This article is reprinted in it's entirety from Car and Driver December 1987. If you like this article please consider subscribing to the magazine.
Photography by Aaron Kiley
When you first approach the Toyota MR2 Supercharged, you don't get much warning of what awaits you under the hood.The new wheels don't jump out at you, the louvers on the engine cover aren't very obvious. Small "Supercharged" decals on the doors and the deck lid are the only other clues that this is anything but your olf friend Mister Two.
Slip inside and you're sure to notice that the cabin is different, though only slightly. In 1987 the MR2 got a three spoke steering wheel and a revised climate control panel. In addition, the hand brake was moved to the passengers side of the center tunnel and the ashtray to the driver's side. The seats look the same as always, but the door trim appears to be new. The instruments don't look any different at all - until finally you notice that the tachometer has a little red-striped sector above 7500 rpm, plus a little green light, labled "Supercharger". And a switch for premium or regular unleaded sits just above your right knee.
When you key the engine to life, you notice that it doesn't sound as smooth as the Mister Two you're used to. And as you drive away, you feel a much heftier push in the back. Within the first few hundred yards it's clear that there is a big difference in this MR2 - and it's a;; in the engine compartment.
The difference is the 4A-GZE motor, and it's available only in the new supercharged model of the Toyota MR2 - the first supercharged production car introduced in this country in more then two decades. Given that the 4A-GZE is light, compact, powerful, and economical, we expect to see it in other Toyotas in years to come.
Toyota decided on a Roots-type supercharger, which it manufactures in house. Together with an air-to-air intercooler, the new blower boosts the MR2's peak outputs to 145 hp at 6400 rpm and 140 pound-feet of torque at 4000 rpm - improvments of 29 and 44 percent over the naturally aspirated motor. (The base MR2's 4A-GE powerplant has gained about three percent in both horsepower and torque for 1988.)
The blown engine incorporates several modifications designed to compensate for the power increase. It's compression ratio has been reduced from 9.4 to 8.0:1 by means of larger depressionsin the piston crowns, and it's pistons are made of forged rather then die-cast aluminum, making them lighter, stronger, and more resistant to heat. It's cylinder head gasket is made of a high strength carbon material, with steel wires around the bores for better sealing against the higher presuures. A high speed electronic knock sensor helps to optimize the ignition timing. A new camshaft design provides revised valve timing, and the fuel injection system has been recalibrated. To round out the development, the intake and exhaust tracts have also been tailored to the new engine.
On the move, the supercharged MR2 is deceptivley quick. The most impressive aspects of its performance are its fast engine response and high torque output. The 4A-GZE powerplant is both gutsy and flexible. If you're cuising along with the supercharger disengaged, a little pressure on the accelerator will summon full boost in just a fraction of a second. The supercharger makes the little motor feel much bigger than it is, from the liftoff right up to the 7500 rpm redline. Compared with the normally asporated engine, the blown motor has only one drawback (besides higher fuel consumption, of course); a groan from the supercharger at any speed beyond 4000 rpm. The melody just isn't as sweet.
But that's a small price to play for performance that is both effortless and quite sufficient to embarass many higher priced sports cars. At the test track, our supercharged MR2 turned in a 0-to-60-mph time of 6.5 seconds and ran the quartermile in 15.0 seconds at 91 mph. That's enough to hold off a Porsche 944S in the stoplight drags. And there isn't another 1.6 liter car in the land that can beat Super Two's 130 mph top speed. In
comparison, the base MR2 is slower by 1.7 seconds to 60, by 1.2 seconds and 6 mph in the quarter, and by 10 mph in top speed. The supercharged MR2 is 210 pounds heavier then it's stablemate, so it obviously has much more usable power on tap.
Toyota recommends the use of premium unleaded in the supercharged MR2, but the fuel selector switch enables you to protect the engine when only regular is available. Unable to resist the opprotunity to test the effect of the switch, we found that the regular-unleaded position slows the car by about half a second in the quarter-mile.
As with the base MR2, Toyota offers a choice of two transmissions in the supercharged model: a new five speed manual and a beefed up version of the previous four speed automatic. The five speed incorporates a 9.0 inch clutch (up from 8.5 inches), wider gear ratios, and a built in oil pump. Both transaxles drive the rear wheels through uprated driveshafts.
Our test car was fitted with the five speed. Although it was generally smooth and positive, it didn't shift as nicely as the unit in our long term MR2 (June 1986). Engaging reverse from a standstill and downshifting from third to second often required more effort than we expected. And during quick upshifts we occasionally beat the syncronizers and crunched the gears. At least the clutch engaged progressively, was nicley weighted, and had a comfortable throw.
