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The Twister, Part I

Testing GM Performance Parts’ HT 383 Crate Engine

Jeff Smith Jun 1, 2002

Step By Step

This is the way the engine will show up on your doorstep when you order a GM Performance Parts HT 383. The engine comes as an assembled long-block complete with an intake manifold and all the necessary tin.

As you probably know by now, the Vortec heads require a specific eight-bolt intake manifold. This is the GM Performance Parts aluminum dual-plane outfitted with a Quick Fuel Technology–modified 650 Holley carb. Rounding out the top end is a stock HEI distributor and Moroso plug wires.

The first thing to do is add an oil filter, fill the engine with oil, and pressure lube it to ensure the oil has lubed everything before you start the engine.

We used Valvoline 10W-40 oil and a quality filter. After the first dyno session, we drained the oil and changed the filter as a normal precaution.

We also added a FilterMag to the engine to prevent errant metal particles from re-entering the engine and doing some damage. The FilterMag’s powerful magnets attract even the tiniest metal particles in the oil.

The iron exhaust manifolds performed admirably in the torque department, but not surprisingly suffered when it came to horsepower. The iron combo made 427 lb-ft of torque but only 314 hp. The iron exhaust manifolds were plumbed into a pair of 2-1/2-inch DynoMax Super Turbo mufflers.

Adding a set of 1-5/8-inch Hedman long-tube headers was worth a bunch of torque and horsepower. We also tested a pair of intermediate-length headers that traded midrange torque for better horsepower. If all you do is add a set of headers and a pair of DynoMax Super Turbo 2-1/2-inch mufflers, you can make 439 lb-ft of torque and 329 hp with the longer headers. The intermediate length headers made 338 hp.

To help the wimpy hydraulic roller cam, Taylor added a set of GM Performance Parts 1.6:1 roller rockers. Combined with the headers and good exhaust system, the HT 383’s power grunted to a torquey 447 lb-ft at 3,100 while cranking an admirable 341 hp at 4,700 rpm. If the motor carried the power out to a higher rpm, the stock valvesprings would have given up for sure. Our next step is a set of LT4 valvesprings and the GM Performance Parts Hot hydraulic roller cam.

The HT 383 uses a late-model, one-piece rear-main seal four-bolt main block that requires the smaller-diameter flange flywheel or flexplate. The engine comes with an externally balanced, one-piece flange flexplate. The engine also comes with an externally balanced torsional damper.

The stock valvesprings (left) will handle the additional lift of the 1.6 roller rockers, but the best move is to install a set of GM Performance Parts LT4 valvesprings (right). These stiffer springs will bolt in place of the stockers with no machining required and can handle the lift of the GM Performance Parts Hot hydraulic roller cam that we’ll install next month.

In the best Tim Allen tradition, the secret password around the Chevy High Performance offices is: More Power! Back in the Sept. ’99 issue, we started a seven-part Goodwrench Quest series(The Goodwrench Quest, Part 1: Dyno-Flogging GM’s Budget 350 Crate Engine; September, 1999) that eventually took us over 400 hp with a stone-stock 350ci short-block. That series was so popular that we decided to do it again. All we needed was a fitting candidate.

Recently, GM Performance Parts released another gem-in-a-crate in the form of the HT 383. This stroker small-block was created as a torquey small-block for heavier street cars and trucks without the bulk or expense of a Rat motor. GM Performance Parts merely mated a long-stroke crank with Vortec iron heads, slipped in a mild hydraulic roller cam, and topped it off with an aluminum dual-plane intake manifold.

Being the hard-core power junkies that we are, we knew the potential of the Vortec heads but realized we had never tested them on a 383. So we quickly called our local GM Performance Parts dealer and the “truck” engine showed up on our doorstep. The engine comes completely assembled with an oil pan, timing chain cover, torsional balancer, iron water pump, valve covers, and an aluminum intake. You even get a dipstick. All you have to do is add oil, pressure lube the engine, drop in a distributor and a carburetor, add some water pump pulleys, and you’re ready to go. You get all this for around $3,895 plus shipping.

This engine promised excellent torque based on the success we witnessed with Tim Moore’s 355ci Vortec heads and Hot hydraulic-roller cam–equipped small-block, which made 422 hp with a single-plane intake (“Hot, Hot, Hot Part II,” Nov. ’01). Moore’s 355 made 412 hp with the GM Performance Parts dual-plane and 423 lb-ft of torque. Dividing 423 lb-ft of torque by 355 ci equals 1.191 lb-ft per cubic inch. Multiplying 1.191 by 383 ci shows a potential for 456 lb-ft of torque! That friends, is serious torque. That’s why we decided to call this engine the Twister. Our next decision after we did the math was to test one of these GM Performance Parts motors.

