Another used part in the Cheap Street engine is the forged steel crank. Reconditioned and
"We wanted to do anything within the rules to lighten the rotating mass of the assembly," he says. "The Diamond pistons have a Teflon coating to reduce friction; it all contributes to speeding up the rotating assembly."
Another trick employed was sealing off the oil return passages at the top of the block, which keeps oil at the bottom of the engine. Within the block, of course, turns the camshaft. We tried with flattery, beer, and money, but couldn't pry its specs from the engine builder.
"It's a Thomson special grind," he says with a grin. "It's the key to this engine's power."
In fact, all we could get out of the tight-lipped Thomson was, "it's a flat-tappet cam."
Um, yeah...We read the "no roller cam" section of the rules, too, Brian.
He also was mum about the compression ratio. The engine uses Vortec iron cylinder heads (GM PN 12558060), which aren't particularly known for their high-rpm flow characteristics, but do have relatively efficient combustion chambers.
The rules say a wet sump oiling system must be used, so a deep-sump, yet lightweight, alum
Because head milling is permitted, Thomson touched the bottoms of the Vortecs, but doesn't want to reveal how much because doing so, he says, would reveal the approximate combustion chamber volume and, thus, the compression.
"There's no technical limit on compression in the rules, but the flat-top pistons effectively limit it," he says. "It's a delicate balance between higher compression and valve interference; the cam also has something to do with it. Next question, please."
Uh, not so fast. According to our copy of the GM Performance Parts catalog, the maximum lift for a camshaft used with the Vortec heads is .475-inch. Those of you who are good with a slide rule can now make some calculations regarding the cam's specs and the engine's compression ratio.
Nothing exotic here: just an inexpensive double-row timing chain setup from the GM Perform
We'll give you a hint to get started: Most engines in this class will have compression ratios from around 10:1 to 12:1.
Minimal SqueezeAlthough mum about the cam and compression, Thomson was chatty when it came to the engine's induction and fuel delivery, which is a simple, single-plain high-rpm manifold topped with one of Barry Grant's Demon 750 carburetor-a Mighty Demon, to be precise.
"Works terrific for high-rpm power," says Thomson. "With the heads being somewhat restrictive in their flow, it was important to get as much air through them as they could take. The Demon is also pretty easy to adjust in the pits."
Indeed, it was designed for between-rounds tweaks, according to Barry Grant Inc.'s Steve Cole.
"It's the perfect kind of carburetor for Cheap Street because it was designed for the types of adjustments racers make at the track," he says.
And with a $500 claim for the carb in Cheap Street, the carb can be purchased for just about that price.
"The Mighty Demon really is hybrid of the street-based Speed Demon carb and the Race Demon," he says. "Actually, once the jet extensions are added and the floats swapped, the Mighty Demon has the features of the Race Demon. For this type of competition, it's really the logical choice."
Between the carb and intake sits a half-inch plate for a Nitrous Works laughing gas system, fitted with the rules-specific 0.063-inch jet. The rules also state the system can only have one fuel spray bar and one nitrous spray bar.
"It's pretty small and the rules are very specific about it," says Thomson of the nitrous jet. "To make it [the nitrous system] the most effective, you have to really get the bottle pressure up. Way up."
All of Thomson's tricks, including those he would and would not tell us, seem to have paid off. On the dyno, the engine produced 479 hp and 447 ft-lb of torque without nitrous.
The broad power curves of the 4,000-6,800-rpm band, where the engine will operate most of the time, show more than 400 ft-lb of torque by 4,400 rpm and holding above that mark until at least 6,300 rpm. In fact, the engine makes 380 ft-lb at 6,600 rpm.