[Mike Smith]

I need help. Just got back a 1969 Totally original GTO. Totally restoring it. Drive train first. I recall this engine sports the 10:75 Compression. I was just wondering Is there a way to cam this engine to bleed off some compression to thwart detonation problems. This car has been in the family all its life. I remember the detonation problems then and I can only figure the problems have gottten worse with todays fuel. My brother made the mistake , years ago...high school..., putting the RA-4 cam in it. Never ran right. This car is totally numbers matching, but, I want to be able to drive it regularly. Any help from you guys would be greatly appreciated. The car is an automatic with 3:55 gears. Just the standard 400 for that model. Nothing fancy. I see a lot of rec. for the summitt cam. 2801. Thanks for your help. You guys are awsome.

[lust4speed]

Actually the RAIV cam was a good choice to bleed off compression, but as you found out there are negatives with going to a larger cam. Anything smaller than a RAIV won't be bleeding off much compression. You need to keep the original heads, so the best course of action is for a set of dished pistons to set the compression ratio right where you want it. Going the piston route allows the engine externals to be 100% correct, and allow you to enjoy the car without mixing in race gas. My choice would be nothing over 9.3:1 to insure it will run in the middle of Summer and with a tank of marginal gas. Most of our cars sit more than they are driven and the fuel will drop to a little lower octane level sitting over a month or so.

[android 211]

I owned a totally stock 3.55 geared '69 convertible with 135,000 miles indicated on the odometer. It ran pretty well on 93 with the stock 067 cam. There was some light ping at full throttle on a hot day but I never even started messing with it to reduce the problem before I parked it and eventually sold it. The actual compression ratio is probably under 10:1. The 10.75 figure is optimistic. I say zero deck it, install the 068 or the 744 cam and change to a big 4 core or aluminum radiator and a 7 blade clutch fan if it doesn't already have one. Those two cams have wide LSAs which have a positive effect on detonation. Avoid cams with tight LSAs.

[screamingchief]

As mentioned,,,the actual CR likely is'nt 10.75:1...

The factory typically over-rated the CR of these engines by at least .5 point of CR.

And most virgin heads will rarely ever measure right @ their "nominal" cc specs,so that tends to drop the CR just a bit more.

I tend to agree that were you to cc the heads and measure the deck height and all the other pertinent things that go into determining the CR,you would be lucky if it were even an honest 10.0:1 CR.

Decent cams for combos like that,the Summit 2802 comes to mind right off the bat,or there is a Crane cam that is a near duplicate of that Summit cam.

And then there are always custom ground cams too.

FWIW

Bret P.