The boostedengine is good news, but we have nothing so favorable to report about the MR2's handling and braking. Although the only brake system change for 1988 is the addition of a second vacuum booster to reduce brake pedal effort, our supercharged MR2 required 199 feet to stop from 70 mph, up from the 175 foot performance of our long term test car. We found that the new car's front wheels tended to lock prematurely. althoug the modulation was good. On the skidpas, the supercharged Two generated only 0.78 g. Our long term car on it's original tires, cornered at 0.80 g.
The biggest letdown was our supercharged MR2's instability. It felt little like the MR2s we have known and loved. Our test car darted badly on all but the smoothest pavement, and we felt an enormous amount of kick in it's steering wheel. On smooth highways it tended to wander, especially in crosswinds. Some testers also expressed concern about it's behavior on wet pavement, where it required more concentratin then expected. Another annoyance was the high steering effort required at parking speeds.
The MR2's handling balance has also changed for the worse. It's understeer and body roll are up, and it's sharpness and over the road poise are down. The supercharged MR2 floats over pavement waves and responds lazily to quick steering motions. It's twitchy when you combine heavy braking or full throttle with cornering or rough pavement. It's lift throttle oversteer is more pronounced now.
The cause of this change in chassis dynamics is that Toyota has recalibrated the MR2's front suspension. The spring rates are now eleven percent higher, and the diameter of the antiroll bar has been increased by three percent. The purpose of these changes was to increase understeer and reduce the abruptness of lift throttle oversteer, but the overall effect is not an improvement.
One benefit of the recalibrated suspension is better ride over over ridges and ruts. The suspension doesn't bottom as easilynow, and it transmits less noise. But we would gladly trade the slightly improved compliance and comfort for the old response - especially since the ride wasn't bad before.
There has been some backsliding in the cabin too: in the placement of the cigarette lighter and the pedals. The lighter is now located under the ashtray lid on the center tunnel, which makes using a radar detector very inconvenient. First of all, the power cord has to be routed around the gear lever so as not to interfere with shifting. Even worse, the ashtray lid opens to the left, where it gets in the way of your right arm - and if you're tall, even your right leg. As for the pedals, we found them much more difficult to heal and toe then they were in earlier MR2s, though Toyota insists that their placement is identical.
Apart from these two shortcomings, the MR2 accommodates as well as ever. This little Toyota is easier to get into and out of that many larger cars, and only the tallest of drivers will find it's headroom marginal. Our test car's fit and finish were up to Toyota's usual high standards. and we heard no squeaks or rattles from the standard equipment T-roof or anywhere else.
Toyota plans to import 14,400 supercharged MR2's thes year, at a base price of about $16,500. In addition to the T-roof, Super Two is outfitted with alloy wheels and body colored aero add-ons. Air conditioning, power windows and other conveniences are available as options.
With the supercharged engine, the MR2 has matured into a more powerful but also heavier and less wieldy machine. We love the engine, but we aren't too keen on the chassis. Mister Two has muscles now, but he's no longer the light, throw around friend we used to enjoy so much. We lament, and long for, the nimbleness of the original design.
Although Louis Renault invented the supercharger at about the turn of the century, the technology wasn't applied to automobiles until Willy Haupt fitted a supercharger to his racer in 1909. Strictly speaking, supercharging is the compression of an engines intake charge above atmospheric pressure by mechanical means. Over the years, however, the term "supercharger" has come to denote an air pump driven by a crankshaft, while an exhaust driven air pump is called a turbocharger.
Both means of compressing intake air have advantages and disadvantages. A turbocharger promises greater efficiency, because it draws no power from an engine's crankshaft; however, it cannot match the almost instant response of a mechanically driven supercharger. A supercharger can provide boost not only faster then a turbo but over a much broader engine rpm range as well. In addition, a supercharger operates at much lower temperatures, and it rotates at approximately engine speed, while the typical turbo spins at more then 100,000 rpm; these advantages promise lower material costs, simpler cooling and lubrication requirements, and generally greater reliability. (An intercooler is benificial with both supercharging and turbocharging, however, since compressing air by any means raises it's temperature). The main disadvantages of supercharging are higher power demands, more mechanical noise, and more complex control requirements.
After weighing all these factors, Toyota concluded that the MR2 would benefit more from a supercharger than from a turbo. Mister Two's positive displacement, Roots-type blower weighs 23.8 pounds and is driven by a ribbed V-belt from a crankshaft pullet, at 1.25 times engine speed. An electromagnetic clutch is regulated by the engine computer to engage the supercharger when more power is demanded.
