Project RX-7 TurboII
Project RX-7 TurboII, Part 1
Ever since the new-style RX-7 was introduced as an '86 model, we've wanted to tinker with one. Now that the long-awaited turbo version of the RX-7 has finally made its debut, our desire to experiment has become undeniable. The new RX-7 Turbo has been touted by the automotive press as a wonderful car that has lots of unrealized potential lying within its turbocharged rotary 13B motor. Enthusiasts eagerly anticipated this model for years, but were greeted with a car offering only 36 more horsepower than its non-Turbo counterpart (the Turbo boosted power from 146 bhp at 6500 rpm to 182 bhp at 6500 rpm). This is surely an improvement - but there could be so much more! The unrealized power lying within the Turbo has cause our desire to tinker to grow into a fixation with this potentially wonderful car.
What if we gave it the blistering speed and close to three hundred horsepower that Mazda's turbo boosting could have given the RX-7 if the company had not needed to consider potential liability and warranty problems - not to mention good social manners?
And then (care we be so bold), what if we could do all this and not ruin a brand new $25,000 car by destroying that brand new engine or making it a bear to drive on the street?
Can we do all this? As it turns out, we really can - and we'll share how in this series of articles.
Before we begin, let's establish a couple of ground rules. First, while this car is legal in SCCA's Solo II ASP class, it probably won't be the hot setup for this class. Our goal with this project car is not to build the ultimate car for this class, We just want to make our RX-7 more fun for both street and track driving. After all, the RX-7 Turbo weighs some 2,850 pounds; but it competes, by some quirk in the SCCA's classing system, against cars that have similar amounts of horsepower but weigh only 1600 to 1800 lbs, If we were looking for ultimate competitiveness in the class, we would probably pull the cover off the old Elan resting behind our office or maybe work at a little more frenzied pace on our Griffith project. But today we're talking RX-7 Turbo.
Ground rule number two: This is not exactly a low budget project. This one is on the high side of the typical grassroots racer's budget, but a lot of our readers are driving this car and they deserve an occasional bit of indulgence. Since this is not a low budget project, we will not approach it in a low budget manner, either. We're going first class on this one, using all the best parts and taking no shortcuts.
ANALYZING & BASELINING
As with any project car, the first thing we will do is analyze and set our baseline. When ordering this car through the more than helpful folks at Mazda East, we were very specific about what we wanted. First of all, we wanted an '88 Mazda RX-7 Turbo. We liked the subtle new rear spoiler and the more aggressive front end treatment on the new model. We liked it so much, in fact, that we decided not to add the aftermarket body kit we had previously considered.
When the car arrived at our local dealer, Mark Kennedy Volkswagen-Audi-Mazda, we were warned that we would need sunglasses to look at the car. Its "arrest-me" Sunrise Red paint and the extra-special detail job done by the dealership made for a glaringly beautiful car.
We donned out shades, picked up our new Mazda and headed out to the Auto-X test course for baseline testing. Our test drivers lapped the track as many times as they felt necessary to get a true handle on the car's attributes. After a thorough analysis, we determined five things. First, this is probably the nicest car Mazda has ever built. But the new RX-7 Turbo would be even nicer with a hundred more horsepower, and it needs a bit more tire and wheel to bring its handling up to speed. In addition, we felt that it definitely needed better brake pads for any serious competition work. Finally, the car's body roll was excessive, to the point that it adversely affected transient response as well as steady state cornering ability.
On our new Auto-X course, we recorded an average 46.454 seconds with the bone-stock car (the MR2 Supercharged holds the course record for FTD with a 44.6). We also recorded a best 0-60 time of 7.25 seconds in 370 feet, and 60-0 time of 3.1 seconds in 120 feet for the RX-7 Turbo.
Obviously, these are impressive times for a bone-stock car, but we feel the number do leave some room for improvement. Next month, we'll tweak the suspension by installing Suspension Techniques springs and bars and mounting bigger Weds wheels with stickier Yokohama tires. We will also explore some camber changes and other suspension settings. A Dino steering wheel will be installed to make that steering feel even quicker. We'll also look around for some better brake pads that can handle the demands of serious competition.
