In our first windswept episode of the continuing adventures Project Twister, we left our almost-stock GM Performance Parts (GMPP) HT 383 crate engine making serious power. If you missed last month's epic, we added little more than a set of Hedman 15/8-inch street headers, a Quick Fuel Technology-modified 650-cfm Holley carb, GMPP 1.6:1 roller rockers, and DynoMax mufflers. The Twister made an axle-bending 447 lb-ft of torque at 3,100 rpm and a respectable 341 hp at 4,700 rpm. All of this from an engine with a rock-solid idle and 403 lb-ft of torque at 2,500 rpm!
That combination would make a gutsy tow engine that you wouldn't have to spin over 4,500 rpm and a plenty-fun engine for a Chevelle or Camaro. But for you restless enthusiasts looking for more power, the HT 383 offers far greater potential. GMPP targeted this engine as a torquey small-block that makes as much grunt as a Rat motor without the heft. But GM also included a decent set of rods and an excellent 4340 forged-steel crankshaft. That means this engine has the bottom-end heft to withstand more rpm with a bigger cam. And that's exactly our plan for this installment.
We actually didn't have to look any farther than the GM Performance Parts catalog to come up with a great cam. The HT 383 is a hydraulic-roller cam engine, so it took about a nanosecond to decide to add the GM Performance Parts Hot cam. We've had very good luck with this cam on a carbureted 355ci with Vortec heads ("Hot, Hot, Hot," Nov. '01) so we figured it would be a natural for the 383. The beauty of this cam is that its mild specs (see Cam Specs) produce a very smooth idle that masks its powerful potential.
Dyno-man Ed Taylor actually installed the better LT4 springs before the first series of tests so we could test the 1.6:1 rockers. >> This made the Hot cam swap that much easier. This requires slight machining of the pushrod slot to clear the pushrod when adding 1.6:1 rockers. Now we had a much stronger hydraulic-roller camshaft, better valvesprings, and the GMPP 1.6:1 roller rockers. It was time to go back on the dyno.
Test Time
After the Twister was suitably warmed up and we gave the new cam a chance to break in, it was time to make noise. We've called this Test 2 on the chart. The only change was the Hot cam and a set of Borla XR-1 21/2-inch mufflers. The rest of the engine remained the same with the original GMPP dual-plane intake, Quick Fuel-modified Holley 650-cfm carburetor, Hedman 15/8-inch headers, stock Vortec heads with the LT4 valvesprings, and stock HEI distributor.
After a few pulls to establish the optimal ignition timing of 36 degrees and to ensure our fuel curve was close, Taylor loaded the water brake against the 383 at 2,500 and let the motor grunt its way up to 5,700 rpm. As expected, the longer-duration cam hurt the low-speed torque up through 3,200, losing as much as 21 lb-ft of torque at 2,500. But the larger cam more than made up for this slight loss with 40-70 lb-ft gains and a maximum horsepower boost at the top of 82 hp at 5,500. That's huge considering all we did was change the cam and add better springs. That's outstanding value. The complete Hot roller cam, roller rockers, and spring kit goes for $500 through Scoggin-Dickey.
But we felt there was more to be gained by helping the breathing out at the higher rpm levels. Our next step was to add an Edelbrock Performer RPM Air Gap intake, a Holley 750-cfm double-pumper carburetor and a set of 1-3/4-inch Hedman headers. We felt that the combination of these three components would offer increased airflow to feed those big, hungry cylinders.
With the headers, intake, and carb in place and a quick jet and timing test completed, Taylor again pulled the handle on the dyno and downloaded the power curve into Duttweiler's dyno memory banks. The bigger headers and manifold lost barely a touch of torque at the very bottom of the power curve compared to the GM manifold and smaller headers, but it was well on its way to cranking out more torque by 2,800 rpm. The torque gains were mostly single-digit until the very top where the three amigos combined to produce 10 more horsepower at 5,600 rpm. Overall, the combination of the GM Performance Parts Hot cam and valvesprings with the Edelbrock Performer RPM Air Gap, Holley 750-cfm carb, and Hedman 13/4-inch headers was worth an astonishing 100 hp and an eyeball-flattening 95 lb-ft of torque over last month's best. Of course, this will add over $950 to the price of the crate HT 383. If you're looking for bang for the buck, put your money on the cam, springs, and rockers and you've still got a thumpin' 383 that makes 454 lb-ft of torque and virtually 400 hp. That's enough power to put you into the high 12s with a 3,600-pound street car with decent traction.
So are we done with this midsize Mouse? If you think the answer is yes, then you don't know us well enough yet. Next month we'll turn our torque wrench on the Twister and bolt on an even bigger camshaft. Remember, we're still using stock iron Vortec heads! Stamp these power numbers on your forehead and wait until next time when we'll twist up with even more power. And pay attention to which way the wind's blowin'.
 The Twister started life as...  The Twister started life as a mild-mannered HT 383 from GM Performance Parts. We turned it into a powerful street beast with a few carefully selected bolt-on parts. An engine like this can be extremely powerful yet mild-mannered, which is a great combination. |
 The key to this installment's...  The key to this installment's power play can be found within the lobes of this GM Performance Parts hydraulic-roller cam. The Hot cam, along with a set of 1.6 roller rockers and a set of LT4 valvesprings, transformed our dust devil into a powerful tornado. |
 If you plan on only using...  If you plan on only using the Hot cam or something smaller, the GMPP LT4 valvesprings will bolt on without machining. If you use a bigger cam, then you must machine the head and use a larger and stronger valvespring that can handle the lift requirements. |
 The 650-cfm Holley modified...  The 650-cfm Holley modified by Quick Fuel Technology really performed on the Twister 383. If you're looking for torque and throttle response, the Hot cam with this carb is a good choice. |
 The 750 Holley is aimed more...  The 750 Holley is aimed more at peak power with excellent midrange torque. All testing was with 91-octane fuel. |
 We used the Edelbrock Performer...  We used the Edelbrock Performer RPM Air Gap (right) because we've heard good things about the design. We didn't see a huge power gain with the manifold, but it is still worth the investment. |
 Smaller headers like the 1-5/8-inch...  Smaller headers like the 1-5/8-inch Hedman's we tried often fit better in the chassis and tend to pump up the midrange and low-speed torque. Adding larger 1-3/4-inch headers tends to increase peak power while costing a little torque below 4,000 rpm. Our testing used Hedman headers for both 1-5/8- and 1-3/4-inch primary pipe sizes. |
 We also retained the GMPP...  We also retained the GMPP 1.6:1 guided rockers from last month's test for use with the Hot cam. These rockers should not be used with guideplates, since the combination of guided rockers and guideplates could bind the rockers and cause problems. |
The Twister, Part I
The Twister, Part III
The Twister, Part IV