Further Considerations

There were a few other things that we integrated into our testing that may or may not apply to a street application.

HPP wanted to identify the discharge temperature of the air from the supercharger and also in the plenum of the intake manifold. We would know the dyno room temperature, and then with the other data, we would be able to confirm if the blower was working in an acceptable adiabatic range. The influence of an intercooler was also one of the test protocols. Craig placed the second temperature probe under the divider in the plenum of the intake manifold so that it would be accurate and not disturb fuel or airflow.

Our plan was to run the engine without any cooling effect for the air and then introduce two different methods. The first would be a Snow Performance Stage 2 Boost Cooler methanol injection system. It includes a 3-quart reservoir, UHO (Ultra High Ouput) electric pump, different flow rate nozzles, a programmable Digital Variable Controller to tune the amount of injection, installation hardware and instructions, and an 8-ounce bottle of Boost Juice concentrate (mix is 50/50 water and methanol).

The theory of the methanol injection is to provide a cooling effect to the charge through the latent heat of vaporization. Methanol and water will evaporate, and this process removes heat from the air. It is the same basic theory that allows a living being to perspire and be cooled on a hot summer day. The controller allows the amount of methanol sprayed to be varied by the duty-cycle of the pump, along with a set pressure for boost.

A traditional air-to-air heat exchanger plumbed in between the blower discharge and the carburetor hat was to be the next method of intercooling. The methanol injection system would be turned off. Gear Head Designs provided a CXRacing intercooler that measures 24 inches wide by 12 inches tall by 4 inches thick.

We felt it was important to try two different cooling methods and to be able to identify an actual heat drop with a thermocouple. In our use, the engine would only see short bursts of peak power on the dyno. A street/strip Pontiac with this kit would have to live under more severe conditions with a closed hood and high ambient temperature. This would almost make some style of cooling necessary for the best results, but based on Gear Heads data, the system can be used safely without any charge air cooling.

The Results

The Mule fired right up with the QFT blow-through carburetor and idled perfectly with the load of the Paxton Novi 1500 blower. With the engine running, we visually checked the belt tension and all the brackets for movement. Everything looked fine, but subsequently the belt came loose as it stretched and needed to be tightened during our dyno session. This is not unusual with a new belt on a fresh installation.

RaceKrafters did install a linear air/fuel meter, since we would not be able to monitor brake specific fuel consumption (BSFC) due to the need for two flow meters when using a return fuel system. The boost sensitive regulator employed a return line to the fuel cell that a standard fuel pressure regulator does not.

We did immediately notice that the supercharger carburetor was rich at idle with an 11.3:1 air/fuel ratio. We were told by QFT that it was set this way, and Craig knew he would be able to lean out the idle circuit if it were a street engine. For the dyno, the rich mixture was just fine. We were not going to idle it long enough to do any harm or foul the plugs.

Our first test would have the 4.00-inch-diameter pulley (the least boost), no air cooling, and 92-octane pump gas. The specification predicted 5 psi of boost, so the timing was retarded from its previous 32 degrees BTDC to 27 degrees BTDC. Another full pull would be done with this setup, and then any tuning changes or decisions would be made.

The third protocol would be to introduce the methanol injection with this calibration, and then shut it off and move up to the next smaller pulley (3.25 inches). The smaller pulley was supposed to produce between 8 and 10 psi of boost. It would be tested with and without the chemical intercooler. Depending on the results and the engine’s health after these tests, we could then go to the smallest pulley and perform all the steps over again.

With the first full pull, it was easy to see the Mule loved the Gear Heads supercharger. It made a peak torque of 632.1 lb-ft and 596.1 hp on pump gas with an average boost pressure of 3.609 psi and a maximum of 5.66 psi. The QFT carburetor was very rich (which means safe) under full load with an air/fuel ratio of around 9.7:1. The engine had 27 degrees of total timing.

The maximum blower discharge temperature was 148 degrees F with a dyno cell of 96 degrees F. With no cooling, the plenum temperature was 88 degrees F. The rich air/fuel ratio was acting as a chemical intercooler and doing an excellent job of removing heat from the compressed air. This is the real benefit of a carburetor over an EFI system. The phase change of the fuel in the plenum provides cooler charge air. The temperature of the charge was actually 8 degrees F lower than the ambient temperature in the dyno room.

After a backup pull, with such sterling results and nothing broken, we decided to leave the tune alone. The Pontiac made more power than we had estimated, and we were very happy with the rich mixture. We did not want to risk hurting a piston by going too lean.

The next step was to evoke the methanol injection system. With it on and timing bumped to 30 degrees, the air/charge temperature only dropped about 2 degrees F to about 86 degree F, and the power stayed about the same. This is no reflection on the system, but is a result of the efficient annular discharge booster in the QFT carburetor and the extremely rich mixture. If we would have leaned the carburetor, then the methanol injection could be used to bring the air temperature back down, and a more aggressive tune-up and higher power could be realized. But we felt it unnecessary to push the envelope for just a few more ponies.

Satisfied with the performance, we moved to the 3.25-inch-diameter pulley and higher boost. For this test, Craig drained the fuel cell and poured in 100-octane Rockett Brand street gas. We did not know how much boost the smaller pulley would make and we wanted an insurance policy.

We thought the Mule was stout before, but the new pulley ratio really woke it up. With this setup, the discharge temperature from the blower went up to 208 degrees, and with no chemical intercooler, the plenum air temperature went up to just around 90 degrees F.

The cooling effect of the rich mixture was still in play, but the engine did pop a little at around 4,500 rpm since it went even richer than before. We could not get a real clean pull. Craig decided that we would need to lean the carburetor slightly and replaced the 74/82 jets with 72/80 jets. The timing was still at 30 degrees and the methanol was turned on for safety with the elevated discharge temperature from the blower.

Project Pure Poncho returned the favor of a slightly leaner mixture (10.5:1) with peak torque of 680.2 ft-lb and 647.4 hp! This is from a stock ’75 block, 6X head 455. The plenum temperature with the Snow Performance injection came down to 82 degrees F, and the blower discharge went up to around 212 degrees F.

Concerned about the cylinder pressure and the stress on the crankshaft, we did not try the smaller pulley. The final test was to install the air-to-air intercooler with the 3.25-inch-diameter pulley.

With everything plumbed in place, we started the pull, and then at 4,000 rpm, the engine went lean (16:1) and started to buck and pop. Bob was running the dyno and he quickly got out of the throttle. After spending an hour trying different things, the engine would not take any load with the intercooler in place. Craig put it back to the way it was and it made 647 hp again with no fuss.

Paxton and Gear Head Designs believe the resistance from the intercooler was robbing boost. Their recommendation is to run a smaller pulley to increase the boost and/or run a smaller intercooler. Gear Head Designs has a smaller CXRacing intercooler on the way for a future test.

Regardless, we had a solid day of testing with very good numbers and no breakage. There is no doubt that the Gear Heads supercharger kit is an excellent system. When installed on a properly prepared engine, it can produce power and reliability that was previously unheard of. Regarding the Mule, it allowed an already great performing engine put out an additional 150-plus horsepower and 125-plus lb-ft of torque on a conservative tune—quite an accomplishment for a bolt-on kit.