The following month, we'll show you the quickest way to add almost one hundred horsepower to this car: a Cartech air to water intercooler upgrade kit. We'll tell you why this system works and prove its effectiveness on the track.
So stay tuned next month, as we bring you Part II of Project RX-7 Turbo.
Project RX-7 TurboII, Part 2 (Grassroots Motorsports, September 1988) face="times" size="3">
PART II: C STOCK SUSPENSION PREPARATION
This month we begin the suspension modifications on our 1988 RX-7 Turbo. But before we jump right into tire task of creating a street prepared screamer for SCCA's ASP Solo II class, we've asked Contributing Editor John Rasfetter to first make a few subtle modifications which will allow our Mazda to remain in stock class competition.
One of the best approaches to modifying a car for grassroots motorsports competition is to move through the classes slowly, modifying one thing at a time and learning and testing the car as yet, go. So we will first build our Turbo RX-7 to file limits of C Stock specifications, then modify it for A Street Prepared competition.
The Mazda RX-7s have long been the front runners in C Stock SCCA Solo II competition. The first-generation RX-7s proved themselves to be nearly neutral handling cars that loved autocross competition; it's not surprising that they were loved in turn by the drivers that chose them.
When Mazda introduced its all-new RX-7 sports car in 1986, RX-7 competitors seemed to adopt a "wait and see" attitude. Many felt that one of the new RX-7 models might be faster than the older versions of the car; but which of the new models would it be? Would the GXL version, with its 146 hp rotary engine, power steering and limited slip differential, be the new hot ticket? Or would it be the lightweight Sport version, which had the same horsepower but no limited slip differential? Regardless of which would emerge on top, everyone knew that these second generation cars would take some time to develop enough to dethrone the early RX-7s.
Then Mazda introduced the RX-7 Turbo as a 1987 model, and the SCCA classed it in C Stock with all the other RX-7s. Now the championship battles promised to get very competitive, very quickly; and, as the record book shows, they did. The new RX-7 Turbo finished second at the 1987 Solo II Nationals, just one place behind the first generation winner.
As we outlined last month in Part One of this project, the car we selected for modification is a 1988 RX-7 Turbo. The major specifications for the '88 RX-7 Turbo are the same as the 87's. Both are equipped with a turbocharged rotary engine that puts out 182 hp @ 6,500 rpm, share the same 5 speed transmission that has a closer ratio than the non-turbo models, and are equipped with a 4.100:1 limited slip differential. The RX 7 Turbo weighs in at approximately 2,850 lbs, or about two hundred pounds heavier than the lightest Sport version.
As we noted last month, the only options selected for the car included Mazda's power-assisted steering and anti-lock brake system (ABS). The power assisted steering was selected because it features a 15.2:1 steering ratio (compared to the standard of 20.3:1) and 2.7 turns lock-to-lock (compared to the standard of 3.6 turns). This will make it easier to negotiate quick changes in direction that could sometimes be too much for the older model's steering.
Unfortunately, the required steering effort with the new power steering is too light; this means that it takes a driver some time to get used to the subtleties of steering feel and tire feedback. The anti-lock brake system was selected because it better controls single inside wheel luck up, which often occurs when trail braking deep into a corner; besides, sooner or later it's going to rain, and ABS provides a definite advantage in the wet.
Mazda's RX-7 uses a strut-type independent front suspension with coil springs and stabilizer bar, The only thing this suspension lacked was sufficient camber and caster adjustments for competition. Our first step was to adjust tire front suspension to set as much negative camber and positive caster as possible.
The RX-7 shock absorber's top rubber mounting block is eccentric; by re-positioning it, one can slightly adjust camber and caster. To do this, raise the front of the vehicle and support it with jack stands. Remove the rubber cap on the suspension tower over the shock absorber, and remove the four nuts holding the shock's top mounting block to the suspension tower. The dot indicator is usually pointed toward the inside front of the engine compartment. Turn the mounting block until the dot indicator is pointed to the inside rear of the engine compartment. This will increase caster by 30' to 45', changing it from tire factory setting of 4degrees 40' to about 5degrees 30'.
