500 LB-FT Small Block in Six Easy Steps cont'd

—courtesy, Hot Rod magazine

Step Two: 1.6:1 Rockers

With just 196 / 207 duration at 0.050, the HT383’s original cam is about the tiniest we’ve seen in a crate engine (the same as a Ram Jet 350’s). We’ve flow-tested the Vortec heads in the past and knew they’d enjoy more lift, and we suspected that a simple change to 1.6:1 roller rocker arms would give us more power at every rpm point with zero sacrifice. In addition to the quicker rate of lift provided by the 1.6:1 rockers, they would also increase total lift to 0.459 / 0.481 as compared to 0.431 / 0.451 with the stock 1.52:1 rockers. Sure enough, a set of Comp Cams Pro Magnum 1.6:1 rollers gained 17 hp and 8.2 lb-ft at peak and didn’t lose power anywhere in the curve. The fact that the rpm at peak increased by 100 and 300 rpm, respectively, told us that the engine would still like more cam.

Later on in the test, when we switched to a bigger camshaft (see Step Five), we tried a set of Comp’s similar 1.5:1 rollers, thinking perhaps the new cam had enough lift to not need the help of 1.6:1 rockers. We were wrong: This engine preferred the 1.6:1 Pro Magnums every step of the way.

The Pro Magnum rockers are lightweight stainless steel and feature roller tips and trunions. The HT383 requires late-model-style guided rocker arms; because there are no pushrod guide plates or guide holes in the heads, the rockers need a grooved tip to center themselves on the valve tips. Comp now offers guided roller rockers in both 1.52:1 and 1.6:1 ratios for Chevy V-6s and small-block V-8s. The only problem with the rockers is that their width caused interference with the internal supports of the center-bolt-style factory valve covers. You’ll either need to buy cast-aluminum covers with no brackets inside, or do like us and cut up the stockers with tin snips.

Description	Part Number	Price
Comp Cams Pro Magnum 1.6:1 roller rockers	1318-16	$322.69 at Summit

Power
Peak HP	355 @ 4,700
Peak Torque	452.5 @ 3,500
Average HP 2,500-4,800	300.3
Average Torque 2,500-4,800	434.3
Torque At 2,500	436.3

Step Three: RPM Air Gap Intake

We’ve yet to test a street-performance small-block Chevy that didn’t run best with an Edelbrock Performer RPM intake, and the Air Gap versions might even be a bit better. Compared to Edelbrock’s other manifolds, the RPM and RPM Air Gap seem to provide nearly the same peak horsepower as a Victor Jr. single-plane, but with significantly more torque than even the standard Performer dual-plane. It’s a best-of-both-worlds deal, a claim supported by the results on the HT383.

The crate engine is delivered with a medium-rise dual-plane intake with EGR provision. Traditional thinking is that smaller intake volume tends to boost low-end torque, yet a swap to the high-rise RPM Air Gap dual-plane made 10-20 more lb-ft and 5-40 extra horsepower. The 70 / 76 jetting in the Road Demon still seemed optimal.

Also, check this out: We’ve found that a 1-inch carb spacer is generally worth a dollop of horsepower, but not on this particular combo. We tried both four-hole and open spacers on the HT383, with both the carbs we tested and both the camshafts, and they weren’t ever worth squat. In fact, the four-hole killed a whopping 25 lb-ft in one test. So for this engine, don’t use a spacer.

Here’s the GM EGR intake (front) compared to the Edelbrock Air Gap, which is about ½ -inch taller. The RPM Air Gap manifolds have runners that are separated from the engine’s valley pan to keep hot oil off of them. That’s more of a benefit in the car than on the dyno, but we’ve found the Air Gaps to be just slightly more powerful than the standard Performer RPMs in either case. A Vortec-style in-take manifold is required for the HT383 engine, but we were able to reuse the original intake gaskets.

Description	Part Number	Price
Edelbrock RPM Air Gap Vortec	7516	$219.95 at Summit

Power
Peak HP	370.0 @ 4,600
Peak Torque	466.6 @ 3,700
Average HP 2,500-4,800	309.9
Average Torque 2,500-4,800	447.1
Torque At 2,500	442.5

Step Four: Demon 750 Carb

GM suggests a 750-cfm carb for the HT383; we second-guessed that and used a 625-cfm Road Demon up to this point. We’ve seen the Road Demon support more than 400 hp on a 400-plus-ci engine, so we still don’t think it’s a bad choice. However, we felt obligated to try the 750 Speed Demon (this one with mechanical secondaries) and were rewarded with another nice power gain. As with every mod we’d done so far, we gained torque at every single point in the power curve. This time the power peaks increased by a pleasant 9.4 and 7.7 lb-ft. With the 750 atop the 383, we had a brag-worthy 450 lb-ft at a meager 2,500 hp. That’s street-smokin’ power.

As for jetting, it’s interesting to note that the bigger carburetor liked 81 / 88 jetting compared to the 625’s 70 / 76 setup. Yet, judging by the Superflow dyno, both carbs used the same pounds-per-hour of fuel throughout the rpm curve. It seems that the smaller venturii of the 625 Road Demon create more of a pressure drop, thereby sucking harder on the jets and pulling more fuel. The more open throats of the 750 have less airspeed, thereby requiring bigger jets to move the same amount of fuel. Consider that during your next carb swap.

Pricewise, the 750 Speed Demon with mechanical secondaries is $439, about $194 more than the 625 Road Demon. The 750 would likely be a fine first choice for the HT383, and clearly more flexible at bigger power levels. The vacuum-secondary 750 Speed Demon VE, at $399, would also be a good pick.

Description	Part Number	Price
750 Speed Demon, mechanical secondaries	1402010	$439.00

Power
Peak HP	379.4 @ 4,600
Peak Torque	474.3 @ 3,700
Average HP 2,500-4,500	316.3
Average Torque 2,500-4,500	456.3
Torque At 2,500	449.9

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