990 heads on 402?

[luzl78]

hey guys, technical question.i have a nos set of 990 heads. i am gonna have my 70 402-330 hp motor rebuilt. i am picking up a 589 manifold and matching holley carb and was wondering if this is to much head for the motor. its gonna be rebuilt with 11-1 pistons and a hot torque roller hydraulic cam. should i use this open chambered head or find some 291 heads which are closed chamber. its going in a 70 monte with a th 400 and 331 rear. cruiser car

[VintageMusclecar]

I don't think there are any off the shelf 11-1 pistons for an open chamber 396/402??-- you may have to go with a custom set.

Given the weight and gearing of the car, I'd stick with an oval port head. I would also advise skipping the 11-1 compression ratio and shoot for 9.5 to 10-1. The power difference would be minimal and you'll have a significantly larger tuning window to help keep the engine out of detonation.

The 569 intake will work fine on the oval ports, been done countless times before. Which Holley carb are you getting to go along with the 569 intake?

[luzl78]

Eric. It’s a 4492. It’s rich peters carb and manifold

[VintageMusclecar]

I double-checked and KB does offer an open chamber piston for the 396/402. They should result in ~10.5 compression with a 118 c.c head--but I'd still advise against the 990's. I'd go with an 049 or 781 head.

[luzl78]

Should I just stick with my original 290 heads. Same valve size as the other later model heads.

[VintageMusclecar]

You can go either way. The 781 or 049 will flow better than the 290's but require the dome for compression. The 290's use the short dome piston for 10.25-1 compression. For what you're looking to build, stock valve sizes will be fine either way.