What is the deal with 18 Degree heads?

[Nutsy]

Ok, i know they flow better and cost more. Why? What are the disadvantages to using them other than cost? Why would i choose 23 degree heads over 18 degree heads in a turbo application?

Trev

[Steve Chryssos]

Trev,
This specification refers to the inclination of the valves relative to the deck. Small block Chevy engines were originally designed with 23 degree valve angles. Shallow valve angles are an evolutionary feature. First it was determined that the original small block Chevy cylinder head could benefit from raised port locations. Raised ports provide a straight shot towards the valves. Then engine builders realized that the raised port designs could be further enhanced by "rolling" the valve angle back towards zero.

So there really isn't any case where 23 degree heads are better except for price. Furthermore the "secret" to LS-x performace potential is exactly the fact that the heads feature extremely shallow valve angles and raised ports. GM simply applied the existing GEN1 shallow angle/raised port technology to their production engines.

To install 18 degree heads on a GEN 1 small block, you must also convert the valvetrain, intake manifold and headers as well. In some cases you may need different pistons (i.e. a BUICK symmetrical port conversion). And 18 degree castings are generally sold in semi-finished form. So add the cost of professional porting as well.

In short, an 18 degree (or 14 degree) conversion will cost you as much as a stout LS-x engine. Unless you absolutely, positively must have a shallow angle/raised port GEN1 small block, the cost for conversion is prohibitive.

[Nutsy]

Well here is my dillema... i have a bee in my bonnet to have a 700hp ish in daily driven street trim, 1000hp with race gas aluminum block setup. LSx's engines AFAIK are not reliably able to make that kind of power. I could go with an IRON block LSx, however, i really want to take advantage of the weight savings of an aluminum block.

If someone could tell me that i could have an 700-800hp twin turbo LSx Aluminum based setup that could be daily driven, i would consider it. Since i have not found anyone that can tell me that is the case, then i am now thinking about going with a world or dart aluminum small block setup possible with AFR 18 degree heads and a hogan's manifold.

Does 18 degree heads make it any easier to build turbo headers? I would think that since the angle is closer to the deck of the block, there would be slightly more clearance between the inner fenderwell and the exaust ports???

Maybe for my use, 18 degree heads are way overkill... can that kind of power be achieved with a nicely massaged set of 23 degree SBC heads?

[XcYZ]

Trevor, 23* heads will work great on a turbo motor. Remember, you're pushing, not pulling. Shaun's (TwinSting) twin turbo 427 SBC with 235cc heads made 1,500hp with 25psi of boost and 850hp with 8psi of boost. It doesn't appear that the old 23* heads hurt it too much.

[Nutsy]

Good to know. Good point on the pushing vs. pulling.

Trev