The gasoline in your bike is hot. Not when it burns, but as it sits. Exposed to direct sunlight, in a black gas tank or red gas can, premix can reach temperatures in excess of 100-degrees. Even at its best, your gasoline will have the same temperature as the outside air (on a 95-degree day that means 95-degrees).
It has been proven in Formula 1, drag racing and airplanes that cold gas produces more power than warm gas. But, would motocross be different. The MXA wrecking crew went to the dyno to find out.
DYNO RUNS ON WARM GAS
The MXA test crew decided to test the difference between hot and cold gas by running them on the dyno. On a pleasantly warm day, we ran a 2002 KTM 125SX on the dyno with ambient temperature gasoline. We allowed the bike to warm up for a long time and ran several test passes on the dyno to insure accuracy. Using a liquid thermometer, we checked the temperature of the fuel before each run. It was 86-degrees.
Here are the baseline numbers using 86-degree gasoline:
6000 rpmÿ9.2 hpÿ
8000 rpmÿ19.9 hp
10,000 rpmÿ30.0 hp
11,000 rpmÿ32.7 hp
11,500 rpmÿ31.8 hp
12,000 rpmÿ28.7 hp
12,500 rpmÿ23.4 hp
13,000 rpmÿ16.6 hp
Most significant about the first set of dyno runs was that the KTM 125SX produced power above 30 horsepower for a range of 1800 rpm (from 10,000 rpm to 11,800 rpm). That is an impressive amount of power (especially for a warm and humid day).
DYNO RUNS ON COLD GAS
Immediately after the warm gas baseline runs, the gas tank and float bowl were drained and 50-degree gasoline was poured in for the next test. To get the gasoline cold, we packed it in an ice chest and left it there for approximately two hours. The temperature difference between the warm gas and cold gas was 36-degrees.
Here are the baseline numbers (and warm gas comparisons) using 50-degree gasoline:
6000 rpmÿ9.2ÿ………(9.2 warm)ÿ
8000 rpmÿ20.0ÿ(19.9 warm)
10,000 rpmÿ30.0ÿ(30.0 warm)
11,000 rpmÿ32.7ÿ(32.7 warm)
11,500 rpmÿ32.6ÿ(31.8 warm)
12,000 rpmÿ29.0ÿ(28.7 warm)
12,500 rpmÿ23.7ÿ(23.4 warm)
13,000 rpmÿ16.4ÿ(16.6 warm)
The cold gas produced horsepower above 30 horsepower for the exact same 1800 rpm range as the warm gas (from 10,000 rpm to 11,800 rpm). Additionally, both the cold and warm gas made the same peak horsepower (32.7).
Looking at those figures, you would think that cold gas didn’t produce any significant power difference, but you’re wrong!
With 50-degree gasoline in its tank, the 2002 KTM 125SX produced over 32 horsepower from 10,600 rpm to 11,600 rpm. That means that for 1000 rpm in the meat of the powerband, the cold gas was exceeding 32 ponies. By comparison, the warm gas only exceeded 32 horsepower for 700 rpm (and for the 300 extra rpm that the cold gas was pumping out 32-plus, it was making as much as one horsepower more than the warm gas fed engine).
WHAT DOES IT ALL MEAN?
There are three conclusions to be drawn from MXA’s gasoline refrigeration tests:
(1) Cooling the gasoline did nada for the horsepower from 6000 to 11,000 rpm. It added nothing to the bottom or midrange.
(2) Cold gasoline works best at heat ranges where fuel is most unstable (high rpm). Once the KTM 125SX reached peak power and rpm, the cold gas stabilized the power and maintained it longer and stronger.
(3) Unfortunately, because there is no such thing as a refrigerated gas tank, cold gas will return to ambient temperature within ten minutes (aided by heat generated below it by the top-end and exhaust pipe). Thus, keeping your gas as cold as feasible before the start of a moto will only offer a small horsepower advantage on the start and for the first few laps. However, it will be an advantage.