Too Much Compression..... - Pontiac GTO Forum
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post #1 of 3 (permalink) Old 01-07-2019, 03:07 AM Thread Starter
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Too Much Compression.....

I have a 72 lemans I bought with a 455 bored to a 472. I don't have any part numbers for the internals. I was just told the engine is at 11.5:1 with iron heads. Performer intake. 750cfm edelbrock carb. Hooker headers. No idea what pistons/cam/rods/crank etc. I've tried running 93 in it and the engine sounds and revs fine until it warms up. Then at anything over idle it sounds like caribbean drums going off in my engine. I'm assuming the fuel is detonating due to compression. I've had the timing looked at and told it was fine at 32 or so degrees. I've come to the conclusion using the Pontiac V8 Cylinder Heads chart the heads are 1968 16 heads. Those seem to be 72cc 10.5:1 heads. I'm left with three options I suppose. Convert to e85 preferably with a fitech. Buy aluminum heads and hope they won't detonate. Or lastly get bigger iron heads. What is going to be my best option? All 3 seem to be 1500+ options. I plan to go efi regardless. Any suggestions? Just looking for a reliable street car with decent power.

I thought this car was close to being complete but the more I get in it the more surprises I find and the lighter my wallet seems to get.
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post #2 of 3 (permalink) Old 01-07-2019, 02:38 PM
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85-87cc alum heads should fix it, if those are in your budget. Looks like it'll take between $2600 & $3200 for heads, bolts, & shipping.

Kauffman Racing Equipment

Usually costs maybe $1200-$1500 to buy good 6x-8 iron head cores, then have 'em set up correctly, using 1-piece stainless valves, new guides, etc.

Of course, the other high cost option is rebuilding with dish pistons.

Can also mix in some Torco Accelerator octane booster. It's a little trouble. But hey, $1500 will buy quite a bit of it. No more miles than some drive these old cars, octane booster is probably the best way to go. Might be worth buying a case to try. It might solve your problem & allow you keep the set-up you have.
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post #3 of 3 (permalink) Old 01-17-2019, 12:49 PM
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Big D nailed it. I ended up installing 87cc iron heads on my '67 GTO so I can run pump has the stock 400. For your car, you'd want heads in the 100cc range for pump gas. My '65 GTO has 64cc iron heads which have been milled for 11:1 compression (I built the engine in 1981). I use Octane Supreme 130 or race gas in that engine to be able to avoid detonation. It won't run on pump gas at operating temps without detonation. I don't drive it much for this reason, but it sure runs hard!
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, Lemans & GTO Complete Engine Compartment Discussion

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