It's common knowledge that most modifications made to an engine involve a tradeoff somewhere in the overall equation. You might want a higher compression ratio, but the trade-off here is a need for higher octane fuels. A single-plane manifold might help make more high-rpm power, but you'll give up some at the bottom. The examples here could go on and on, but the most obvious case of this kind of compromise usually involves the camshaft. Most of the time, a bigger camshaft will mean more peak power, but the cost is generally clear, with an increase in idle chop, poor low-speed performance, and a drop in idle vacuum. Sure, for a lot of guys the uncivilized roughness of a big, nasty stick is actually a plus. This is especially true if the engine is to be used solely as a Saturday night street brawler, or a purpose-minded race car. For those looking for good road manners and drivability, camshaft selection is a tough balance to strike while looking for power.
Generally, the factors that have the greatest effect on a given cam's idle and low-rpm characteristics are related to the camshafts's duration, lobe separation, and overlap, though they are all interrelated specifications. What all these have in common, is that they relate to the cam's timing points. As you start piling on the duration and overlap, power up the rpm range will build, though low-speed operation and idle quality suffer. While the above mentioned specifications have a pronounced effect on an engine's operational characteristics, the one key-specification omitted until now is lift. Generally, a more aggressive lift-profile will increase power output by adding more area under the lift curve, but unlike added duration, the increased power will usually come with a minimal penalty at low engine speed.
We had a freshly built 383 on hand, that is destined for install in a true street driver. Here is an application where we certainly did not find it acceptable for the engine to idle like a Pro Stocker, but we did want to optimize the overall power curve in a reliable operating range of just off idle, to a maximum of 6,000 rpm. In fact, we really wanted to approach a dead-smooth idle quality. The initial camshaft choice was very conservative. It was a hydraulic flat-tappet with 214/224-degrees of duration at .050-inch lift, a lobe separation angle of 112-degrees, and .442/.465-inch lift with stock 1.5:1 rockers. This profile is one that should be familiar to old-timers in the hobby, having been a popular grind for decades under numerous private labels. In fact, it is still sold by large mail-order houses today. By any definition, this is a mild camshaft, though it is considerably hotter than most older factory flat-tappet passenger car cams, with specifications comparable to mid-range OEM performance cams of yesterday.
Our overall engine combination (See Sidebar: The Engine Combo) was that of a mild street performance engine. It's something definitely carrying more punch than stock, but tame enough to comfortably claim daily-driver status. We knew going in, that the camshaft would provide a very nice idle-quality, but we had to wonder if we might be leaving a little power on the table. The lobes on our baseline camshaft are designed for a minimum tappet diameter of .842-inch, which makes the profiles generic for most popular engine types. With flat-tappet camshaft lobes, the tappet diameter determines the maximum velocity achievable, and since Mopar engines have a much larger .904-inch tappet diameter, a cam designed for the larger lifter, could be designed with more lift for a given amount of duration.
To settle the question of how much potential was locked away in the engine, we decided a test including a modern, more aggressive .904-inch-tappet specific camshaft would be worth looking at. Taking the bigger picture into account, we could see that with a very mild 214/224-degree duration, our baseline camshaft choice was skewed more in favor of smooth low-speed operation, than what you might find in a typical street machine. With all of the above in mind, we decided that it would be worthwhile to run the engine with a camshaft that is much more representative of a hotter performance application, and use a fast-rate .904-tappet profile. We settled on Comp Cams' XE275HL, also a hydraulic flat-tappet, but substantially healthier in specs, with 231/237-degrees duration at .050-inch tappet rise (275/287 advertised duration), .525-inch lift, and a 110-degree lobe separation. With 17 degrees more intake duration and .080-inch more lift, this cam is right smack in range for a hot-street application.
1. We brought a freshly built 383 to the dyno, packing an extraordinarily mild hydraulic flat-tappet camshaft. We knew that this cam—with only 214/224-degrees duration and .442/465-inch lift—would be leaving some power behind, but it does offer the near-stock idle quality we wanted.
2. After the initial fire-up and run-in, we looped the timing to find just what the engine wanted. The engine responded best with 38-degrees total advance, a setting we retained for all further testing.
3. We equipped the engine with a vacuum-secondary Holley 770 Street Avenger carb with the goal of trouble-free performance on the street. The carb performed flawlessly on the dyno. We carefully monitored air/fuel ratio throughout the test session, though once jetted for the baseline combination, we found no further gains in revising the carb even with the cam changes.
4. While running the very small cam, we were rewarded with 18.5 In-Hg of manifold vacuum, 402 horsepower, and 441 lb-ft of torque. Actually, considering the smooth idle, strong low-end torque and an rpm range that isn’t dead until over 6,000 rpm, this combination was pretty good.
5. For the next configuration, the cam was stepped up to a Comp XE275HL, a very fast-rate .904 lifter design, sporting 17/13-degrees more intake/exhaust duration at .050-inch lift than the baseline cam, .083/.060-inch more lift, and 2-degrees tighter lobe separation. Make no mistake, this is a lot more cam. A new set of Comp’s 822-16 lifters went in with the new camshaft. These are their standard hydraulic lifters.