The last time Project Pure Pontiac (aka The Mule) was on the dyno at RaceKrafters it made approximately 647 horsepower with the 3.6-inch (medium sized) pulley on the Gear Head Designs supercharger. If you recall we had no luck getting the engine to run with the intercooler. The carburetor went very lean and thus, the engine bucked and could not take the load. Bob Wise was running the dyno and quickly aborted the pull.
Since the concept of cooler charge air is valid, we wanted to go back to the dyno and try again with another CX Racing intercooler. Before we get into what HPP found during our latest test session, it would be prudent to apply some theory as to what was happening with the engine.
Much to the chagrin of the EFI crowd, in many ways the lowly carburetor is the ultimate air flow meter. When calibrated properly it does a remarkable job of delivering the proper amount of fuel in lock step with the incoming air throughput. As airflow increases so does fuel flow.
When discussing carburetor theory, the term signal is often used. It describes the low-pressure region that is created in the booster that allows fuel to flow from the float bowl and through the delivery circuits. The greater the difference in the atmospheric pressure, or in the case of a blow-through carburetor such as our Quick Fuel unit boost pressure on the float bowl and the low-pressure region in the booster, the stronger the signal. A strong signal allows for more fuel to be delivered but more important, improved atomization. A weak signal usually means less fuel delivered for a given jet and air bleed size, and poor atomization, a term that describes the fuel being broken into small particles but still in a liquid form. A practical example of this is the discharge from a spray bottle or aerosol canit is still in liquid form but very minute particles.
The best example of a poor signal is an engine with too large a carburetor. Contrary to what many believe, if the carburetor throttle plates and venturi are too large, the engine will go lean and not rich. This is due to the area being too great for the airflow into the engine and the resultant poor signal quality. Thus, little fuel is discharged.
What usually happens is the enthusiast does not have a full grasp on what is occurring and keeps increasing the jet size to fuel the engine. They then make the false assumption that the fuel pump is too small since no matter how large a jet is installed, the mixture can never be made rich enough under full power. So a 400 horsepower Pontiac ends up with huge jets and a fuel pump that can support a Pro Stock engine to compensate for too large a carburetor and poor signal strength. This was not the case with The Mule, but a twist on the theory.
When the original intercooler was installed the engine ran poorly and the air/fuel ratio went to 16:1. The key was the leaning of the mixture. That meant the signal in the carburetor went away. We knew exactly what was happening but not why, and the printing presses were waiting. The mystery would need to be solved another day.
In subsequent discussions with Gear Heads, we came to understand that we were actually performing the beta test for the possible integration of an intercooler into the companys Pontiac supercharger kit. To this point, the system was only offered with a chemical intercooler (methanol injection).
When our results were presented, the theory of using a smaller CX Racing intercooler, along with the smallest supercharger pulley (more boost), was suggested. We had refrained from going to the smaller pulley since the author was concerned with the load on the snout of the crankshaft. To reiterate, Craig Wise, who built the engine, did not share that concern. As we had mentioned in the previous installment, The Mule was not broken, and 647 hp at a mild boost level was quite an accomplishment.
But as hot rodders, we wanted to push the envelope. So a smaller intercooler was sent from Gear Heads to RaceKrafters. HPP made travel plans to be there when The Mule would once again be bolted to what was becoming an old friendthe Stuska water brake.
Craig filled the dyno fuel cell with what was left of our 100-octane Rockett Brand unleaded gasoline. It was the same fuel we used when we aborted our testing previously. HPP and RaceKrafters wanted to keep as many aspects of the test constant as possible.
The one thing we could not control was the weather. Whereas the last session had the dyno room ambient temperature at nearly 96 degrees F on our test day the weather was much better. The dyno cell was about twenty degrees cooler at around 74 degrees F.
Our protocol would be to attach the smaller intercooler with the medium size pulley and no methanol injection. Then based upon the results we would make a decision from there.
The Mule made a full pull with this combination but did not sound as clean, and was a little on the choppy side. Its mixture did lean out to an average of 13.483:1 for the entire run. That was about three air/fuel ratios leaner than the best previous pull (647 hp) without any intercooler. This was pointing to a loss of signal, but was much richer than the 16:1 we experienced with the larger intercooler. No jet changes were made to the carburetor.
With this setup, the engine was down substantially from the previous Snow Performance chemical-only intercoolerwe lost about 72 hp. The average boost pressure throughout the run was now 3.02 psi; before it was 4.84 psi.
With confidence that the engine would take the load, Craig now switched to the 3.25-inch pulley, the smallest one (we had not used this one before) and made a pull. The Mule had the small intercooler with the highest boost pulley. This would be the most aggressive combination we could employ while still using an intercooler.
The average boost pressure went to 4.40 psi and the peak horsepower climbed to 59255 ponies shy of the chemical intercooler and lower boost pulley. The average air/fuel ratio was about the same at 13.63:1, showing a flow loss.
Since that was the best we could do with the intercooler, the wrenches came out and the system was restored to its previous best state but this time with the high-boost pulley and Snow Performance methanol injection with the #3 nozzle. The Mule really liked this with the average boost pressure going to 6.648 psi and a peak of 8.62 psi at 5,100 rpm. Our Pontiac with factory casting ported 6X cylinder heads, a smog block with an over bore, and a cast-aluminum intake manifold rewarded us with an astounding 673 hp and 740 lb-ft of torque! The average air/fuel ratio was 10.03:1. And the best part of the whole thing was The Mule sung like an opera star. No muss, no fuss 673 hp from a pure Pontiac with no signs of discontent in any way whatsoever.
Where Did The Signal Go?
With the results now in hand, we can explore where the signal to the carburetor went along with the boost pressure.
To a student of engineering, air is considered a gas and follows some basic laws. The most dominant one being that it does not like to make turns, and the longer the flow path the greater the frictional loss. A simple tape measure solved the mystery of the lost signal and boost.
1 The original intercooler measured 24 inches across and was 12 inches high. It was not too large, but the amount of pipe to install it made it an excessive flow loss.
2 The replacement intercooler shared the same height but was 7-inches narrower.
3 The three pulley overall outside diameters offered by Gear Heads are 3.25-inch, 3.6-inch, and 4.00 inch. Where the belt actually rides on the grooves the diameters are 2.95-, 3.25-, and 3.8-inch respectively. For this story and last months, the pulleys are identified by their overall outside diameters. In the last issue, we mistakenly referred to the middle-sized pulley as having an overall diameter of 3.25-inch. We apologize for the confusion.
4 Craig confirmed the pulley diameter before each test, since it would have been easy to grab the wrong one.
5 The intercooler was hung on the wall and the dyno exhaust fan drew a good deal of air across it.
6 As you can clearly see, the pipe length to include the intercooler was very long.
7 The Mule loved the high-boost pulley and Snow Performance methanol injection, and produced 673 horsepower.
8 A larger supercharger would be required for a big cubic-inch Pontiac engine if the intercooler were desired.