Compression test question
Race Director
Joined: Jan 2003
Posts: 17,683
Likes: 3,678
From: Central Arkansas
I will re-run the leak down and see if I get the same results. This was my procedure, let me know if it's correct.
Started with #1 at TDC by the timing mark and also can see the piston at TDC through the plug hole. The intake manifold is off so I can also see both valves are closed. Set the pressure to the tester to 100# connect to the cylinder and read the pressure that the cylinder holds.
Continue cylinder by cylinder by firing order.
Started with #1 at TDC by the timing mark and also can see the piston at TDC through the plug hole. The intake manifold is off so I can also see both valves are closed. Set the pressure to the tester to 100# connect to the cylinder and read the pressure that the cylinder holds.
Continue cylinder by cylinder by firing order.
I haven't used my gage in years. Is this a single or double gage leakdown tester? I've only used a double gage unit. If you have forged pistons and loose clearances you probably won't get great numbers on a cold engine - but they should be a lot better than most of those.
I still doubt you have a significant engine problem worth going inside over.
Melting Slicks
Joined: Oct 2002
Posts: 2,317
Likes: 111
From: St Louis MO
The leak down test should be a good indicator of ring problems if it's conducted properly and the tester is clean and in good working order. It is also possible that debris is restricting the orifice and causing a greater than normal pressure drop and therefore large leakdown readings.
Have you verified that the orifice in the tester is clear?
Is the air to the tester passed through an inline filter?
As far as #7 is concerned, if the piston won't stay at TDC, it never was at TDC.
Have you verified that the orifice in the tester is clear?
Is the air to the tester passed through an inline filter?
As far as #7 is concerned, if the piston won't stay at TDC, it never was at TDC.
Team Owner
Joined: Oct 2000
Posts: 38,897
Likes: 1,926
From: Washington Michigan
Melting Slicks
Joined: Oct 2002
Posts: 2,317
Likes: 111
From: St Louis MO
Looking to see if both valves are closed on #1 is difficult because if it's not at firing, it is in overlap and both valves will appear closed but really aren't. Checking the cylinder three positions later in the firing order (18436572) will have the intake valve nearly full open which makes it a lot easier to see!
Thread Starter
Drifting
Joined: Oct 2001
Posts: 1,550
Likes: 652
From: Richmond VA
I going to re-do the leak down but first I'm going to try to get another gauge so all bases are covered. I was quite sure I was at TDC on all cylinders, valve covers and intake is off so it's easy to see when valves are closed but it was late so will check it one more time.
Le Mans Master
Joined: Nov 2008
Posts: 5,691
Likes: 12
From: columbus oh
I going to re-do the leak down but first I'm going to try to get another gauge so all bases are covered. I was quite sure I was at TDC on all cylinders, valve covers and intake is off so it's easy to see when valves are closed but it was late so will check it one more time.
Race Director
Joined: Jan 2003
Posts: 17,683
Likes: 3,678
From: Central Arkansas
A very simple way I used the other day to determine when a cylinder was approaching TDC on the compression cycle was to clean the plug seat and stick a piece of masking tape over the plug hole, sealing it tight. As the piston approaches TDC building compression the tape will let go with a very audible "pop". Hard to get much simpler.
Thread Starter
Drifting
Joined: Oct 2001
Posts: 1,550
Likes: 652
From: Richmond VA
OK, new day new leak down test. I used a different gauge and triple checked TDC on all cylinders. Finger over the spark plug hole to feel when piston is coming up on compression, check that both valves are closed, look in plug hole to see piston and rock crank and watch to make sure it's TDC.
#1 - 70% leak, all air coming through exhaust.
#2 - 40% leak, all air going into the crank case.
#3 - 45% leak, air going into the crank case and a little through exhaust.
#4 - 30% leak, all air going into the crank case.
#5 - 30% leak, all air going into the crank case.
#6 - 40% leak, all air going into the crank case.
#7 - 40% leak, air going into the crank case and through exhaust.
#8 - 40% leak, all air coming through exhaust.
On the cylinder with leaks through the exhaust, I bumped the exhaust valves a couple of times incase there was a piece of carbon keeping them from seating but there was no difference found.
#1 - 70% leak, all air coming through exhaust.
#2 - 40% leak, all air going into the crank case.
#3 - 45% leak, air going into the crank case and a little through exhaust.
#4 - 30% leak, all air going into the crank case.
#5 - 30% leak, all air going into the crank case.
#6 - 40% leak, all air going into the crank case.
#7 - 40% leak, air going into the crank case and through exhaust.
#8 - 40% leak, all air coming through exhaust.
On the cylinder with leaks through the exhaust, I bumped the exhaust valves a couple of times incase there was a piece of carbon keeping them from seating but there was no difference found.
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: Feb 2008
Posts: 6,483
Likes: 1,918
From: Folsom CA
If your valves are not sealing you certainly can improve performance by fixing your valve seal and you need to pull the heads as noted.
GM made many double hump heads and some had small valves while others had all the way up to the 2.02 intake size. Find out what you have before deciding what to do next.
GM made many double hump heads and some had small valves while others had all the way up to the 2.02 intake size. Find out what you have before deciding what to do next.
