1976 Chevrolet Camaro Intake Manifold Install - Stepping It Up

When we last visited the subject of performance with Project g/28, we supertuned the engine combination on the chassis dyno at Westech Performance Group. We concluded by noting that we had pretty much played out the initial performance objectives of the project, and by most accounts had met the project's targets, as initially defined. The "as initially defined" verbage here refers to some of the self-imposed constraints we decided to work within. In terms of the engine combination, the most obvious constraint was that our 383 mill was bolted together with equipment that carries a California Air Resources Board exemption order certificate. We were playing clean and by the book, at least to the extent to which we were able to interpret the legality of the modifications performed-and believe us, we are not lawyers. With this approach, we had just a hair over 400 hp at the crank, and tuned-up, running through a single exhaust with a cat, squeezed 292 hp at the wheels.

Satisfied with the results, we considered dropping the curtain there. We stated that there was a good chance that we would re-visit the project to try things that were not part of the original plan. So opens the next phase of g/28's modification effort. While a major theme of the initial build was keeping within stringent California emissions rules, we realize that such an approach is entirely moot for many other geographical areas. In many regions, this criteria is utterly irrelevant. So, we've demonstrated what could realistically be done under those constraints. With this series of modifications, we will explore another approach, looking for good street performance, but unconstrained by our local rules. Note that throughout, we used the term "rules," not standards. Herein lies a big distinction, since the build was strictly governed by rules-rather than more sensibly, standards. We don't generally toss garbage in the streets, and likewise aren't looking to foul the air we breathe. The intent here is to build up a street power combination as though the rules do not apply, with the goal of a significant improvement in output. It may be well outside the rules for this type vehicle in California, but we fully expect to make much more power before all is said and done-and quite likely remain within the original standards for a '76 Camaro. Free-Breathing Induction Our first mod was to swap out the rules-abiding intake manifold and carburetor combination to a more performance-oriented arrangement. Our original build spec'd a Weiand Action +Plus 8000 EGR-equipped intake, and 600-cfm 4160 vacuum-secondary carb. This system bore the rules-decreed EO number, but the current thinking in performance intakes suggests that a divorced-runner style manifold is the better bet for outright power. Recently, Holley introduced its Stealth Air Strike two-plane air-gap style manifold (PN 8501, street price $189.99), and it seemed like the right choice for our power pursuits. This intake features an air space separating the runners from the valley portion of the manifold, which along with no exhaust crossover should result in a power gain over the Action +Plus. While we were at it, we also swapped the 600-cfm vacuum secondary carb to a Holley 4150 HP (PN 0-82751, $499.95, street price) 750-cfm mechanical-secondary piece. Previous testing on the chassis dyno showed only 0.8 in-Hg of manifold vacuum, so we knew that at this level of modification the smaller 600-cfm carb was actually adequate, and costing very little power. However, we also realized that today's test was just the beginning of our future plans, so we took the opportunity to make the jump to a larger carb.

Before the parts were swapped, we strapped the old combination to Westech's SuperFlow chassis dyno to record some baseline figures. Initially, we had 285 hp at 4,600 rpm, and a power curve that became a little squiggly after that. A fresh set of Denso platinum plugs improved on that considerably, bringing our baseline power up to 298 hp at 4,800 rpm. That's a pretty impressive number considering the mild combination. We quickly got to work lifting the induction system and substituted the Air Force manifold and 750 carb waiting in the wings. The induction change definitely showed a benefit, boosting power to 311 hp at 4,600 rpm. While 13 hp may not seem huge, what was more intriguing to us was the fact that the peak power rpm was actually 200 rpm lower. It's worth noting that the new induction made more power throughout the recorded test range, from 2,300 rpm to the top of our test range at over 5,000 rpm, but at the very top of the curve, the gap narrowed. Intuition would suggest that the new induction should have had its maximum advantage at peak rpm, but it didn't. The logical question was, why not?

We had a pretty good idea on just what might be capping the top-end output, right where the new induction should have shown its greatest advantage. To investigate our suspicions, a pressure gage was tapped into the exhaust system, and it told us what we needed to know: there was nearly 6 psi of back pressure developing at the top of our power pulls. The single exhaust with a cat was definitely crimping peak output. We uncorked the exhaust, and power surged to 326 hp at 4,600 rpm. Considering the mild emission-legal cam with 216 degree @ 0.050-duration and 0.454-inch lift, that's pretty stout.

Our final experiment was to add a 1-inch open spacer. We had found an advantage of about 10 hp by adding the spacer to the Weiand Action +Plus intake, and it was a part of the baseline setup in our current testing. We added the spacer with the exhaust still open, and found that it added just 1 hp when all was said and done, however, it did allow the engine to hang on at a slight power advantage for an additional 300 rpm. The upshot here was that the added volume of the spacer was a big benefit to the more restricted Action +Plus intake, but with the higher-flowing Air Force intake, it just wasn't needed. We decided to finish the day by re-installing the exhaust, and then going back into the carb to jet it appropriately to deal with the 6 psi of exhaust gas pressure. Power predictably dropped to 314 at 4,600 rpm, which still represented a decent gain over our baseline numbers, but down noticeably from our open-exhaust achievements. So, where might you guess will we take g/28 next? If you guessed a more free-flowing exhaust, then stay tuned.