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.
GROUND RULES
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.
INTERCOOLER EFFICIENCY
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.
Results
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!
