Part VII already? That's right. Nearly one year has passed since we first got our hands dirty with Mazda's extraordinary third-generation RX-7. While we've lived through both the good and the bad, we like to think we've learned something during every step of the way.
Before we continue our goal of building the ultimate streetable race car, let's summarize what has been done so far. In Part I, we evaluated our then- stock, '93 RX-7 and offered a "buyers guide" for potential owners. Then, in Part II, we addressed the car's inherent shortcomings while beginning the early stages of our comprehensive build-up. Part III covered the vitally important issue of thermal management with the installation of a high-efficiency radiator and a very trick heat dissipating, metal matrix composite brake upgrade.
In Part IV, we turned up the wick even further when we began our series of horsepower-enhancing upgrades from M2 Performance. Additionally, we also installed a few goodies from the, now defunct, Mazdaspeed. Unfortunately, in Part V, things came to a crashing halt when a local tire shop drove Project RX-7 into a parked BMW. Making lemonade out of lemons, we documented the repairs in Part VI while additionally installing a modified front frascia from the Japan-spec 1999 RX-7. Resisting the temptation to rest on our rotary-powered laurels, we upped the ante even further by installing and testing two popular intercooler upgrades. With the larger of the two intercoolers now installed under its exquisite vented hood, our Project RX-7 is stronger than ever as evident during its most recent trip to the Dynojet, when it logged a tire-scorching 301 wheel horsepower run with just under 13 lbs of boost.
When Brakes Break
There are a few things in the world more stressful for a car than having it driven by a professional rally car driver on a closed track. With four heavy braking zones situated in a short, one-mile road course which is dispatched in just under one minute, brake rotors are given almost no opportunity to cool their overheating heels. Unfortunately, the very nature of the Cooltech's MMC rotor material didn't quite match our needs. Within a few hot laps, the brakes completely faded and a rotor eventually cracked.
Why? Our best guess is the MMC material needed more airflow to make use of its extraordinary heat dissipating properties. This would explain why the brakes worked well at the larger road course such as Thunderhill and Laguna Seca. Perhaps with the use of dedicated cooling ducts, the brakes could have shed heat as quickly as it was generated, and in the process, stayed intact. Unfortunately, we'll never know for sure. But in all fairness, when operating within their temperature range, the brakes worked wonderfully. With colossal 14x1.25-inch rotors and massive Porsche "Big Red" calipers, the braking system offered a mechanical advantage untouched by most modern performance cars. Capable of engaging the ABS system at nearly 90 mph (while rolling on sticky R-compound tires!), their velocity reducing abilities were never in doubt. However, as we witnessed on the small track, they failed as soon as they reached their maximum heat capacity (which is dictated, in part, by their low mass). That's too bad. With each rotor weighing less than 8 lbs, they appealed to the mass-reducing obsessive in all of us. Unfortunately, it appears the MMC rotors' most obvious strength was, in fact, their fatal flaw. That may sound overly dramatic, but certainly not as dramatic as approaching Turn 1 at 90 mph with no brakes. 'Nuff said.
Building a Better Braking System
At 2,800 lbs, the third-generation RX-7 is exceptionally light by production car standards. Through Mazda's fanatical weight reduction program (they even shortened the spark plug wires!), the RX-7 is an alarming 700-1000 lbs lighter than its comparatively adipose competition (e.g. Toyota Supra, Nissan 300ZX, and Mitsubishi 3000GT). In fact, it's nearly 300 lbs more feathery than Acura's all-aluminum NSX. As we all know, less mass means less momentum. And less momentum means happier brakes. However, as fast as this car is on a track, it will eventually be even faster. While 300 rear wheel hp is nothing to sneeze at, just the idea of another 50 ponies makes us giggle with anticipation. This and the fact we almost always run gummy race rubber would suggest there is no such thing as too much braking capability. But where do we look?
Fortunately, Brian Richards at M2 Performance has spent the last several months developing a system that could potentially fill our big brown bag of needs. As we established early in our series, everything we install on our car must maintain a stock-like streetability while offering exceptional performance. A "streetable" brake system, for instance, must be just as reliable, noise-free, and low-maintenance as the stock braking system. Installing a rackety race caliper that is designed to be rebuilt after every track testing session is not acceptable. Understanding our needs, while designing in room for growth, Richards has developed a unique big brake upgrade that should quench our thirst for decellerative g-forces. Consisting of a road-going version of AP Racing's four-piston race caliper and a 13-inch AP Racing rotor, M2 Performance's big brake upgrade is the next logical choice for our Project RX-7.
