I rebuilt a 302 a little bit ago, .030" over block. I used flat top hyperutectic pistons, E7 heads, and a .047 compressed thickness head gasket. The block had a 0 deck height as measured the redneck way (straightedge across the bore). Assuming 62 cc combustion chambers, this combo should be in the area of 9.5:1 static compression.

But I am having trouble with detonation under heavy load. I am currently only running 4 degrees of initial advance, and though I haven't measured recently, if I recall correctly that should be 32 degrees total. This is on the 87 octane left in the tank.

I can try higher octane fuels, but wonder what the effect of changing to thinner head gaskets would be. What I have read tonight is that it would improve the quench affect, and promote better mixture of the fuel/air, thus improving resistance to detonation. A switch to a .039 thick gasket would also yield a .2 increase in static compression.

What do you all think? Would it be worth the cost to change to better/thinner head gaskets?

 

Since a lack of responses here, I'll summarize what came up on a couple other forums.

I got a few people that say I should run a thicker head gasket (not sure that exists)

And this from Zziggy on a local dragstrip forum:

"You are right, the proper quench is imperative in a wedge head motor.
The static compression ratio of 10:1 with the proper quench will not ping while a 7.5:1 with no regard toward the proper quench will be a pinging nightmare.

Tighten the quench to 0.040" and actually measure your deck height, just to be sure."

 

What cam and heads are you running this with ? I doubt the head gaskets are the problem if you truly have zero deck flat top pistons.

 

Before messing with a new head gasket, check to see if your vacuum advance is adjustable. Most DSII vacuum cans have an allen head screw inside that allow you to change when the vacuum advance kicks in. If not, you may need to recurve the dizzy to bring the mechanical advance in at higher rpm.

Thicker head gaskets do exist. I've got an expensive .060" MLS gasket in the garage I ended up swapping for an even pricier .080" MLS gasket to keep piston to head clearance up and the compression down on a 351W build (the pistons ended up ~.02" above deck).

 

Baddad - E7 heads with a comp 252H cam. That cam is mild, .433 lift, 206 degrees duration at .050" if I remember right.

And Mr Baron sir, Yeah the vacuum advance is adjustable. I don't remember what it is set to currently, but in an effort to be thorough, I disconnected it and plugged the tube, no change.

At 9.5 compression, with 89 octane and a high octane additive, I can't even ALMOST get away with 10 initial and 36 total in by 3000 rpm. That is the curve I have pretty well always run on 87 octane previously, and should be reasonable. 4 initial and 30 total just doesn't seem right...

I'm all for a solution that isn't the HG's though!!! I appreciate the input.

 

What carb do you have? With the timing that retarded it's got to be running lean. Maybe stiffer step-up springs or a higher numbered power valve might help?

 

I'd check the balancer to see if the outer ring has slipped. It should run fine with 12* initial timing with those heads and pistons. Also check the fuel mixture, if it's lean it'll do the same.

 

I'm running a holley 600 cfm jetted as it came from holley at 66 primaries, stock metering plate in the secondaries.

I checked my total timing tonight, and mechanical advance is 28 degrees on top of initial. So at 10 initial, that's 38 total. After reving the engine, I got a decent tick that sounded like a lifter. I hadn't noticed that before. It went away after a moment of idling. I checked vacuum levels, and its rock solid at 22 inches at idle.

Then I pulled all the spark plugs to double check that its not lean, and found something interesting. 1 looks a little too clean. 2,3,4 look to be rich. 5,6,7,8 look good. I also have AFR gauge that reads from an O2 sensor in the drivers side header - I don't trust it too much, but it shows being slightly rich.

Does this make sense to anyone? My thought is that a bad lifter in #1 may be preventing the exhaust gas from escaping, creating a lean mix. Or I guess it could be the the intake valve. Before I tear in to anything I'm gonna check compression to see if I get anything interesting there.

I really appreciate you guys helping me think this out!

 

If the #1 exhaust lobe is going flat, then that cylinder will at some point build up enough pressure to cause a backfire when the intake valve is opened. I've had two cam lobe failures, both were exhaust lobes (on two different cams) and both exhibited this same behavior. As for the AFR readings, I've never used one, I just read the plugs, the plugs don't lie. I've seen enough guys on the net pull their hair out trying to get EFI like AFR's from carbs, that's simply not going to happen.

 

The best article I have found explaining Ford small block timing is here:

Carbed Ford Message Board :: View topic - The Ultimate Duraspark Ignition Timing Guide

Starting with the total advance is the best way to set timing. This eliminates the unknown of mechanical advance at idle. Weak springs, wear in parts etc. can alter the behaviour of the mechanical advance.

As Baddad suggests, double check that the timing marks are correct as well.