Here's what the HT 383 looks like out of the crate, with the GMPP-recommended Quadrajet bo
Stroker motors are all the rage these days, especially stroked small-blocks. One of the most popular combinations is the 383 small-block, which traditionally combines a 0.030-over 350 block with the 3.75-inch crank from a 400 small-block (and some ancillary machine work).
If done right, the results generally produce a small-block with big-block-like torque. Until recently, however, building a 383 meant re-building an existing 350 engine. But, with core engines and 400 cranks becoming increasingly scarce, as well as the machine work necessary to make room for the higher-swinging reciprocating assembly, building a 383 isn't the cheapest engine option.
GM Performance Parts is changing that with the new HT 383. It's a factory-built 383, assembled with a brand-newengine block and components. We discovered that you can buy a complete 383 crate motor-rated at 340 hp and 435 ft-lbs of torque-for less than $4,000.
Sounds to us like an economical upgrade candidate for just about any low-po small-block-powered Vette. With such ideas percolating in our mind, we headed to suburban-Detroit's Thomson Automotive, where technicians were doing some testing on the 383 at GMPP's request. While there, we were able to inspect the engine's new internal components and check out its performance on the engine dyno.
The GM Performance 383's foundation is a regular production 350 iron-block, with four-bolt
Our impression? Two words: Torque monster. On Thomson's SuperFlow dynamometer, the 383 was already making 367 ft-lbs at 2,000 rpm; and by only 2,400 rpm, the twisting power had risen to 385 ft-lbs. From there, the torque curve remained as flat as an Arizona mesa.
Any drawbacks? Yes. The motor was out of breath by 5,000 rpm. This is because the original intent of the engine was a retro-fit for trucks, so the relatively low-lift camshaft and small valves in the iron Vortec heads limit high-rpm performance-it was designed for pulling, not racing.
A higher-lift cam and bigger valves would do wonders, according to Thomson Automotive, but the Vortec heads won't handle a cam with gross lift beyond .480, or the seals will crush.
Still, for those more interested in a cruiser, the horsepower and torque are more than adequate, and off-the-line performance is neck-tugging, to say the least.
"This engine is great for cars," says GM PP's Ernie Callard. "It's got a lot of useable horsepower. But it's the torque you feel, and this engine is all about torque. Better still, with all that torque you don't need a lot of gear to get the car going."
That's a good point, as taller highway gears will likely save a few bucks on gas during that Carlisle round trip.
Perhaps the most unique components of this new engine assembly are the connecting rods. Fo
Notched Pan RailsGMPP's 383 crate engine is constructed unlike conventional 383 rebuilds. To achieve the desired displacement, a 3.80-inch-stroke crankshaft swings the pistons in the small-block's standard 4.00-inch bores. Regardless of the engine's price point, GM didn't skimp on the important parts. The crank, for example, is 4340 forged steel.
"We couldn't see any reason for boring brand-new blocks," says Callard. "There's room for a 3.80-inch stroke, so that's where we got the cubic inches. And since the engine was designed for the hard life cycle of a truck, we needed super-strong internal components."
Although GM doesn't overbore the blocks, it's still necessary to clearance the oil pan rails to make room for the high-swinging reciprocating assembly. And that brings us to the other unique aspect of the new 383-custom connecting rods.
Callard says when GM first began testing the engine, conventional rods (with corners machined to clear the block) were used, "Our durability tests showed that the rod bolt shanks were pulling off the heads after a while."
According to Callard, those durability tests included 50-hour constant-duty cycles on a 550-horse version of the engine, with stops only for oil changes. For one minute of each of those 50 hours, the engine was taken to 7,500 rpm.
In the iron head version of the 383, the camshaft selection is all about low-end grunt. It
To solve the rod bolt problem, GM engineers designed a special rod for the engine, which was designed to be corner-clearanced and incorporates tapered, screw-in studs rather than bolts.
"The studs hold great," says Callard. "When we put the new rods in an engine and ran the durability cycle, we pulled them apart and they were perfect. No wear or stretching."
