Muscle Mustangs & Fast Fords
1996 Ford Mustang GT - Project Redheaded Step Child Part 8
Retaining the low- and midrange power meant not resorting to the easy way out in terms of intake design. When it comes to making an intake fit under the hood, there is no easier way than to build a manifold with short runners. The old single-plane intakes for the 5-liter Ford (like the Edelbrock Victor Jr.) are a good example of the short-runner design. While easy to manu-facture (even fabricate), the short-runners dramatically reduce midrange torque. On many applications, the difference in power production between a long- and short-runner intake is less pronounced at 2,500 rpm, since neither design is optimized for torque production this low.
According to the calculations, the ideal runner length (including head port) for optimum VE (torque production) at 2,500 rpm would be 37 inches long (just imagine a manifold with runners that long). Naturally, neither a 6-inch runner nor a 19-inch runner would be of much help here (though the longer the better), but the difference between the torque production of a 19-inch runner and a 6-inch runner from 3,500 to 4,500 rpm (actually all the way to 6,000 rpm) increases with engine speed. For any given runner diameter (cross section), a longer runner will increase the volumetric efficiency percentage and decrease the engine speed where peak torque occurs.
Naturally, all combinations of runner length and cross section will result in some sort of trade-off between peak torque and horsepower production, the key is selecting (and producing) a design that provides the best compromise for the given combination.
Given our affinity for optimized (as opposed to the generic terms long and short) runner length, it is not surprising that a great deal of attention went into finding the proper runner length for this modified non-PI application. Without divulging the exact specifications, we can tell you that the combination of runner length, cross section, and (surprisingly important) plenum volume was chosen to help the limited-flow potential of the ported non-PI heads produce impressive torque. That this motor managed to produce a peak of 327 lb-ft of torque is a testament to the work that went into finding the proper runner design.
Every bit as important is the fact that the VRI intake helped the motor exceed 300 lb-ft of torque from 3,300 rpm to 5,200 rpm. Despite the use of the aggressive XE274H cams, the modified non-PI motor still exceeded 250 lb-ft of torque (the peak torque offered by the stock motor) from 2,000 rpm all the way to 6,000 rpm. Note that this engine made peak power (302 rwhp) at just 5,400 rpm. There is no need to wind this motor to the moon in an effort to extract the big number. A peak power number at just 5,400 rpm means there is always plenty of thrust for everyday driving. The swell of torque from 3,500 rpm to 5,000 makes passing maneuvers and getting onto the freeway something to be enjoyed rather than feared.
As evident by the photos, the VRI intake featured what appeared to be a rear entry. In reality, the 3.5-inch inlet tube entered from the top (at the back of the lid to the plenum). Unlike most compromised factory designs, all eight of the runners were identical in both length and cross section. This allowed the manifold to provide the same airflow (and resonance tuning) to all eight cylinders. Even flow and resonance distribution to all eight cylinders was one of the little tricks used to help maximize power production.