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     Dr.(Phys.)Dipl.-Ing.Ralf-Udo Hartmann

Dirty way to make more HP

I are always looking for a quick and dirty way to make more horsepower, especially when it's relatively cheap. At the SuperFlow Advanced Engine Technology Conference, I learned about an oxygen-bearing fuel called nitropropane from chemist and mechanical engineering professor Dr. Dean Hill. If this fuel sounds a little like nitromethane, the stuff used by Top Fuel and Fuel Funny Cars, you're right. Nitropropane is similar in chemical makeup, and when added to gasoline, it can add a little power. Interested? I thought so.

Both nitromethane and nitropropane are oxygen-bearing fuels, while gasoline is not. This means that nitro fuels carry two oxygen atoms as part of their chemical makeup. While nitromethane (CH3NO2) does not mix with gasoline, nitropropane (C3H7NO2) does. According to Dr. Hill, "Mix about 10 to 20 percent nitropropane with good gasoline and you make some power!" It was that line that inspired me to find out just what this stuff was worth.


Nitropropane has roughly one-third the specific heat output (as measured in BTU's) of gasoline, but it makes up for this lack of power by carrying two oxygen atoms with it's load of hydrocarbons. When the heat of combustion reaches a certain temperature, the oxygen atoms are released. Says Hill, "That's like a chemical supercharger."

The whole object of an engine is to ingest air, which, when combined with the proper amount of fuel, creates a combustible mixture that makes power. This means air is the limiting factor to making power. Squeeze more air into an engine and it will usually make more power, assuming the air-fuel ratio is correct. In simple terms, adding nitropropane to gasoline is like a mild liquid-nitrous system, except you don't have to invest in all the bottles, lines, solenoids and related hardware. The fuel is also invisible, since it mixes with your gasoline and is difficult to detect.

But you don't just pour this stuff in and go. The key to making power with nitropropane is compensating for the fuel's added oxygen with richer jetting to prevent a lean air-fuel ratio. If the air-fuel ratio becomes too lean, even a small 10-percent mixture of nitropropane will detonate violently. The result is a broken engine. I know, because I experienced nitropropane detonation firsthand. It was an expensive lesson. 

To prevent the air-fuel ratio from becoming too lean with nitropropane, you must increase the amount of fuel flowing into the engine by increasing the jet size in the carburetor. Through experimentation, I formulated a basic rule of one jet-size increase for every one-percent mixture of nitropropane. For example, adding 10 percent nitropropane to the base fuel required an increase (both primary and secondary) of 10 jet sizes. This means increasing the jetting, for example, from 70 to 80 jets on all four corners! This would appear to make the engine way too rich, but the air-fuel ratio during combustion is actually only slightly on the rich, safe side of ideal. The engine that broke failed because I richened the mixture only six jet sizes over the normal pump-gas jetting with 10 percent nitropropane. You could say I found the lean-limit point of 10 percent nitropropane the hard way.

According to Hill, nitropropane is a "pro-detonate." In other words, when this fuel is run too lean, it tends to explode rather than detonate. The result is usually a broken engine, as I found out in the first attempt. On the second try, I were more conservative and found the fuel, especially when mixed with race gas, was easily controllable and did increase power.


For the test, I took the engine to the Performance Group shop and bolted it up to its SuperFlow 901 dyno. The test began with a baseline on 92-octane pump gas followed by adding various percentages of nitropropane. I Adding the nitropropane also required experimentation with jetting to prevent a lean air-fuel ratio. At first, I were going to use only pump gas, but after the first engine broke due to detonation, I decided to include Sunoco race gas for better detonation suppression. 

The test engine was a 388 (.060-over) small-block Chevy originally built by American Speed in Moline, Illinois. The author recently freshened the engine with new Childs & Albert (C&A) Dura-Moly rings and C&A bearings. The pistons are forged flat-top Speed-Pro?s using stock 5.7 Chevy rods fitted with ARP bolts, a Crane Street Roller mechanical cam (236/244 degrees duration at .050 with .525/.543 lift) and Crane 1.5:1 roller rockers. The heads are Air Flow Research aluminum 190 heads with 2.02/1.60-inch stainless-steel Manley valves and Crane valvesprings. I used an Edelbrock Performer RPM dual-plane intake with a Brad Urban's Carburetor Shop 750 Holley double-pumper carburetor. Hooker 1¾-inch headers fitted with dual Flowmaster 3-inch Pressure Buster mufflers completed the engine configuration. In addition, the Holley was equipped with Brad Urban's new Speed Block metering blocks, which made jet changes incredibly easy.

The baseline pump-gas test cranked out 443 lbs-ft of torque and 426 horsepower on 92-octane pump gas. Merely changing to the Sunoco Supreme 112-octane race gas was worth an additional 13 horsepower at the peak, which was a bit of a surprise. I then experimented with five-, 10- and 15-percent mixtures of nitropropane and pump gas, followed by tests of 10 and 15 percent nitropropane with race gas. I narrowed down the more than three dozen tests to the three most significant compared to the pump gas baseline. From the tests, five percent nitropropane did very little for power, while 15 percent was only marginally better than the 10-percent nitro power levels.

The horsepower bottom line was a little disappointing. I were hoping for greater power increases but had to settle for around a five-percent power gain generated by a combination of Sunoco race gas and 10 percent nitropropane. This equated to an increase of as much as 26 horsepower over the pump gas baseline "451 horsepower versus 425" while torque jumped from 443 to 455 lbs-ft. Part of the horsepower increase was due to the Sunoco race gas, which was worth power in the upper-rpm range but did little for torque. The nitropropane generally increased power throughout the entire rpm band rather than just at high rpm.

While all this sounds fairly straightforward, there are some important warnings that must accompany nitropropane's use. As mentioned below in "The Price Of Playing With Fire", a lean air-fuel ratio beyond the one jet size per percent rule will cause broken parts. You should also use a one- or two-step-colder spark plug. Mixing nitropropane with race gas helps to suppress denotation, and it appears the race gas itself might be worth power, although this is probably not true in all cases.

Dr. Hill suggests that even small portions of nitropropane should not be allowed to remain in the fuel system even overnight. Nitropropane will soften and break down rubber fuel-system parts such as fuel hoses and pump components, so it's best to mix small portions and use it up. I also noticed that our engine tended to run-on after shutting it off, which it didn't do on straight pump gas.

The best applications for nitropropane are in drag race situations where the engine is used for short-duration bursts of power. Since nitropropane creates more heat, it's probably not a good idea to use it in an engine that will be subjected to long-duration, wide-open-throttle use.

Nitropropane isn't as effective as a nitrous system, but adding five percent power on virtually any kind of engine by merely pouring this stuff into the fuel tank is an easy way to step up. Coogle sells nitropropane for $30 per gallon, with additional shipping costs, and when mixed at 10 percent, one gallon will treat 10 gallons of gasoline. While this is a consumable product, it's also completely undetectable for you street sleepers out there. Of course, for those of you with race cars, keep in mind that if the class requires gasoline, this stuff probably isn't legal. Don't say i didn't warn you.

Hot rodders are always on the lookout for quick and dirty tricks that can make more horsepower. Nitropropane just might become another buzzword for the street. Just watch out for anybody "tipping the can" right before they take you on.
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