Here's what the HT 383 looks...
Here's what the HT 383 looks like out of the crate, with the GMPP-recommended Quadrajet bolted in place. It even comes with a dipstick and later-style water pump (with long mounting legs). On the dyno at Thomson Automotive, the 383 proved to be a stump puller, registering 367 ft-lbs at just 2,000 rpm. Reflecting its truck-based intent, the engine's rev range is limited by the cam to about 5,000 rpm. Buying the short-block assembly and adding a high-flow set of heads is one way to get a higher-revving, deeper-breathing 383. Still, it's hard to fault the complete engine for around $3,800.
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...
The GM Performance 383's foundation is a regular production 350 iron-block, with four-bolt mains. It's a '86-and-later-style block, meaning it uses a one-piece rear main seal. Although common 383 build-up methods include a 0.030-inch overbore, this 383 retains its stock 4.00-inch cylinder bores. GM machines the pan rails to provide clearance room for the high-swinging connecting rods. Production-style four-bolt iron main caps are used to secure the crankshaft.
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...
Perhaps the most unique components of this new engine assembly are the connecting rods. For the sake of durability and long engine life, GM uses powdered metal rods with tapered, screw-in studs rather than bolts. Studs are used with the 5.70-inch rods after GM testing showed typical rod bolt shanks tended to pull away from the heads at higher rpm, particularly on the side of the rods that were machined to clear the engine block. These new rods are designed for this 383 engine, including the clearanced corners.
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.