More HP!

Hi guys, I have a 1973 formula 350. The 350 is very tired & not very fast. I have a 1975 400 60 over. I also have a set of ported # 16 heads & flat top pistons. I am starting to build the 400 & I want to make sure I dont make a huge mistake. I have the Jim Hand Pontiac performance book & a recipe in the back of the book has a combo with the same heads & claims it will require 93 octane. One recipe has dished pistons & one does not. This is a street car & I dont mind the 93 I just dont want to run any kind of race gas. I bought the motor with the heads on it cheap. So if I need another head thats ok. The 16s need a valve job anyway. They do have hard seats & are ported. If I need dished pistons to avoid problems thats ok too. Any feed back would help. Thanks

9.5:1 is considered the practical limit for 93 octane and iron heads. We (CVMS) always "dish" the piston for more volume. The heads must first be measured ("CC" the chambers). Between the chamber and dish, total volume should be no less than 84 CCs. That does NOT include the "dead area" (head gasket). A 3 1/2" dish, .080" deep usually "does the trick".

It can also depend on which piston is used. Are they true "flat tops"? That would mean 4 valve releifs. Or are there 8? Some replacement pistons have both the 12 and 19 degree valve reliefs in them to enable use of early heads or later heads. The 16s are "later" (12 degree).

Cam selection can also have a significant affect on fuel requirements. A "smaller" cam (short duration) can increase cylinder pressure at lower engine speeds, creating a situation where detonation can occur. A longer duration cam can "bleed off" SOME of the cylinder pressure at lower engine speed, but does nothing for it at higher speeds. We've found the Comp XE series to be the most effective at making power in lower compression (under 9.8:1) engines.

I wrote "Chapter 2: Building Your Shortblock" and "Dishing and Detonation" in Jim's book. I can help a lot if you let me. Many of those recipes are mine.

When choosing a compression ratio, it's better to "err" on the side of caution. That is, the little gain from 9.3:1 to 9.8:1 (about 2%) is NOT worth the hassle of proper fuel and tuning for the higher ratio. "Pressing the edge" is only good for "class racers", where every inch-ounce of power is called for. 5 HP won't make a difference in a street or bracket car.

Good luck with the Poncho. And welcome to the higher side of GM performance!

Jim

Thanks, Jim
The recipe is D by Jim Lehart. The pistons are true flat tops & have 4 releifs. Can any machine shop dish them .080? Is it cheaper than new dished pistons? The engine was in very good shape other than the blown head gasket. The rods are stock with ARP bolts. I would like to use the stock rockers with new cam & lifters. I also have a B&M 400 trans to replace the 350 in the car now. Do you think the 350 will hold up? It is rebuilt with B&M shift kit. The car has a 79 T/A 308 8.5" rear. I also have a 67 GTO 400 with stock bore. It is a lot thicker than the 7os 400 & the crank is a larger. I would think the 70s will be lighter. I dont have the pistons or rods for the 67.
Thanks,
Tony

Tony,

It has been my experience, TH350 will last for a WHILE behind a strong Pontiac. Not forever. TH400 (M-40 in a Pontiac) is FAR superior. The slight amount of parasitic loss over the 350 is of no concern as the Pontiac more than makes "enough" torque.

We (CVMS) charge $80 to "dish" a set of pistons. That's a "simple" dish (a circle). Some shops have the capability of milling a "D-shaped" dish. That is better, but only marginally so. It must also be "deeper" due to missing part of the circle. The pistons must be removed from the rods, so the services of a competent machinist are required.

No matter what you do to this engine, REPLACE the connecting rods. The factory rods are made of cast steel and were inadequate when new. They're now at least 35 years old with a bazillion "cycles" on them. The "minimum" we recommend are the RPM forgings. They cost about $250 a set, depending on who you buy them from. The Eagle H-beam is the best rod for the money (IMO).

Yes, that's one of my recipes... But bear in mind, that book is now 6 years old. We DO learn, so there are some things we do today you won't find in there.

Jim

Thanks, Jim I thought that might be your recipe. I have had that book for about 3 years & have returned to it for information countless times. I will replace the rods. I am looking for a local shop that will dish the pistons for me. If you have time after the holidays I would be greatfull if you could give me some advice on my cam choice. Do you have any idea what kind of HP I can get out of this build? I would like to stay away from roller type lifters & stick to the stock rockers for now. The 350 has a knock so I would like to get the 400 built. The car is set up to corner & run road course. I dont know if that makes any difference in cam selection. I have the Holley Pro-jection 950 system to keep it strong around the corners.
Thank you so much for your help!
Tony