So in my Firebird I have a slightly modifed original 350. Heads are stock, engine is 30 over (now a 360) cam is XE256H. Has headers and a two barrel. I also picked up a 557 block, machined for 9 to 1 cr. 10 over, new rotating assy when rebuilt, heads are ported and polished 6X 8 and are milled. Cam is a summit 2801. I bought the 400 carb to pan with headers and all accessories for 550 bucks. I know i can turn this around for an easy profit. Now here is the dilema, if i put a Qjet on the 350, how different will the two motors be? I tried deskstop dyno, And the 350 made slightly more HP on a wider rpm range. ???Torque was slightly lower. i have a hard time believing that, and want your opinions. Thanks!
P.s. Also posted on Firebird Nation
1968 Firebird, 350 4V, TH350 with trans-go, 69 slap shifter, 8.5 inch 10 bolt 3.42 posi
Are you sure that 400 motor has 9:1 CR with 6X-8 heads?? The 6X-8 heads have a pretty big combustion chamber and even with flat top pistons and the milling, I'd be shocked if you are truly at 9:1.
I've always thought that the Pontiac 350 was a very underrated engine. Years ago, I had a 71 Lemans Sport convertible with a 350, 4bbl, and a set of 3:23 gears. That car ran strong. The 350 had great street manners and pulled hard through the gears. I beat quite a few 5.0 Mustangs in some stoplight racing. Of course the hot rodder in me wanted more and the stock 350 was replaced with a mild 455 but that's a story for another day.
The Summit 2801 is very similar to the factory 068 grind cam which is a great street cam. I'd say that 400 would run strong in your car. If you're looking for a little more oompf then drop it in and hold onto the 350.
Are you sure that 400 motor has 9:1 CR with 6X-8 heads?? The 6X-8 heads have a pretty big combustion chamber and even with flat top pistons and the milling, I'd be shocked if you are truly at 9:1.
Yes, I'm pretty sure. The seller Is an honest guy. He spent $3500 on the engine 10 years ago. The block has been zero decked and the heads milled. I agree with you on the 350, it is very powerful and has gobs of torque. Definitely will not sell it.
1968 Firebird, 350 4V, TH350 with trans-go, 69 slap shifter, 8.5 inch 10 bolt 3.42 posi
Over on the PY site most people frown on using the 557 block because they are a thin casting in the mains and considered very week.
As stated above the 6X-4 350 (and 77-79 T/A 6.6), 93cc, 2.11/1.66" 6X-8 400, 101cc, 2.11/1.66"
If the 6X-8 heads were milled .050 to get 91cc and 0 deck would be about 8.8 compression with a 041 gasket. With a .027 head gasket you would get 9.02 compression.