Next, finger tighten the top shock mounting block nuts. Now, loosen the ball joint to lower control arm bolts and nuts, the knuckle-to-shock absorber bolts and nuts, and the two bolts holding the rear bushing of the front lower control arms. Then pull outward at the bottom of the tire and push in at the top. While you're holding that under light tension, tighten all the bolts and nuts to the following torque values: ball joint to lower arm, 69 to 86 ft-lb; knuckle to shock absorber, 69 to 86 ft lb; rear bushing of the front lower control arms to cross member, 43 to 54 ft-lb; and shock mounting block to suspension tower, 17 to 22 it lb. This process will put all of the factory tolerances in your favor, and give you maximum legal negative camber.
The RX-7's front suspension alignment specifications call for a total toe in of 3 mm, plus or minus 3 mm (0.12", +/0.12"). We found that the steering response and cornering performance of the car improved with a setting of zero toe.
The rear suspension of the RX-7 is of the independent, multi-link, semi-trailing arm type with coil springs and a stabilizer bar. The rear suspension alignment specifications call for negative 0degrees 44' of camber and a toe setting of zero, plus or minus 3 mm (+/- 0.12"). We found the standard rear camber setting to be sufficient, but eventually adjusted the rear toe setting to toe-out of 3 mm (+ 0. 12"). The torque value for the rear control arm to subframe was 46 to 70 it lb. This setting helped reduce understeer on the skidpad without upsetting the RX-7's balance on the handling oval or autocross course.
Our next step was to try to get a little wider and more efficient tire footprint on the ground. The standard tire size used on the RX-7 Turbo is the 205/55VR16 mounted on a standard seven inch wide wheel with a positive offset of 40 mm. The 1987 RX-7 Turbo could not fit a wider tire on tire standard wheels; but the SCCA Solo II Stock Class rules do allow competitors to change wheels, as long as the new wheels have the same dimensions as the car's original equipment wheels and don't change the offset by more than plus or minus 1/4" (6.35 mm). We found it difficult to get alternate wheels for the '88 RX-7 Turbo, because only a few companies build a 16" wheel with anything near a 40 mm offset.
We finally selected a new DP 5 Motorsport Wheel from A.S. Auto Mechanicca in Seattle, Washington. This which, which is reminiscent of a Porsche 959-style wheel, is available for the RX-7 Turbo in a 16" x 7" size with a 35 mm positive offset. The 35 mm offset allowed us to install 225/50VRI6 Yokohama A008-R tires and still remain SCCA legal. The same wheel and tire combination was used front and rear to allow our Mazda to accelerate sooner and harder out of cor
Project RX-7 TurboII, Part 3(Grassroots Motorsports, October 1988)
PART III: STREET PREPARING THE SUSPENSION
The key to modifying any car, especially a brand new $26,000 car, is to do it very carefully. In today's maze of aftermarket performance parts, it's very easy to end up adding pieces to your car which will not increase performance. We've seen Street Prepared cars that actually performed worse than when they were stock!
Research and planning are the answers to properly preparing your vehicle for competition. The first step in the planning process is to decide what you want your car to do. Last month, we illustrated the best way to prepare our new Mazda RX-7 Turbo for SCCA's Solo II C Stock class competition, keeping in mind all the while the fact that we planned to move up to the A Street Prepared class later. Now we're ready to modify our Project RX-7 for Street Prepared action.
But there's more to the planning than simply choosing to prepare a car for the Street Prepared category. In today's motorsports world, two distinctly different approaches to street preparing cars are emerging. The first type of street preparation is a no-holds-barred level of competition modifications that take the vehicle to the limits of the rules outlined in the Street Prepared section of SCCA's Solo II rule book. This usually means that the car is given a bone-jarringly stiff suspension, and tires that are not designed for road use. This type of preparation is fine for a competition only vehicle; but what we're after here is a different kind of Street Prepared.
