Inhaling coolness

» Posted by on Jul 6, 2012 in Current projects and Tech Notes | 1 comment

Inhaling coolness

The power that an engine produces varies with the temperature and pressure of the air going in. Lower temperature and higher pressure result in more power. The pressure increase from a ram air intake hardly makes any difference in power output below 150 mph or so, and the drag that it creates wipes out some of the power gain unless it is located and shaped really well. That leaves temperature and the pressure drop through the air filter as the two things that are worth doing something about.

The AIM EVO4 data logger in Freddie records the inlet air temp sensor output and the data logger internal temperature. The logger is located low in the car, forward of the dash bulkhead. Because it is in the shade, the data logger temp is a reasonable substitute for ambient air temp. I ran the March driver school without any ducting to the air filter because of time constraints, so the engine mostly inhaled air that had been heated by the radiator. Like that, the inlet air temp stabilized at 42°F above ambient. That cost me about 7.2% of the power that the engine could produce if it were breathing in ambient air. That’s a big loss! Fortunately, the school wasn’t about going fast.

I calculated that power loss by simplifying the math to: Power is proportional to absolute air temperature. Air temp in degrees Fahrenheit + 460 = absolute air temperature.

For the Buttonwillow race in May, I installed a nicely made fiberglass inlet duct from Motorsports Composites that connected the inlet in the roll hoop fairing to the air filter. My fairing is distorted at the bottom of the inlet opening, so there is a 1/4″ gap that allowed some of the heated air from the radiator to get inside the duct. Like that, the inlet air temp was 6°F above ambient. So, the duct was a huge improvement, but there was still a 1.1% power loss due to that temperature increase.

For the Fontana race in June, I covered the inlet duct with an absurdly thin gold mylar insulation film and taped the duct to the roll hoop fairing with yellow racer tape. The tape sealed the gap, so there was no more leakage from the engine bay into the inlet duct. Like that, the data showed that the inlet air temp and the ambient air temp were the same. Victory! It doesn’t get any better than that. So, tape those gaps!

1 Comment

  1. Are you far enough below your weight limit to take a bottle of liquid nitrogen along for the ride?