It's been awhile since the last installment of Project Miata Gen II. "Why the delay?" you ask. Simple. Our car was stolen.
Don't worry it was recovered. But it was plenty beat. The soft-top was severed and almost every performance enhancement was ripped from the car. Our Jackson Racing supercharger, TEC-II programmable engine management computer, RC Engineering fuel injectors and Enkei alloy wheels are probably on eBay. Even our Hard Dog roll bar is missing.
Plus, the engine took a beating during the four weeks and 400 miles the car was missing. Once mildly boosted and transferring nearly 200 hp to the wheels, the 1.8-liter inline four showed 12- to 15-percent leakdown on cylinders two and four. So our once-tight engine is now impaired. While compression tests look fine and oil consumption is essentially nil, there'll be a complete engine rebuild in our Miata's near future.
But back to the here and now. While we'd love to strap on another Jackson Racing supercharger, our most recent dyno-testing sessions suggested we were approaching the upper limits of the small M45 blower. Therefore, we're going to install a bigger blower--the M62.
As the numbers suggest, the M62 offers nearly 50 percent more displacement than the M45. This extra flow capacity makes it the supercharger of choice for Jackson Racing's supercharger systems on high-output VTEC engines. What we lose with the absence of variable valve timing, we plan to make up with extra boost. After all, the knock resistance of the Miata engine should more than make up for its lack of high-rpm breathing efficiency.
However, to our dismay, Jackson Racing only offers ready-made Miata supercharger kits in M45 variants. Luckily, Oscar Jackson offered to cobble together an M62-based system for us to play with. As far as we know, our Miata going to be the first ever to employ such a large displacement blower.
But before we could move the car, we needed wheels. Upon its recovery, our Miata was riding on four spares. Our wheel requirements are always simple: lightweight, rugged construction, gobs of brake caliper clearance and good looks. Luckily, we have The Tire Rack with near-limitless combinations of wheel and tires to choose from. The Tire Rack suggested 16x7-inch SSR Competition wheels wrapped in Nitto NT-555 rubber, which arrived in two days fully balanced and ready for mounting.
Next, we wanted to fix the interior damage. After an insanely expensive quote from the local Mazda dealership, we looked to Project Rally Beater for inspiration. We borrowed a pick-up truck and headed due north toward Rancho Cordova Specialized Recyclers--junkyard of the gods. There we flashed a list of the parts we needed and moved the truck to the loading bays. We walked away with a glove box, an entire plastic center console, a nifty wind-stop, a tenth-anniversary Miata fancy two-tone leather Nardi steering wheel and an entire Sport Package Bilstein suspension. With the money left over, we even bought a new, relatively low-mileage (30,000) engine to build in our spare time.
Then we took the car to Jackson Racing in Westminster, Calif. A few weeks later, we got the call. Our prototype M62 supercharger system was buttoned up.
But we were worried. With serious boost levels and no intercooler, the journey to big-boost heaven was going to be rough.
As always, Jackson Racing did an excellent job shoehorning the M62 in the place usually taken by the M45. Eaton blowers suck air through an intake port in the rear of the blower. Normally, the throttle body bolts to this port, but with the longer M62, that would jam the throttle body right into the firewall. Instead, Jackson fabricated a U-shaped inlet pipe, which allowed it to mount next to the blower. While this extra piping adds some additional throttle volume, it deftly dances around all interference problems associated with installing an M62 under the hood of a Miata.
The next problem was the large, rectangular outlet on top of the blower. The standard casting used to gather pressurized air from the little M45 was too small. To remedy this, Jackson Racing simply cut it in half, added a couple of inches of material in the middle, and welded it back together again. Voila. An instant, extra-long M62 outlet casting.
As for the supercharger-mounting bracket, Jackson adapted the standard M45 piece to work with the M62 by using a few carefully sized spacers and extra long bolts--a surprisingly simple and elegant solution.
Nothing comes easily, however. Using the Dynojet at Jackson Racing, our initial, non-intercooled results were less than ideal. With a pulley arrangement yielding approximately 10 psi of boost and with some judicious tuning of our TEC-II engine management system, the Miata spun the rollers to 196 wheel hp. Just 4 hp higher than the 7 psi wheezing out of the smaller M45 supercharger. What the heck is going on?
