Detonation Problems
 

:gmc2:

Team,

I have a sb350 (3970010) hooked to a TH400 (auto) from a 77 Chevy in place in my 67 GMC. We are running 305 heads (54cc) and have a decked block (not sure how far, but the vin plate isn't there anymore). We have been having detonation issues since we got the truck. Based on the block/head combo we are probably running ridiculous compression #'s but I don't know if that is contributing to it or not. Nothing under 2500rpm or so, but only over and at heavy load on the motor. Carb is new, dizzy and rotor new, plugs and wires new, and fuel pump new. We also run 93 octane and include booster in every tank. Everything seems to be set correctly according to specs. We are total timing of around 33. The issue is this, we have a high plane manifold (aftermarket of course) on the truck and the vacuum advance from the dizzy hits the manifold limiting the amount of spin we can put on the dizzy. Is there a way to get rid of that vacuum advance and switch to something more efficient w/o going to a comp controlled system? Any input would be grateful. We recently put in a new pressure guage on the fuel line and found that we are consitently running around 10psi when we should be at 6-7. I don't think that would have anything to do with the detonation issues though. The mixture on the carb was set with a vacuum guage and we are standing idle at around 900rpm. Not looking for a high end race truck, but would like to get on it every now and then so I'd like to get this worked out so we can finally move on to body work. Thanks,
Steve

Re: Detonation Problems
 

Yes if your Compression is too high it will cause detonation especially at your altitude. I'm sure there are better people here that can help too.
s/t

Re: Detonation Problems
 

Well, there is no way to know the actual CR if you don't know the type of pistons and the deck clearance. 54 cc heads will probably send your CR off the chart, especially if they are cast iron. I'd stop now, before you have nice melted holes in your pistons. Tear it down, measure, re-measure and see what combo (heads and pistons) you need. Dynamic compression ratio is a better figure than static compression ratio. Go to Keith Black's web site for the calculators. Good luck!

Re: Detonation Problems
 

I had a problem at 0* until I put in some R45TS plugs. It help a lot but didn't slove the issue at least I can run at 8* now still on 93

Re: Detonation Problems
 

The timing issue can be solved easily.Pull the distributor and rotate it one tooth.Then retime to spec.This should give you room to adjust enough.You might also look at you cam specs.More overlap can reduce compression since there will be some bleed off.I'm no "Guru" on cams but a cam tech line should be able to help.

Re: Detonation Problems
 

What's your compression pressure as measured with a compression gauge ? Although not a real gauge of compression ratio, it might help to know. You might have too much vacuum advance, about 10 deg vac advance may be all you can tolerate with high compression. You can buy a little cam thing that mounts on the vac adv screw to limit the vac advance amount. Unhook the vac advance and see what happens.

Re: Detonation Problems
 

Thanks for the help. I think if we pull the dist and rotate it, that may give us the extra little bit to kill the detonation issue. When retarding the timing you can hear the motor knock, and it seems that we just can't get far enough with the limit from the advance sticking out. I'm going to try the easier of the two first by disconnecting the vac line from the carb to the advance and then proceed with pulling the dist.
Thanks for all the replies team. I greatly appreciate it.
BTW- We are running 45TS plugs as recommended from spec.