Off The Shelf
But while the engine's reciprocating assembly is new and noteworthy, the remaining components comprise a collection of the best from the Performance Parts catalog. The somewhat restrictive heads, for example, are out-of-the-box iron Vortecs, with 64cc combustion chambers and 1.94/1.50 valves. Combined with the engine's dished, hypereutectic pistons, the 383 has a compression ratio of 9.1:1-perfect for just about any grade of pump gas you might find on that road trip in the middle of nowhere.
To fit the Vortec heads, the engine also comes with a dual-pattern, dual-plane intake manifold. Its design is similar to the proven Edelbrock Performer RPM intake and features ports for EGR and other emissions equipment. GM does not include a carburetor with the 383 engine, but not surprisingly, recommends a Quadrajet.
The depth of this new stroker's displacement comes from this unique 3.80-inch stroke crank
Brian Thomson, of Thomson Automotive, says the Q-jet is a good performer on the 383.
"Once it's tuned correctly, it can make a lot of power," he says. "We've been experimenting with the Quadrajet on the this engine, even testing a few different cams, and it has performed remarkably well."If you must use a Holley, Callard says a 750 is the right one.
The Bottom Line
While well executed, the GM HT 383 still requires the buyer to supply a distributor, plug wires, a fuel pump and the previously mentioned carburetor. Front accessories and headers, of course, will have to be sourced, too. And, if we were putting this motor under the hood of our Vette, we'd swap out the cheap, stamped-steel valve covers for those with a little more flash.
Nothing to fancy here-pistons are off-the-shelf hypereutectic parts, clearanced for the Vo
But what about those restrictive heads? As if anticipating this question, a follow-up call to GMPP informed us that GM has released an assembled short-block version of the motor. You get the cubic inches, but supply your own heads and induction system. Perfect. (Don't be surprised if GM releases a higher-revving, higher-horsepower version of the 383 this year. We won't.)
Still, for its under-$4,000 list price, the HT 383 (GM PN 12497317) represents an unqualified performance value. Our time at Thomson Automotive proved it not only lives up to its advertised promises, but also holds the potential for even greater performance.
Or, get the short block (PN 12498332) and drop on your own heads and induction system-are you listening L98 and LT1 owners?
Either way, with 383ci of small-block under the hood (prepare yourself for a cheesy, story-ending pun), your Vette will undoubtedly be the "torque" of the town.
To keep the 383 engine's cost below the $4,000 level, GMPP drops on a set of production-style iron Vortec heads. For street performance, the heads are somewhat restrictive and limit high rpm performance. Bigger valves would improve flow considerably. Regardless, they work well enough and help the engine maintain smooth, straight power bands.
Another cost-saving, off-the-shelf item included with the engine is the dual-pattern, dual-plane intake manifold designed to the Vortec head. It's plumbed for EGR compatibility and has a mild high-rise design, similar to Edelbrock's venerable Performer RPM.
The intake manifold even includes an EGR block-off plate for those enthusiasts whose Vette needn't be saddled with emissions equipment.
The efficiency of the Vortec heads comes from the "Fast Burn"-style combustion chambers. The bean-shaped chambers allow a more complete and powerful burn of the mixture. On this 383 package, the heads boast 64cc chambers. The valves measure 1.94-inches on the intake side and 1.50-inches on the exhaust. The 383's valvetrain includes stainless valves and 110 pound single springs with dampers, along with 1.5 rockers.
Unique to the 383 is its 8-inch damper, which was specifically for the engine. A flywheel is included, too. Interestingly, the flywheel is a zero-balanced, '86-and-later style part. When combined with the engine's damper, the parts comprise a combination of external and internal balance. But it works very well.
Just about the only part not included with the 383 crate engine is the carburetor. GMPP naturally recommends a Quadrajet, which engine tester Brian Thomson says performs "remarkably well" on the 383. The dual-pattern intake manifold, however, accepts a Holley carb, too. If going the Holley route, GM's Ernie Callard says a 750 is perfect size-go smaller and the engine could lean out at the top end; go bigger and it's just too much carb.