However, just a cursory look at the intake reveals several problems:
- The mass air flow sensor, a very precise instrument used to measure the volume of incoming air into the engine, is placed between two bends in the pipe. The sensor therefore receives a turbulent, uneven flow of air, for which the MAF is not calibrated.
- The diameter of the intake is smaller than stock, causing air to flow faster through the tube. The mass air flow sensor thus reads higher, causing the engine to think more air is going into the engine than really is. As a result, the car will add too much fuel to the air/fuel mixture.
- The intake draws in warm air from the engine bay, whereas the stock airbox draws in cold air from outside the hood. Warm air results is less dense with oxygen molecules and therefore contains less energy potential. This results in less power potential.
- The intake uses a cotton filter. The filtering qualities of cotton filters are questionable versus the OE paper unit, however less debatable is the the additional servicing a cotton filter imposes. The oil used in the cotton can dirty the mass air flow sensor, requiring that it be cleaned (a delicate process) every 10,000 miles or so.
Now, I'm just some John Doe on the internet and a far cry away from the engineer types I envision developing these products. However, these flaws are still pretty obvious to me, and they're flaws that could have been easily (and cheaply) circumvented. If I can see these problems, how can SPT not see them? How did Subaru approve it?
These very questions are what lead many of SPT's customers to disregard their better sense and ignore their doubts. Subaru's stamp of approval implied that all these seemingly obvious flaw were either imaginary or overcome. Besides, SPT claimed a horsepower gain, not a loss.
Cobb Tuning, a rival company well-known for Subaru aftermarket products, echo'd the comments of others and assured the intake would result in power loss and a poorer running engine. Upon request, Cobb tested the SPT intake and released this document showing their results:
http://www.cobbtuning.com/tech/instructions/SPTLegacyGT1.01.pdf
While the document should be taken as propaganda (it is from a rival company, after all), the results were rather expected. Some tests have been since repeated independently by users, adding validity to Cobb's document. In short, every assumed shortcoming of the SPT intake turned out to be valid.