With 25 years of racing success, AP Racing literally builds the highest quality braking components in the world. For instance, the very same rotor we are using on our Project RX-7 is also used in street course racing Champ (formerly known as Indy) Car applications. While Champ Cars are hundreds of pounds lighter than our RX-7, they are designed, from the ground up, to provide positively staggering performance numbers. Running alarmingly wide wheels which are wrapped with the gummiest slicks imaginable, and armed to the hilt with mass multiplying down-force devices, the braking loads experienced by these vehicles is beyond the realm of comprehension. Imagine approaching a hairpin turn at 180 mph, standing on the brakes, and WHHOOOMMMPHHH.... by the time it takes you to read the first few words of this sentence, you are strolling into the turn at a leisurely 45 mph. Lap after lap. No time for fade. Good God man.
Designed with 48 curved veins for maximum heat dissipation and made of high-quality cast iron, the slotted rotor is heat treated, then ground to a fine tolerance, and finally pre-bedded for immediate abuse. (They are, after all, designed for Champ Cars.) The result is an unquestionably stalwart 13x1.2-inch rotor that weighs, along with its anodized aluminum hat and titanium fasteners, a surprisingly light 12.5 lbs. By comparison, the smaller 12.7x1.25-inch rotors from the Porsche 911 Turbo weigh a massive 21 lbs. Even a relatively unconvincing 11.6x0.85-inch stock rotor is heavier at 14.6 lbs. AP Racing's all-aluminum race calipers are just as over-built. Equipped with anti-rattle springs that hold the brake pads firmly in place, there should be none of the squealing and clunking typically associated with full-on race calipers. And unlike the previous brake system's top-of-the-line Brembo "Big Red" calipers, AP's version has a completely recessed dust seals which tend last longer than their bellows-type counterparts (since they do not come in contact with the backside of the hot brake pad).
Like the Brembo calipers, the AP unit also uses differential bore pistons which are designed to reduce the effects of taper wear (when the leading edge of the pad wears faster than the trailing edge.) The first brake pad we will evaluate with the new brake system is the Hawk HPS pad. Designed as a high-performance street pad, the HPS is the most conservative pad we will likely evaluate. As time goes by, we will work our way upward, experimenting with pads of increased fade resistance. With some luck, we'll eventually find a performance pad which fulfills our wildest, fade-free, trail-braking fantasies while still working acceptably when cold. Enough of theory. How does it work on the track? Like usual, we took the car back to Thunderhill Raceway to find out.
It's 10:30 a.m. in Willows, Calif. when I check my brake fluid, confirm my tire pressures (32psi cold according to the ever-entertaining SmarTire system) and head off to pre-grid. By 10:35 a.m., I'm on the track. I begin my warm-up lap, remembering to stay completely off the brakes. Tire pressures are slowly rising and coolant temperatures reach a steady 180 degrees F. after three-quarters of a lap. The car is warm, but the brakes are stone cold.
Minutes later and I'm slowly exiting Turn 8 in second gear, gradually laying on the throttle. I short-shift to third gear. Back to even throttle. I exit 8A at 3000 rpm and plant my right foot to the floor. Boost gauge needle immediately jumps to an indicated 13 psi. Exhaust note deepens. The rate of acceleration begins to swell. A cacophony of hissing noises fills the cabin, piercing my helmet. At 4800 rpm, exhaust notes drop an octave and I now hear whooshing.
I feel like a stone that has been launched from a slingshot. Heading towards the far end of the long and narrow front straight, I grab fourth gear as the scenery begins to blur. 5500 rpm comes and goes. I look at the speedometer and see 100 mph and rising quickly. Against every primal instinct of self-preservation, I keep the throttle floored. All in the name of good journalism, I remind myself. The cone chicane at the end of the straight is looming closer and closer. I try to remember some old kinetic energy equations from high school and fail miserably. Cones are getting big. Real big cones ahead. I begin to count to myself. Three...110 mph. Two....115 mph. One....120 mph. Hit the brake. Now. I'm going to hit the cones. Now. [STOMP!!!] WHOOOOOOOOMMMMMMPH...
The car comes to a complete stop. Blood rushes back to my body--away from my palpitating extremities. The chicane stands before me. Four car-lengths before me. Damn, Champ Car brakes are nice.
With brake pads that were so frightfully effective when cold, I expected a fade problem during prolonged hot lapping. After all, brake fade is a fact of life. It happens with any and every braking system. Deal with it. But to my surprise, fade only became an issue after four or five laps of heavier-than-normal braking. In fact, I repeatedly found myself expecting too little and braking too early. Pretty remarkable for Hawk's softest Ferro Carbon street pad. But still, it would be best to use a more aggressive pad for our purposes. Fortunately, with several pad models to choice from, all fitting our AP caliper, we should have little trouble finding our pad of choice in an upcoming installment.
