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

Various people advanced theories for the loss of turbo torque in the higher-rpm ranges. One was that turbine energy was insufficient to drive the big T76 compressor wheel in this range. Another was that we had some kind of mismatch between the turbine and compressor in terms of available exhaust energy vs. turbine speed. Yet another was, for some reason, ignition energy was degrading in the upper speed-density ranges (coincidental to the removal of the blower).

Without installing a turbo-shaft tachometer on the T76, we were shooting in the dark. But, in any case, we decided to focus first on improving turbo-only low-end spooling, and see what this did to the disappearing torque above 5000 rpm. We began looking for ways we might improve turbo-only spool-up in the 2000 to 3000-rpm range without devastating compounded performance in the higher end.

After considering a compressor change to a smaller wheel to reduce inertia and speed up the turbocharger, Majestic recommended swapping turbine housings to a .70 A/R and reducing spooling inertia with a patent-pending Majestic fastener in place of the standard nut on the turbine end of the turbo shaft.

In fact, the .70 housing is a twin-entry unit, and we are currently considering redesigning the exhaust feed to maintain separate exhaust pipes from the two banks all the way to the twin-entry turbine housing.

In the meantime, Ignition Solutions shipped us a set of six Plasma-direct performance coils, which can be installed as coil-on-plug devices, or with plug wires. According to Ignition Solutions, the Plasma Direct replacement ignition coil is designed "with built-in booster technology to amplify secondary spark current by 100 percent (two times as much as OEM) and release about four times as much energy at the initial moment of spark."

As of the writing of this article, we have not yet generated dyno results with the new ignition and turbo equipment. We are looking forward to further development of the turbo-only and compounded engine configurations, to measuring power losses in the blower-only configuration, to measuring turbine power losses of the turbocharger due to backpressure or other factors in the turbo-only configuration, to logging turbine speed, and to measuring temperature rise and intercooling efficiency associated with the turbo and the supercharger at various levels of boost.

We expect soon to get dyno runs of the turbo and compound engine at 1-psi increments to 20 psi with 93-octane Texas pump gasoline and/or unleaded race fuel. Stay tuned.

Pros & Cons of Roots Blower, Turbos & Compound SuperchargingRoots Advantages
* Instant boost on throttle application
* Good low-rpm boost (with by-pass valve) zero charge heating at idle and high-vacuum cruise
* Good and linear volumetric efficiency

Roots Disadvantages
* Excessive heating at higher boost (over 8-10 psi) due to reversion-pulsed turbulence of non-compressed air from the supercharger encountering higher-pressure previously compressed air in the intake
* May be inconvenient or impossible to intercool
* Robs some of the power it produces from the crankshaft
* Can be noisy

Turbocharger Advantages
* Excellent thermal efficiency to high-boost pressures
* Not limited to crankshaft speed (may lag behind, but can surge ahead with sufficient exhaust energy)

Turbo Disadvantages
* At least minimal delay in onset of boost
* Boost inaccessible at lowest rpm ranges

Compound Advantages
* All of the above
* Supercharger produces excellent low-rpm exhaust energy to spool turbo

Compound Disadvantages
* May be inconvenient or impossible to intercool supercharger section