hello everybody My daily river is a 65 Grand Prix, 389 low compression (8.6:1) tripower. I picked up a set of #77 heads super cheap as I wanted to get a little more power out of the engine. The #77 heads are rated at 10.5:1 on 389's, and 10.75:1 on 421's I believe. I have been told that the pontiacs achieve their compression ratios by way of the combustion chamber and that all mid 60's 389s used the same piston. I was wondering if anyone had experience with low compression 389's as I would think the piston needs to be dished in order to get the 8.6:1 CR? I need to run pump gas, but the car runs like a top now so I don't want to go pulling the heads off just to check, hence why I'm looking for some solid advice. My hope is that with the original pistons and the #77 heads I would be down closer to 9.5:1 or so, and then I would swap the heads and cam. Thanks in advance!!
The 1965 8.6:1 389 used casting # 75 heads, the 10.5:1 389 & 421 engines used #76 heads and finally the 10.75:1 389 & 421 engines all used #77 heads. In 1965 the low-compression 389 had it's own specific cylinder head with a larger combustion chamber. No low-compression 421s were ever built. Swapping to the # 77 heads will likely give you at the very least a true 10:1 compression, pretty marginal on today's pump gas especially when using a small street cam. The reason the 421 and 389 had the same compression ratio with the same head was because the 421 pistons had deeper valve reliefs to add some volume, about another 4ccs to keep the compression equal with the 389s. 1964 389 piston on the right, 1964 421 piston on the left.
Do you really need to mess with it? 8.6 on a Pontiac is just fine. Got any pics of your Grand Prix?? Welcome to the HAMB.
Hi guys thanks for the responses. The motor is the original 2 bbrl. With the original cam heads etc. I got the heads cheap enough so thought it might be nice. I'd rather not be over 9.5:1 so think I'll wait until I have the time to take it off the road and rebuild it then. I just wanted to wake it up some, it is fine as is, but I'm always tinkering... I'll get some pics of the car later this afternoon.
I didn't realize the 65 GP had that engine available. It does show up in the NHRA class guide. Most were hi comp ,4 bbls. My experience has been that the Tri Power doesn't work so good on the low comp. engines. You'd need some compression and at least an 068 cam. You are right to wait and tear it apart, but check the deck height before you do. It may need to be bored anyway ,which adds a little bit of compression too. You could push it up near 10 to 1 with premium gas, with some distributor work .
I put the tripower on. low cr pontiacs had the 2 brrl. Pontiac tripower uses the center carb which is mostly identical to the original carb for most work, until you hit about 75% throttle, at which time the outboard carbs come in. Apart from the shorter runners with the tripower manifold, its almost an identical set up to the original engine running on the center carb. the car feels the same if i take the outboard carbs offline.
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