Much more and there would be trouble, but they revved so eagerly, with a 1.75 rod ratio and small and reltively mediocre heads, it was a great match. The 390 really didn't get on in street trim. Way too conservative on cam choices, and aluminum intakes like a CJ from the start with such cam and early heads with CJ valves would have made the 66 Fairlane right withthe GTO, they could rev and they had a really good rod ratio to port cross section, the unibody chassis should have been designed for at least CJ manifolds and had them standard on all GT cars. A 390 or 410 CJ with a cam like the Pontiacs had which was always in the 230ish range @.050 would have changed the story a lot, but woulda should coulda... HFII was just worried about 16 year olds wrapping them around phone poles, the insurance, and beating the world racing, he missed the parade. I have lost a few friends in those GM and Mopar missles. Morally he was right, but darn it its America, we should have had the choice.