1997 Mazda MX-5 Miata: Kraftwerks Supercharger Install, Pt. 1
December 09, 2010
When it comes to modifying cars that are well-seasoned like Project Miata, our 125k-mile longterm 1997 Mazda Miata, it helps to begin at the beginning. Go back to basics.
Oscar Jackson, owner of Kraftwerks Performance Group, had reminded me of this before we even met in person to install the company's Rotrex supercharger kit. I made a mental note of it. Thing is, my memory is spongelike -- a few things soak in but there are a lot of holes.
Our well-maintained, strong-running, no-oil-burning, issue-free Miata appeared in normal working order, especially as it was completely stock and previously owned solely by a well-to-do senior gentleman. I'd earlier made sure the ignition timing was correct, and the engine made as-new compression numbers, plus the car drives with as much or more pep down low as these things ever did. Pretty much a best-case scenario for bolting on a forced induction thingie, right?
Yeah, that hole-y spongelike thing. There was one thing I didn't check, and we found out what it was when we threw it on Kraftwerks' Superflow chassis dyno for a baseline performance check. Oscar immediately noted something was amiss. Peak power was down quite a bit compared to other stock Miatas he'd tested and, more tellingly, power after peak nosed over more aggressively than it should. Come to think of it, it did feel tepid in the upper reaches of the rev range...
We both suspected the cam timing was off. Pick up a well-used car and it's a sure bet that many, many people -- with varying mechanical skill levels -- have mucked about with the greasy bits.
In Project Miata's case, the last mechanic to change the timing belt was a primate of some kind and he buttoned it up with the intake cam off by two teeth and the exhaust cam off by one tooth. I like orangutans and all, but their attention to detail in mechanical matters could be better.
Once the cam timing was re-set to stock, another dyno check showed our Miata was right on the money for peak power and picked up power everywhere else. Dynos rule.
Now we can throw on the Rotrex supercharger with confidence.
Kraftwerks' Rotrex supercharger is not a traditional Roots blower or even a traditional centrifugal supercharger. Okay, it is a centrifugal supercharger (we'll shorten this to CS for brevity's sake) in that it has a radial outflow compressor wheel just like that of a turbocharger.
Rotrex's twist on the CS is the high drive ratio (12.7:1) of its planetary
gearset traction drive (it employs rollers, not gears), which allows higher shaft speeds than those found in a conventional CS. The Rotrex's shaft speed capability means it doesn't need to rely on as large a compressor wheel to generate meaningful boost. The smaller compressor wheels better match the engine's breathing characteristics and package better to boot. Also, the planetary drive system doesn't have the gear whine of conventional CSs.
The smaller wheel means the Rotrex doesn't have the banana-shaped boost profile often associated with CSs, either. Instead the Rotrex's boost characteristic is basically a straight line that rises from zero at idle and peaks (at around 6 psi in the Miata) at redline.
This low-boost approach (plus the fact that the whole shebang is self-contained) makes the Rotrex well-suited as an affordable bolt-on kit to a stock engine. If this approach sounds like deja vu, it's probably because you remember the popular Jackson Racing blower kits from years ago which took a similar tack.
Yes, Jackson Racing was Oscar Jackson's business before he decided to retire several years ago. It was the Rotrex supercharger that compelled him out of retirement to form Kraftwerks.
Naturally, 6 psi isn't going to make huge dyno queen numbers, but the Kraftwerks' 160-ish whp will be a nice bump from the stock 100-ish whp.
Jason Kavanagh, Engineering Editor