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1996 Ford Mustang GT - Project Redheaded Step Child Part 8
Well, it has been a long time coming-and believe us, it was no cakewalk-but Project Redheaded Step Child (a.k.a. our '96 Mustang GT) has officially cracked the 300hp barrier. A huge thanks goes out to all the companies that helped with the normally aspirated portion of the project, but especially Steve Ridout from Powertrain Dynamics. Ridout had to suffer through the endless testing performed during the development of my new intake manifold.
For the final round of dyno runs, he was kind enough to come back and reopen his shop after thinking he made it safely home without having to spend another moment with that damn red project car. Like a trooper, Ridout allowed yours truly to make those last few runs (naturally with tuning involved) to provide the necessary air/fuel and timing curves for the new non-PI combination. It was his dedication to the project (or maybe we just wore him down with all our begging) that allowed us to finally surpass the magical (and for some time seemingly unreachable) 300hp mark.
Before getting into the specifics of the new intake manifold that allowed our early non-PI motor to eclipse the 300hp mark, we should review what transpired to get us this far.
In Part 1, the modifications included a C&L plenum and 75mm throttle body. To that we added a set of BBK underdrive pulleys and a custom Powertrain tune, and were rewarded with gains of 14-15 hp.
Part 2 included just maintenance items (plugs, 180-degree thermostat and MSD ignition components), but we improved the power in Part 3 with a complete MagnaFlow after-cat exhaust and x pipe system (with cats). The exhaust mods showed gains of 15 hp, but things really got exciting after installing the XE262H Comp cams in Part 4. The cams allowed us to finally surpass the power output of a stock PI motor. After a complete Maximum Motorsports suspension upgrade in Part 5, we got crazy and swapped out the tired (200,000-mile) short-block for a fresh setup from Coast High Performance. Topped with a set of CNC-ported heads from Ford Performance Solutions and a set of even larger XE274H cams, the new combo brought us to roughly 275 rwhp. The nitrous upped that tem-porarily to 377 hp, but in all-engine trim, we were stuck in the 275-rwhp zone.
With nowhere else to turn in terms of viable street bolt on for our non-PI motor, I decided to take it upon myself to design a new intake for the early 4.6. With little else to choose from, I knew there was a great deal more power to be had from this combination with the right manifold. Previous testing on the engine dyno with a prototype unearthed a great deal of power, but now the trick was to get that combination to fit under the hood and be able to accept all the factory vacuum lines, EGR and IAC connections.
Not surprisingly, such an endeavor took some time. I can honestly say I have become an expert in the removal and replacement of the '96 intake system, having performed the task no fewer than 20 times during testing and development of the new intake. The design difficulty was not so much reaching a given power output, as eclipsing the 300hp mark would actually be easy on this combination. The real trick was managing to dramatically increase the peak power number without sacrificing all that wonderful low-speed torque production the factory intake had to offer. I'm a firm believer that losing power all the way up to 6,000 rpm is a poor trade-off for an extra 10 hp at the power peak (or beyond). Getting an intake design to provide a significant gain in peak power without sacrificing the low and midrange power is the real trick.
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.
Not surprisingly, the 3.5-inch inlet tube eliminated any and all airflow restrictions. In retrospect, we should have replaced the factory mass air meter with the C&L unit we tested in Part 1. Given the power level, we suspect the factory meter was now the restriction in the inlet system. The inlet tube was fed by the same 75mm Accufab throttle body used on the stock intake. It should be noted that the VRI was tested against the stock intake using the Accufab throttle body and C&L plenum, which was worth considerable power compared to the stock throttle body and plenum at this power level. Thus the power gains offered by the VRI would have been even greater had we run them against the stock intake, throttle body and plenum. The 75mm throttle body was fed by a 3.5-inch inlet tube (positioned between the throttle body and MAF). The tube featured provisions for the PCV line from the driver-side valve cover and the IAC motor. The second PCV from the passenger's side cover was fed behind the throttle body.
The equalized runner length and cross section required a new fuel rail. The billet fuel rail (in prototype form) was configured to work with the stock 19-pound injectors. That this motor managed to exceed 300-wheel hp with just 19-pound injectors (and a stock pump) shows just how much power the stock stuff will handle. The custom fuel rail required an external (adjustable) fuel-pressure regulator, but a new design is in the works to allow use of the stock fuel rail and regulator. Note from the photos that alternator, thermostat housing, and ECT sensor are all intact. Having realized our goal of producing 300 wheel horsepower with a non-PI motor, we can now turn our attention to bigger and better things, namely forced induction. We enjoyed dealing with all the normally aspirated modifications, but the power available with boost has to be experienced to be believed. While we kicked and scratched to get a few horsepower here and there (and spent endless hours on a new intake design), these gains pale in comparison to those available with a simple pulley change on a Kenne Bell supercharger. It's amazing what a few extra pounds of boost will do for the power curve.
Now that we run the gamut of normally aspirated mods for the non-PI motor, we can step back and take a realistic look at the buildup. Was it difficult? Sure, coaxing this kind of power from the pathetic non-PI motor took every trick in the book. Was it worthwhile? Sure, it's always beneficial (certainly to the readers) to examine and (accurately) test the merits of different performance components. Would we recommend this route to our readers with '96-'98 Mustangs? Probably not, unless (like us) they are looking to prove a point. It would make much more sense (and power) to simply equip the engine bay with the later PI components, the most important of which would be the PI cylinder heads.
Even in ported form, the early non-PI heads simply do not flow as much as a well-done set of PI heads. The flow difference can be as much as 30-40 cfm per runner, especially since the later PI heads can be run with 0.550-lift (or greater) cams while the early heads must make do with 0.500-lift cams. Limiting the lift to just 0.500 is the same as limiting airflow since either head (in ported form) flows more at 0.550 than it does at 0.500. We would expect an easy 40-50 hp were this combination run with ported PI heads and attending cams instead of the non-PI stuff.