I've been chasing this issue for the better part of a year now and have made little progress since this is my daily driver, I will first describe the issue and the steps I've taken to solve it, how it's changed over time and what theories I have.

My engine is a gmpp 350 300hp 9.1:1 compression running r45ts with an EPS intake, AVS2 carb and 8362 streetfire distributor paired with a 700r4 automatic and 2.73 rear gears. The problem I am having is I get detonation above 4500 RPM really bad, originally when I had the car on the road for the first two months I never had any detonation noise at all, I set the distributor timing to 12 degrees BTDC which gives me a total (supposedly) of 34 degrees, running vacuum advance on ported it was fine on 87 octane gas, I had no issues or hints of detonation. Once the weather started to get warmer I noticed that slight detonation noise when cruising around town so I upped the gas to 89 and bought an adjustable vacuum advance can which got rid of the part throttle detonation but I still had it at wide open, so then I ran 93 and it seemed like it solved the issue, until it started happening again, this time it would only happen above 4500 RPM on WOT pulls, so I set the timing back from 12 degrees to 8 which helped for the time being, until it came back AGAIN, leaving me to set the timing to 6 degrees a month or two later and that's where it's stayed for a few months. I was never able to truly verify total timing because both of the timing lights I used seemed to show the timing retarding itself above 4000 RPM for whatever reason when the engine dictated opposite.

I thought maybe at first it was a fuel delivery issue since I was pulling through the factory pump in the tank but I removed that, used a very good mechanical pump and verified fuel pressure with a gauge, it was fine at WOT. So I dug into the carburetor maybe thinking that the secondary circuit was jetted too lean, it wasn't, I jetted it up one stage with the calibration kit and it made no difference, I never bothered going any higher than one stage since it wouldn't make sense given it a 350 considering what other people usually run for secondary jets on the AVS2, so I crossed fuel delivery off the list for now.

Next I tried to tackle the advance mechanism in the distributor, maybe there was a problem with it, I ended up making my own stop screw that I tapped into where the rotor mounts to limit the advance to a true 18 degrees of mechanical advance, so it starts at 14 and ends at 32 and THIS actually made a difference, I did a WOT pull with 93 octane in the tank and I had no detonation whatsoever, just a nice clean hard pull. I put 89 octane in the tank a few days later and did the same pull and in first gear there was absolutely zero detonation however as soon as it shifted into 2nd gear, the detonation was back. It’s weird since there’s NO way this thing is advanced more than 32 degrees at wide open.

at this point I am just at a loss and that's why I came here to see if maybe you guys have had something like this happen, some threads I've seen have mentioned intake manifold gaskets (I do have oil smoke on startup and a slight amount of smoke when I rev the engine in park when warm but I think that's valve seals), others have mentioned the gear set being the soul cause. I honestly do not know. Let me know what you guys think it is. I’m very tired of paying for 93 at the pump every time I fill the car up when I used to be able to run 87 no problem.


These are the specs for the engine