Subjectively, the new braking system is unreproachable. Pedal feel is easily superior to that of the Cooltech system--firmer with less travel. It offers incredible feedback as well. When pressed hard, one can actually feel (through the pedal) the slotted rotors sliding against the pads. If that isn't a testament to the caliper's outstanding rigidity, we don't know what is. Also readily apparent is the fact the caliper pistons are sized correctly, making the system perfectly compatible with the stock master cylinder and brake proportioning.
The previous big brake system, on the other hand, tended to use the front brakes harder than the rears, forcing the ABS to act as an electronic proportioning device. The M2 Performance brake upgrade fits like a glove, looks like a winner, delivers all the goods, works like a charm, and whatever cliche of unabashed praise we can think of if given enough time.
As discussed earlier in this series, it isn't unusual for us to install and evaluate a product for several months before offering one word of editorial print. Sometimes it's a matter of scheduling. Other times it's because the necessary photographs were misplaced. However on some occasions, we actually hold off judgment for more professional reasons. And such an occasion is now.
Three months ago, we removed the RS*R coilover system in an effort to install something that would come closer to meeting our needs. While nothing was intrinsically wrong with the RS*R set-up, we did find it lacked the overall balance we were looking for. As we mentioned in Part IV, the car tended toward oversteer, making it difficult to get the power to the ground while exiting a turn. That, coupled with a tendency to wag its tail during high-speed lift throttle doesn't quite meet our goal of building a fast, safe, and enjoyable track car. In its place we experimented with M2 Performance's height adjustable coilover suspension system. Consisting of a set of eight-way, high-performance shock absorbers, threaded aluminum spring perches, and a vast selection of linear rate race springs, we hoped we could stumble upon something that would perfectly meet our needs of providing a vice-free, comfortable street ride while being able to withstand our idea of no-holds-barred track flogging. Little did we know our needs would change on a regular basis. Unlike our efforts with, say, Project Subaru Impreza, it isn't fair for us to criticize a car that, quite frankly, is better than any of us. Even in stock configuration, anyone short of a professional race car driver would be hard pressed to distinguish between the car's on-track strengths and weaknesses.
Instead, most (including us) are just overwhelmed by how damn capable it is. Hardly a good position from which to judge a car. Fortunately for us, Richards is also a professional race car driver/builder. With over a decade of Mazda RX-7 racing experience, Richards is a walking, talking treasure chest of knowledge. According to Richards, the third-generation RX-7 is a misunderstood sports car. He states, "The first thing most drivers do is mistake power-on oversteer for a general oversteer problem. Then they make changes in the car in an effort to improve the perceived imbalance.
"What they end up with is an imbalanced car that still suffers from excessive power-on oversteer. What the car really needs is the right kind of front-to-rear weight transfer. It needs to be able to squat down on its rear haunches slightly while biting into the tarmac. It's almost like stepping away from the road racing mentality and thinking along the lines of a drag racer." Richards continues, "Another problem with the stock suspension set up occurs during hard steady-state cornering. With stock soft spring rates, the car will lean over so much it comes very close to compressing its rubber bump stops. Once under power, the car squats back even more and actually starts to ride on them. This transition tends to make the car very tail-happy, especially on bumpy surfaces. Not surprisingly, direct replacement lowering springs make matters even worse."
From what we learned during our months of tutorial and self-indulgent spring-swapping, Richards' comments seem to be spot-on. Clearly, the only way to improve upon the stock suspension would be to install a high spring rate, height-adjustable, coilover system. This would allow us to lower the car appreciably without the problems typically associated with lowering springs. It would also completely eliminate the troublesome bump stops which were only designed to keep the overly soft stock suspension from bottoming out under heavy load.
The system would also enable us to carefully pick and chose the spring rates which best meet our particular performance needs and driving style. As per Richards' recommendation, we also integrated a three-way adjustable front anti-roll bar into the new suspension system. The anti-roll bar comes complete with heavy-duty, adjustable end-links. The stiffer front bar allowed us to slightly reduce the spring rates up front while still keeping the rear rates relatively high. This, in turn, allowed us to improve ride quality while keeping body roll to a minimum.
