Chevrolet HT 383 Engine - The Twister, Part III

449 hp And Monster Torque From The HT 383

Jeff Smith Aug 1, 2002 0 Comment(s)

Step By Step

The only external change to our Twister engine was the switch to a Barry Grant 750-cfm Speed Demon mechanical-secondary carburetor. The carburetor created an excellent fuel curve to match our hotter hydraulic-roller cam. We retained the Edelbrock Performer RPM Air Gap Vortec intake.

Despite the big power numbers, we’re still lighting the fire with a stock HEI distributor and a set of 8mm MSD plug wires.

The most significant change this time around was the switch to the Comp Cams Xtreme Energy HR282 hydraulic-roller cam and better springs. Since the HT 383 comes with a hydraulic roller cam, all you need is the cam and the bigger valvesprings, retainers, and keepers.

Installing the new Comp Cams Xtreme Energy hydraulic roller is fairly easy. Taylor also degreed the cam to ensure it was installed as specified on the timing card. The factory roller-cam blocks use a thrust plate to limit cam travel and prevent ignition retard.

This is what the stock pushrods looked like after last month’s test with the 1.6 rocker arms and the Hot cam. The 1.6 rockers position the pushrod cup closer inboard, which scuffed about half of the pushrods on the cylinder head. You must slightly machine the pushrod holes to prevent this interference.

The far left spring is the stock Vortec iron-head valvespring. Next right is the GM Performance Parts LT4 valvespring that we used with the 1.6:1 rocker arms. Third from the left is the Comp PN 986 dual spring that Comp recommends for the XE282HR cam. The far right spring is the Comp 977, which offers a higher rate.

Dyno-testing the HT383 at Duttweiler Performance revealed that despite the big cam, the engine preferred the Hedman 15/8-inch headers to the larger 1-3/4-inch headers.

Taylor also tried a set of Hedman Tork-Step headers on our 383 that feature an initial 1-1/2-inch primary pipe that steps up to a larger 1-5/8-inch pipe roughly halfway down the tube. In this configuration, the HT 383 preferred the standard 1-5/8-inch headers by a slight margin.

Every once in a while, we run across an engine combination that makes amazing power. Combine tractor-like torque with big horsepower and even a barge-like 3,600-pound street Chevy can run low 12s without much of a gear. Sound too good to be true? Not if you configure your GM Performance Parts HT 383 like we did.

Check this out.

This is Part III of our buildup and dyno flog of the GM Performance Parts HT 383 small-block Chevy. As a quick review, we started by testing this midsize Mouse in a standard towing-type application with just headers and a 650-cfm carb and it made great low-speed torque and decent horsepower. Then in the last installment, dyno tech Ed Taylor dropped in a GM Performance Parts Hot hydraulic-roller cam, Edelbrock RPM Air Gap intake, and a Holley 750-cfm carb and made even more power. This time, we dialed in a larger Comp Cams Xtreme Energy XE 282 cam, a Speed Demon 750-cfm carb, and a torquey set of 1-5/8-inch Hedman headers. This last combination produced an outstanding 486 lb-ft of torque at a reasonable 4,600 rpm with 449 hp at a mere 5,300 rpm.

But these are just the peak numbers. To get the full effect, look at the torque curve and you’ll really appreciate the powerful story this motor tells. Remember, this is a crate 383ci small-block right off the shelf complete with hypereutectic pistons and a set of iron 1.94/1.50-inch heads! Granted, the cam is lumpy, barely making 9 inches of manifold vacuum at 850 rpm. But if you can live with an unstable idle, this is killer power. But enough hype. Here’s how we did it.

Cam, Intake, and Exhaust

The key to this HT 383 is combining a great cam with the Vortec iron heads. These heads offer outstanding mid-lift flow potential. Adding a cam to take advantage of this potential really wakes up a medium-inch engine. Still, fitted with the stock 1.94/1.50-inch valves, dyno guru Taylor pulled out the mild GM Performance Parts Hot cam from last time and gently slid in a much more aggressive Comp Cams Xtreme Energy XE282 hydraulic roller. The Cam Specs chart compares the timing figures, but the big thing is that with around 0.550-inch lift on both the intake and exhaust, this baby is ready to rock.

However, we also knew that the stock iron Vortec heads would need some work to accommodate the much more aggressive cam. The first order of business was to machine the valvespring seats for the larger 1.43-inch diameter dual springs. This is necessary because the stock spring pads are designed for a 1.25-inch diameter spring. The stock guides must also be cut down to clear the additional lift.

With the new Comp Cams roller in place and the otherwise stock Vortec heads back on the engine, we decided to retain the Edelbrock RPM Air Gap dual-plane intake, but switched to a Barry Grant Speed Demon 750-cfm mechanical-secondary carburetor. We also tried several different header combinations in search of maximum power.

Bending the Needle

Once Taylor had the HT 383 back up on Ken Duttweiler’s dyno, it didn’t take long to find out what >> worked the best. Given the 383’s displacement, we thought the 1-¾-inch headers would work the best, but the smaller 1-5/8-inch Hedmans produced the best overall power. Taylor even tested the Hedman Tork-Step headers, but in our case the standard 15/8-inch headers produced the best results.

The amazing part of this engine is its incredible torque curve. This thumper makes more than 400 lb-ft of torque from 2,300 to 5,800 rpm. But the really impressive part is the pool-table flat area where the 383 makes over 450 lb-ft from 3,000 to 5,200 rpm! Considering that the 1,000-rpm band cranks out over 480 lb-ft, this sounds more like a description of a big-block than a small-block. We eventually want to plug this motor into our ’66 Chevelle test mule and beat on it for awhile. It should be too much fun.

This motor makes so much power that we’ve come up with even more ideas for next time. There just may be some aluminum heads in this engine’s future.

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