Compound Forced Induction, Part 2 Turbocharging The Ultimate "Mr6" Power-Adder Test-Bed - 1991 Toyota MR2

With these super-duty parts in the engine, we were confident the 1MZ would survive a lot of boost. But we also recognized that sooner or later the stock 1MZ valve springs would float and that valves and intake ports and cam profiles that flowed well on a 200-hp, all-motor engine were going to be a major restriction on a 500-plus hp compoundsupercharged engine.

In the meantime, we were eager to get some road-test results without grenading the engine, so we settled for a wastegate setting and a "Compound" MoTeC calibration that reliably delivered a reliable 425 flywheel horsepower.

In approximately 8,000 miles of driving, ranging from south Texas in the scorching heat of summer to northern California in the chill of fall nights, we found the compound-supercharged system completely reliable and streetable. Bottom-end felt big-block massive yet top-end developed with a turbo rush. The compound super-turbo MR6 even had its own sound.

According to Jerry Magnusen, who designed the 1MZ TRD blower kit, when a compound turbocharger is operating at full-howl-forcing tons of boost downstream into the TRD blower-turbo boost actually begins to drive the blower's rotor assembly like exhaust gases driving a turbine, actually putting power back into the supercharger drive belt.

And, indeed, we observed that as the turbo component of boost spooled up above 5-psi boost, the blower whine of the Eaton fades and what you hear is purely the whisper of turbo power. A lingering and unresolved question is what the overdriving turbo does to the parasitic drag of the supercharger if turbo boost is pushing energy back into the blower belt.

Turbo-Only Testing and Optimization
After we'd driven the modified car around Texas, we decided it was time to test the MR6 in turbo-only mode.

As you might expect, in each of the possible permutations of power-adder modes (none, turbo only, blower only, compounded with both) there were unique areas of the MoTeC air-fuel map the engine would encounter that weren't reachable in certain other modes.

For example, the turbo makes lower boost at full-throttle and 2000 rpm than you'll ever see with the supercharger at the same engine speed, yet the optimal turbo calibration in this range is not precisely the same as the N/A engine. The loading required to drive the supercharger has an effect, as does the exhaust backpressure of a turbocharger.

All of which meant that Bob Norwood had some dyno work to optimize turbo-only performance after we pulled the blower.

The first turbo-only dyno runs were U-shaped; the T-76 with .81 A/R was sluggish in the bottom end, and turbo-only torque did not surpass blower-only torque until 3500 rpm. At 4500 rpm, the turbo-only torque temporarily surpassed the compounded torque, and by 5700 rpm, turbo-only torque had fallen below that of blower-only torque, neither of which we'd expected to see.

With the blower off the MR6, we drove 1,000 miles from Dallas to Tucson, Ariz., in the turbo-only configuration, at which point we reinstalled the blower and continued on a road trip to L.A., after which we again removed the supercharger for a drive to Silicon Valley. We eventually returned 2,000 miles to Austin, Texas, uneventfully.

Throughout the trip, all systems were up to the best abuse various magazine editors and I could extract from on-highway flogging. The Clutch Masters multi-disc clutch performed equally well stuck in L.A. freeway traffic or under hard, race-type flogging.

Although the turbo was sluggish in the 2000 to 3000-rpm range, the 1MZ motor is not sluggish, and people who drove the turbo-only car loved the low-end all-motor torque combined with the rush of additional power and torque as the turbo spooled up to make 10 psi boost in the mid-range.