AMC Cylinder HeadsAMC cylinder heads flow pretty well in stock form. There are differences in exhaust port design from pre 70 and 70 up. The later "dog-leg" design flows much better than the earlier rectangular port. Heads for 290 and 304 engines had smaller valves than those for 343 and up displacement. 1970 390 heads (casting number 319 6291) seem to flow the best even though all the later heads have pretty much the same port design. Typical flow numbers at 0.500 lift and 28" of H20 are 225 cfm intake, 155 exhaust for a 1970 head with 2.025 intake and 1.62 exhaust valve diameters. With street strip porting and a good valve job, 235 to 240 is typical. A rule of thumb is that 1 cfm is equivalent to 2 horsepower, so this is good for about 30 horsepower. To take advantage of more aggressive porting, the valve sizes need to be enlarged. SS AMX cars ran 2.08 intake and 1.74 exhaust valves, from big block Chrysler wedge heads. Flow numbers for heads with these valves range from 250 up to 300 in a typical modern day SS head. There is the potential for 150 horsepower over a stock set of heads at this level. The AMC publication "Performance American Style" provides precise recommendations for modifying heads with stock and oversize valves. Most experts today do not modify the combustion chamber to the "open chamber" configuration illustrated. The directions also suggest a 45 degree intake seat but with stock valves the stock 30 degree seat angle will flow a little better and require less machine work. When using Chrysler or after market oversize valves intended for 45 degree seats the seat can be change to 45 degrees since the seat must be re-cut to accommodate the larger diameter valves. The first two steps in the instructions say to spot face to the valve seat and bore the valve pocket throats. These machining operations take a lot of setup, and are often overlooked. The oversize spot face is important to unshroud the valves. Note that for installation of 2.08 diameter valves, the seat outer diameter will need to be scaled up to 2.06 and it may be desirable to increase the spotface diameter to as much as 2.50 but use the head gasket as a guide and limit spotface diameter accordingly. The valves can be unshrouded with a grinder, but if you can, try to find a machinist that will use the correct spotface cutter. Boring the valve pocket throats is also important, yet also often done with a grinder. If you can’t find a shop that will do this preparatory machining make sure your porter knows what the desired dimensions are. Most shops that do performance work should be able to handle the valve and seat and guide work for installing oversize valves. The directions for valve and port modifications are also very good. Pay particular attention that the ports are opened up in the middle of the port to get the really big flow numbers. Unlike Chevrolet heads, the restriction is not primarily in the valve bowl or "pocket". On the intake the port is widened until you almost break through the wall on the pushrod side, or even to the point the wall must be repaired with epoxy. On the exhaust the restriction is the lump for the exhaust manifold bolt. If you reach in with your finger and pull it out along the wall where the bolt is, you can feel the restriction. The lump needs to be smoothed out as much as possible, so that your finger can slide along the wall. There is one more thing to consider. One expert quipped " I don’t know how someone can take money for porting without a flow bench. You just can’t tell by looking". Make sure your porter has experience and insist on flow results to show that the ports are all the same. Ralph Winslow |