Craig and Bob Wise prepare the Mule for its supercharged dyno test.
Now that we have discussed the theory behind supercharging and introduced you to Gear Head Designs’ supercharger kit for Pontiacs in the primer article, we can move on to preparing our Pontiac engine for supercharging, setting up the dyno, and dyno testing.
When contemplating upgrading to a supercharger, many aspects need to be considered, not the least of which is that the engine needs to be blower friendly. Here are a few points to consider before you bolt on a supercharger kit, and how we approached them with the Mule:
Is the Pontiac mechanically sound enough to take the added cylinder pressure?
Prior to bolting on the blower we had performed a leakdown, cranking and running compression tests, and a bubble test on the Mule, so we knew it was healthy.
What is the compression ratio?
Project Pure Poncho was built with a 9.1:1 compression ratio, so in theory it would be able to accept moderate levels of boost. If your engine is 10:1 or greater with iron heads, then it most likely would not be the best idea to put a supercharger on it, even if you limited the boost to 5 psi. The only exception would be if your Pontiac had a bulletproof rotating assembly and you were willing to use 100-octane fuel.
Is the rotating assembly (pistons, rods and crankshaft) up to the task?
A supercharger places more load on the snout of the crankshaft and the front main cap. It could take up to 100 hp or more to turn a blower at high rpm and boost levels, and this needs to be recognized. Any weakness there will become apparent very quickly, as the crankshaft will see a load in a region it’s not designed for.
Our forged rotating assembly was not a source of trepidation. It’s comprised of stout pieces, and though it’s meant for a normally aspirated application, we felt confident in its durability when exposed to moderate boost pressure. The only concern is that when bad things happen, they do so very quickly in a forced-induction engine. If the tune-up wanders near the line, it would be very possible that a ring land would disconnect from a piston and wreck the engine. So keep this in mind when using forced induction—safety trumps power when it comes to the tune.
Is the cam profile good for a blower?
Superchargers are sensitive to the overlap of the camshaft. If both valves are kept open, the boost pressure goes out the exhaust instead of being trapped in the cylinder. A cam profile that works great on a drag engine will usually have poor results with a supercharger. The Mule was designed with a cam grind of minimal overlap, since it is meant to be street friendly. It should be able to build boost easily.
1. A normally aspirated carburetor...
1. A normally aspirated carburetor fuel pressure regulator makes no provision for a rise in output under boost, so an Aeromotive boost-sensitive fuel pressure regulator was employed and the output set at 7 psi. (A boost-referenced fuel pressure regulator does come with the Gear Heads kit, but we already had one for the dyno, so we used it instead.)
2. An Aeromotive high-flow...
2. An Aeromotive high-flow fuel pump that could supply a sufficient volume at the higher pressure was also fitted.
3. Corey Porter did the plumbing,...
3. Corey Porter did the plumbing, since a return line to the fuel cell from the regulator was required.
Is the fuel delivery stout enough?
A typical dyno fuel system was not going to be sufficient for a forced-induction application. The fuel pressure regulator needs to be able to add one pound of fuel pressure for each pound of boost. Likewise, the fuel pump must to be able to maintain the volume at the higher pressure. A standard carburetor uses 6-7 psi of fuel pressure and according to Gear Head Designs, the three pulleys they sent us would be approximately 5, 10 and 15 psi. Thus, a fuel pump that would maintain flow up to 22 psi (7 plus 15) would be required, along with a boost sensitive pressure regulator. We installed both.
Will a new carburetor be required?
The Gear Heads kit is a blow-through design—the carburetor is pressurized, so a standard version will not work without a good deal of modification. (This is in contrast to a draw-through system that places a plenum for the supercharger to provide forced air under the carburetor.)
To resolve this issue, we turned to Quick Fuel Technology (QFT) and the company’s 750-cfm blow-through street/strip carburetor. It features blower- specific annular discharge boosters with additional holes to guarantee enough fuel flow. The metering block is machined to accommodate larger main wells. Other changes are to the throttle body. It has relief slots to direct fuel to the intake manifold and not out the shaft bushing. In addition, the throttle shafts are round instead of the normal flat stock. This is to prevent bending under boost pressure. A Nitrofil float replaces a brass style that may crush under pressure. The power valve has no back-pressure blow-out protection, since that would not allow it to work with forced induction.
Will the ignition need to be upgraded?
The ignition system needs to be strong enough to arc the spark plug under the higher cylinder pressure. A street car would require a boost-retard system that we did not employ on the dyno, since we were only testing under full load. It’s not just to keep abnormal combustion at bay, but also as the VE is increased, the turbulence in the cylinder improves and less timing is required to make peak power. A boost retard system would allow the engine, when used on the street, to enjoy an efficient spark advance curve under vacuum to atmospheric pressure, and then remove a minimum of 1 degree advance for every pound of boost.
Do I need a different spark plug heat range?
You want the spark plug to not hold an excessive amount of heat, and be responsible for igniting the gasoline prior to the discharge of the ignition coil. Identify the heat range of the spark plug that is in the Pontiac now, and then go a few steps colder to help keep abnormal combustion away.
Craig used a special tool to confirm the heat range of the plug that was already in the Mule and then went two heat ranges colder. The Pontiac had Autolite 26 plugs; he switched to Autolite 24, and tightened the gap to 0.035 from 0.045. The wider gap makes the engine more prone to misfire due to the added turbulence and pressure from the blower.
4. The Snow Performance Stage...
4. The Snow Performance Stage 2 Boost Cooler water-methanol injection system was mounted on the wall of the dyno cell. It employs “manifold boost pressure to determine when and how much fluid to inject,” according to the company. This improves cooling efficiency, which results in better performance over a broader rpm band.
5. The Snow digital variable...
5. The Snow digital variable controller allows the user to tune the injection delivery and boost pressure for it to evoke.
6. The kit comes with three...
6. The kit comes with three injector nozzle sizes 3, 5, and 6. We used the nozzle marked number 3, which is rated for 175 ml/min.