The following was written by George Pence, one of the luminaries in the Pantera and 351C communities. He addresses one of the modern issues with stroker 351C engines, but you can extrapolate this to any engine, really.
I think he makes some interesting points.
Why I’ve Glossed Over the Subject of Stroker Crankshaft Kits and Very High Horsepower Street Motors
I used to think 500 horsepower was a good goal for a 351C powered performance car; the 351C 4V was designed as a 500 horsepower race motor and it is rather easy to assemble it to achieve that power output, so why not take advantage of the motor’s capabilities? I also thought stroker kits were great because they made that 500 horsepower goal attainable at lower rpm. But over the years I’ve changed my opinion regarding the importance of having a 500 horsepower street motor; and I’ve changed my opinon regarding the benefit of a crankshaft with a longer stroke.
I know a fellow who had purchased a brand new Z06 Corvette a few years ago. It was equipped with the 427 cubic inch LS7 motor with a 4" stroke crankshaft, 505 horsepower, big back tires, low center of gravity, and a modern sophisticated chassis. He had owned the car about a month, when one sunny Saturday afternoon he found himself cruising down a freeway with very light traffic. He thought it would be a good opportunity to feel what the car was like with the electronic traction control switched off. So he turned it off, and he pushed the acelerator pedal toward the floor while crusing at about 70 mph. The rear end of the 'Vette immediately lost adhesion and went side ways. The car began spinning in uncontrollable circles across the freeway and crashed into the barrier in the middle. The owner was able to crawl out of the wreckage and only sustained minor injuries, but the one month old 'Vette was totaled.
That story is what convinced me to change my opinion on stroker motors for older street cars that aren’t equipped with traction control. Considering my position with Pantera International I have a responsibility to the public to offer advice that has their safety at heart. Being in this position I also hear about the problems owners don’t share with anyone else. There seems to be one or two Pantera owners every year who quietly replace their expensive high output motors with motors that are a little more sedate, having found their high output motors were too much for the chassis to handle and were no fun to drive on the street.
My Pantera “sports car” weighs at least 3200 pounds, it is equipped with 325/25ZR20 Michelin PS2 rear tires on 12" wide wheels, about the best street tires money can buy. The stock stroke 351C which powers my Pantera can light up the rear wheels at will, and there have been a few occasions when it has kicked the rear-end out in corners when I didn’t want it to. I personally wouldn’t want the motor to have any more crank arm leverage than what it already has. In fact I think a 3.25" stroke crank would be better, I think my Pantera would negotiate challenging roads faster if the motor had less crank arm leverage.
There are many aspects we must consider when we decide to modify an automobile. For instance, will the car be just a “fun car” or will it be used as serious transportation? Will the car be operated on the controlled road surfaces found at race tracks, or will it be operated on public roads having unpredictable road surfaces? Will the car be operated strictly in warm dry conditions, or will the car be operated on cold, wet or icy surfaces? We must consider the traction limits of both the tires and the chassis, and in some instances even the cooling capacity of the radiator can be a limitation (Cobras).
My comments below are geared towards older cars which aren’t equipped with electronic traction control, operating on public roads with street tires; such as sports cars and high performance street cars, Mustangs, etc.
Things got out of hand in the 1960s. The automakers involved in the horsepower wars installed motors in production cars with so much torque it dangerously overwhelmed the tires, chassis and brakes of the vehicles they propelled. Introduction of the 351C 4V marked Ford’s return to sound engineering practices, the automaker looked ahead to performance cars with balanced performance. Indeed, the 1971 Boss 351 had the quickest acceleration times of any show room stock Mustang until the introduction of the 1999 Cobra Mustang. The Boss 302 was a fantastic performance car, but it was hindered by a 6150 rpm rev limiter and 3.50:1 factory gears. With the rev limiter removed and lower gearing the Boss 302 would have given the Boss 351 stiff competition. The big block 428s & 429s made too much horsepower at low engine speeds, they overwhelmed their showroom stock chassis and street tires. The 428 had far too much “crank arm leverage” (crankshaft stroke) for a vehicle as light as the Mustang. When you’re spinning your tires you aren’t accelerating. The mid-size 351 was a better balanced package, exactly what Ford had intended it to be. The Boss 351 didn’t loose the honors of fastest Mustang to a stable mate with a larger motor either, the 4.6 liter motor in the Cobra Mustang is smaller; but the 1999 Cobra Mustang was a better balanced package than the Boss 351.
Consider two Panteras, the first weighing 3000 pounds and powered by a 500 horsepower motor; the second weighing 2700 pounds and powered by a 450 horsepower motor. Both Panteras have a weight to power ratio of 6 pounds per horsepower. All else being equal the two cars should accelerate at the same rate of speed … but all else isn’t equal. Traction will be easier to control in the lighter less powerful car. It will handle and brake better too. The lighter Pantera would be a better balanced package.
Building a 351C based stroker motor is contrary to the spirit of the 351C. It returns to the excesses that Ford designed the 351C to leave behind. I’m convinced if a high performance street, sports car or road racing enthusiast wants a 500 bhp engine the standard displacement 351 is a better choice than a stroker displacing 383, 393 or 408 cubic inches. A smaller motor builds power more gradually as the engine speed rises, it doesn’t make 400+ foot/pounds of torque abruptly at 2000 rpm like a stroker does, the power is delivered in a manner that is easier for the average driver to control, or to put it another way, in a manner making it harder for the tires to loose adhesion.
Choosing balanced performance over brute force is my recommendation. Build a 400 to 450 horsepower 351C, a task which is not difficult or exorbitantly expensive; keep the power delivery controllable; equip the car with the lightest wheels, the best tires, performance gears, lightweight drivetrain parts (drive shaft and axles), the best shocks and the best brakes you can afford; lower the chassis; and lighten the vehicle to improve the power to weight ratio. Truly going fast on street tires is not a matter of how big or powerful the motor is, but how well balanced the package is.
Copyright © 2011 by G.Pence/Pantera International