We want our Project RX-7 Turbo to be what we'll call street Street Prepared, with the emphasis on the word "street". What this means is that we want to build a true dual purpose, street-driven autocross car that is quite comfortable and fast, both on the street and at the track. This type of preparation is, in effect, a compromise; but as you will see, it does not require as much compromise as you might think.
Whichever philosophy you decide to follow, deciding what kind of car you want before you start ripping into it is critical! Plan your purchases and talk to people who have built cars like yours. Decide who is going to do the work: Do you really have the time? Do you really have the money? Do you really have the tools or the ability. Would it be better for you to stay in the Stock class for one more season? Think about what you plan to do; don't ever dive into something completely blind to the perils that be ahead.
The main differences between Street Prepared and street Street Prepared involve the tires and suspension stiffness. Obviously, for our purposes we wanted to choose tires that could be used both an the street and at the track; we chose the non-shaved Yokohama AOO8R. The unique tread pattern of the Yokohama doesn't seem to "chunk" as badly as most full-tread depth tires when used for competition. We have found the A008R's performance to be top-notch, and tread life is also comparable to the best of the current "super" tires. While this is certainly not a 40,000 mile tire, the 008R can safely be used on the street (though it is not recommended for those who do a lot of wet-weather driving).
The Hoosier tire is also very popular among Street Prepared autocrossers, but is by the manufacturer's own admission not designed for extensive street use.
We used 225-50-16 tires on 16" x 8" front and 16" x 9" rear Weds V/R wheels. There are those who say that this tire/wheel combination will not fit under the new RX-7; but we've encountered no tire rubbing at any speeds,
The Weds V/R style wheel and Yokohama A008R tire combination looks truly stunning. Even better, on our test track we shaved more than a second off our lap times by adding this tire and wheel combination.
Our average lap time went from 46.45 seconds (stock car on OEM Goodyears) to 45.24 seconds with our new wheels and tires. We found that the best tire pressures for track use were 42 psi front and 32 psi rear - the same as those used earlier in this series when the car was run on the stock wheels with Yokohama tires.
Next, We turned our attention to the suspension system. As we mentioned last month, we had adjusted the suspension alignment settings to crank in as much negative camber as was practical for street driving while remaining within the factory specifications. We also turned the eccentric top rubber mounting block on the front chock absorbers to add a bit more caster. We kept the settings fairly moderate, because we wanted our car to remain comfortable streetable.
A stock RX-7 handles very well, since it features an excellent strut-type independent suspension and an independent, multi-link trailing arm rear suspension. We wanted to be very careful not to destroy what has been engineered into this fine automobile; but our car did still exhibit a lot of body roll due to its nearly 3000 lbs weight and its soft, comfortable springing. To help limit the car's body roll, we replaced Mazda's stock 24 mm front and 14 mm rear sway bars with Suspension Techniques' 28 mm front and 19 mm rear bars for the RX-7.
These bars, which included excellent directions and high quality hardware, went on quite easily. However, the RX-7's delicate radiator heat sensor is located right over the front bar; it should be removed to avoid breakage when installing the front bar. Also, while the Suspension Techniques front swaybar end link is adjustable to three positions (a very nice feature) we found it impossible to make the bar fit into any but the middle position. A spherical link would have made it easier to adjust the bar.
We then tested the effects of tire new sway bars using a six-cone slalom with constant 15 yard spacing. Using two drivers, we went from 3.76 second average times with the OEM sway bars to 3.54 seconds with the new sway bars. This represents an improvement of 5.8%; not bad for a simple bolt-on modification!
Next, we turned our attention to the springs. The good folks at Mark Kennedy Volkswagen/Audi/Mazda offered us the use of their shop to change the springs on our RX-7 Turbo. Needless to say, it's much easier to do a job like this when you have a lift and all the right tools, (A factory-trained Mazda mechanic looking over your shoulder doesn't hurt either.) We changed all four springs in less than two hours! Remember, though: this was a new car with no dirt, grime or rusty bolts. If you're going to attempt this job yourself, follow the shop manual carefully, use jack stands or a lift to raise your car, and please use a high quality spring compressor, and use it carefully.
