What matters more than horsepower? In our minds, in our dreams, in the TV and magazine ads, at the track, in the endless bench-racing sessions with our friends and rivals online, it’s all about horsepower. Hate to break it to you—it’s not. And, unlike what your Plymouth Road Runner–driving, old-fart, step-uncle-in-law tells you, it’s not even about torque. Both are important on some level, of course, but unless you’re bench-racing, the magic lay in an equation: pounds per horsepower. Dropping weight gets you free power.
Ten pounds per horsepower is a most magic figure—for both performance fans and the insurance industry. Virtually nothing certainly nothing most of us can afford, unless you’re in the market for a new Lexus LFA or Nissan GT-R—is built up (or down, depending) to that figure. Anything that dips below that arbitrary 10-pounds-per line will get hit with an insurance surcharge that looks more like a car payment. Generating power doesn’t seem to be the problem; with 10 airbags and more computing power than the Apollo command module in even the smallest cars these days, plus strict regulations about crashworthiness, it’s lightness that seems to be at a premium. Could (for example) Mazda build a 240hp, 2,400-pound Mazda2 for public consumption? Probably. But the target market (that’s us) wouldn’t be able to afford the insurance and the car at the same time.
Older cars are lighter, not to say flimsier, just less packed with airbags and wires and accessories and junk to chuck out when you want to go fast. To wit: an ’82 Mazda RX-7 had a factory curb weight right around 2,500 pounds, (and that’s with the leather-lined GSL package, which included rear disc brakes, cruise control, power windows, a sunroof and a kickin’ four-speaker Clarion AM/FM stereo), give or take a meal at Jack in the Box, right around what today’s Mazda2 weighs. Thirty years ago, the RX-7 scratched every sports car owner’s itch, except for outright speed: a 0–60 time below 10 seconds out of the 100hp, 2-rotor 12A wasn’t lighting anyone’s short hairs on fire, even in the dismal days of 1982. It doesn’t take a calculator to suggest that 25 pounds per horsepower is never going to feel like 10 pounds per horsepower.
Let’s look at a slightly madder machine, the R32-generation Nissan Skyline GT-R. A screamer of the first order, the twin-turbo, detuned-race-mill RB26DETT is advertised at 276hp, thanks to that dopey gentlemen’s agreement all of the car companies had at the time, and is dropped in a 3,256-pound curb-weight all-wheel-drive machine. (To be honest, both of those figures sound a little low, but we’ll roll with it for the sake of argument.) Crunch the numbers, and you’re at 11.8 pounds per horsepower, still not at the magic 10 pounds per horsepower. And if you’ve ever driven or ridden in an R32 GT-R, even a stock one, you’ll know that that’s a rickockulous amount of power.
Now, for the sake of argument, let’s say you were to combine the two: take the light, lithe, long-nose lancet of an early RX-7, and cram it full of RB26DETT power. Forget the fiddly details, momentarily, about fitting six-cylinders in an engine bay that previously housed none and look at the numbers. Even if you believe Nissan’s numbers, and history indicates that there’s no reason on Earth why you should, you’re looking at 9.05 pounds per horsepower. Get real, bump the RB’s output to a more palatable 350 horses with a couple of exhaust tweaks and you’re looking at an eye-opening 7.14 pounds per horsepower. “I don’t drag race for time,” the owner, Chris Rothrum says, quite possibly because he’s quick enough that he’d be banished from every sanctioned racing facility in the land if he tried his hand at tripping the beams—but consider this: the ’12 Nissan GT-R clocks in at 7.2 horses per pound and hits 60 mph in 2.9 seconds.
But then you start futzing around with the numbers. The RB has to weigh a couple of hundred pounds more than the yanked-out 12A. The weight of a rollcage should probably be factored in to keep the unit-body floor pans straight at launch. But then you install racing seats, a bunch lighter than the puffy leather chairs it came with, and take out unnecessary junk like the stereo. Let’s say you gain 100 pounds. You’re still ahead of the curve. Moving the battery to the storage compartments behind the seat allows you to redistribute some of that weight, something as much of interest to the sports car fan as straight-line speed.