That's right, we can pontificate all we want about how enjoyable it is to drive our lovely emerald green Miata along the serpentine roads of Pacific Coast Highway, but it's still one of the least potent cars in our project car fleet.
Generating only 127 hp at its rear wheels, the short-legged roadster can only hope to offer straight-line performance on par with a six-cylinder Mustang, which is--to say the least--simply unacceptable. And if our "Battle of the Project Cars" challenge ever pans out, our Miata, in its current state, would get pummeled in all but the tightest cone-dodging autocross tests. In order to stand up to the heavy-hitting likes of our RX-7, 300ZX and Impreza project cars, it is evident that our darling little roadster needs a hefty dose of horsepower. For that, we need forced induction.
Supercharging vs. Turbocharging
As our technical project series have illustrated in the past, when it comes to making more power, there is more than one way to skin a cat. But one avenue taken far more often than the others is that of turbocharging.
For instance, under the collective hoods of our RX-7, 300ZX and Impreza project cars, we can count five heavy-breathing turbochargers. And rumor has it at least two more SCC project cars will soon be outfitted with turbos! And what about SCC's first-generation Project Miata, our most natural competitor? It appears our older sibling will soon be sporting one as well! While this may sound like a perfect example of utter redundancy, there is actually some logic and reason behind all this maddening turbo ubiquity: Simply put, when it comes to compressing air, turbochargers are a tough act to follow.
Unlike belt-driven superchargers, turbos rely on otherwise wasted exhaust energy for motivation. Right off the bat, the absence of belt-drive parasitic losses means that turbo engines enjoy a 10 to 15 percent power advantage over similarly boosted supercharged engines. And contrary to popular belief, turbos by virtue of their internal compression ratio, also offer more thermally efficient compressor characteristics than any positive-displacement blower.
This means they heat the air less during the compression stage. Cooler air means more oxygen content, which, as we all know, means more power!
So does this mean that turbocharging is always the best form of forced induction? A turbo-lover would insist, "Of course!" while spewing out dyno results, outlet temperature measurements and references to every winning abnormally aspirated race car of the last few decades.
The supercharger-phile, on the other hand, would take much offense, citing the existence of dreaded turbocharger lag, slam-bam power delivery and unacceptable exhaust restrictions. But the truth is far more beguiling than most people think.
First and foremost, we need to look at our particular application and carefully list (in no particular order) our performance objectives.
At Least 220 Wheel Hp
That would be more than enough grunt to humble any stock Porsche 911 in any show of straight-line acceleration. Also enough to hold its own against the more powerful SCC project cars. While we could certainly make more power (as others have done), we would do so at the ever-increasing risk of overheating, susceptibility to octane fluctuations, potential for transmission failure and other stress-related concerns. Unlike most other high-powered Miatas, our project car will spend a fair amount of time on the racetrack, pushed to its limits for extended periods of time. That, in itself, means we need to maintain a healthy margin of safety when it comes to tuning for power.
While it is easy to get blinded by sheer peak horsepower figures, it is extremely important to retain a forgiving torque curve. In a perfect world, it would be possible to keep the engine spinning deep within its torque reverse. But in reality, where it's not always possible to keep the engine on its boiling point, some low-end torque would be much appreciated.
A Torque Curve That Complements the Stock Gear RatiosOften overlooked but never under-appreciated, well-matched gear ratios make use of every last drop of performance an engine can muster.
Likewise, poorly matched ratios can destroy both performance and drivability--making an engine that looks good on paper and on the dyno, feel anemic and unhappy in the real world. A good example of a synergistic powerband/gear ratio combo is that of the Integra Type R. Drive it like you stole it and you will find that each successive up-shift drops you smack-dab in the middle of the big, torquey secondary camshaft. Whereas with, say, a Celica GT-S, each up-shift is greeted with a temporary--and very unwelcome--torque lag before the second cam kicks in. Guess which car is faster in the real world.
Good Power Modularity
Anyone who has driven a stock Miata can attest to the fact that it is one of the easiest cars around to push right up to, and beyond, its cornering limits. As comfortable sliding sidewise around a chicane as it is hammering down a highway, the Miata simply begs to be driven hard. And perhaps the only thing that could upset this endearing trait is a less-than-perfectly controllable power delivery. Remember, we aren't building a drag car. Instead, we're putting together a high-performance sports car that can be comfortably throttle-steered by even the most ham-fisted of automotive journalists. The last thing we want is a chassis-disrupting, on/off power rush.
With such criteria in mind, supercharging seems to be our most viable form of forced induction. First, our power output requirements, while not exactly modest, should be within reach of a well-designed, moderately boosted and intercooled supercharger system. Secondly, when it comes to low-end power response, a positive-displacement supercharger will stomp all over a turbocharger. Third, such a supercharger system should, theoretically at least, maintain the general shape of the stock torque curve. This means no low-end lag, mid-range torque spike or top-end roll-off. This alone will maintain the effectiveness of the stock, short-legged, five-speed gearbox. And lastly, when it comes to linear power response and the absence of feedback-induced anomalies, a positive-displacement supercharger is in a class all by itself.
