Intake Modification

The stock air intake system on the 1994 Dodge Stealth R/T Twin Turbo and Mitsubishi 3000GT VR-4 is on the far left of this diagram from Corky Bell's excellent book, Maximum Boost.  A common upgrade is to replace the restrictive airbox pictured with an open element filter, such as the K&N Filter Injection Performance Kit (FIPK), shown installed below.

Unfortunately, the air in the engine compartment is hot, which reduces the density of the air entering the intake, forfeiting horsepower.  Also, the engine must suck the air into the intake, slowing turbo spoolup and reducing efficiency.  Below is a simple but effective air intake system described in Joe Pettitt's excellent High Performance Honda Builder's Handbook.  This system provides fresh, cool air to the intake from the front of the car, avoiding the hot air in the engine compartment.  At speed, this setup also acts as a simple ram-air system, improving efficiency.

One day, after changing the oil in my 1994 Stealth (only mods were K&N FIPK and A'PEXi SAVC-R), I took the car out for some spirited driving and noticed that the turbos were spooling up faster, higher boost pressures were maintained at high RPMs, and the engine seemed stronger throughout the entire rev band.  This didn't make sense, as all I had done was change the oil!  When I returned home, I noticed that the hood was safety latched but not closed, leaving a gap through which cool air could easily enter the engine compartment.  I securely closed the hood and went for another test drive, and the car was back to normal.  Therefore, it would seem that enhancing the engine's ability to freely ingest cool air results in a noticable improvement in performance on these cars.

As the engine compartment is well sealed and the engine bay cramped, it will be a challenge to find a way to provide outside air to the FIPK.  One idea would be to add a hood scoop just before the FIPK, but the body work would be expensive, the car could not be returned to 100% stock condition, and there is the problem of preventing water and excessive dirt from entering the system.  Another idea I am considering is reinstalling the bottom portion of the stock airbox beneath the FIPK, attaching a hose to the intake snout, and running it down past the coolant overflow tank to a custom-made scoop beneath the car.  This would scavange air from the high-pressure area beneath the front of the car and would minimize water or dirt contamination.  Construction of an effective scoop should not be difficult; below is an example from Maximum Boost of an air scoop for a horizontally mounted intercooler (not on a Stealth/3000GT!).  My idea is to create a similar scoop that forces air into the hose running to the FIPK.

The more complicated problem is running the hose from the bottom scoop to the stock airbox snout.  The airbox snout is already well-positioned beneath the fuse box, but the coolant overflow bottle may need to be relocated.  Care must also be taken to route the hose around the passenger-side intercooler piping.

If all goes well, cool air will be blown at the FIPK when under speed.  The final question is whether or not to use the top of the stock airbox to enclose the FIPK, completely isolating the intake from the warm air in the engine compartment.  While this would ensure that only cool, dense air is being used, requiring the intake to suck air through an additional meter of hose may reduce effeciency to the point that the purpose of this project is defeated.  Only testing will tell!

Again, there are three pictures above from the following excellent books, Maximum Boost, by Corky Bell, and High Performance Honda Builder's Handbook, by Joe Pettitt, both highly recommended!:


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