Fuel line size is important....
Fuel line size is important. That dinky 5/16-inch line that Ford put on your Mustang from the factory isn't large enough if your engine is making 450-500 hp. Mark is using a 3/8-inch line. When power rises above 500, you need a 1/2-inch line.
One dyno test involved switching from a dual-plane Edelbrock Per-former RPM Air Gap intake manifold to a high-rise Parker single-plane manifold, available from Trans Am Racing. A dual-plane intake has long runners, which translates into good low- and mid-range torque, which is needed on the street. The Parker single-plane high-rise manifold has shorter runners, which changes torque and adds horsepower at high rpm.
With the Parker intake, horse-power went from 485.2 to 495.1, a gain of 9.9 hp in the same 6,500-rpm range. However, we lost torque-some 8.9 lb-ft at 5,200 rpm. Torque comes on strong at the same rpm, which means the single-plane Parker loses very little twist.
"After installing the Parker manifold, our fuel requirements changed," Mark comments. "Normally, a dual-plane like the Edelbrock Air Gap requires a slightly leaner mixture, which is fine for fuel economy. Our first pass with the Parker gave us an air/fuel ratio of 14.1 to 14.5:1. This put us right back where we started." This is when Mark went up three jet sizes for the primaries and up two for the secondaries. After the jet swap, the air/fuel ratio went back to 12.6:1-perfect for dyno testing. With Mark's objective being 500 hp, he was close at 495.1 hp at 6,500 rpm.
Mark experiments with carburetor...
Mark experiments with carburetor spacers and gaskets to see what happens to power throughout dyno testing. Sometimes, the results are disappointing. Typically, thicker spacers yield more torque because of the indirectly extended runner length, which increases velocity.
A day at the dyno is more than just flogging an engine under load at high rpm. It's an opportunity to find out what an engine is made of. If an engine is going to fail, it will fail during jet-check or the first solid pull. During a dyno pull, an engine works harder than it ever will in your Mustang.
When Mark began his Westech dyno session with our 347 stroker, peak horsepower was at 479.2 at 6,500 rpm, with torque coming in at 416.5 lb-ft at 5,200 rpm. With fine-tuning that included rocker arm swaps, going from a dual-plane to a single-plane high-rise manifold, and jet swaps to ensure proper fuel mixture, Mark was able to close the gap, achieving 502.4 hp at 6,500 rpm. Peak torque rose to 425.1 lb-ft at 5,200 rpm.
For a street stroker, peak torque should occur around 3,800-4,200 rpm. However, when that happens, peak horsepower isn't going to be 400-500. This is the trade-off between horsepower and torque. If you want high-rpm horsepower, you're not going to have good low- to mid-range torque. If you want strong low- to mid-range torque for the street, you're going to sacrifice horsepower. And fuel economy? Don't kid yourself. At 400-500 hp, you're not going to have fuel economy no matter how gently you drive.
Before the first pull, Mark...
Before the first pull, Mark did a jet check starting out at 0.073-inch. A plug reading confirmed he needed to go richer at 0.076-inch, which solved fuel mixture issues.
What we learned from Mark's dyno experience is to know your engine's mission. This is not a mild-mannered small-block, but decidedly aggressive at over 500 hp with the Parker single-plane and roughly 475 with the Performer RPM Air Gap. Your 347ci stroker doesn't have to be this aggressive and you don't really need 500 hp in a weekend cruiser. With a milder hydraulic roller cam, Edelbrock Performer RPM cylinder heads for 351W, Performer RPM Air Gap, and a 750- to 830-cfm carburetor, you can still bank on more than 400 hp at 6,000 rpm and roughly the same amount of torque at 4,200 rpm.