The progressively wound Suspension Techniques springs lowered the ride height of our project car by only 18 mm (5/8") in the front and 28 mm(1 1/8") in the rear. After the installation, we went out for a test drive. We were impressed with the fact that the ride quality had not deteriorated significantly while the car felt firmer, the ride was not at all objectionable - even on rough surfaces. The end result was a car you could live with an a day-to-day basis.
The next thing we noticed was that the body roll had been all but eliminated. We did experience more understeer, which was eliminated by adding more air to the front tires and using the throttle more aggressively while cornering. These two corrections made for much faster, smoother transitions.
The car also felt better in the slalom test; our times confirmed the improvement. Our two drivers negotiated our slalom in an average of just 3.42 seconds; this represented a 3.4% improvement over the car with just sway bars, and a 9.0% improvement in the slalom over the stock car with just the Yokohama tires and Weds wheels added.
We made only one modification to the interior of the car: we added a smaller 370 mm Model 22 steering wheel from Dino's "Prestige" line. The addition of a new wheel is always an inexpensive, easy and satisfying modification. I must admit that, unlike most of our project cars, we were hard pressed to find a nicer-looking steering wheel than the RX-7's stock wheel. But as usual, Dino came through for us and we were very satisfied with the results.
That wraps up the suspension modifications for our foray into A Street Prepared competition. Next month, things get even more exciting as we finish our Project RX-7 Turbo by exploring the art and science of turbo upgrades.
Project RX-7 TurboII, Part 3(Grassroots Motorsports, November 1988)
PART IV: UPGRADING THE TURBO SYSTEM
This is the fourth and final installment in our Project RX-7 Turbo series. In the past three months, we've optimized the performance of our 1988 Mazda for SCCA C Stock Solo II competition, then upgraded the suspension for A Street Prepared action. The final step is to upgrade straight line performance for Street Prepared competition. In keeping with the nature of our project, we want to increase power without sacrificing new car reliability; the simplest way to do this is to increase the turbo's efficiency by adding Cartech Performance Systems' intercooler upgrade kit.
In order to develop products for upgrading the Mazda Turbo II, Cartech spent ten months living with the factory turbo under various conditions and at varying levels of modification. As with all their development projects, Cartech development manager Todd Wilson went through an extensive exercise in determining baseline turbo, intercooler, fuel, air inlet, ignition and exhaust system efficiency prior to making any changes. This included instrumenting and measuring most or all of the following on the stock vehicle operating on 92 octane fuel:
- Ambient air temperature
- Barometric pressure
- Air/fuel ratio*
- Vacuum ahead of compressor inlet*
- Compressor discharge pressure (intercooler inlet)*
- Compressor discharge temperature (intercooler inlet)*
- Intercooler discharge pressure (before throttle)*
- Actual pressure in intake after throttle plate
- Exhaust manifold pressure before turbo*
- Exhaust backpressure after turbo, ahead of all converters and mufflers, plus individual readings ahead of each individual point of restriction
- Exhaust gas temperature*
- Tailpipe temperature*
- Actual cooling system temperature
- Underhood temperatures at air filter inlet, behind radiator, air inlet to i.c., air discharge from i.c.
- Timing changes related to boost pressure - at varying levls betwen 0 & stock max boost
Once these baselies are complete, a plan is outlined to concentrate effort an those areas weakest in the base car's various systems. In the case of the Mazda RX-7 Turbo II, the results showed that the following major areas could be improved.
PROBLEM AREAS & SOLUTIONS - EXHAUST BACKPRESSURE
The stock exhaust has four points of restriction. First and second are the two monolith convertors on the turbo outlet; third is the main catalytic, and fourth are the dual rear mufflers. Starting from the rear, the measured amount of pressure ahead of the mufflers showed that the factory produced a good-flowing muffler with only 2 psi of restrictions. The main cat showed a tolerable 3.5 psi restriction, and the two monoliths rendered an additional 5 psi restriction.
So what does backpressure have to do with performance? Simply put: If the burned exhaust can't flow out the tallpipe, then a reversion of outward flowing gas occurs at the combustion area, along with not allowing maximum inlet flow, this increases combustion temperatures and lowers horsepower. Even more directly, lowering this backpressure in a turbo car results in a noticeable increase in boost response.
