If a large compressor is turning too slowly but making too much boost, air can suddenly change direction and explode backwards through the compressor. That momentarily releases the head of pressure produced in the compressor discharge, sounding like a series of minor explosions and air disruptions.
Anything, like an overly restrictive air cleaner/intake tube, that decreases air pressure at the compressor inlet can intensify any tendency to surge. We realized the 6x5-inch conical K&N air cleaner we'd jammed into the MR2's driver's side engine air intake was more appropriate for a 225-hp naturally aspirated engine than a 600-hp compound monster.
Final Dyno Testing
On the Alamo Dynojet, with five gallons of 118 octane in the tank, we installed a Majestic T-61 turbocharger. Since the T-70 suffered surge problems and was operating within 5 percent of the surge line at high boost in the 5000- to 6000-rpm range, we needed a smaller turbo. The candidates were the T-66 and T-61. Limited to a threshold of 65-percent thermal efficiency, the T-61 was good for 640 flywheel hp and the T-66 for 680 hp. Limited to 60-percent efficiency, the T-61 could manage 670 hp, while the T-66 could do about 710 hp.
With the MR6's excellent air-to-water intercooler, on the day we could return compressed-air temperatures to that of the tap water we were using for cooling, we decided to go with the T-61, which would provide excellent response and fast spooling.
We hooked onto the dyno again with the turbo-only application. In one and a half days, we ran 46 dyno runs, with Alamo owner and chief tuner, Brice Yingling, at the laptop searching for the optimal fuel and timing. With an additional spring in the Majestic wastegate, the first run (no electronic wastegate controls) made 325 whp at 9 psi boost, which is an estimated 375 to 410 flywheel hp.
We methodically worked our way to 22 psi of boost, working with the dyno, the dyno's wideband air/fuel logging, and the Motec M48's internal logging capability, and making changes to fuel, timing, boost and other factors.
At 22 psi, we still didn't have enough wastegate spring to keep the gate closed against exhaust pressure as boost increased further, and we were forced to again begin adjusting the wastegate spring compressor bolt to clamp harder on the spring (simultaneously limiting wastegate travel). With Brice's tuning, the 1MZ managed 455 whp at 22 psi, an estimated 525 to 575 flywheel hp.
Pushing on further, the trick was to dive under the car on the dyno, adding additional turns to the wastegate adjustment bolt, and then make a run with fingers crossed, ready to jump off the throttle on the next dyno run if boost ran away.
Boost never ran away. As we pushed to 28 psi boost, the turbo was out of juice, and we stalled at just below 500 whp. So we disconnected the air-cleaner/air-intake system entirely from the turbo, directing a fan to blow cool air toward the compressor inlet blades under the car. After a few more tuning runs, we clamped on the wastegate bolt, mashed the gas and recorded the results.
The car made a mighty groan as rpm pushed through 3000, and then the engine suddenly went nuclear and torque increased 300 lb-ft in the next 2000 rpm. When the run was over, we looked on the Dynojet computer monitor, and the mighty turbo 1MZ had managed 537 whp at 5400 rpm on 28.5 psi of boost.
We bolted on the TRD blower kit with the wastegate still clamped shut. This time the engine was much stronger, ahead by 20 to 40 whp most of the way up the speed range, until compound power stabbed steeply through 500 whp just above 4500, and Brice aborted by 5000.
Backing out on the wastegate bolt, we did several more runs, managing 524 hp across a broad range, from 5500 to nearly 7000 rpm-and doing it on 23 psi of boost.
Then we substituted the Random Tech cat and Borla muffler. On the dyno, the power was down from 525 to 478 whp. That was 47 hp, or less than 10 percent, with most of the loss due to the cat. Five or 10 percent wasn't bad, leaving the car still capable of 550 to 600 flywheel hp at 23 to 26 psi of boost.