But we’re jumping ahead of our story. We plan to run this motor through four installments to be sure that Ed Taylor’s Ventura Motorsports will squeeze every last ounce of torque and horsepower out of this rascal. To do that, Part 1 will consist of a baseline test to ensure that our engine lives up its claims. Our experience has been that GM Performance Parts engines are often slightly underrated, which means that often these mail-order crate engines can pull a few more lb-ft out of their hats.

Testing One, Two...

Since this HT 383 engine was intended as a mild-mannered torque mixer, we began testing with a set of cast-iron exhaust manifolds, a true 2-½-inch exhaust system with a pair of DynoMax Super Turbo mufflers, and a torquey 650-cfm vacuum-secondary Holley, modified by Quick Fuel Technology. After bolting the motor onto the dyno, Test 1 revealed the 383 made an amazing 427 lb-ft of torque at 3,100 rpm. With iron exhaust manifolds, good horsepower is difficult, but the 383 still managed 314 hp at 4,700 rpm. GM Performance Parts advertises the HT 383 at 415 lb-ft of torque and 325 hp, but this is with the engine configured with a throttle-body fuel-injection unit (in essence, an electronic carburetor) and a set of headers. That our engine made more torque but less horsepower with ugly iron exhaust manifolds is actually very impressive.

With the baseline test established, the first obvious change was to move up to a better exhaust with a Hedman full-length header and a pair of 2½-inch DynoMax Ultra-Flo mufflers. This is always the first place you should look for power, and the addition of the Hedman headers >> didn’t disappoint us at all. Torque increased immediately across the entire power band. Peak power came in at an impressive 439 lb-ft of torque at the same 3,100 rpm, while the horsepower improved to a respectable 329 hp at 4,900. Remember that we have yet to remove a valve cover on this engine. With the addition of a simple set of Hedman headers and good DynoMax mufflers, this crate 383 was already looking strong.

We felt that the small hydraulic roller cam was responsible for holding this engine back from even greater accomplishments. But rather than jump right in with a cam change, we decided to keep this first series of tests simple. Engine-builder Ed Taylor bolted on a set of GM Performance Parts 1.6:1 roller rockers. This required no more effort than removing the stock stamped rockers and installing the rollers with a ½-turn preload on the lifters.

After the quick rocker swap, it was time to pull the handle once more on Ken Duttweiler’s dyno. The rpm peaks didn’t move very much, but the torque jumped to an awesome 447 lb-ft at a grunting 3,100 rpm while horsepower surged to 341 ponies at 4,700. This simple rocker change generated additional power throughout the entire rpm band by as much as 36 lb-ft of torque and 29 hp over the stock combination. All of this was still with a streetable 650-cfm carb that would offer outstanding throttle response and driveability.

These are excellent power numbers for a conservative small-block that you could easily drive everyday. To get an idea of what this power is worth in a typical street car, we plugged all three power curves into Racing Systems Analysis’ Quarter Pro dragstrip simulation program. For our baseline vehicle, we spec’d a 3,600-pound Chevelle with a Turbo 350 automatic, 2,400-stall converter, 3.08 gears, and a solid suspension that would plant all this torque through a set of sticky 10x26 tires. The details on all three combinations are in the Quarter Results sidebar, but with just headers and a set of roller rockers, the Twister is capable of a 12.85/103.50-mph pass! This is based on ideal weather conditions, but clearly this combination is worth low 13s at 100 mph in anything short of a torrential downpour. Remember, this is with a set of 3.08 gears! Can you say, “torque is my friend”?

We could stop right here and have a winner street motor that anyone could have fun driving, but we’ve only begun to spin our Twister motor. Next month, we’ll screw in a GM Performance Parts Hot cam, better valvesprings, and a bigger Holley carb and see what that’s worth. There’s even more planned for the Twister, but we don’t want to divulge all our plans just yet. So stick with us as we bend the needle on the dyno in search of more power.


Surprise, AZ 85374
Hedman Hedders
Whittier, CA 90602
Valvoline Co.
Lexington, KY 40512
Ventura Motorsports (Ed Taylor)
Ventura, CA 93002

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