Cartech offers the autocrosser, for off-highway use, a high flow header from the back of the turbo to the "Y" intersection of the stock exhaust. This bolt in/out replacement eliminated over 80% of the total exhaust restriction, and wakes up the boost response dramatically.
The most critical concerns In any turbo design are maintaining a proper air to fuel mix in the combustion chamber, and keeping the combustion temperatures low enough so that the air/fuel mix does not explode ahead of the flame front (i.e. ignition spark). When this explosion due to too small a fuel versus air mix or too high a combustion temperature occurs, it is called 'detonation.' Detonation is several hundred times more violent than normal spark-ignited, controlled combustion the results of this violence can, in instants, necessitate several thousand dollars worth of overhaul repairs.
Even though the stock car operates at a very modest 5.5 psi of boost pressure, the RX-7 benefits from a sophisticated electronic feedback sensor capable of listening for detonation and adjusting the ignition advance/retard level to protect the engine several hundred times every revolution. Cartech found that the large injectors and the aggressive fuel curve afforded by the stock EFI rendered a correct air/fuel mixture to a level of about 8 psi without lowering discharge temperatures; with other improvements, it was possible to run safely to the 11 psi range before additional modification to the fuel system was required.
In measuring the intercooler efficiency the stock car showed a heat rejection of 68% of the incoming temperature from the turbo above ambient, and a pressure restriction of 1 psi at 5.5 psi of boost. The pressure restriction is important, since for every psi of boost lost the turbo must produce that much more to equal the desired boost at the engine; and for every psi of work the Mazda produces, there is a rise in compressor discharge temperatures before the intercooler of approximately 22 degrees. So, in the case of the Mazda, the turbo is called on to produce a temperature equivalent to that at 6.5 psi boost in order to achieve a 5.5 psi pressure in the intake manifold. In addition, a pressure restriction is a percentage of total boost; so as boost rises, so does the restriction, which further exacerbates the condition.
The reasons for the stock intercooler's lack of efficiency are two-fold. First, the heat rejection surface area is located at a less-than-optimum position for air flow. In fact, according to a drawing Mazda supplied to Rotary Rocket (a magazine for Mazda owners), which that magazine published in its December '85 Collectors' Edition, the designers would have done much better to point the air intake for the intercooler back toward the windshield. The area they did select receives very poor flow, because it is smack in the middle of a pressure zone where the air is moving upward. The optimum location would be ahead of the radiator; however, we can understand Mazda's reluctance to locate it there due to the massive complications that location would create. The second area of inefficiency, pressure restriction, is even more fundamental; the intercooler lacks internal flow area. This area, which on the stock intercooler measures right at six square inches, is again forced by the location and resultant physical size. There are two common ways to increase the flow area on an air to air unit: one is to increase the size of the complete intercooler; the other is to change the internal aerodynamic shape of the heat transfer core. A third solution involves redesigning the system as an air to water unit.
Cartech has built air to water intercoolers for a number of years, but the typical single path air to water unit has an efficiency that is generally accepted to be in the mid-70% range. This is good, considerlng that the given flow area in this method of intercooling is much greater, and the result is typically a much lower pressure loss (a norm of about . 1 psi). At 70%, however it is not good enough to suggest that the owner of a new Mazda Turbo II should scrap his or her original unit,
So the Cartech engineers arrived at the idea of producing an intercooler plenum that bolted into the position of the stock intercooler, but had two completely separate pathways (two separate intercooler core units) inside. The twin-pathway design would offer the user a combined efficiency of 86%.
Air to water units have an air to air intercooler welded into a plenum. These units flow the water through the pathway normally carrying the pressurized air, while blowing the pressurized air across what is normally the frontal or heat rejection surface area. A small electric pump then moves the water from a reservoir to the front of the car where the heat rejection coolers are located. So the twin pathway design contains two intercoolers in the plenum, two reservoirs, two pumps and two coolers running one after the other.
