Aftermarket Parts - Are Premium Parts Worth The Price?

The RPM Air Gap system is... 

   

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The RPM Air Gap system is sold as a complete kit and came with everything we needed to bolt it in place including a simple linkage package. The Air Gap kit uses twin 500-cfm Edelbrock carburetors, and it's clear that Edelbrock has tuned this package since the jetting was very close right out of the box.

Dual-Plane Vs. Single-Plane Vs. Tunnel-Ram
Much of the action that happens on the street is influenced by dragstrip chic-of-the-week. That includes such audacious displays of horsepower as 8-71 blowers and tunnel-ram intake manifolds protruding through the hood like a real-world version of a Rat Fink cartoon. Much of this is done strictly in the name of fashion, but there are those hard-core car crafters who believe in tall-runner manifolds. We wanted to see for ourselves, so we fashioned a quick comparison test for our Dart-headed Lunati 383. Configured with the Dart Pro 1 Platinum 215 heads and 280 Comp roller cam, this engine was ripe for an intake manifold face-off. Summit has long advertised a budget-based Weiand tunnel-ram intake configured with a pair of 600-cfm Holley carbs at an affordable price, so we tagged that as our eight-barrel heavy-hitter. For comparison purposes, we decided on a good single-plane intake and came up with one of Holley's Keith Dorton-designed single-planes. Then just to make it even more interesting, we decided to throw down one of Weiand's new Stealth Air Strike dual-plane intakes as the baseline. This way we would test the entire range of intakes from dual-plane to single-plane to tunnel-ram.

Dual-plane manifolds are known to enhance torque based on a longer intake-runner path compared to the shorter intake length of a single-plane manifold. The tunnel-ram offers the best of both worlds with longer intake runners like a dual-plane combined with a straight shot between the carburetors and the intake ports. The main reason tunnel-rams aren't more popular for the street is the simple issue of hood clearance. Not everybody wants to carve holes in their hood.

The Edelbrock 500-cfm AVS... 

   

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The Edelbrock 500-cfm AVS carburetors use larger-diameter primary boosters to decrease flow and increase velocity, making the carbs very responsive even with eight barrels plumbed to the plenum. One advantage the Edelbrock carbs enjoy is that it's easy to make part-throttle fuel-curve changes with metering rods that you can change without having to pull the lid.

As for our test, theory dictates that the dual-plane should make the most torque, given the smaller plenum and stronger signal to the carburetor. The single-plane should make more peak horsepower than the dual-plane but at the sacrifice of midrange torque. The tunnel-ram should best the single-plane in peak power and in theory generate a solid torque curve as well. The only hiccup in this concept is the size of the tunnel-ram's pair of carburetors and dialing in a decent fuel curve.

After we were done making all kinds of noise, we immediately noticed that even with a strong 480hp small-block, the dual-plane intake still made the most overall power of all three manifolds. Weiand's Stealth Air Strike dual-plane beat both the single-plane and the tunnel-ram in torque, and the only place the single-plane intake combo made more power was at the top of the rpm curve. Given that scenario, our money is on the dual-plane every time with a combination like this.

After bolting on the tunnel-ram with a pair of 0-1850 Holley carbs right out of the box, we discovered that perhaps the twin 600-cfm carbs were a bit too much for this engine combination. It became impossible to load the dyno below 3,500 rpm at wide-open throttle, necessitating testing the tunnel-ram system from 3,700 rpm and above. Plus it appeared from the testing that the engine really wasn't happy until at least 4,500 rpm, where the induction system seemed a bit more stable. But only around peak horsepower did this system show any kind of advantage over the dual-plane, and then only by a couple of horsepower. Part of this disadvantage came from the fact that the 4160-style Holley carbs use metering plates in the secondaries rather than metering blocks. Without additional metering plates, we couldn't adjust the secondary metering. This is something you should put on your shopping list should you decide to add one of these tunnel-ram kits to your small-block. We couldn't convert to metering blocks since the fore-aft carbs would then be too long to fit on the tunnel-ram. It would have required placing them sideways, but we didn't have the proper linkage to accomplish that.

Based on input from guys like Pontiac expert Ken Crocie, a choice of the smaller 450-cfm Holley carburetors would have resulted in a much cleaner-running engine and more powerful dyno curve that would have allowed us to start the test at a lower rpm and generate stronger torque numbers as well. It's possible that fuel metering was not quite as good as it should be because the flow velocity through the larger 600 carbs was just too slow to accurately meter the fuel. Another advantage of the 450 carbs is that they are slightly less expensive. Since this test was in the midst of our mad dyno thrash, we didn't have the luxury of time to dial in the carburetors, but with a little tuning time we could have made this combination work much better, especially in the midrange and lower-rpm levels.