# 1968 GTO 400 Motor Spark knock



## larrw (Nov 11, 2011)

Newly rebuilt stock 1968 400 motor, with HEI ing. continues to spark knock, under accecleration with premium fuel and octane boost. Have reduced timing to 6 degrees BTC. Any suggestions on how to solve this?


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## BearGFR (Aug 25, 2008)

Hi Larry,
Need a little more information sir - do you know which heads you have? Which pistons? Head gaskets? How's your cooling system?

If it's going into detonation because compression is too high, then your solutions are going to be limited, I'm afraid. I wouldn't drive it until you get it figured out though. If it truly is detonation, it won't take long at all for it to hammer all the bearings to the point where you'll need another rebuild. 

It's probably not possible to add enough octane booster to get past the problem. Racing fuel, or perhaps 100 octane low lead aviation fuel would work, if you can get it --- but it's pretty expensive.
Depending on which heads you have, another solution would be to run dished pistons to get the compression down - but I'm sure you don't want to hear that either since you've just paid for a rebuild. Whoever did the work for you didn't do you any favors by not discussing this with you before doing the work, I'm afraid.

Hate to rain on your parade, but it's better to be cautious with these things. 


Bear


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