TURBO COMPRESSOR EFFICIENCY*
The rotary engine is an interesting application for turbo use, in that it produces an extremely high amount of exhaust heat and velocity. This is largely due to the two cycle function and the associated firing of the combustion chamber at closer intervals than are found in four cycle piston engines. Cartech learned to exploit this high exhaust gas velocity by utilizing very large turbos on applications for the non-turbo Mazda; they also worked hard to find ways to modify the stock Mazda turbo unit for a similar advantage.
The reason it was felt that a larger turbo would be beneficial is that the stock turbo has a measured efficiency of only 49% at the peak flow rate of the motor. A 49% rating shows a little better pumping efficiency than that of a positive displacement supercharger (commonly considered to be peaked out at about 42%); this is nowhere near the current 70 to 75% which is considered "state of the art" in terms of turbos.
Just what is compressor efficiency? This is the difference between the actual temperature discharged from the compressor at the maximum flow rate of the engine, and the calculated thermodynamic ideal. This ideal is arrived at by an accepted thermal formula. So, to put it another way, efficiency = discharge temp. ideal + discharge temp. actual.
The next question is: why doesn't the factory install a larger turbo - or, more exactly, why do they tolerate such a low thermodynamic efficiency? To answer this question, we can only guess that Mazda's choice of turbos was probably the result of a combination of searching for instant low speed response and compromising efficiency in order to utilize a turbo size already available from their vendor.
In answer to the question of response, Cartech has found that the properly sized, 70+ % efficient turbos used in their own applications provide a boost response target allows full boost by 3,000 rpm and continues to pull strongly all tile way to 7,500 rpm. Conversely, the Mazda factory turbo develops full boost by 2,600 rpm, but falls off noticeably by 5,000 rpm. This is due to the compressor dropping off the efficiency curve, and the associated rise in exhaust manifold back pressure as the turbo requires more shaft speed to produce the same boost pressure in proportion to the rise in CFM of the engine at higher rpm. Note that as boost pressure is raised above the factory level of 5.5 psi, the ability to perceive the turbo "falling off" is increased.
What Cartech offers as their solution is an increase in the compressor size. This solution was selected instead of a complete turbo change, in order to keep costs down and to maintain the low speed response offered by the stock twin scroll exhaust manifold ports. The compressor change results in an increase of compressor efficiency to about 66%, which is enough to afford a reduction in temperature of about two degrees per psi. Although this reduction seems small, it is enough to justify raising the boost pressure from 9 psi to 10 psi without an increase in the temperature seen at the engine.
Although this modification involves altering the compressor or 'cold' side only, SCCA Solo Board Chairman Gregg Lee did say in a telephone conversation that he would not consider it legal for Street Prepared competition. This is a gray area, however, so check your local club rates. At any rate, this step in the upgrade process is worth an additional 10 hp or so at most, and can be omitted.
Chassis dyno testing of Cartech's compressor upgrade, thier dual path intercooler system and an off-road exhaust system produced a rear-wheel horsepower of 239. Before modification the stock car tested ran just 151 hp! This would tend to validate Cartech's conservative estimate of 265 bhp.
Of course, all this technical data is great, but it doesn't answer the most important question: How does this upgrade kit perform in the real world? With the aforementioned modifications in place, our Project RX-7 Turbo is cornpletely docile in traffic. It doesn't idle differently from the factory car. It's not appreciably louder than the original car. It doesn't start hard. It doesn't run hot . This is truly a remarkable "hop-up" modification. High tech performance is very different in the eighties, compared to the relatively crude modifications made twenty years ago.
On the Auto-X test course, we did even better. We started with an average lap time of 46.454 seconds for the stock car; we ended tip with an average time of 43.455.
The Cartech approach to improved turbo performance is to lower the thermal stresses, and then raise the boost back to a point at or below the stock thermal load. The company feels that this approach is the only logical way in which factory-type durability can be maintained when turning up the boost. Some turbo upgrades may promise more, but few manufacturers have invested the thought and planning required to assure engine safety. The Cartech system is not cheap, but look at it this way: the price of this upgrade does not have to include the cost of regular engine overhauls!