Q: I own a classic 1970 Mopar with a 440 engine.

The engine is bored .040 inch over, has a forged crank, flat top pistons, and ‘452’ cast heads—the original ‘906’ cast heads were eaten up by unleaded fuel. It also has a Weiand Action Plus intake, a Holley 750 vacuum secondary carb, a stock distributor, a 272/282 duration cam (at .050 inches), and Thorley headers.

I’m having problems with detonation. I use super unleaded, and it sounds like popping corn under acceleration. When I bought the rig, some exhaust leaks covered up the detonation sound, and I blamed the poor performance on the chewed-up valve seats. Having fixed the leaks and replaced the heads, I can hear the sound now.

I have retarded the timing back so far the engine barely runs, which has reduced the pinging, but I can’t tow or drive around much in the mountains. I have even replaced the distributor with another stock unit and a performance model, to no avail. Do you think there is enough difference in the size of the heads to matter?

a chrysler 440 big block v8 under the hood of a classics muscle car mopar

A: According to our manuals, the 906 cast heads will give you a chamber size between 79-81cc, while the 452’s will give you between 86-92cc. So, switching to the 452 cast heads should have lowered your compression ratio, but we can’t really say exactly how much, due to the variation.

Another consideration is which pistons you have. Most 440 replacement pistons provide low compression, even with the 906 castings. Providing that your pistons are at least .020 inches down in the bore (they are probably a lot more), compression should not be a problem.

You didn’t mention any cooling issues, but an overheating engine can behave in the manner you described. An inefficient cooling system and/or a lean fuel condition can cause excessive engine temperatures.

Speaking of lean fuel, that condition can cause detonation. Our advice would be to set total timing between 32 inches and 36 inches, and adjust the jet size in your carburetor until you have a light brownish-colored spark plug. We’d also advise you to install a mechanical water temperature gauge: engine operating temperature should be in the 180-200 degree (Fahrenheit) range.

This is another in a series of weekly Q&A Mailbag sessions with Summit Racing’s tech department, in which there are hundreds more. Click here to see them all.