Dale Amy
June 2, 2010

With crude oil being a finite and mostly imported resource, the search is well and truly on these days for alternative fuels to serve our energy needs. While some take an environmentally zealous, politically pious, or unrealistically utopian view about the preferred types of alternatives to gasoline, the more levelheaded amongst us would surely rank availability, viability, and affordability at the top of the criteria list when considering any such alternatives.

Those three pragmatic criteria have made propane pretty popular not just for heating our homes and cooking our steaks, but also as a transportation fuel, where it ranks third behind only gasoline and diesel. That said, we've never before seen propane used to fuel a drag car, but that's about to change.

For the 2010 NMCA and NMRA season, Susan Roush McClenaghan's She-Devil Motorsports-part of the Roush Drag Team-is fielding a freshly constructed '10 Mustang powered by a naturally aspirated Four-Valve modular fueled by liquid propane.

Why propane? "I was trying to find a way," Susan explains, "to tie our drag racing into the other parts of the [Roush] business, and this project demonstrates a number of those pieces-our engine build services, our liquid propane technology, paint and body services... The other reason for looking at propane is I want to take our race program and really have it stand out in terms of current events, so to speak, with emphasis on alternative fuels and concern for being ecologically responsible."

In the foregoing quote, Susan is referring to Roush Industries' recent forays into the business of engineering and supplying liquid-propane fuel-injection systems for the conversion of fleet vehicles (currently various Ford pickup and van platforms). The research and development invested in that fleet conversion program were harnessed in building her new race car.

The use of propane-more formally, liquified propane gas, or LPG-as a motor fuel has been around for decades, but Roush's fleet and race programs harness the efficiencies and drivability of liquid propane injection systems (older-technology agricultural and fleet propane conversions were vapor-based systems saddled with vaporizers/mixers and carburetors). Check out our sidebar The Science of Propane for more on the fuel itself.

Our mid-winter visit to the Roush race shops coincided with the first trial merging of the project's development engine and chassis, the latter having started off as a '10 Mustang body-in-white, now constructed to NHRA Super Stock specs. By comparison, Susan's previous two New Edge drag 'Stangs were simply converted production cars, so this purpose-built racer is a serious step up. Expect to see it in the NMRA's Modular Muscle class and some NMCA events.

The complete development engine in our photos is based upon a stock-displacement Ford GT aluminum block and heads, and is essentially an R&D tool used to develop, package, and test the propane fuel system. This engine has been run on the Roush brake dynos in two forms: initially with a normal gasoline fuel system in place; then, otherwise unaltered aside from recalibration of its FAST XFI processor, with the new liquid-propane injection hardware onboard.

Showing the viability of propane as a gasoline substitute, the engine produced similar power figures when fed either fuel-between 625 and 635 hp. By the time you read this, a second engine will be in place-the real race engine. It will have a similar aluminum 5.4-liter basis, but will be bored and stroked to about 374 cubes, with a target horsepower in naturally aspirated form of somewhere around 700. If the program proves successful, supercharging may be added for next season.

Our captions contain much more detail, but overall, the only real challenges to adaptation of a liquid-propane injection system for race purposes were in the specifics of the fuel system itself. That's to say that the engine internals-crank, rods, piston, compression, cams, and so on-are no different than if the modular were built to run on gasoline. Of course, the Roush team also needed to develop an efficient means of fueling (and de-fueling) the car at the racetrack, and even for fueling the engine on the dyno stand, as our photos show.

Will we ever see widespread use of propane in the Ford drag racing scene? We would guess it unlikely at least in the near term, but racing, like hot-rodding, is all about experimentation, so who knows?

Horse Sense: Roush Industries reports that propane combustion produces roughly 20 percent less nitrogen oxides and 60 percent less carbon monoxide than gasoline combustion.

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The Science of Propane
At normal room temperatures, propane is in a gaseous state, since its boiling point is a chilly minus 44 degrees (boiling point simply being the temp at which any given liquid changes "phase" to vapor). But much as automotive cooling systems are pressurized to raise the boiling point of their water/antifreeze mix, so is propane pressurized to keep it in a liquid state at typical ambient temperatures. Release it from that pressure and propane immediately vaporizes, expanding and cooling in the process. (Think of what happens when you open the valve to see what's left in your barbecue's propane tank.)

On older propane conversions for carbureted vehicles, that phase change to a gaseous state had to occur at the mouth of the carburetor, after which the mix of air and propane gas had to travel the length of the intake manifold runners and cylinder-head ports before reaching the combustion chamber. Without getting too buried in the science, these systems could be hampered by cold-start issues and had other inefficiencies.

Today's liquid propane injection systems, as employed by Roush, keep the propane in a liquid state right up until it exits the tip of the (specialized) fuel injector, immediately upstream of the intake valve. Like any form of fuel injection, this improves fuel efficiency but also helps make propane cold-start issues a thing of the past.

More propane tidbits:

  • Propane holds only about 86 percent of the energy per volume of gasoline, but this is partially compensated by its higher stoichiometric air/fuel ratio of 15.6:1 versus 14.7:1 for gasoline.
  • About 90 percent of U.S. propane is domestically produced; most of the balance comes from Canada.
  • Propane is a byproduct of both the processing of natural gas and petroleum refining.
  • Propane in gaseous form is heavier than air (about 1.5 times as dense) and therefore sinks and pools at the ground. However, liquid propane is significantly lighter than gasoline.
  • Propane has an octane rating of roughly 105 to 110, so will theoretically support higher compression ratios and more spark timing.
  • Propane burns cleaner than gasoline, meaning both fewer emissions-especially carbon monoxide-and cleaner engine internals.
  • Government incentives/tax credits help defray the cost of vehicular propane conversion.