Looking at the torque numbers, it's clear our car wants to make big horsepower. At 5000, the Dynojet results reveal a whopping 175 lb-ft of torque--some 16 lb-ft higher than where we last left off with the smaller, and now stolen, M45 blower. Clearly, our big blower'd system is moving some serious air. By 5300 rpm, however, the torque curve takes a nosedive, suggesting that excessive inlet temperatures are getting the best of us. And sure enough, with our trusty thermocouple installed just after the supercharger outlet, our peak intake temperatures quickly ramped up to 250 degrees Fahrenheit.
In the back of our minds, however, we were questioning the integrity of our stolen and abused engine, which we suspect was subjected to serious and prolonged bouts of detonation. But we decided to cast those unpleasant notions aside and focus on optimizing the current system.
The quest for cool air
One of the many nice things about Miatas is the big opening in the front nosepiece. There's plenty of room to mount a big, fat air-to-air intercooler. However, the problems associated with front-mount intercoolers are two-fold.
First, Miatas with front-mount intercoolers tend to overheat. With the radiator, A/C heat exchanger and intercooler all stacked on top of each other and sharing the same flow of ambient air, there's only so much cooling potential to go around.
Another problem associated with large front-mount intercoolers is pressure loss. This is especially problematic in Miata applications, where typical intercooler plumbing has to run across the front of the engine. Then down. Then under the radiator. Then upwards. Then turn 90 degrees into the intercooler end tank, make a 180 degree turn in the core itself, back out and down, under the radiator, turn sharply up and finally back into the intake manifold. In total, these intercooler designs often suffer pressure losses in excess of 2 to 3 psi. If such a system were employed in our supercharged application, the pressure loss will offset any improvements we'd make through cooling gains.
Instead of routing our piping around the radiator, we opted to tilt the radiator's upper end tank backwards a couple of inches. To do this, we lengthened the upper radiator mounts approximately two inches--with the help of a local fabricator, of course. With the radiator tilted backwards, there still wasn't enough room to run 2.25-inch intercooler piping through the gap on each side.
To remedy this, we pulled out the industrial-strength Dremel tool and got to work. Within minutes, we removed just enough material to pass our imaginary intercooler piping around both sides of the radiator. Then the A/C heat exchanger was in the way. With it there, we barely had any room for a well-designed intercooler. But living in temperate northern California, there's little need for in-car temperature regulation. Out it goes, revealing gobs of space for any reasonably sized intercooler.
Next, we measured the space and relayed the dimensions to Johnny Wang at Spearco. Within a few days, he had an intercooler, built exactly to our specifications--28-inches wide, 18-inches tall, and 2.25-inches thick. To make the installation sanitary, we mounted the intercooler using the factory A/C heat exchanger mounting tabs. To do so, we welded on two small mounting brackets on each side of our intercooler.
Now, we just had to make our intercooler piping. Our trusty fabricator revealed his collection of aluminum U-bends and started taking measurements. Within hours, he fabricated our intercooler piping--right down to the hose-retaining beads on the ends. And more impressively, the piping cleared the power-steering reservoir and upper radiator hose with room to spare. Good fabricators kick ass. When the dust cleared, we had the most elegant intercooler system ever bolted onto a Miata. Check out Project Miata, Gen I if you don't believe us.
Back to the dyno
After a few tuning runs, the roadster was generating a healthy 218 wheel hp--only 2 hp shy of our output goal--with nary a ping in sight. With peak intake temps within 20 degrees Fahrenheit of ambient, our 10 psi big-blower Miata already exceeded the output on many similarly boosted turbo Miatas. And we're just getting started.
Additional power can be had with careful exhaust system matching, a new header design and a larger throttle body--all of which we hope to install and test in the immediate future.
For now, our jury-rigged, intercooled M62 supercharger system is driving strong and safe with a healthy margin of safety against detonation. Judging by how easily we achieved 218 wheel hp--26 hp more than our M45-based system--another 20 or 30 hp shouldn't be too hard to find. Stand back turbocharger snobs, our Miata is back.
Fuel Injectors 550cc/min (Peak-and-Hold type)
Custom M62-based Supercharger
Moss Motors/Jackson Racing
Headers and Cat-back Exhaust
The Tire Rack
Wheels and Tires: 16x7-inch SSR Competition with 205-45ZR-16 Nitto NT-555
EBC Green brake pads
Hard Dog Fabrication
Rancho Cordova Specialized Recyclers
Interior Replacement trimGlove box, center console, etc.