Pontiac 427 heads and combo

helping a buddy with his motor . has pontiac 427 heads with casting number 10093385 in them. can anyone tell me anything about these heads. one has a broken valve in it right now will have to be fixed. what size combustion chamber do thay have ruffly? his pistons are dished about 100 thou deep and a 4.5 bore. they are venolia and the only number on them is 485. teying to figure out where he kinda is on compersion ratio. had a 80 thou head gastet to.also how much timing can you run with one of these. has a 14/71 blower at 50 over. and a 44 mag. i think they tried 36 degrees. now we have smashed bearings and broken valve. thank you for any help.

 

The 385 head is a 15 degree valve angle. They are not a 427. They are very similar, but the 427's are an 18 degree. Also, the 385s were only available in a raw casting. So they have been fully done by someone. (Most likely for pro stock or that era pro mod) So, there is no way to know what the chamber is but to measure it. Probably about mid 60's to a low 70. Depending on how sunk the vlaves are, and how heart shaped they machined the chamber.
And as for timing, let me say; "HOLY CRAP!!! HOW DID THE BLOWER NOT COME OFF AT 36?!!" These chambers are so shallow, that you are going to want to take 10 off from a standard BBC type of deal to start. These chambers are THAT fast. And then you add 50 over? 36 is very far from safe. VERY FAR.
Venolia is a great and durable piston. Also, a very good-ole-boy company to deal with. So, you can just send them off a piston and they can go from there to make you anything new, and right away. The 485 is their forging number. The 80 thou head gasket is actually hurting your performance, but that is another story.
As for the compression, we need to know the stroke of the engine to tell compression with that. But I would say around a low 11:1 with a 3.75, up to a mid to high 12 with a stronger stroke.
Those heads will make power without much blower. I can tell you that.

 

thank you for the imfo. it was a 4.5 stroke also.

 

Head

WANNABE:

You mention a .080" gasket is hurting performance. Could you explain this further? I run a very similar thickness gasket in my Chevy and wonder if I am leaving somethng on the table?

Thanks
Corey

 

The reason that chevys are more efficient is the swirl and tumble in the equation. And that with the squish area. Keeping your squish tight is the key to keeping the fuel suspended in the air, and quick flame travel. This is why you will never see a real hemi run with pro stock or comp cars. (Or NASCAR or any of that)
The magic number for squish is pretty much as close as you can get it without hitting things when the pistons are cold and rocking all over the place. RPM's are also part of the equation. Add about 10 for aluminum rods. In a blown alcohol application staying under 8 grand with aluminum rods, you are looking at a minimum of mid 50's with your set-up. But with higher rpms, then low to mid 60's. And that is actual piston to head.
So, if you are total zero decked, you are giving up lots of squish on an .080. And more if you are in the hole.

 

chevy

we just had our chevy combustion chambers machined. The quench area is sunk in and angled. It gained clearance but apparently makes power. I guess it's fairly common in the n20 world.