[geeteeohguy]

70Dream, I have a '67 GTO convertible with TH400 and the original 400 engine that I've driven about 125,000 miles in the past 28 years. I've taken this car all over the US. I drive it a lot. Here's my take: in the early '80's, with the stock, never-been-touched 400, it would ping very slightly at WOT on 94 octane leaded gas. By the mid '80's, as octane dropped to 92 out here in CA, it ran worse. Pinged on light load, too. I rebuilt the engine at 173,000 miles because it had a burn exhaust valve. I bolted on a set of freshened up #12 '70 Ram Air heads. The car ran strong, but still pinged. I tried water injection, recurving the advance, fattening the carb, etc.etc. The problem? This car would not run without pinging with 92 octane and stock compression ratio. The car had a 3.36 gear, too. I ended up putting some huge chambered heads on the engine at about 190k miles, and it ran without pinging, but was sluggish. Compression factored out to 7.25 to 1. Stock grind, 067 cam this whole time. I ended up with an 068 cam and installed that. Ran a little better (turned out the 067 was keyed about 8 degrees retarded!). Finally, about 3 years ago, at 235,000 miles, I installed a set of 87cc #15 heads off of a '70 455. I reworked the heads (screw in studs, etc) and ended up with right on about 9 to 1 compression. I kept the 068 cam. Gas in CA is now 91 octane, and over $3.50 a gallon. The car ran like it dropped 1000 pounds of dead weight. It was fast, light on its feet, and a joy to drive. I next slipped in a 2.56 rear end (it was a posi and it was a freebee) and I lost a little accelleration off the line....but it was worth it. I've gotten over 20mpg with this set-up, and it will do it on 87-89 octane without pinging. It cruises like a dream, and the fact that it sips gas (for a GTO) allows me to drive it a lot without feeling depressed and broke. I applaud your choice of a 2.93 gear....It's a perfect ratio for a TH400 GTO. My long-winded point is, drop your compression to 9 to 1 or thereabouts, and enjoy the car. I have had a lot of stock Pontiacs over the years, and NONE of them ran without pinging on less than 94- plus octane fuel, which you can't get at the pump any more. My '65 GTO has stock HC heads that have been milled a little, and a huge cam....it needs race gas to run. I don't drive it very much at all for just that reason.

[70 Dream]

Originally Posted by geeteeohguy

Finally, about 3 years ago, at 235,000 miles, I installed a set of 87cc #15 heads off of a '70 455. I reworked the heads (screw in studs, etc) and ended up with right on about 9 to 1 compression. I kept the 068 cam. Gas in CA is now 91 octane, and over $3.50 a gallon. The car ran like it dropped 1000 pounds of dead weight. It was fast, light on its feet, and a joy to drive. I next slipped in a 2.56 rear end (it was a posi and it was a freebee) and I lost a little accelleration off the line....but it was worth it. I've gotten over 20mpg with this set-up, and it will do it on 87-89 octane without pinging. It cruises like a dream, and the fact that it sips gas (for a GTO) allows me to drive it a lot without feeling depressed and broke. I applaud your choice of a 2.93 gear....It's a perfect ratio for a TH400 GTO. My long-winded point is, drop your compression to 9 to 1 or thereabouts, and enjoy the car. I have had a lot of stock Pontiacs over the years, and NONE of them ran without pinging on less than 94- plus octane fuel, which you can't get at the pump any more.

Thanks for your post, GTOGuy, I am going to stick with my plan and do what you did, and this includes trying to find 2.93s or similar.

Won't know the final cc on my heads until the machine work is done, I suppose. I am going to start a new thread on cam selection, maybe you an help me with my cam choice?