10:1 compression on regular gas ??
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Senior User
Joined: Jul 2003
Posts: 204
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From: Montana
10:1 compression on regular gas ??
For a 390, is anyone running 10:1 flat top pistons with 428 Cj heads, and getting away with regular gas? I've been reading about the quench area of these heads. The heads have larger combustion chambers than 390 heads (with smaller valves), so this lowers compression a little. I've also read that making use of the quench area with 10:1 flat top pistons has less detonation problems than 9.5:1 compression.
So with the CJ larger combustion chamber dropping the 10:1 ratio down to what, around 9.7:1 (??). Combine this with less detonation by using the quench area (0.040 inch thick head gasket, piston tops level with top of block), altitude above 4500 feet, intake valve closing at 60 deg ABDC, and play with fuel mixture and ignition timing, will the engine run well on regular gas?
What does everyone think?
So with the CJ larger combustion chamber dropping the 10:1 ratio down to what, around 9.7:1 (??). Combine this with less detonation by using the quench area (0.040 inch thick head gasket, piston tops level with top of block), altitude above 4500 feet, intake valve closing at 60 deg ABDC, and play with fuel mixture and ignition timing, will the engine run well on regular gas?
What does everyone think?
Hotshot
Joined: May 2003
Posts: 11,141
Likes: 25
From: south louisiana
I can't say for sure about your combo, but the 390 in my 68 Monterey, runs fine on 87 octane ( with a little CD2 lead substute in it) My combo is a std bore and stock 10.5 to 1 cast flattops, heads are C8AE-H's, cam is a Crane 272 degree Energizer, timing is set at 12 degrees initial with the original distributor that cam ewith the car, but does have a Pertronix unit in it. Can't remember what the plugs are though. Head gaskets have been changed when I over hauled it last winter, instead of the .020 thick steel shims it came with, it's now got FelPro Permatorque Blue's which I think are .040 thick, so the comp ratio is down slightly as a result, but shouldn't be down by more than a 1/4 to 1/2 point though. The cam specs you need to observe in my opinion are the overlap. not when the intake valve closes, a cam with a small overlap will have a higher working comp ratio than one with a longer overlap. I've heard that if I'd have gone with a Comp Cams high energy 268H, I'd have had plenty of detonation problems with this motor's piston combo.
Posting Guru
Joined: Aug 2001
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From: Evanston, WY
Keep in mind that at 4500 feet you are effectively reducing your compression ratio by 9 tenths of a point. In other words, if you have 10:1 static compression ratio, it will act like a 9.1:1 compression engine would at sea level. You will likely have no problems. The thing will probably run on 85 octane.
-Scouder
-Scouder
More Turbo
Joined: Mar 2003
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From: Green Bay Wi
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Posting Guru
Joined: Jul 2003
Posts: 2,232
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From: Colville, Washington
I run 87 octane in my 10:1, measured, 428.I seldom have problems with pinging. Elevation is about 2500 to 3000 feet.
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