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Rebuilt my 68 L79. Never really seemed right to me. Converted to roller cam and rockers. 11:1 CR with forged pistons. Correct Qudrajet rebuilt by Lars. Here's what I found when I scoped the cylinders. I plan on pulling the motor after 700 miles a rebuild again and don't want to over look something I'm not seeing. Any options are appreciated. This is #4 . I also had a over heated rocker on the intake valve for the cylinder. It turned blue. You can see a piece has broken off the piston. This is a representative of what all other pistons look like only after 700 miles? I've been burning 93 octane fuel. This was the cleanest piston. This just shows a spot in the oil coading inconsistency.
Where is the timing set at?
What doesn't seem "right" about it?
At a true 11 to 1...you would have to pull so much total timing the engine would run like **** and not burn completely each stroke.
Do a compression test and see if you are at 180 or higher.....
Jebby
Last edited by Jebbysan; Oct 12, 2020 at 09:59 AM.
From: At my Bar drinking and wrenching in Lafayette Colorado
You need to get that compression dropped down so you can run adequate timing to make the engine run right. Your XR276 cam needs a bunch of timing to run right, and you can't run the correct amount of timing with 11:1 compression. So you end up running retarded, which grossly elevates your combustion temperatures - a really bad recipe for a good-running engine...
Looks like more than just a timing / lean mixture / detonation issue to me. You should have heard the excessive pinging. I believe this is a mechanical clearance issue.
Some questions come to mind:
Was the blocked decked?
Clearance between top of piston & head? (head gasket thickness)
Was one cylinder sleeved?
Ring-end gap set too tight?
Piston to valve head clearance?
You need to get that compression dropped down so you can run adequate timing to make the engine run right. Your XR276 cam needs a bunch of timing to run right, and you can't run the correct amount of timing with 11:1 compression. So you end up running retarded, which grossly elevates your combustion temperatures - a really bad recipe for a good-running engine...
Looks like more than just a timing / lean mixture / detonation issue to me. You should have heard the excessive pinging. I believe this is a mechanical clearance issue.
Some questions come to mind:
Was the blocked decked?
Clearance between top of piston & head? (head gasket thickness)
Was one cylinder sleeved?
Ring-end gap set too tight?
Piston to valve head clearance?
no pinging at all? No sleved cylinder. The block was not decked due to orginal motor and I did not want to cut the numbers from the pad. The block was checked for square before this decision. The other questions I do not have answers to.
You need to get that compression dropped down so you can run adequate timing to make the engine run right. Your XR276 cam needs a bunch of timing to run right, and you can't run the correct amount of timing with 11:1 compression. So you end up running retarded, which grossly elevates your combustion temperatures - a really bad recipe for a good-running engine...
Lars
this motor was rated 11:1 from factory. Is it the fuel? I sure motors are running high compression ratios today.. please educate me.. i may not be understanding or i missing something.
this motor was rated 11:1 from factory. Is it the fuel? I sure motors are running high compression ratios today.. please educate me.. i may not be understanding or i missing something.
My wifes Equinox 2.4l 4 banger runs 12 to 1.....but it also has 50 years of chamber design evolution and sprays the fuel in the cylinder at 50,000psi. No way you can compare todays anything with older units. My own LS powered Silverado runs 10 to 1......EFI, chamber design and several other factors allow this.
11 to 1 L-79 327 with a correct timing curve and 36 degrees total timing will knock on 93 octane fuel.....I know this for fact. My 66' in the Avatar did.....they all will. That said.....if yours does not, then there are other things at play.....maybe it is not a true 11 to 1....maybe the curve is lazy in the distributor....etc......
Unless the L-79 is blueprinted to a 9" zero deck.....with a true 64cc head that is checked and the exact replacement piston.....it is not 11 to 1.
If you broke a piece off the piston.....it is detonating.....hard. I can't tell from your first photo as it is not clear enough.
No expert on detonation, but seems like if that were the issue, why only one piston broken? Should be indication signs on all eight, right?
In my former post, I was leaning towards an actual strike between piston & valve. In that photo, it would appear that's the area where the exhaust valve could strike depending on clearance. And that should be around 0.080 min.
Also, a too tight of piston ring gap that heats up could expand to the point of breaking a piston ring land. H-m-m-m-m-m-m- Especially if nitrous is involved.
If the timing was causing detonation issues along with really high compression ratio, trying to start this engine would have been an adventure. Often times they actually kick back and damage the starter.
Last edited by HeadsU.P.; Oct 14, 2020 at 08:08 PM.
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Are you seeing any black or blue smoke? Are you using a lot of oil? I was thinking your ring gap was too tight and you buckled a ring to get that to pop. But looking at the clean piston I'm thinking you may be sucking in too much oil either through a bad intake seal or too much oil in through the pcv. Your engine should be fine with 10.5 or 11:1. Are you running aluminum heads.
I think you have so much oil caked and crusted on top of your piston it looks like a broken piston. I had the same issue on my 396 and using a sealant on both sides of the intake gasket cured it.
How is it running? How is a compression check? Turn it over until the needle stops.
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Are you seeing any black or blue smoke? Are you using a lot of oil? I was thinking your ring gap was too tight and you buckled a ring to get that to pop. But looking at the clean piston I'm thinking you may be sucking in too much oil either through a bad intake seal or too much oil in through the pcv. Your engine should be fine with 10.5 or 11:1. Are you running aluminum heads.
I think you have so much oil caked and crusted on top of your piston it looks like a broken piston. I had the same issue on my 396 and using a sealant on both sides of the intake gasket cured it.
How is it running? How is a compression check? Turn it over until the needle stops.
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
You should run your engine and spray some sea foam or a squirt bottle of water and mist water into carb to steam clean the pistons and valves while it's running to seecwhat the tops look like before dismantling
What may be going here
