This Toyota MR2 is Twin-Turbo and Supercharged
There is a replacement for displacement, and that’s boatloads of boost.
What has five gears, four cylinders, three air compressors, two driven wheels, and one crazy builder? The supercharged, twin-turbo 1989 Toyota MR2 built by a fella named Mike Townzen, who refers to the engine he built as a 4A-GTTZE.
The car started out life as a factory-stock supercharged MR2, with the optional 1.6-liter supercharged 4A-GZE inline four. Townzen, who has owned the car for more than a decade, became dissatisfied in time with the car's 145 factory horsepower, and despite increasing output with a supercharger overdrive pulley, he found himself wanting still more power.
Townzen initially built the car with a simpler twincharged setup, with a single turbo and the car's factory supercharger. A trip to the drag strip yielded a 13.9 second pass, more than reasonable for a 1.6L engine, despite "some terrible 60 foot times." His craving for horsepower was still unsatisfied and ultimately led him to integrate a second turbo into the 4A-GTZE's plumbing, which brought with it a variety of supporting modifications.
Air enters the engine through the larger of the two turbochargers, which feeds directly into the overdriven supercharger, compressing the intake charge for a second time. The air is then condensed by a combined water and alcohol injection system before feeding through an intercooler chilled by a pusher fan. Fuel dosed by an aftermarket ECU and provided by an upgraded pump and 680 cc injectors then joins the charge air in the intake manifold—where pressures can reach 30 psi below 3,000 rpm—before being sucked down to the intake runners and into the cylinders.
Spark is provided by a coil-on-plug system adapted from a Celica, igniting the mixture, which presses down on an entirely stock bottom end and upward on a cylinder head secured by upgraded studs. Power goes to the ground through the car's stock E51 five-speed transaxle and 245/45R16 rear tires, and exhaust gases depart through the smaller of the two turbochargers, which uses its own boost to directly drive the other turbocharger in a system known as compound turbocharging. Once through both turbos, exhaust gases exit rear and the cycle will repeat freely up to the 8,000 rpm limiter.
The limits of Townzen's so-called 4A-GTTZE have not yet been found. He has seen intake manifold pressures as extreme as 36.5 psi, which were immediately followed by clutch slip. Though it has not yet seen a dynamometer to confirm peak power levels, Townzen estimates the larger turbo can produce enough boost to push the MR2 to 450 horsepower on 40 to 45 pounds of boost—more than a handful in a compact 80s sports car that weighs just over one ton. Before he can push the little Toyota's limits for sure, he suspects the axle cages will need reinforcement, and is already laying plans to upgrade the supercharge to either a Sprintex unit or one from a Mazda Millennia.
Looking at his MR2's exterior, you would never guess you gaze at a car that gulps boost like an anaconda swallowing a mule. The car's rear bumper comes from what he suspects to be a junkyard Honda Accord (it's meant to hide the second turbo) and its headlight housings are homebuilt, holding HIDs. Only the fender flares were in any sense meant to fit the car, and even they required the factory body be chopped up to fit.
We're not looking to throw out any platitudes about looks being deceiving or big things in small packages; we just admire Townzen's dedication to his car and the craftsmanship he has put into it. We hope we can see the car on the dyno one day, and hear both the turbos and superchargers shriek in tandem under full boost.