Project Miata Gen 2: Part 3

One of the nicer things about Miatas is their sheer ubiquity. For a magazine project car, the ability to blend into its surroundings is a welcome trait. After all, driving a car that is constantly yelling "Look-At-Me!" tends to bring its share of speeding tickets and unwelcome glances from less-than-savory folks.

Apart from a sense of good taste and dignity, this is why we usually don't dress our daily-driven project cars with obnoxious decals, wailing exhausts and disco-strobing neon strips. Yet, as fate would have it, all our efforts at camouflaging Project Miata were for naught.

Yes, halfway through Part III, our story takes a very unfortunate turn. Project Miata was stolen; taken from a well-lit, previously safe parking garage in San Francisco. Nestled between a flurry of BMWs, Mercedes Benzes and other European elite-mobiles, our innocuous little green Mazda came to the attention of some indecent miscreant who decided that our property was his for the taking. And what property it was; for the last few weeks of our time with Project Miata, we made some serious progress towards our goal of building the ultimate street Miata. How does nearly 200 wheel hp sound?

Yep, we thought that would get your attention.

The car has been recovered, though with significantly less performance than it had when it departed our company. It may take us a little while to get back to speed, but our accomplishments with the car before it was stolen were significant enough to warrant a full update--something to keep your appetite whetted while we rebuild the beast.

Making More Power
While Project Miata responded well to Jackson Racing's supercharger system, with 5 psi of boost and 153 ponies at the wheels, it was a long way from meeting our ultimate power goal of 220 wheel hp.

While one less-than-methodical tuning approach would have been to swap pulleys, raise the boost and see what happens, we chose to focus on system efficiency. We all knew that the Eaton M45 blower doesn't enjoy the same compressor efficiency as a properly sized turbocharger. However, we were quite certain that we were flowing more than 153 hp worth of air.

To get an idea of how much power our system was capable of generating, we installed an Electromotive TEC-II engine management system. Yes, our Miata became the second SCC project car to run a TEC-II. And yes, there are many other programmable engine management systems available. Of course, as mentioned in the last installment, we are planning on using Project Miata as the test bed for a giant engine management technical series, comparing and contrasting a whole slew of different systems. But to reach our horsepower goal as quickly as possible, we decided to start with the system we were most familiar with.

With the TEC-II installed and tuned on our Miata, peak output immediately climbed from 153 to 165 wheel hp--a whopping 8 percent gain just through ignition and fuel recalibration.

Most impressive is the fact that these gains were realized while still using stock injectors (running at higher-than-stock fuel pressures). Without appropriately sized injectors, we were somewhat limited with respect to fuel tuning. In fact, above 6000 rpm, we found ourselves flirting with the duty cycles at which our injectors go wide open, or static. Once that occurred, the air/fuel mixture richened drastically, immediately killing engine torque and, as a result, high-rpm horsepower. To prevent this over-fueling, we were forced to keep the injectors from operating over 82 percent duty cycle, a condition that actually yielded a slightly leaner-than-desired air/fuel mixture. As a result, in order to stay clear of knock, we had to be extra conservative with ignition advance. But deep down, we know that we could have made more power had we been given generously sized fuel injectors.

To test our theory, we took the next logical step and looked for oversized injectors. On first-generation Miatas, the fuel system is equipped with a fuel pressure regulator on the fuel rail to keep fuel pressure constant, relative to manifold pressure. On second-generation Miatas, Mazda opted for a returnless fuel system that maintains a constant fuel pressure relative to the outside world.

It does this by using an in-tank fuel pressure regulator, but unlike a traditional fuel pressure regulator, the in-tank unit is not equipped with a vacuum reference to the manifold. What this means to us is that fuel pressure will not linearly increase with boost pressure. In other words, increases in boost pressure will reduce the effective flow rate of the injectors because the pressure drop from the fuel rail to the manifold will be lower. To determine what size injectors we will need for our application, we have to do some careful injector math.

In order to determine the necessary injector for our application, we need to make an assumption of what boost pressure is necessary to attain our goal of approximately 270 engine hp. Judging by our earlier Dynojet results, it would seem that we could reach this output with 10 psi of intercooled boost. Since we know that fuel rail pressure in the second-generation Miata is regulated to a constant 45 psi, we know that at 10 psi of boost, the pressure differential between the injector and the intake manifold is just 35 psi. If you deal with standard manifold-referenced fuel systems all the time, it's easy to take for granted that injector flow rates, which are measured at a standard 43 psi, are always accurate for your application. Remembering the formula for injector flow at different fuel pressures, we see that with the lower-than-standard fuel pressure drop, our injectors will flow roughly 88 percent of what they are rated to flow under standard conditions. This means that, at the end of our injector calculations, we must remember to oversize our injectors by 1/0.88, or roughly 11 percent.

The injector math continues...
Being conservative, we'll assume our supercharged Miata will make 270 hp at the flywheel, operate at a Brake Specific Fuel Consumption (BSFC) of 0.55, which is typical for a moderately boosted engine, and that our injectors shouldn't operate above 80 percent duty cycle. Using these assumptions, we can calculate the necessary injector size. Pulling another formula out of the memory banks, and plugging in our numbers gives us 46.4 lb/hr. Multiplying by 10.5, we get 490 cc/min. But let's not forget that, because of the unusual flow fuel rail pressures, we have to oversize by the aforementioned 11 percent, bringing our actual needed injector flow rating to a whopping 540 cc/min. Since 540 injectors aren't commonly available, we'll settle for 550 cc/min injectors. But where are we going to find 550 cc/min, low-impedance, direct drop-in fuel injectors for a Miata? RC Engineering, of course.

