BB stock compression ratios
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Nominally 10.25. That's if everything was set to specs. Most check actually closer to 9.8 or so until you do some "optimizing" and putting things to the min/max values on head chambers, deck height, pistons, head gasket et.
JIM
JIM
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Cam choice has a HUGE impact on what fuel you can run...as well as tune, temps, altitude and overall engine efficiency. Don't underestimate any of those. I ran 93 in my 555 with 11.56 compression ratio on street, track and dyno. Never an issue. But it had aluminum heads, real tight quench and tuned well. I've done a lot of iron head engines with 10+...and even some at 11.0...but you have to watch whole combo.
JIM
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Distributor timing is also important. The original vacuum advance can on a 69 427 is an MS 360 12 and unavailable now. It only produced 12 degrees of vacuum advance so with a modern can that produces 16 degrees it's easy to get a quick off idle ping when you step on it before the can catches up and releases. People then blame the octane rating when just using a vacuum advance restrict plate would solve the problem instead of higher octane gas or dialing back the initial distributor timing.
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Racer
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Cam choice has a HUGE impact on what fuel you can run...as well as tune, temps, altitude and overall engine efficiency. Don't underestimate any of those. I ran 93 in my 555 with 11.56 compression ratio on street, track and dyno. Never an issue. But it had aluminum heads, real tight quench and tuned well. I've done a lot of iron head engines with 10+...and even some at 11.0...but you have to watch whole combo.
JIM
JIM
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CC your heads. The Chambers can be bigger or smaller than specs say. I use a 4 buck 30ml (cc) Walmart flavor shooter for injecting gravy into meat and a piece of Plexi about 6 inches square with a 3/8ths hole near one side.
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I have the engine currently in a rebuild shop and they have checked and re-checked by whatever method they use. I'm coming in around 10.5 and a little higher. I didn't have a detonation problem at all prior but the mechanic says that is because I've been running an extremely rich mixture and that is obvious because the passages in the head are covered with carbon and a gas oily mixture that is rather gummy. He says if you correct that problem you will be having some pre-ignition issues staying at 10.6. Therefore I should bring the compression back to about 9.5 and try to run the fuel mixture much leaner to prevent fouling plugs and gumming up the internals again. The cylinder bores were about .003 oversize for some reason so maybe that was the reason i was losing oil too fast. We're trying to determine what went wrong the engine and parts don't show much wear overall. Thanks for the comment, I'll ask about that method.
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"The cylinder bores were about .003 oversize for some reason so maybe that was the reason i was losing oil too fast. We're trying to determine what went wrong the engine and parts don't show much wear overall".
Could be the prior owner installed a motor built for a performance boat into your Corvette.
Back when I had drag boats it was common for guys to sell their hulls and save their high $$$ big blocks for a car project.
Boat motors are built a bit looser than a street motor and it's perfectly ok to run a marine motor on the street, however you don't want to run a street motor in a marine application.
Could be the prior owner installed a motor built for a performance boat into your Corvette.
Back when I had drag boats it was common for guys to sell their hulls and save their high $$$ big blocks for a car project.
Boat motors are built a bit looser than a street motor and it's perfectly ok to run a marine motor on the street, however you don't want to run a street motor in a marine application.
Last edited by OldCarBum; Dec 7, 2018 at 01:39 PM.
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As mentioned...a LOT depends on tune. IF it's tuned properly...you can run 10.5 on good pump gas all day. IF there are issues....it's better to run less compression and more timing than it is to run higher and pull timing out of it. If you have oil control issues,,,.that will cause problems also. Oil in the intake stream doesn't help. A mega-rich tune will wash the cylinder walls and cause ring/piston/bore wear. Having EFI on new engines is a prime reason as to why new engines go 200K+ with no issues.
Try to keep engine as cool as possible...select the right cam, block heat crossover in the intake, dial in timing curve etc.
Most machine shops err to the side of caution. You can't blame them...they have no control once it leaves their shop. Some owners will do things correctly and some won't. They try to make sure you don't have issues even if t gives up some power/drivability.
What numbers are you getting? Deck height? Piston cc, head CC?
JIM
Try to keep engine as cool as possible...select the right cam, block heat crossover in the intake, dial in timing curve etc.
Most machine shops err to the side of caution. You can't blame them...they have no control once it leaves their shop. Some owners will do things correctly and some won't. They try to make sure you don't have issues even if t gives up some power/drivability.
What numbers are you getting? Deck height? Piston cc, head CC?
JIM
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Racer
Joined: Aug 2011
Posts: 429
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From: Charlotte NC
Could be the prior owner installed a motor built for a performance boat into your Corvette.
Back when I had drag boats it was common for guys to sell their hulls and save their high $$$ big blocks for a car project.
Boat motors are built a bit looser than a street motor and it's perfectly ok to run a marine motor on the street, however you don't want to run a street motor in a marine application.
Back when I had drag boats it was common for guys to sell their hulls and save their high $$$ big blocks for a car project.
Boat motors are built a bit looser than a street motor and it's perfectly ok to run a marine motor on the street, however you don't want to run a street motor in a marine application.
Thread Starter
Racer
Joined: Aug 2011
Posts: 429
Likes: 24
From: Charlotte NC
As mentioned...a LOT depends on tune. IF it's tuned properly...you can run 10.5 on good pump gas all day. IF there are issues....it's better to run less compression and more timing than it is to run higher and pull timing out of it. If you have oil control issues,,,.that will cause problems also. Oil in the intake stream doesn't help. A mega-rich tune will wash the cylinder walls and cause ring/piston/bore wear. Having EFI on new engines is a prime reason as to why new engines go 200K+ with no issues.
Try to keep engine as cool as possible...select the right cam, block heat crossover in the intake, dial in timing curve etc.
Most machine shops err to the side of caution. You can't blame them...they have no control once it leaves their shop. Some owners will do things correctly and some won't. They try to make sure you don't have issues even if t gives up some power/drivability.
What numbers are you getting? Deck height? Piston cc, head CC?
JIM
Try to keep engine as cool as possible...select the right cam, block heat crossover in the intake, dial in timing curve etc.
Most machine shops err to the side of caution. You can't blame them...they have no control once it leaves their shop. Some owners will do things correctly and some won't. They try to make sure you don't have issues even if t gives up some power/drivability.
What numbers are you getting? Deck height? Piston cc, head CC?
JIM
I’ll probably go to .040 over if that’s enough to clean up and use the recommended tolerance for the pistons I choose. Might be only removing about .008 though so may need to go to .060 to have good clean up.
He said he hasn’t checked the deck height yet. I think I’ll take his advise and stay around 9.5 and bump the cam up slightly. Stay with the L88 sized iron heads and I bet I can do a lot better for using as a DD. Thanks.