Jackson Racing To The Rescue
OK, so what hasn't been said about Jackson Racing? Simply put, we love its products. We've found them to work, fit well, and exhibit exemplary reliablity--three traits not found too often, all at the same time, in the aftermarket. And, from the looks of it, its second-generation (M2) Miata supercharger kit is no different. Although subjected to some initial engine- management-related growing pains, Jackson suggested the final production version of the Eaton M45-based supercharger system is ready for prime time.
First and foremost, let us say this particular kit will not enable us to meet our ultimate power goals. The absence of an intercooler and the use of the relatively small 45 cubic inch blower imparts some serious power limitations. Oscar Jackson suggests that Miatas fitted with this basic 5-psi supercharger kit typically generate somewhere between 150 and 160 hp to their rear wheels.
Without getting too picky, that's a good 60-70 wheel hp shy of our long-term power goals. What on earth will we do? For starters, we can work with Jackson Racing to put together a higher boost, intercooled supercharger system based on a larger displacement M62 Eaton blower. From our initial calculations, such a system, coupled with the appropriate engine management electronics, should provide the performance we are looking for. But until that system is ready to be bolted into our project car, we're going to see how far we can take the little M45-based kit before it runs out of breath.
Anyone familiar with Jackson Racing products won't be surprised when we say that all the supplied parts fit perfectly, exhibiting the quality control typically found in OEM applications. The supercharger's air discharge casting, for example, was carefully shaped to accommodate the lack of space between the supercharger and the ribbed aluminum hood.
Also nicely integrated into the discharge casting is a load-actuated supercharger bypass that relieves the engine of blower drag during light-load operations such as cruise and idle. The blower itself mounts to the exhaust side of the engine on a high-quality, laser-cut steel bracket designed to distribute the weight of the blower evenly throughout the head. Likewise, the idler pulley/belt tensioning assembly was expertly engineered to provide OEM functionality. All in all, the level of fit and finish associated with Jackson's supercharger system is rarely exhibited in the automotive aftermarket.
Fuel and Timing Tricks
Any schmoe can bolt on a blower and run gobs of boost. But not everyone can put together a functional engine management system that keeps the engine in one piece. And even fewer can design one and still be rewarded a CARB EO number. But that's exactly what Jackson Racing did.
However, there are some characteristics, specific to the second-generation Miata, which made Jackson's work a little more difficult. For one, the fuel system is returnless. That means there is no fuel return line in which to install a manifold-referenced, rising-rate fuel pressure regulator. That means on-boost fuel enrichment must be achieved through some other form of trickery.
At 5 psi of boost, we need approximately 35 percent more fuel. How do we know that? Simple: Since we know atmospheric pressure (1 bar) accounts for 14.7 psi of absolute pressure, another 5 psi accounts for roughly 35 percent more pressure. Since we have increased pressure by 35 percent, we have increased airflow through the engine by roughly 35 percent. Likewise, fuel demands have increased by roughly the same amount. While we have not accounted for certain minor volumetric efficiency changes that would naturally accompany changes in manifold pressure, our basic fuel demand estimates are good enough for our purposes. We aren't exactly sending a manned spacecraft into orbit.
Thirty five percent is a lot of extra fuel to find without an auxillary regulator. But who said that's the only way to bump fuel pressures? In this application, Jackson chose to drop a small restrictor pill in the stock, in-tank regulator's return line, bringing static fuel pressures from 42-50 psi to 62-65 psi. To properly support the increase in fuel pressure, Jackson also includes a replacement high-pressure, high-flow (255 liters per hour!) fuel pump by Walbro.
This moderate jump in fuel pressure accounts for roughly 15 percent more fuel flow.
A 15 percent jump in fuel flow is all well and good, but we're still at least 20 percent shy of our fuel demands. Worry not, for Jackson has more tricks up his sleeve.
One trick takes the form of a small intake manifold pressure switch. Once it detects positive manifold pressure, it grounds the stock air intake temperature sensor's signal, fooling the stock ECU into thinking the air temperatures are much cooler than they actually are. This sensor trick urges the stock ECU to extend injector on-times, ensuring an additional 5 to 8 percent fuel enrichment. The last 12 to 15 percent extra fuel is achieved through less elegant, but equally effective means: injector inefficiency.
The stock fuel injectors, like any fuel injector, can only pulse to a certain duty cycle without going wide open or static. Typically, this maximum duty cycle is 80 to 85 percent. Any more fuel demand results in what is essentially an injector failure. While the injector itself doesn't actually fail, it does fail to close, losing its ability to operate at any duty cycles between 85 and 100 percent. And at a 100 percent duty cycle, we are rewarded with nearly 20 percent more fuel flow. Eureka! We now have enough fuel.
