Here we see the full compound forced induction 1MZ V6 installed in the MR2. The engine's V
We consulted with forced-induction experts Bob Norwood and Corky Bell. Bell ran some pressure-ratio and density-ratio equations and suggested some criteria for the right compressor and turbine for turbo-assisted supercharging. Norwood discussed compounded engines he'd worked with in the past, and made suggestions on compressor trim and turbine A/R.
We also consulted with Majestic Turbo in Waco, Texas regarding selection of specific turbocharging equipment. The TRD positive-displacement blower would knock out full boost the instant the throttle opened, and this would immediately generate enhanced exhaust energy and heat to spool a turbo. Of course, to achieve 500 whp, you'd need to make more like 575 to 625 hp at the crankshaft, which at a ratio of 10 hp per pound of air implies roughly 60 pounds of air per minute airflow required from the turbo compressor. The smallest Garrett-type compressor that'll possibly achieve this airflow is a GT-61 trim unit.
Centrifugal compressor performance is fairly predictable using compressor maps, but it can be difficult or impossible to predict turbine performance on a particular engine without a certain amount of testing. Unfortunately, it's largely turbine performance that determines the onset of measurable boost.
Corky Bell pointed out that 70 to 80 percent of the energy required to drive a turbine comes from the heat in the exhaust rather than the pressure. The energy that can go down the shaft to drive the compressor is a function of the absolute Turbine Inlet Temperature to the fourth power, minus the absolute Turbine Outlet Temperature to the fourth power.
Since the temperature drop through the turbine nozzle can easily be 200 degrees, there is prodigious heat energy available to drive a centrifugal compressor.
So we see that the turbine energy available to do work is dependent in a small way on exhaust pressure, and in a big way on exhaust heat (which is why insulating or coating exhaust plumbing on the way to a turbine can yield dividends in faster turbine response. It takes great energy to overcome the inertia of a turbo and to accelerate it extremely rapidly to full working speed.
Majestic Turbo of Waco, Texas, supplied the critical turbo system components for the compo
What's more, because centrifugal compressor air flow increases exponentially with compressor speed, the compressor must be turning 30,000-40,000 rpm before it's capable of significant pumping at all. A T-76 compressor, for example, can begin to make boost at as low as 36,000 rpm, whereas a T04E-60, for example, requires as much as 46,000 rpm shaft speed to make viable boost pressure.
Majestic Turbo supplied a honkin' huge T-76 turbocharger to make boost and a delta-type Racegate to limit total maximum boost delivered by both the blower and the turbo. The initial turbine setup was a Majestic P-trim wheel in a .81 A/R housing. Majestic's T-76 is a large turbo capable of delivering 90 lb/min of air at a pressure ratio between 2.6 and 3.2-good for supplying enough air for as about 900 horsepower. One would not necessarily expect such a big air pump to deliver much low-end boost, but the idea was to let the blower deliver the low-end boost.
Fabricating the Turbo SystemOne fine day in the late spring of 2002 we drove the supercharged MR6 onto a lift at Alamo Autosports in Arlington, Texas to begin construction of the turbo conversion-and immediately developed some new muscles hefting the T-76 into various possible placement locations in the Toyota's engine room.
Initially, we considered locating the turbo in the upper engine compartment above the transaxle and piping exhaust to it directly from both exhaust banks. We eventually decided to locate it behind the transaxle to the left of the engine near the rear-bank exhaust.
This made fabricating the exhaust system a much easier task for Alamo, while providing enough room behind the V6 for a 3-inch turbine discharge pipe that would plumb into a muffler and/or catalytic converter with passenger-side entry.