What is a chopped/cut/hacked aircan/airbox?

A 'chopped' aircan is simply one where a significant part of the can has been cut away - in other words, there is a big hole in it. The cut-away portion can be from 1/4 to 3/4 the size of the air can. Some people remove the airbox altogether, and use other methods to attach the air filter to the mass airflow sensor (MAS).

The theory is simple - the stock airbox, with its 'horn' or 'snorkel' air vent at the front, is a significant restriction to the free flow of air into the engine. Cutting or removing the can eliminates the restriction, allowing more air to flow into the engine. There are several FAQs on how to modify or remove the air can.

In recent years some controversy has arisen concerning this modification. The airbox snorkel may be restrictive, but it also ensures the air drawn into the engine comes from outside the engine compartment, where the air is relatively cool. Removing the snorkel allows hot underhood air to enter the engine, and since hot air is less dense than cool air, this is (theoretically) bad for engine performance. There is no doubt that hotter air is bad; according to Bill McGoldrick in his Jan 01, 1999 posting on the subject:

"A drop in charge air temperature of 20 degrees F yields a 1 motor octane drop in octane requirement. Example: a naturally aspirated engine that needed 93 octane at 90 degrees F could run on 91 octane at 50 degrees F. Turbos and intercoolers complicate things a bit but the formula still holds. By what ever means you lower inlet charge temperature, it's cold out today, big intercooler, more efficient turbo, etc., you'll effectively gain one motor octane number for every 20 degrees F you lower charge temperature."

Proponents of modified air cans point out that it is unlikely underhood temperatures will be significantly different from the outside air temperatures while the car is in motion. They argue that any performance drop will occur while the car is at rest, and will quickly disappear as soon as the car gets moving. There is some evidence that this may be true.

Still, it is possible that the performance gain obtained by removing the intake restriction is offset my a nearly equal loss of performance caused by the introduction of hotter air into the engine. Unfortunately, no one has been able to say for sure if this is the case. In the meantime, modding the air can continues to be a very popular modification on DSMs, as it 'has been done before' and nobody has ever reported decreased performance.

Tom Stangl addresses the hot-air problem in his VFAQ on how to do a ram-air intake. The ram air setup includes panels to block hot underhood air from entering the air intake. This type of air blocking panel/ducting is also a common practice among owners of non-turbocharged cars, who have long recognized the disadvantages of allowing hot air into their engines.

Tom also lists some brief information regarding air intake temperatures, but more information on the effect of the stock air horn can be found in the December 21, 1998 Digest in a post by Gary Schmitz (not yet in the archives). Gary found his air intake temperatures 20 degrees Farenheit hotter without the air horn, as compared to the same setup with the air horn at 60 mph. Howard Draper ran similar tests on his GVR4, and found a 60 degree difference between having the horn and not while the car was in motion. 

More opinions on why modified aircans may not be so great come from Greg Campbell, Victor Del Col, Donal Lavoie and Mac Crossett in the Jan 01, 1999 Digest (edited):

"A couple of us did some testing this summer when the temperatures hovered around 100deg [Farenheit] for several weeks.... With the 14B turbo and an open filter (K&N FIPK), temperatures at the MAS were around 135-140degF, or 35-40degF over the ambient air. With 15lbs of boost and the stock intercooler, temperatures at the throttle body exceeded 220degF at WOT! (and maxing out our temperature sensor)

Replacing the [FIPK] with the stock airbox and a drop-in K&N filter, temperatures at the MAS were reduced to ~105degF, or only 5degF over ambient... Temperatures at the TB were reduced to around 185degF at WOT, a significant improvement. With the addition of a front-mount intercooler from a '88 Starion, TB temps dropped to around 145-155degF."

Owing to the continuing debate and somewhat conflicting evidence/results, it is fair to say that the dis/advantages of modified aircans may vary from car to car. In other words, YMMV (your mileage may vary). However, the bulk of real-world testing shows that inlet air temperatures increase when using modified aircans. The debate over the pressure drop induced by the aircan will likely continue for some time.