With a set of TFS 215cc aluminum heads and a Comp 282 hydraulic roller cam, our HT 383 eng
Ryan Strohecker; Irmo, SC: I have an '81 GMC truck with a 305 small-block, an SM 465 four-speed transmission, 4.10 gears, and 35.5-inch-tall tires. I'd like to build a high-torque, low-speed engine to power it. I had planned to build a copy of the HT383 using the same cam, Vortec heads, 1.6:1 roller rockers and a dual-plane Air-Gap intake with a Holley 670 truck Avenger carburetor. Due to chassis/drivetrain restrictions, I'm going to be running 1 5/8-inch block hugger headers into 2 1/4-inch pipes that Y into a 3-inch single exhaust. I wish I could fit some long-tube headers and duals, but there just isn't enough room under the truck. I'd really like to do better than the factory HT383 rating, especially on the torque, which I would like to get into the 450 to 475-lb-ft range. I think I could get there with a bump in compression, but I'm unsure about what would happen with such a small cam that I'm sure is designed to build low-speed cylinder pressure. What would happen if I were to try and run 11.5 or 12:1 compression on this engine? I'd do it with a thin head gasket, flat-top pistons, and careful attention to compression height, chamber volumes, and so on. I'm toying with the idea of trying to reverse-cool the engine much like the Gen II LT-1s. If I were to run aluminum heads, reverse-cool the engine, and pay close attention to the ignition timing, do you think I could get there? Any chance I could make this happen on 91-octane gas? Has anyone ever tried to reverse-cool SBCs with any successful results? Could I reach my torque goals with a more conventional compression ratio?
Jeff Smith: These are some great questions, Ryan. It's obvious you read Car Craft and that you've paid attention to some of our tech stories. Most of your plan is right on target. You can certainly get to your goal of 450 to 475 lb-ft of torque with conventional compression. Let's start with a review of the HT383 GM Performance Parts engine. The base configuration is intended for applications like Ryan's truck using a steel crank, hypereutectic pistons, 9.1:1 compression, a mild hydraulic roller that specs at 196/206 degrees of duration at 0.050 with 0.431/0.451-inch lift using a pair of Vortec iron heads and a GMPP dual-plane intake. GMPP rates the engine at 340 hp at 4,500 and 435 lb-ft of torque at 4,000. Several years ago when I was the editor of Chevy High Performance magazine, Ed Taylor and I did an extensive dyno series on this engine. The combination that makes the most sense for your truck is the addition of a GMPP Hot cam kit (PN 24502586, $224.95, Summit Racing) that specs out with 218/228 degrees of duration with 0.492-inch lift using a 1.5:1 rocker ratio. This will require going to beehive springs like the Comp PN 26915 as a quick solution to accommodate the Hot cam's additional lift. With just the addition of the Hot cam and the springs with 1 5/8-inch long-tube headers, this engine made an amazing 486 lb-ft of torque at 4,600 rpm and 416 hp. That is exactly your goal, Ryan, and this was with the HT383's stock 9.1:1 compression.
I do find it odd that you don't have room in a fullsize truck for long-tube headers. Unless your pickup is slammed on the ground, there should be plenty of room for a set. While shorty headers fit much nicer, a longer primary-tube-length header is a major reason our HT383 test was so torquey. When the primary tube length is shortened, torque is the first victim. While I've never done this test, it could cost as much as 20 to 30 lb-ft, especially in the midrange where you will most likely need it. If you must use a shorter header, keep in mind that there are midlength headers available that might fit your truck and at least add a little more primary pipe length compared with the block hugger headers. A 3-inch collector and Y-pipe into a single muffler is also a better configuration.