McKeown 4.6L Mod Motor - Final Answer?

The debate usually goes something like this: "Carburetors cool the engine and injectors let hot air in."

"Yeah, well injectors are more precise and can shoot the gas right into the combustion chamber."

"Yeah, well carburetors are cheaper."

"Yeah, well injectors can run even if the car is vertical or upside down!"

"Yeah, well where is the oil in your engine if the car is upside down?"

"Yeah, well my dad can beat up your dad."

And so it goes.

Both fuel delivery systems have had many face-lifts over the years, but the question remains: Which is better? While building an engine for the 2009 Engine Masters Challenge, Mark and Heather McKeown of McKeown Motorsport Engineering decided to tackle that question once and for all.

The engine that MME used as their testbed was one of the millions of two-valve 4.6L Ford engines that was produced for Mustangs, Town Cars, trucks, and cop cars from 1991 through the present day. The exact specimen began its life powering a van through 200,000-plus miles of reliable service, and was thoroughly seasoned. Ford cast their blocks at two different plants and designated the skeletons as either a Romeo or Windsor casting. Different main bearings, timing gears, crank flange bolt pattern, valve covers, and cam cap designs individualize the engines, but both are equally capable in terms of durability in the 500-600hp range where most high-performance 4.6-liters draw the line from badass street/strip car to serious race engine. The McKeowns' block was bored, torque-plate honed, and fitted to a set of matching Probe H-beam rods and forged pistons via Clevite rod and main bearings.

Taking advantage of Ford's R&D efforts, Mark used the factory crank blessed with undercut and rolled fillets that are proven to reduce stress risers and increase overall strength. Many high-performance 4.6L builds use a Cobra oil pump with its steel gears and high-volume design, but with the factory standard-volume pump producing over 80 psi of pressure throughout the dyno pulls, Mark felt justified not upgrading his pump. He does stress, however, that good oil pressure is critical on 4.6L builds to let the hydraulic cam followers (i.e. lifters) work correctly.

For the carb setup, MME used a Pro-Systems modified Holley 950 HP. During testing, they wanted to check all the variables so they made sure to record inlet temp as well as fuel consumption, oil pressure, and water temp. Using these measurements, they could get a better view of the efficiency of the combination.

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.

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.

Asking Mark what the overall difference was between the carburetor and fuel injection, he says: "You know what, I didn't find anything between the two of them. You can overlay the carbureted version with the injected version, and it is so close to identical you could hardly tell the difference from one to the other. Even the air/fuel ratio was the same." Looking back at previous engines and data, Mark suggested that differences in the two setups and more specifically comparing individual cylinders' air/fuel ratios compared to power output, it was not so much whether or not the engines used a carburetor or EFI, but rather the design of the intake and exhaust systems and the way that they correspond to cam timing events. The pulses of air through the engine and out the exhaust have more of an effect on the engine's output than the method of fuel delivery.

When asked as a side question whether he had tried to run both a carburetor and fuel injection at the same time to take advantage of both systems' positive inputs, Mark's simple yet slightly evasive answer was "yes." Was it beneficial? "Yes." Take that as food for thought and discuss amongst yourselves.

White or wheat? Soup or salad? Carburetor or fuel injection? The answer might be as simple or complex as you want to make it, but in the end, it may be a matter of personal taste and the confines of your components.