Cam It Up
Eric Rosendahl; San Diego, CA: I have a '67 El Camino with a 0.040-over 350 small-block that uses the later '87 one-piece rear main seal block that accommodates an OEM roller cam. The engine uses a Scat cast crank and 6-inch ARP capscrew rods, Edelbrock Performer aluminum heads, Speed Pro hypereutectic pistons, and a Summit Stage 3 dual-plane intake similar to an Edelbrock Air Gap. The present cam is a custom Schneider Racing Cams step-nose roller that uses the OEM retainers and spider. The cam specs are 214/222 degrees at 0.050 (270/280 advertised) with 0.480-inch lift and a lobe-separation angle of 112 degrees installed at 110 degrees, and I'm running a 1.5:1 rocker ratio. The headers are 15⁄8-inch primaries with 18-inch collectors, an X-pipe, and DynoMax Super Turbo mufflers. The rear gears are 3.36:1, and the trans is a 700-R4. Given the internals and my cylinder heads, what sort of hydraulic roller cam specs using the factory spider and retainers would you recommend to increase performance? I'm looking for fairly civilized manners but with a nice boost in power. I suspect the heads are likely my limiting factor. I would love to hear your thoughts.
Jeff Smith: There's no question that a slightly bigger camshaft will help power, Eric, but it's worth discussing what you will give up by adding duration and lift. I promise to keep this short. You are correct in thinking that power is more closely tied to cylinder head flow. The Edelbrock heads flow around 250 cfm at 0.500-inch lift and 225 cfm at 0.400-inch lift. These mid-lift flow numbers are generally more important than maximum-lift flow because the valve is at that mid-lift point twice, on the opening and closing sides of the lift curve. As a rule of thumb, given a choice between two cylinder heads, I always go for the one with the stronger mid-lift curve on the intake side—especially if it's combined with a strong exhaust port. You can evaluate exhaust flow by dividing the exhaust flow by the intake at the same valve lift. So if the intake flow is 225 cfm at 0.400 and the exhaust flows 146 at the same lift, then the exhaust flows roughly 65 percent. These are the actual published Edelbrock Performer flow numbers. A good number to look for is 70 to 75 percent exhaust-to-intake (E/I). With less than 70 percent, you will want a dual-pattern camshaft. This is defined as a cam with more duration on the exhaust side than on the intake—as with your current Schneider cam with 8 more degrees on the exhaust. The additional exhaust duration gives the exhaust port more time (in terms of degrees of duration) to evacuate the exhaust gas from the cylinder.
What all this means is that we should use cylinder-head flow data if we want to make an intelligent decision regarding a cam. Since you have the advantage of already knowing how the engine runs in its current configuration, you have solid ground for making a decision on a bigger camshaft. The most important thing to consider is how you will be driving your El Camino. We'll assume it's more of a weekend cruiser, and since you didn't mention drag racing, we'll also assume that while added power is the stated goal, a drastically lower dragstrip e.t. isn't the endgame. I've built several 355ci and 383ci small-block Chevys with cams of around 220 degrees of duration at 0.050 with 9:1 compression, and they work very well. I looked at Schneider's website and found a 278 advertised, 218 degrees at 0.050 cam along with comparable versions from Crane and Comp versions. All three companies offer hydraulic roller cams with duration between 222 and 226 degrees at 0.050 lifter rise with intake-valve lift of around 0.510 inch. Beyond picking the short straw, how can we differentiate these cams? One thing to consider is since the Edelbrock exhaust port could use some help, additional duration might help peak horsepower, so the cam with added exhaust duration might be beneficial. But then you must also consider that added exhaust duration means more overlap, so we also need to look at the lobe-separation angle. If you like the idle quality of your current cam, then we should try to maintain the same amount of overlap with the new cam. As an example, if we add 10 degrees of intake duration to your current cam, we'll assume the intake valve will open 5 degrees earlier and close 5 degrees later. The same is true on the exhaust side. So with a later-closing exhaust valve and an earlier-opening intake, this will increase the amount of overlap by roughly 10 degrees. One way to minimize this is to move the intake and exhaust lobe centerlines farther apart. This angle is expressed as the lobe-separation angle (LSA). If all we do is spread the LSA from 110 to 114 degrees, we reduce the amount of overlap by 4 degrees. This will help produce a smoother idle, yet with the duration, you will still have that lumpy idle quality.
Another crucial point is that adding intake duration pushes the engine's torque peak higher in the engine speed range. Adding roughly 6 to 8 degrees of intake duration will tend to raise the peak torque rpm point by perhaps 300 to 400 rpm. So if the smaller cam's peak torque was at 4,000 rpm, an 8-degree-longer-duration cam might move this point to roughly 4,400 rpm. This tends to help peak horsepower since most engines generate around a 1,500-rpm powerband between peak torque and peak horsepower. So moving peak torque to 4,400 might push the peak-horsepower rpm to 5,900 or perhaps 6,000 rpm. A higher peak horsepower rpm generally will result in more peak horsepower.