While we initially thought that there were no direct drop-in replacements for the "narrow-body" Nippon Denso injectors that came standard in the 1999 Miata, a few careful measurements suggested otherwise. Several minutes later, the gentlemen at RC Engineering were assembling and machining the first set of direct-fit injector upgrades to ever be used on a second-generation Mazda Miata.

Of course, with just 5 psi of positive manifold pressure, we're a long way from testing their full potential. Once installed in the stock fuel rail, we pulled out our trusty laptop computer and recalibrated our fuel maps. After a few tuning runs on the Dynojet, we were pleased to record a very significant 10 hp gain at the wheels, bringing peak power to 175 SAE-corrected hp.

No longer fighting the mechanical limitations of the stock injectors, we were free to get a little creative with our fuel calibrations, enriching just enough to raise our detonation threshold, in turn, allowing us to increase spark advance at higher engine speeds. As a result, we were able to sustain torque higher into the rev range, making peak power at a sky-high 7200 rpm--simply remarkable for an engine that is not exactly known for its high-rpm breathing capabilities.

Even more remarkable are the blower outlet air temperatures which, at peak horsepower, reached nearly 180 degrees F! Despite the boost and hot intake charge, we were still able to run timing advance numbers that one would expect to find on a naturally aspirated engine. While much of advance can be attributed to the TEC-II's super accurate crank-triggered ignition system, a lot has to do with the Miata's extraordinarily detonation resistant engine. As far as we know, our project car has the dubious honor of being the strongest 5-psi boosted Miata in the world.

Even More Power!
If 175 wheel hp can be accomplished at just 5 psi of non-intercooled boost, what would happen if we bumped our boost levels to 7 psi and installed a free-flow exhaust system? Would an increase in intake temperatures and parasitic belt-drive loss offset the increased manifold pressures and reduction of exhaust back-pressure?

To find out, we installed Jackson Racing's crank pulley upgrade and a full 2.25-inch exhaust system from Brainstorm Products. Unfortunately, due to the premature and unexpected departure of our project car, we were unable to conduct Dynojet testing of each individual upgrade. Some readers may also notice that this particular installment of Project Miata is a bit skimpy on photography. Why? Because a few vital rolls of film were stashed in the glovebox at the time of the theft.

But even without pretty color pictures, we can still report that Project Miata's peak power rose (yet again) to an SAE-corrected 192 rear wheel hp with the most significant torque gains occurring throughout the low-end and mid-range. Interestingly, with the added boost, the power peak dropped from 7200 rpm to a relatively low 6500 rpm--no doubt a result of excessive air intake temperatures that measured as high as 210 degrees F at redline!

These scorching intake temperatures have two effects: First, hotter air meant less dense intake charge which, in turn, means less oxygen to burn and less power to make. And second, as intake temperatures increase, so does an engine's tendency to knock. In this case, both additional fuel and extra ignition retard become necessary for safe on-boost operation. Of course, the downside to these approaches is a reduction in engine efficiency and torque output.

On the dyno, the real world effects of 210-degree peak intake charge temperatures are very obvious. Perhaps the most noticeable characteristic of our most recent Dynojet runs was the torque curve which suddenly became lumpy--a far cry from the perfectly smooth torque curve we witnessed when running at 5 psi of boost. Such roughness is almost always indicative of either ignition misfire and/or excessive ignition retard, both of which are the case, given the amount of fuel and ignition retard we were forced to live with on our steamy, non-intercooled 7-psi supercharger system.

Without a doubt, the use of an appropriately designed intercooler, which could reduce our intake temperatures by as much as 100 degrees F, would allow us to get our ignition advance values and air/fuel ratios back to the level that is most conducive to making maximum power. Fate being the prankster it is, Jackson Racing was just about to ship us its promising new water-to-air intercooler for testing before our Miata disappeared.

With it, the new intercooler, we not only hoped to decisively crack the 200 wheel hp barrier, but we also planned to give some of the higher-boosted turbo Miatas a run for their money. As it stands right now, we'll accept the honor of having the most powerful M45-based supercharged Miata that we know of. And we're proud to say that we did so completely through optimized engine management controls, not by ridiculously high boost levels, water injection, race gas or intercooling--although we plan to try most of those tricks when we get everything back together.

Safety Comes First!
While we haven't come right out and said so, I should be quite clear that Project Miata has been fitted with a four-point roll bar. All SCC staffers agree that driving the snot out of souped-up Miatas that offer no form of roll-over protection feels as uncomfortable as swimming in shark-infested waters while wearing a prime rib life vest.

Wasting no time in getting our Miata prepped for high-speed, at-the-limit driving, we placed an order for a Hard Dog's Hard Core Double Diagonal roll bar. Almost identical to the roll bar fitted to our first-generation Project Miata (December 1999), the Hard Dog bar is constructed of heavy-duty, 1.75-inch diameter 1020 DOM carbon steel, providing true roll-over protection (although Hard Dog is careful not to make that claim in its product literature). Sitting up higher than the regular Hard Dog roll bar, the Hard Core version meets the SCCA height requirements for most drivers. The added level of protection also far outweighs the extra 50 lbs. of sprung weight. Perhaps the only downside of this roll bar is that is not compatible with the Mazda hard top. But come on, who actually mounts a hard top on their Miata?

Up Next...
Just in case you're thinking that we would let something as trivial as a car theft prevent us from continuing with our project series, let us assure you that we have no intention of throwing in the towel. Nope, not us. In fact, we're more serious than ever in probing the outer limits of Miata performance. All we have to do is replace all the missing parts and determine what damage was done during the few weeks our car was in the wrong hands and we'll continue with our upgrading ways. As you read these words, rest assured that we are doing just that and are currently in the process of building what will eventually be the ultimate Miata road car. And that's a promise!