Though the lobes look huge,...
Though the lobes look huge, they actually have a fairly small base circle for the design. Any alteration to the base circle forces the engine builder to change the valve tip length in order to maintain proper geometry. The roller followers float on the valve and lifter tips, and if geometry is wrong, the cam will spit them out.
Ignition on a 4.6L engine, like an LS or late-model Hemi, is designed for a coil-on-plug, or coil-near-plug system, but Mark took a different route. Designing his own beltdriven distributor, he used MSD components such as the cap and rotor and HVC ignition coil to send fire through the spark plugs. An MSD Flying Magnet crank trigger sent a signal to the XFI computer where Mark generated the most efficient ignition table. The signal was then sent over to an MSD Digital 7 ignition box where the fire was finally released to the coil. This setup was used for both the carbureted and fuel-injected setups as it provided complete accuracy and repeatability. Another benefit to this design is that with a simple swap to a 36-1 or 60-2 crank trigger wheel (or a cam position sensor) XFI allows for individual cylinder timing and fuel changes to be made for ultra fine-tuning. With fuel injectors plugging the open holes in the intake but not obstructing flow, they ran the engine through its paces making a number of dyno pulls at various rpm levels with Shell 91 octane pump gas.
Edelbrock's Victor Jr. for...
Edelbrock's Victor Jr. for the 4.6-liter was virtually unmodified for the engine. Though its design, by necessity, has some sharp bends in order to have any hood clearance, it clearly did not make the carbureted test suffer as expected.
Since they wanted to use this engine for the Engine Masters Challenge, they chose the 3,000-7,000 rpm range as their final comparator. Over such a broad range, typically people have claimed that a carb might make slightly better peak power but the EFI setup would trump it for overall average torque. Would that be the case here? Time would shortly tell.
The cooling effect of the carburetor was validated as the McKeowns measured a 25-degree drop in temperature from the ambient air in the dyno room to the inlet tract. A drop in temperature like that should show up as an increase in power, as the air/fuel charge would end up denser than the outside air. More oxygen equals more fuel that can be burned, which equals more power. Fuel usage was quite efficient as the carefully tuned engine was dialed in to a 13.5:1 air/fuel ratio. With those numbers and a good baseline torque/horsepower curve in the bank, Mark swapped the fuel line to a high-pressure feed, and hooked up the FAST XFI.
COMP Cams' adjustable cam...
COMP Cams' adjustable cam gears for the 4.6-liter use six bolts to positively hold the gear in place. Fortunately, all it takes is removal of the valve covers to adjust cam timing, and not removing the whole front of the engine.
Like many EFI units, the FAST system controls fuel and spark over the operating range of the engine, but those are just the basics. XFI will allow the user to run up to four pre-programmed Qwik-Tunes that can be swapped out at the flick of a switch, control four stages of nitrous, and has easy Windows-based tuning with their C-Com software. It can be used with standard distributors, coil-on-plug and coil-near-plug setups, crank triggers, and just about any possible engine combination using up to 16 injectors and making up to 4,000 hp.
MME modified their Holley 4150 by adding a Throttle Position Sensor to it so they could use it as their throttle body and eliminate any question of whether the design of the throttle body/carb would alter power output. After inputting the basic dimensions of the engine into the C-Com software, they fired it up and were pleased with how easy it was to get it dialed-in and start making pulls. Typically with EFI, the easiest way to start is with the base map generated by the ECU, which is what they did. This was followed by short pulls to increasing rpm levels while making small changes and verifying that everything was copacetic. By easing into dyno pulls, it was no problem to dial in part-throttle and tip-in performance, which is where most tuners start to run into trouble. With years of EFI tuning and the easy XFI software, it was no problem for Mark. Once the fuel map was roughed in, they spent a good deal of time with full-range rpm pulls getting everything fine-tuned.