Prepping AMC V-8 Heads For Performance Use
There are 2 desirable subtypes of AMC V8 heads. The 1970 to early-71 heads with 51cc chambers (casting number 319-6291), and the mid-70 to mid-73 58cc heads (casting numbers 321-2993 and 321-3948). Both heads have the superior dogleg exhaust port and studded rocker arm mounts. The only real difference between them is a thicker layer of metal inside the combustion chambers. When you compare the two side by side it is very noticeable, especially between the valves.
(Later heads are identical to the 58cc heads except for the paired fulcrum rocker arm setup, which is not adjustable in any way, and requires quite a bit of machine work to convert to an adjustable drivetrain. However, the later heads flow just as well as the earlier ones if you are willing to spend the money to convert them. There are rumours of some kits that help do this, I will try to find out more and include it here.)
Both of these heads were original equipment on 4V 360s, 390s and 401s. Before porting they outflow almost all other small block heads, and with porting they are still competitive with some of the best out there until you move up to aluminum race heads. The design is a product of the factory Trans Am racing effort in the late 60's, and culminated in the winning of the Trans Am Championship in 1972 and 1973.
Which one you choose, of course, depends on what you have available and what kind of pistons you want to use. The 1970 heads, when used with 1970 pistons have a static compression ratio of 10.25:1. This is a little high even for todays 92 and 93 octane fuel. A better choice is the 1970 heads combined with the 1971 or later pistons, for a static compression ratio of about 9.5:1. This is workable on a street and strip car that does not need octane booster with every tank of gas. TRW makes a forged version of both these pistons types (all factory pistons were cast), and it has proven to be a durable, if fairly heavy piston. Unfortunately, they seem to have been out of production for a while, and are difficult to find these days. The stock 1971 heads and pistons are about 8.25:1, and even lower if the combustion chambers have been polished to any significant degree. A good start for a mild turbo or supercharged engine, perhaps.
Replace the valve guides with high quality bronze guides, sized for the narrower stemmed Chevrolet small block valves. This will allow you to buy high quality yet fairly inexpensive racing valves, and keeps the valve weight lower. The stock valves seem to have a tendency to break off at the head, which causes other "Bad Things" to happen, so consider yourself warned. Use good umbrella oil seals , especially if you intend to drive the car on the street. Oil contamination, even tiny amounts, increases the tendency of a motor to detonate. Bronze guides can be run with slightly tighter clearances than stock cast iron guides, if you do this it will help a high compression engine live longer on the street. If you decide to run blueprinted clearances "on the tight side", keep the engine oil clean! Any small trash circulating with the oil will cause a tight engine to wear much faster than normal. All AMC heads were induction hardened at the factory, and do not seem to suffer much from exhaust valve recession when using unleaded gas. You might wish to install hardened exhaust seats, but be careful, the waterjackets are very close to the seats and core shift in your particular heads may cause problems when cutting the heads for their installation.
If you have any knowledge to impart or corrections on the errors of my ways, please let me know.