Iron heads and 91 pump gas

I plan on getting the 424 ci 403 short block together next year. I have ported #6 heads with positive seals, 2"/1.625" valves and filled crossover's. Other than new springs for the roller cam along with new positive seals, I will probably leave them alone. Problem is the 55cc chamber, it puts at 11 to 1 with 0 deck, aiming for about .010" the hole at most. Being a 403, I plan on enlarging the chambers close to 403 gasket around the valves an polishing the chambers to add 10cc. That will drop it to 10 to 1 or slightly less. I plan on running an appropriate sized roller cam, whatever Mark(Cutlassefi) recommends. I went 9.6 to 1 with crappy .065" quench and the small 204/214 cam in an Olds 350 using these heads, it sucked on 91 with day to day driving, pinging, I even put in NGK 5670-8 plugs, extremely cold and I will reuse them. I plan on Chinese Stainless full length 1 3/4" headers, Summit 2.5" X pipe exhaust with either Hooker Maximum Flow or Magnaflow mufflers, a 2500 stall Hughes converter, a junkyard TH350C with a Transgo shift kit and the stock 2.78 open with maybe a Lock Right or similar. Thoughts?
Oct 5, 2018
91 octane with no ethenol and you were still pinging with 9.6:1?
Yes, I had pinging. I don't know if it was the shitty quench combined with the small 204/214 cam, but guessing. It wasn't as fun at part throttle with limited timing but ran Ok at the track.


New member
Oct 3, 2018
I'd be real cautious of going to full 403 bore on the opening of the chamber. Its a delicate dance of lowering compression and loosing quench area. Personally I know you have some money into these heads, but I think you'd be money and time ahead to get a set of stock BB heads. Even J's will probably outflow what you have, have at least the valve you have in the 6's, and will drop the compression significantly allowing a much tighter quench. Just trim them as needed to find the sweet spot for your compression.
My discussions with Bernard Mondello shocked me, but he suggested with a good aftermarket rod to run quench down in the .027 to .030 range. Obviously tighter than I'd ever want to run the junk I put together, but mine lives at 10:1 with .040. With your rod, a 0 deck, and .036 cometics you shouldnt have any issues.
I could use #8 heads, I have a set in good shape, J's are nearly indentical to #8 other than slightly bigger runners and intake valve, the flow numbers are nearly identical stock, chamber size is close and both have that stupid lip under the exhaust valve. But it means some milling, bowl hog cutter at minimum under all the valves, 2" intake valves which I have from the 403 heads, machine work at the top of the guides for positive seals and new springs which I will need anyways. It is an option, these current #6 heads with definitely out flow stock J heads by probably 30 cfm. They CC around 77-78 with 0 deck will equal 9 to 1, close maybe a .020" cut to bump it slightly. I won't be going to track since they closed our local track, closer to 9 to 1 compression will probably be fine.
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Another option is these nearly fresh heads with 2 05"/1.56" valves with the bowls opened. I could do some blending on them, add roller springs, new seals and bring the chamber from 62-64cc to 70cc. That would give me 9.6 to 1 with 0 deck and a .040" gasket, 9.4 to 1 .010" in the hole.