I'm currently in the process of buying parts for a 406 small block build for a 1987 IROC, street/strip car. I am currently looking for an internally-balanced harmonic damper. I have been looking at either ATI #917780 or #917781. The only difference with these two dampers is the size: One is 6.35" and the other is around 7" (also the 7" is about $30 more). I was wondering if I should spring for the 7", or if it wouldn't be any better than the 6.35". Max RPM on the motor will be between 6500-7000. Any help is appreciated.

 

So you're having the 400 crank internally balanced?

I would go with the larger damper. It will more effectively dampen harmonics, and won't contribute significantly to rotating momentum.

Any other details of your build you'd like to share? Like heads, rods, induction, etc.

 

It's a Scat forged crank, so yes it is internally balanced. Block is a 1976 but it's splayed, 6" forged i-beam Rods, Forged aluminum dished pistons, AFR 220cc heads, compression around 10.1:1, naturally aspirated.

 

I've got another question. ATI makes two different 7" balancers: one is 8.75 lbs and the other weights 6.25 lbs. I know the more the balancer weighs, the better it dampens engine harmonics at high RPMs... but for a street/strip 406 @ 7000 RPMs, is 8.75 lbs a little much? Would it make more sense to get the lighter one and gain horsepower that way?