The bolt-on combo-we've all seen it, but what's it worth in the real world? Building horsepower for the dyno environment is one thing, and certainly it has merit in terms of raw power, but what do those numbers mean when the engine is in your car and pushing that performance through spinning components? We decided to answer that question via a dyno blast that would result in a pass-or-fail grade. We proposed to Carlisle Productions that we would bring in the hardware and personnel to acid-test a reader's Mopar. Steve Pilic's mobile Dynotek Dynojet chassis dyno would be there to grade our effort. The idea was simple: select a willing reader's ride and perform the mods right there at the show, complete with a before-and-after dyno test. This sounds easy, but in the publishing world, this kind of live (non-foolproof) project could be akin to suicide. Too many variables, not enough time, no room for errors, and most importantly, no way to cover your rear. Of course, we decided to do it anyhow.
We used Dale Mathews' '71 383 Road Runner, a nicely detailed street resto with a rebuilt resto-stock 383. Since Dale isn't a drag racer, the engine had been rebuilt with stock-level replacement parts. We only required that the engine pass a cranking compression test to make sure we were dealing with eight good holes. We knew a 440 would have more potential for the upside due to the increased displacement, but the 383 fit our criteria of making these changes to a typical musclecar engine. By the way, Dale doesn't own a trailer; he drove the car from North Carolina and planned to drive it home once we were finished.
We were after a hot street combo. Dale indicated the engine was rebuilt with forged replacement SpeedPro pistons, so we figured the short-block was easily reliable to 6,000 rpm. We asked where the pistons ended up at top dead center, and Dale thought the piston deck height was .012-inch down. He agreed to pull out the MP cam-the only internal improvement from stock-and replace it with a bone-stock unit and swap the Holley carburetor for a stock AVS. The Road Runner is equipped with an automatic, the stock converter, and 3.23 gears out back; it had to retain its streetability, so we weren't looking to build a full-tilt drag engine. To find serious extra power, we wanted to science-out airflow and spark: the heads, intake, carb, cam, valvetrain, ignition, headers, and exhaust.
We saw what the new Edelbrock heads could do on the engine dyno (see the Oct. '01 issue) and were impressed. Out of the box, they'll make more power than the stockers, even on a relatively mild combo. Theoretically, at 210cc, the port size won't give away any torque generation at the low end. Previous testing on the engine dyno with a 440 confirmed this, and the Edelbrock heads actually showed more bottom-end torque than the stock heads. At the top end, the head's 291-cfm peak intake flow (Edelbrock's number) significantly outperformed the 383's stock 346 castings; the ports are quite efficient, making them an ideal replacement for the 383. Better yet, we knew from experience they would bolt right on and fit, a critical requirement for this type of weekend jam session. These heads are a closed-chamber design, with chambers coming in at 84cc, slightly smaller than the stock heads' 88cc. Based on the short-block specs we were given, we calculated the 383's compression ratio would come up to 9.22:1 with the replacement heads.
Here's the opening scene:...
Here's the opening scene: One stock 383 '71 Road Runner making 158 hp at the wheels standing by on the Lifts Unlimited four-post lift, a table covered with the latest go-fast hardware, a Mac tool truck standing by outside, and 24 hours to put it all together.
The plan was to strip the...
The plan was to strip the Road Runner to the short-block, bolt on the new Edelbrock heads, follow up with the tti exhaust system, then wrench together the rest of the mill for a dyno test. With the stock cam out, we stripped it and changed directions to start building back up with the fresh goodies. Thanks for the mug shot, guys; Mike Shirley's holding the cam.
Turning the engine over, we...
Turning the engine over, we found the pistons a good .080-inch down the hole, rather than the .012-inch down spec we'd been given. Say good-bye to 3/4 of a point of compression ratio, but we had to work with what we had.