But what about ignition advance? More so than fueling, the precise control of spark timing is critical when it comes to ensuring reliable, long-term performance. While the second-generation Miata is equipped with an electronic knock sensor, it is only capable of minimal timing adjustments and is suspected to become disabled at higher engine speeds in order to avoid mistaking normal engine noises for knock. This means it can only be used to account for mild fluctuations of gasoline octane and ambient temperatures during certain circumstances. Usually, such external variables only require a knock sensor with a piddling 2-3 degrees of authority range.
At higher engine speeds, where the knock sensor is disabled, OE powerplant engineers usually program a very conservative ignition curve. But with 5 psi of non-intercooled, knock-inducing hot air forcing its way through the intake valves, additional ignition retard is bound to be necessary. For this duty, Jackson employed something called a BTM or Boost-Timing Module. Intercepting both the cam/crank angle sensors and throttle position sensor, the BTM introduces an electronic signal delay in the crown sensor's signal when boost and throttle angle increase. This amount of retard can be further manipulated, as the user sees fit, through a small glovebox-mounted knob. It is assumed that the user should be able to detect knock and act accordingly.
It almost goes without saying that knock detection and ignition box tuning should be done with the soft-top closed, windows rolled shut, radio off and ears open. Likewise, it should also be noted that during unusually heavy use, such as on-track hot-lapping, additional ignition retard/knob-turning may be required.
How Does It Work?
Quite impressively, with just 4.5 to 5 psi of non-intercooled boost, our supercharged project car spun the Dynojet rollers with 153 wheel hp with a torque curve as flat as an office desk. Driven casually, our Miata behaved as if it were stock, with the exception of a slightly higher idle speed--it had to be increased by approximately 100 rpm due to blower drag--and a subtle supercharger whine. The only glitch in the system is during cold start when it occasionally acts abnormally sluggish. But this behavior is short-lived; once fully warmed up, the car drives around town as if it were stock.
However, at wide-open throttle, the beast from within is unleashed. And what a noisy beast it is! The blower whine, once faint, becomes as clear as a punch to the head. Not exactly subtle, but nonetheless entertaining in a masochistic sort of way! As one would expect by reviewing the dyno graphs, the additional torque throughout the rev band is much appreciated.
While this 25 percent improvement in power still leaves us quite a distance from our ultimate power goals, we are confident there's more performance to be squeezed out of the system through further system tuning. But alas, that's another project installment in itself. For now, we'll spend the time being beating the snot out of our project car, reveling in its extroverted, banshee-like wail and new-found potency. Those Mustangs better watch out. Yes, even the eight-cylinder ones!
Holdin' Our Horses
Brakes. As much as we don't like to use them, they are there for a reason. And when needed, they are, far and away, the most valuable components in any car. For this reason, we sought to upgrade our binders as the much-abused stock brake pads seem to have given up much of their bite.
One manufacturer which has caught our eye is EBC Brakes, the manufacturer of increasingly popular color-coded brake pads. For street and occasional track use, EBC recommends its green compound. For a slightly more aggressive, but still dual-purpose pad, the red compound fits the bill. For track use only, the ultra-aggressive yellow compound is the material of choice. For the time being, we opted for the least aggressive green pad, supplied to us by our friends at M2 Performance (of Project RX-7 fame). If we find them to be too overworked, we'll immediately opt for the reds.
Once fully bedded, the EBC pads imbued some much-needed life into our once-tired braking system. Due to the greater pad friction coefficient, pedal effort has been greatly reduced. As a result of no longer having to stand on the brakes, pedal travel has been reduced, much to the chagrin of those who like to see us botch heal-toe downshifts. Also improved is the front-to-rear braking bias, making the act of threshold braking, sans ABS, much less daunting. While we have yet to test their merit on the track, we're very satisfied with the EBC pad's street performance.
More power, that's what! Through additional exhaust tweaking, intake tuning and other extraneous fiddling, we hope to get closer to the limits of the M45 supercharger. Not entirely by coincidence, our Project Miata is doing double-duty as SCC's testbed for an upcoming (and on-going) stand-alone, programmable engine management computer series. That being the case, we're sure to learn other ways to further improve the performance of our newest project car. After all, the "Battle of the Project Cars" draws near. From the looks of things, the competition is getting fiercer by the day!
13010 Bradley Ave
Sylmar, CA 91342
(818) 362-5467 (TEL)
(818) 362-2196 (FAX)
Jackson Racing/Moss Motors
(888) 888-4079 (TEL)
(805) 692-2523 (FAX)
125M Mason Circle
Concord, California 94520
(925) 686-9047 (TEL)
(925) 686-9069 (FAX)