Everybody always wants the big score. You know the dream--hit the Lotto for $10 million and never work again. The problem is that your chances of getting hit by lightning in your left ear are better than scoring the Lotto jackpot. So where does that leave us? Back to tweaking and fighting for every lb-ft of torque and every horsepower to push our GM Performance Parts HT 383 to make more power. The good news is that it's actually quite easy.
After last installment's series of grueling dyno flogs, we had a chance to more closely inspect the dyno numbers and huddle up with Comp Cams' designer Billy Godbolt. He suggested a set of stiffer Comp Cams valvesprings on the Vortec iron heads. The Comp Cams PN 977 spring offers more seat pressure and a 50 percent higher rate than the recommended 986 spring. Godbolt suggested the stiffer spring might be worth more power through better valve control. Valve float is the usual term applied to a valvetrain out of control, but the most common result of all this loss of control is that the heavier intake valve bounces off the seat as it closes. This occurs during the compression cycle, which allows cylinder pressure to escape, reducing power without warning.
Based on this discussion, we bolted a set of 977 valvesprings onto the Vortec heads since the valvespring seats had already been modified for larger-diameter springs. With no other changes to the engine, our intrepid dyno man Ed Taylor discovered a significant power increase virtually throughout the 383's entire power band. The most interesting part was that the largest power gains came in around 3,800 rpm (a much lower engine speed than we would have predicted). Peak horsepower also improved, but not nearly as much as the torque. We saw as much as 18 lb-ft at 3,800 rpm, recording a killer 501 lb-ft torque peak at that rpm. Horsepower increases were in the 10hp range with a peak of 459 hp at a conservative 5,400 rpm. Overall, the entire power curve improved with just a valvespring change--amazing.
This left us enthused about bolting on the Trick Flow Specialties 215cc aluminum heads. If you've been reading this magazine for some time, you should already know that we've tested the smaller 195cc heads on several engines with excellent results. The 195cc castings offer excellent flow and are one of the least expensive aluminum heads on the market. A previous quickie test netted us about 20 more horsepower with the smaller HOT cam. We wanted more power, so we opted for the larger 215cc castings.
One advantage of these TFS heads is that they utilize a standard intake manifold bolt pattern. This gives us a broader range of intake manifolds to experiment with. We also used the same 977 valvesprings on the TFS heads in order to keep the testing consistent. Ed Taylor bolted the big TFS heads in place, rechecked the pushrod length, and bolted the rest of the GM Performance Parts HT 383 back together including a new Edelbrock Performer RPM intake, Speed Demon 750-cfm carburetor, and the Hedman 15/8-inch headers. Compression also remained the same since the TFS heads come in a 64cc chamber spec with a similarly shaped combustion chamber.
The TFS intake ports are a solid 45cc's larger than the Vortecs, so we expected to see a slight loss in low-speed torque (below peak torque), followed by a gain in horsepower above the torque peak. The TFS heads didn't disappoint us, losing only single-digit torque values below 3,400 but then charging in with as much as 43 more horsepower over the Vortec iron heads. Overall, we saw as much as a 50hp swing over the iron Vortecs with the 986 valvesprings.
Almost as interesting is the fact that even with the larger cylinder heads, this 383 still doesn't like larger headers. Despite these lofty power levels, this engine is breathing through a stock Performer RPM Air Gap intake and a set of production 1-5/8-inch Hedman street headers and a pair of Borla mufflers. Taylor tried a set of 1-3/4-inch tubes and came away with less power. This is flat-out amazing power from an otherwise off-the-shelf 383ci crate engine. We have never pulled a piston out of its bore or even yanked the oil pan. Taylor bolted in a big Comp Cams hydraulic-roller cam, a set of 215cc TFS aluminum heads, and a dual-plane intake. These are Rat motor numbers in a lightweight package. Idle vacuum isn't all that stout at around 8.5 inches of vacuum at 900 rpm, but the amazing torque is more than enough to live up to its namesake.
We're going to give the Twister a break for a while and return with a test of GM Performance Parts' newly revised Fast Burn aluminum head. Stay tuned.