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

In the last installment, we stuffed a big 1MZ-FE 3.0-liter Toyota V6 into our project '91 MR2 Turbo. We then installed a TRD Eaton-based blower kit and then bitch-slapped the V6 to 325 hp with the help of a MoTeC engine management system.

In Part 2, we will yank the supercharger, add a custom air-water turbocharging system, recalibrate the MoTeC M-48 computer, then hammer it on the dyno turbo with both glamour power-adders making boost-compound forced induction style.

Historically, staged forced induction has been used for super-high boost applications where the required pressure ratio for a single compressor is too high for reasonable thermal efficiency, or where sufficient boost is impossible with one compressor. Achieving, say, 50 psi boost with one turbocharger may require an enormous centrifugal compressor section and may result in excessive charge heating and significantly increased intercooler loading-and could be in great danger of surging, depending on total engine air consumption.

With twin staged compressors, an equal amount of air might be supplied to the same engine at the 50 psi level with greatly reduced heat of compression due to improved thermal efficiency, with the added benefit of improved spool time and responsiveness.

Our plan was not necessarily to run extreme levels of total boost (though we wanted that potential); it was to combine turbocharging with supercharging to get the best of both technologies. The great strength of a root-type Eaton supercharger is it provides instant boost when you open the throttle, avoiding turbo lag entirely.

A positive-displacement root-type supercharger provides linear increases in pumping volume at the cost of maximum boost, achieving reasonable thermal efficiency in the 1 to 8 plus psi range (though inferior to that of the centrifugal compressor in a turbocharger or centrifugal blower).

For our application, a big turbocharger would take up where the Roots blower left off, compounding 5 to 10 psi Stage One supercharger boost to virtually any level of combined boost required-with reasonable thermal efficiency.

Perhaps our biggest compounding challenge was going to be the following: How much boost could the 1MZ engine tolerate without detonating on 93-octane premium pump gasoline? Remember, the supercharger is located inside the intake manifold, and can't, for all practical purposes, be intercooled.

Corky Bell, owner of Bell Experimental Group in San Antonio, Texas, and author of technical books on turbocharging and supercharging "Maximum Boost" and "Supercharged!", calculated that an Eaton supercharger running 5 psi boost (.33 pressure ratio) ideally provides a temperature increase of 8.5 percent; applied to an ambient temperature of 100 degrees F, the unit would heat the 100-degree air to 148 degrees. Correcting for the Eaton running as much as 62-percent efficiency, the blower will actually raise 100-degree ambient air to 177 degrees.

A maximum-performance MR6 requires the intercooled turbo sub-system of the compound forced-induction system compensate for the blower's thermal heating by keeping charge air at or below ambient temperature at 15 to 20 psi boost (prior to supercharging). All this with up to 500 whp worth of airflow and negligible pressure drop through the intercooler.

For this, we were going to need a giant custom air-to-water intercooler. An air-to-water design would allow us to optimize the layout of the air cooler separately from its water-cooling radiator (particularly valuable on a mid/rear engine vehicle).

We decided to locate the intercooler in the MR2 trunk behind the rear engine firewall. Alamo Autosports siamesed two air-air cooler cores and boxed them with sheetmetal to contain liquid coolant. The goal was zero pressure drop at 500 whp with high efficiency.

Sizing turbos and blowers correctly is critical to extracting the maximum performance when compounding two compressors in a series as a compound or staged system.