The principal components of the MR2's supercharger are a pair of twin lobed aluminum rotors mounted on seperate shafts inside an aluminum housing. The v-belt drives the lower shaft, which in turn drives the upper shaft in the opposite directin by means of phasing gears. the rotors are coated with a flouric resin and are press fitted onto their shafts. The shaft-gear-and-rotor mechanisms are attached to an end plate in such a way that the relationship of the components cannot vary. The supercharger has it's own oil supply for lubrication, and it's front and rear bearing channels are vented to the atmosphere by an engine-computer controlled valve to prevent the buildup of rpessure inside the housing. The rotors spin very close to each other and to the wall of the housing, but without contact; the small clearances allow some air leakage, which reduces the blower's overall pumping effieciency, particularly at low rpm. There is also a tapered lip on the housing across the width of the exhaust outlet; by reducing the velocity of the air that flows back into the housing whenever one of the rotors passes the outlet, it alleviates one source of aerodynamic noise within the supercharger.
When supercharging is not needed - when the load is light and the throttle opening small - the clutch disengages the blower from the engine, and an air bypass valve (ABV) allows intake air to flow around it. The ABV is managed by the engine computer by means of a vacuum-switching valve. Under some intermediate-load conditions, the computer routes air through the blower without engaging it's drive clutch. The rotors then freewheel to let the air pass.
When the supercharger is engaged, the rotors suck air into the housing as they spin. The air is trapped above the top rotor and below the bottom rotor and is then pumped around to the exit side of the supercharger. Both volumes of air are exhausted through a common port. During each revolution, the supercharger pumps four seperate volumes of air, with a total displacement of 1.20 liters. Multiplied by the 1.25:1 drive ratio, a total of 1.50 liters of air is pumped for each crankshaft revolution. Since the engines actual intake volume is limited to 0.8 liter per crankshaft revolution (half of the engines displacement), the result is a substantial increase in intake-manifold air pressure.
The engine computer limits the maximum boost pressure to 10.7 psi by opening the ABV as necessary, allowing a backflow of air from the outlet side to the intake side of the supercharger. Since the airflow meter is located upstream of the blower, this backflow does not interfere with it's readings, and the computer continues to administer the appropriate amount of fuel to the engine.
Engineers and customers alike will surley watch this first modern day application of supercharging with great interest. And other manufacturers will soon follow Toyota's lead, so watch this space for a supercharger-versus-supercharger comparison test
The new supercharged MR2 is simultaneously much better and much worse then previous editions of Toyota's mid-engined two seater. The blown motor is brilliant, producing both the output and instant response of a much larger normally aspirated engine. But the chassis that accompanies Mister Two's overacheiving powerplant is a real dissapointment.
The new MR2 feels soft and floppy no matter how gently it's driven. It doesn't like to go down the road straight. The slightest side wind or bump is enough to send it darting one way or the other. The supercharged MR2 doesn't atke to hard driving, either. The closer one pushes the it to it's limits, the less settled and balanced it feels. And neither it's cornering nor it's braking limits are particularly high.
As it stands, the supercharged MR2 is the worst handling Toyota in America. The excellent blown engine deserves much better.
Bravo! And yeeccchhhh! The changes to the newly supercharged MR2 almost leave me speechless...almost. They do wonders for the engine of our beloved "Mistah Two", but they also perpetrate travesties on his chassis.
Toyota, by folding a supercharger into the MR2's engine bay (a good trick in a space about the size of a shaving kit) turns it's harried little engine into a hairy little engine. Potent and all around responsive, this rampaging boost-beastie lights up your life and Mistah Two's tires alike.
But how to put this power to best use? The answer apparently remains a puzzle. Despite the MR2's zippy size and looks, the whippy nippers soft suspension sends it bobbing like a beach ball. If you kick the ball too hard, it bounds around like one of those ka-wobbly orbs that have weights stucj inside to knock their trajectories for a loop-de-loop. With most of the MR2's weight stuffed near it's tail, a beach ball suspension is the last thing you need to help plot a proper trajectory as you go.
Mister Two and I go way back. Toyota's perky two-seater had just introduced in early 1985 when C/D ordered on for long term evaluation. Over the ensuing year, I gleefully racked up a good portion of our MR2's 30,000 flawless miles. Time only made my heart grow fonder; our fiesty MR2 was always a blast to drive.
When I heard that a supercharged MR2 was in the works, I was more than a little interested. After all, the regular MR2's only serious weakness was a lack of power. With a little extra muscle, it was sure to be flat out dazzling.
I guess that's why I'm so disappointed in the supercharged MR2. I expected the new edition to offer all of the old MR2's goodness and a healthy dollop of speed to boot. But instead of being muscular, it feels muscle-bound. The power is there, but the chassis has gone all soft and lazy. And the normally aspirated engine's crisp, exhilarating exhaust note has been replaced by a less then eager moan.
You know, good ol' Mister Two was really fast enough all along.
-Arthur St. Antoine