up a gauge to each spark plug hole (instead of plug) and turning the engine over, and measuring the pressure in each cylinder, without actually firing up the motor, by disconnecting the power lead from HEI distributor?
If that is correct, what are ranges, on lets say a stock 8.2:1 test and how is that ratio obtained (so I can get the correct ratio when I do it with my modified motor).
P.S - I wouldn't be disappointed with +250rwhp, and would give me an opportunity to TUNE it to max to get every bit of hp & torque ....
Sometimes, I wonder if I got myself enough cam ... but hey, that is another story .....
If that is correct, what are ranges, on lets say a stock 8.2:1 test and how is that ratio obtained (so I can get the correct ratio when I do it with my modified motor).
P.S - I wouldn't be disappointed with +250rwhp, and would give me an opportunity to TUNE it to max to get every bit of hp & torque ....
Sometimes, I wonder if I got myself enough cam ... but hey, that is another story .....
Racer
Joined: Dec 2007
Posts: 445
Likes: 2
From: Thomastown Mississippi
When you do the compression test, I would first warm up the engine. Then remove all the spark plugs so the motor spins freely. Then just screw the compression guage in to each spark plug hole and have someone turn the key over a few seconds. The main thing is not really how high it reads on the guage, but that the readings on all 8 cylinders are close to the same. Within 10-15%. My stock 350 tested between 140-160. Number 8 cylinder was the low one, 140, but the reading hasn't changed in over a year. You just don't want a big difference in the readings. If 7 cylinders read 150 and 1 cylinder reads 60, you know you've got a problem.
Thread Starter
Racer
Joined: Jul 2005
Posts: 420
Likes: 0
From: Yerington NV
A) The 140-160 compression number is in PSI?
B) So how is the stock 8.2:1 (on my 80) CR computed from those values?
C) Could you have one/more cylinders that far off (60ish) and NOT NOTICE in driving the car?
D) I am assuming the heads' combustion chamber size, maybe even head gasket thickness plays into the CR value somewhere? Anything else I have not yet thought of?
B) So how is the stock 8.2:1 (on my 80) CR computed from those values?
C) Could you have one/more cylinders that far off (60ish) and NOT NOTICE in driving the car?
D) I am assuming the heads' combustion chamber size, maybe even head gasket thickness plays into the CR value somewhere? Anything else I have not yet thought of?
Team Owner
Joined: Aug 1999
Posts: 42,487
Likes: 47
SPARTAN
CI 3-4-5-6-8-9-10 Vet
CI-9 AutoX Winner
CI-3 Go Kart Champ
St. Jude '03-'04-'05-'06-'07-'08-'09-'10-'11
Compression test results are largely irrelevant as long as they're similar to each other. A high compression engine with a big cam with lots of overlap may well have a lower cylinder pressure in a compression test than a bone stock low-compression engine. As long as the results are within a few percent max to min, you're good. But for a ballpark, 150psi is a decent reference point (i.e. if you only see 100psi with your setup, something is wrong)
I'd also guess something above 250 rwhp. Maybe 275 even.
I'd also guess something above 250 rwhp. Maybe 275 even.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
You can make comparisons on compression tests between engines when they are configured identically. Other than that, you just want to have no more than 15-20 psi difference between any of the cylinders (10 would be better for a recently rebuilt engine). The psi you get as a measurement can't be converted [directly] into CR.
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
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
You can make comparisons on compression tests between engines when they are configured identically. Other than that, you just want to have no more than 15-20 psi difference between any of the cylinders (10 would be better for a recently rebuilt engine). The psi you get as a measurement can't be converted [directly] into CR.Compression ratio is a calculated number. What you really want for a compression test is no more than a 10% variance beteen cylinders. see where each cylinder is on the 1st stroke, then the 5th stroke, then what it makes maximum.