We set about to discover if this was true in a couple of ways. First, we tapped into the stock computer using an OBD-II tool. This allowed us to monitor the temperature the engine itself was seeing via the intake air temperature sensor located in the intake pipe. We also mounted thermo-couples on the car's roof to measure ambient air temperature, under the front bumper on the driver's side to measure the air temperature at road level, and inside the engine compartment near where all the short-ram systems placed their filter to measure underhood temps.
The first difference was obvious. Even at speed, the temperature under the hood was significantly warmer than the outside temperature. On the day we tested, it was a nice, comfortable 78F day. However, under the hood it was positively Phoenix-like, averaging 110.1F (ranging from a high of 116 to a low of 105). Note these temperature readings were taken at 80 mph, with plenty of air circulating around the engine bay, but they were still far above the ambient outside temperature. What's more, when the car was at idle (say, at a stoplight), underhood temperatures would skyrocket, sometimes shooting well over 130F.
How did this translate to the intake temperature itself? Well, pretty much as you'd expect. We tested a representative sample of the short-ram and cold-air intakes (AEM's short-ram and DC Sports CAI, respectively). The designs of the intake systems vary only in details, and we were more interested in the temperature of the air they inhaled, not the power they created at this point, so testing each system would have added needless complexity to the test. The AEM short-ram drew in air at a two-way average of 123.5F, 7.5 warmer than the underhood air itself. The DC Sports CAI, on the other hand, had an intake air temperature of 98F, 10 cooler than the underhood temperature for those runs. In addition, the CAI was much more consistent, running 98F on both runs, while the short-ram varied wildly, averaging 127F on one run and 120F on the second, a 7 difference.
We also tested Injen's intake system because it comes with a stamped-steel heat shield designed to block heat from the nearby exhaust manifold. We bolted it in place, made our passes, then quickly removed it and made two more passes. We were fast enough with the wrench that the outside temperature didn't change at all. However, the results were interesting. The underhood temperature reading was actually higher with the heat shield in place, but the average intake temperature itself was lower by 3F.
So, what do all these temperature readings mean? Well, it turns out cold-air intakes actually do draw in cooler air than underhood short-ram intakes do. So what? Well, as air cools, it gets dense. Denser air means more air getting to your intake manifold. Even with the effects of heat soak on the intake pipe itself, the air from the cold-air intake was still significantly cooler than the short-ram. Remember, we are reading the intake air temperature from the car's sensor, so we are seeing what the car sees. Clearly, as far as intake air temperature goes, the cold airs have an advantage.
However, the short-rams were actually capable of inhaling more air during our flowbench tests. Will this greater flow ability offset the cooler air of the CAIs? Only the dyno will tell.
On The DynoLet it be said here: Testing the RSX Type-S on a chassis dyno is a pain in the ass. Consistency is very difficult to achieve, with the engine computer constantly switching between six different fuel maps, six different ignition maps, and who knows how many different maps for the VTC part of the i-VTEC mechanism. Integras with a B18 were easy: Just strap them down, hit the go pedal, and out pops your reading, time after time. With an RSX, you can be up one minute and down the next without even touching the car.
It is for this reason that so much variation is seen in intake tests. Some systems have been reported to have astronomical numbers; however, the testing procedure itself must be called into question. Like we said, this isn't the same as the B18. If you're going to test an intake system, for example, you can't do your baseline a week in advance. If you have no choice, you must take the RSX's fickle brain into account if you want anything even close to accurate numbers.