Heads & Intake Swap = Low Compression
Thread Starter
Drifting
Joined: Jun 2004
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From: Gilbert, Arizona Currently Own: 1998 Coupe YouTube: My Corvette Life
Heads & Intake Swap = Low Compression
So I completed a heads and intake swap on my 98. Changed to 799 heads and LS6 intake. Connected everything back up, and it wouldn’t start. Low compression.
After a bit of research, we found that changing to the newer heads meant we should have changed to the 7.375” pushrods instead of reusing the 7.425” pushrods. We order up the new pushrods and installed them today.....same result.
Has anyone gone through this LS6 top end swap and come across this? Are we missing something? Need advise.
After a bit of research, we found that changing to the newer heads meant we should have changed to the 7.375” pushrods instead of reusing the 7.425” pushrods. We order up the new pushrods and installed them today.....same result.
Has anyone gone through this LS6 top end swap and come across this? Are we missing something? Need advise.
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are you positive it's due to low compression and not a fuel/spark issue?... did you attach the ground on the back of the driver side head?
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From: South Hill Wa
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Chris, I agree with Neutron. What makes you certain off the bat that you have low compression? I think you need to be investigating other avenues.
I'm sure you'll get this resolved in no time.
I'm sure you'll get this resolved in no time.
Thread Starter
Drifting
Joined: Jun 2004
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From: Gilbert, Arizona Currently Own: 1998 Coupe YouTube: My Corvette Life
it has fuel and spark. We checked that first. Yes, I did attach the ground to the driver’s side head. We did a compression check and it only had 50 pounds.
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50 on all cylinders?... I would pull all the rockers and do a leak down test, that will tell you if they are holding pressure or not... if that checks out then move on to the pushrods, you should buy an adjustable pushrod tool and measure for the correct length
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From: Anthony TX
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Pull the rockers and do a leak down test. You could have bent the intake valves. If pulling the rockers doesn't restore the cylinder pressures, be prepaired to pull the heads back off if pulling the rockers does not restore compression.
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Info about switching to the 7.375” pushrods is probably correct but it may be worth $10-15 to buy a pushrod length check tool; especially if you plan on other engine work later including cam swaps, lifter swaps, etc.
Hopefully the longer pushrods are just keeping the valves from seating and that the valves are not bent.
Good luck and hoping for an inexpensive fix.
Hopefully the longer pushrods are just keeping the valves from seating and that the valves are not bent.
Good luck and hoping for an inexpensive fix.
Pro
Joined: Dec 2017
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From: Volo, Illinois
So I completed a heads and intake swap on my 98. Changed to 799 heads and LS6 intake. Connected everything back up, and it wouldn’t start. Low compression.
After a bit of research, we found that changing to the newer heads meant we should have changed to the 7.375” pushrods instead of reusing the 7.425” pushrods. We order up the new pushrods and installed them today.....same result.
Has anyone gone through this LS6 top end swap and come across this? Are we missing something? Need advise.
After a bit of research, we found that changing to the newer heads meant we should have changed to the 7.375” pushrods instead of reusing the 7.425” pushrods. We order up the new pushrods and installed them today.....same result.
Has anyone gone through this LS6 top end swap and come across this? Are we missing something? Need advise.
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From: Newark Delaware
Interested as well. I watched the video and that car had zero compression. Either every valve was bent (which I doubt since he had the springs off and would have assumed you could see if they were bent that bad), or it's some push rod issue. I would have liked to have seen the engine crank with the valve covers off to see if the rockers were allowing the valves to close. Other than those two things, I can think of nothing else it could be.
Last edited by Robrote; Sep 25, 2018 at 01:23 PM.
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From: South Hill Wa
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As Rob said, after seeing the video that car had zero compression from the get go of cranking it. Either the valves got bent from hand cranking the engine or the valves were damaged before installation.
I don't buy the push rod being too long possibility at this time. They would have to have milled a metric ton off of his 243 heads in order for it to have had that much effect on push rod length. I've run 7.40 push rods on stock 243 heads before with a decent sized cam.
Just rambling here, and dying to know the resolution. Sounds like it might be a bit longer before he can get to it.
I don't buy the push rod being too long possibility at this time. They would have to have milled a metric ton off of his 243 heads in order for it to have had that much effect on push rod length. I've run 7.40 push rods on stock 243 heads before with a decent sized cam.
Just rambling here, and dying to know the resolution. Sounds like it might be a bit longer before he can get to it.