The rear end of the car also got its share of attention. According to Richards, the stock rear suspension bushings were designed with a certain amount of compliance based on the gripping, accelerating, and braking performance of a stock car. Once one improves any of these abilities, the bushings allow a larger degree of compliance, which adversely affects alignment characteristics. Richards believes the best solution is to completely eliminate bushing compliance by replacing the stock trailing arms and toe links with heavy-duty racing counterparts. With essentially no give at either end, these elegant upgrades should theoretically provide more predictable handling under extreme uses. Additionally, they do a remarkable job in eliminating the all-to-common rear suspension "clunks" which tend to plague higher mileage third generation RX-7.
At the end of the day, the suspension we ended up with was nearly identical to Richards' Speedvision Grand Sports race car and just slightly softer than his all-out World Challenge race car. Considering the perfectly acceptable ride quality it provides on the less-than-perfectly smooth roads of Northern California, the new suspension systems easily meets our real-world expectations. With the shock absorbers set on a softer setting, Project RX-7 rides no rougher than a bone stock, R1 package-equipped RX-7. While pot holes and speed bumps are best negotiated at very slow speeds due to the "much higher than stock" spring rates, the overall ride is completely agreeable under almost every real-world condition we came across. While no right-minded passenger would mistake the car for a Lexus in a blind test, its firm but compliant ride is entirely streetable for most performance hunting enthusiasts.
On the track, however, is where Project RX-7 shines the brightest. "These cars are amazing," says Richards, "Compared to other production cars which need to be cut in half, tubed, and heavily modified in order to be competitive on the race circuit, the RX-7 needs nothing more than a few well thought-out bolt-ons. In stock form, the car already has almost everything it needs to be a real race car. It's got an incredibly stiff chassis, solid suspension bushings, all-aluminum A-arms, and light overall weight. Unlike the Saleen Mustang or the Comptech NSX, RX-7 race cars are surprisingly similar to their stock counterparts. What's even more incredible is the fact the Mazda is still the best handler of them all." And what a sweet handler it is. It's a strange feeling when you drive a car that, at first, seems to have unreachable performance limits. However, the real magic is felt the first time you find yourself exceeding those once unobtainable limits while experiencing no unfortunate consequences. Frankly, there is nothing tricky about driving this car hard. Of course, it's so blindingly fast and capable that smooth inputs and quick reactions are needed to keep it from taking an strangely unorthodox line through any given turn.
But, at the same time, it clearly communicates to the driver every gram of weight transfer and every bit of available tire grip. It's almost scary the kind of cornering speeds attainable when driving ineffably hard-core sports cars. Under braking, the car refuses to get squirrelly. While turning-in, the front end remains securely planted. The car tracks effortlessly and neutrally during steady-state cornering. Under power, the rear stays magnetically stuck to the asphalt as the car rockets in a smooth, controllable arc toward its quickly approaching apex. Just incredible.
On-Track Intercooler Tests
With a car that behaves so well on the track, we don't need much of a reason to conduct sessions of continuous hot lapping. In fact, we can even try to make it worthwhile. In Part VI of our series, we tested three intercoolers, the stock unit, a Mazda Competition direct replacement, and an enormous unit from ASP/M2 Performance. Our street testing revealed that, in terms of intake temperature reduction, the smaller units offered nearly identical performance, while the latter intecooler was clearly superior than the rest. With sustained hot-lapping on the track, however, our results could become even more interesting. Let's see how the intercoolers stand up to the punishment. Again, we did all intake temperature measurements with fast-reacting thermocouple/fluke meter device we plumbed just downstream of the intercooler. We tested two nearly identical cars. One car (Project RX-7) was equipped with the ASP/M2 Performance intercooler. The other car was equipped with a stock unit.
Other than intercoolers, both cars were nearly identical in every way. They were also driven by the same driver under similar track conditions. During our testing, ambient air temperature was 85 degrees F. The results were staggering. The intake temperatures in Project RX-7, despite 20 minutes of all-out race conditions, never exceeded 158 degrees F. By comparison, the car with the stock intercooler experienced intake temperatures as high as 248 degrees F. In fact, within 5 to 7 minutes of race conditions, a cool-down period was required to keep the car from overheating. On the other side of the spectrum, Project RX-7 (with the upgraded intercooler and high efficiency radiator) never began to show any signs of heat related stress. In fact, coolant temperatures remained constant at 210 degrees F--right where they were while idling in the paddock! Again, just incredible.
In Part VIII of our series, we will continue to build the ultimate road-going RX-7. Among such hot-ticket items soon to be reviewed is a fully functional roll bar, an auxiliary oil cooler, carbon-fiber brake ducts, a trick F1-style steering wheel, and the adjustable rear wing you see in the pictures. Also in the works is yet another round of horsepower-augmenting engine upgrades. Stay tuned, there's a lot more to come.
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