Camming Up the Pontiac
Scott Brooke, via Car Craft e-mail: My two sons and I have read your magazine for more than 30 years. I would like to know if you could help me pick a camshaft for a '37 Pontiac business coupe that I have spent 10 years building from the frame up. I read your article on the Pontiac 455 buildup quite a while back ("Junker to Thumper: Pontiac 455," June '03). The motor is a 0.040-over 455ci Pontiac. The car weighs about 3,700 pounds, and the 464 has a Torker II intake, 750-cfm Holley, Mallory ignition, Hooker headers, and 3-inch exhaust. The heads are the stock-iron '72 7K3s that I race-ported and also fixed the exhaust-manifold hole problem with Manley 2.11/1.77-inch valves, Crane 1.65:1 roller rockers, and Crane springs with 110 pounds on the seat and 320 open. The heads have been shaved 0.020 inch, which puts the compression ratio around 9.4:1.
The car is a Pro Streeter and has 29x18.50 M/T tires, a Chris Alston's Chassisworks four-link, a Currie 9-Plus case, Truetrac 31-spline Strange axles with 4.10 gears, a Turbo 400, and a 10-inch TCI 2,800-stall converter. I would like to use this as a street/strip car. I am more interested in a good performance motor than it being totally streetable. I would like your opinion on the Comp Cams XE274H and the XE284H or even the Crower 60211. I originally set up this car for the XE284H, but now I'm wondering if I would get more low-end torque and better midrange street performance with the XE274. I am thinking that with the stock iron heads, even ported this motor probably won't twist over 5,700. Do you think the XE274 will be enough cam for this motor and for what I want to do with this car? Would I lose low- and midrange torque using the XE284, and what would I gain by using the bigger XE284? And what about idle? I would like to have a fairly rough idle and a good performance sound.
Jeff Smith: Thanks for your letter, Scott. I have a reputation for being somewhat conservative in my cam recommendations, but in your case I think your initial consideration of the Comp Cams XE284 is right on target. The XE284 flat-tappet hydraulic offers 240/246 degrees of duration at 0.050-inch tappet lift with 0.507/0.510 inches of valve lift with a lobe-separation angle (LSA) of 110 degrees. While this sounds a bit long on the duration side, keep in mind that you're working with a 464ci engine with a long 4.210-inch stroke. Since the heads are also somewhat restrictive in terms of airflow, even with your port work a little more duration and lift will help the top-end power a bit. The next-smaller XE274 cam reduces the duration by 10 degrees and the lift by a solid 0.020 inch. The Crower 60211 cam specs out at 236/242 degrees at 0.050 with 0.509/0.516-inch lift ground with an LSA of 108 degrees. In terms of duration, the Crower cam splits the difference between the two Comp cams and offers roughly the same lift as the larger Comp cam.
What might make the Crower cam sound aggressive is the tighter LSA of 108 degrees. The LSA refers to the angle (in cam degrees) between the intake- and exhaust-lobe centerlines. As the angle between the two centerlines is reduced (from 110 to 108 degrees in this example), overlap increases. This is the amount of time (in crankshaft degrees) that both the intake and exhaust valves are open at the same time. Increasing overlap reduces manifold idle vacuum and produces that distinctive lopey or choppy idle that everyone seems to prefer in a performance engine. Widening the LSA from 110 to 114 degrees, for example, will produce the opposite effect and reduce the amount of overlap and make for a stable idle. A smoother idle is why the Gen III LS1 family of engines uses an LSA of 116 to 118 degrees. There are many other effects from changing the LSA, but these are the most important. There won't be a huge difference in idle quality between the Comp XE274 and the Crower cam, but if you're looking for a raspy idle, go with the Comp because it still has more overlap due to the longer duration.
Pontiacs can make good power and outstanding torque. They just cost a little more than your typical small-block Chevy. This is an engine built by Butler Performance in Leoma, Tennessee.