Just had a new 455 motor built for my GTO. Engine has a roller cam .510 lift, roller lifters, 9.1:1 compression and I have pinging between 1800-2500 under light throttle. The builder broke in the motor on the dyno and full advance set at 33 degrees. Not sure why he couldn't hear a pinging issue. Anyhow, I've disconnected the vacuum advance and have the heaviest springs installed on the mechanical advance. The only way to get rid of the pinging is to back the timing off to 8 degrees initial advance, total advance is 28. Is there a chance the cam was installed retarded? Any other ideas are welcome.  

Are you sure the C.R. is only 9:1 what is the head casting numbers and what type of piston are you using was the block or the heads shaved and if so how much.  

Yes, they're 6S heads which are 135cc. Flat top forged pistons, block was not decked.  

Heads weren't shaved either.  

Bad gas would ping like crazy.  

Good gas,91 octane and the same issue on gas from different filling stations. (I don't go to the cheap gas stations either)  

Have you checked the balancer to see if it is spun giving you a false timing reading.  

Good idea, I'll check it.  

Checked it, right on the money. Number 1 piston at top when mark hits "0".  

What is the pumping compression? It should be around 170psi.  

Compression is about 135psi.  

135PSI???  Ok, so it has a roller cam, but still...   Usually a tired 350CID 7.6:1 will crank 135psi.  You definitely have a weird situation there.

Do you have a copy of the Dyno sheet?  It should have the A/F and EGT on it.  

One more thought...

Put an fuel pressure gauge between the fuel pump outlet and the carb inlet and temporarily mount it where you can see it when you drive, but NOT in the passenger compartment (tape it to the hood or whatever).  Maybe the fuel pressure is nosediving causing the A/F is leaning out under actual operation as opposed to on the Dyno.  

Yep, I've got a dyno sheet. A/F range is between 10.5 and 11.2 between 2800 to 5800 RPM. NO EGT on it. I also run a fuel pressure gauge, although it's under the hood... but with it carb'd it doesn't make sense to me that pressure would be the problem especially if I have no issue's at upper RPM and backing off the timing has shut it up too why could it be fuel pressure? Don't get me wrong, I'm not an expert just trying to wrap my head around this issue.  

Hey, just found my paper work from the machine shop that cc'd the heads. Chamber cc's in head are 100 even. According to the spec sheet they provided motor punched .030 which yields it a 464.4 I should have a compression ratio of 9.14:1, this accounted for the valve reliefs in the pistons which was 6 cc's.