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.
Chris isn’t one for bench racing or theories. He’s more of a hands-on sort, which the customers at his shop, Real Street of Blaine, MN, appreciate. “I enjoyed my Mazda in stock form,” he tells us. “It was a solid, clean, 12A-powered FB. And it was the slowest thing I owned! In a fleet consisting of an R32 Skyline GT-R and a ’92 Mitsubishi 3000GT VR4, a friend asked if I could install the RB in my Mazda.” That sounded suspiciously like a dare, but apparently, the answer was yes.
That’s not to say it was simple. Oh, no. “I built it over a sporadic 10 years,” he claims. “The engine swap started in 2008, after the car was completely refurbished throughout. Most of the process was in making one-off parts, complex work or custom installation. Function and fitment is determined by the skills of the person doing the work. There are no conversion parts, no how-to swap you can follow along in a magazine for this.” Having friends with skills, Chris admits, is helpful. It’s not everyone who has the talent and equipment to bend a custom sway bar or hand-wire an entire harness that plugs right in to the Mazda’s factory systems.
Yet, all of this hand-fabrication, and the endless hours of trial and error, weren’t the toughest part. “The problem was the budget. I didn’t divert company funds to build it, and material costs were high,” Chris said. “Keeping things rolling was a challenge.” He figures everything you see here cost him $20,000; 10 grand for the basic RX-7 and getting it up to snuff prior to the swap, and another 10 grand in parts to get it in the frenzied state you see it here. “With labor charges,” he admits, “who knows what it would have cost.” Yeah, but even with labor charges added in the mix, the result would cost half of a new GT-R. And that was using only parts that would get the job done, i.e., stock replacement parts, not the latest double-throwdown components build to make the paper racers and the fanwankers in the cheap seats go “ooh” and “aah.” Take the oil pan (to pick a part at random off the tech sheet), it’s a Nissan RB25DET pan, a perfect fit for a rear-drive-only application for the Godzilla mill.
To have the power is one thing—to wield it judiciously is quite another. When you’re chucking a pipe bomb, there are really two ways to go: to be seen or to not be seen. Total show or total stealth. Chris chooses the latter: “For the inter-city night driving I enjoy most after work, the flat-black coat and low profile suit me well.” Stealthy, and the 15-inch wheels in the openings (2 inches up from the standard, diminutive 13-inch rollers! Such was the state of things 30 years ago) don’t even give force the unwary to look askance. A body kit? Please, that’s just dead weight, and a little too flashy. Even popping the hood, it’s very tone-on-tone in there—anything that was painted or anodized a color other than silver or black was stripped and refinished. Very stealthy, well, at least as stealthy as an RB26DETT can be while crammed in the nose of an SA-generation RX-7.
Sure, the heavier engine in the nose might upset the stock RX-7’s chuckable handling properties. Sure, driving around without creature comforts might get old. But, did we mention? It’s got the power-to-weight ratio of a new Nissan GT-R and costs as much as a Sentra SE-R. We’re no math whizzes, but we like that equation very much indeed.
1982 Mazda RX-7 GSL
Owner Chris Rothrum
Hometown Minneapolis, MN
Occupation Shop Owner
Engine 1991 Nissan RB26DETT; ARP head bolts; Tomei top-end metal gasket kit; custom V-mount intercooler; 3" intercooler piping; custom 2-into-1 downpipe; GReddy RX blow-off valve; custom-built 3" steel exhaust with Vibrant Street power muffler, all wrapped in DEI black wrap; NGK Iridium plugs; custom urethane puck mounts; RB25DET oil pan
Drivetrain Nissan RB25DET rear-drive 5-speed manual transmission; stock replacement clutch; Nismo shift knob; custom one-piece driveshaft by Real Street Performance; Torsen LSD with 3.91 gears
Footwork & Chassis Custom front coilovers using Skunk II springs and Tokico HP-series struts; custom double-dip 28mm front sway bar with Energy Suspension endlinks; Eibach Pro rear springs; Tokico HP series rear shocks; stock 15mm rear sway bar
Brakes Stock Mazda four-wheel discs; Brembo drilled rotors; Hawk HPS pads; Racing Beat braided stainless lines; DOT4 brake fluid
Wheels & Tires 15x7" Kosei K1 wheels; Nitto NT01 205/50R15 tires
Exterior Custom cooling vents; rolled fender lips; PPG flat black paint
Interior Mazdaspeed seats; custom gauge cluster with Auto Meter ES-series guages (tach, oil pressure, oil temp, water temp and manifold pressure); stereo removed “to make room for all the badass RB26 noise”
Thanks You Ricky Hunt for planting the seeds to my project, and the club guys from S.I.O. for lending a hand during the final assembly.
WWW greddy.com; nittotire.com; racingbeat.com; realstreetonline.com; tokicoperformanceshocks.com; tomeisua.com