From issue "10/2005"
Letter titled - "They" say dumb things
Reader question:
I just bought a 2004 SRT-4 and I wanted to change the factory intake. I was thinking about a cold-air intake, but I have heard from a few people that a cold-air will work the same as a short ram because the turbo heats the air to a constant temperature when it passes through anyway. Is this true? If you could give me some advice I would appreciate it. SCC is sick.
Dave's response:
"That hot-air-equals-cold-air theory is a steaming pile of cat poo wrapped in a thin veil of truth. the only true part is the turbo will heat the air anyway. The steaming cat poo comes right about the time "they" start saying the air coming out of the turbo is the same temperature regarless of the temperature going in."
"In the simplest terms, the air temperature actually increases a fixed amount going through the turbo. So if the turbo normally sucks in 60-degree air and spits out 160-degree air, sucking in 100-degree air will make it spit out 200-degree air."
"In reality, it's slightly worse than that. Actual temperature change across a compressor can be calculated with this arcane formula:"
Temperature Rise = (Tin*[pressure ratio^0.238 -1])/Compressor efficiency
"To make the arcane formula work you have to use absolute temperature on the Rankine scale for the inlet temp, which means you take Fahrenheit and add 460 degrees. So 60 degrees F is 520 degrees R."
"Pressure ratio is absolute pressure (boost plus 14.7 psi atmoshperic) divided by atmospheric pressure. So if you run 14.7 psi of boost, the pressure ratio will be 2.0. Let's use that 'cause it's easy. Then let's use 0.73 as the compressor efficiency, since I was born in 1973."
"In that particular case, the arcane forumla insists that air will come out of the turbo 128 degrees hotter than it went in (for a total of 188-degrees F). Now, if we increase intake air temperature to 100 degrees, the temperature rise is 138 degrees (again, a total of 238 degrees F). Air in went up 40 degrees, air out went up 50 degrees."
"In other words, cold air is important in any car, but it's even more important on a boosted one."
