The other day I came across something interesting while reading some info on Garretts website about choosing the correct filter size for a turbocharger. They have a few nice formulas that you can use to determine flow and this led me over to K&N's website.
Apparently, according to K&N, their panel filter flows better then the typhoon intake for the Legacy GT. Filter 33-2304 (panel filter) flows 459.8 cfm and RP-5113 (the cone filter the typhoon uses) flows 330.1 cfm. I remembered reading this a year or so ago and actually mentioned this to my tuner. I did my mods backwards to most and after upgrading turbos, I was contemplating getting an intake as my next one. He dismissed the idea completely. He said that there was no way that a panel filter could out flow a short ram intake. I let it go, but always thought that my car seemed fastest with the stock airbox over the typhoon.
After seeing all this again, I decided to do the math. According to garrett, a turbo that flows around 44lbs (HTA68) at my elevation, requires a filter that flows around 725cfm without restriction. The math for the panel filter proved correct when compared to K&N's flow numbers. The math for the cone said it should flow slightly better then what K&N said, but it's close enough.
All this leads me to three questions.
#1) Are people wasting money on the typhoon, when a K&N panel filter is better?
#2) Are people with larger then stock turbos being held back by sub-par CAI/SR filters?
#3) Is the rumor that the stock airbox is good for up to and around 300whp true?
We all know that leading up to the turbo, the intake is always under vacuum. But how much would different restrictions change the amount of vacuum?
After a bit of research, I decided that the best way to proceed with this test was to get a manometer and test the increase in vacuum with the different intakes. This would show how much restriction there physically was on the intake. A manometer tests the difference in pressure in relation to the external pressure, or the outside air (But at a more sensitive level then the normal PSI that we are used to seeing). The measurement I would be using was inches of water. 1 psi of vacuum is the equivalent of 27.7 inches of water. Basically the amount of pressure it takes to move water a given distance in a U-shaped tube with a vacuum on one end.
I hooked the manometer directly to the port where my three port BCS normally vents back into the intake (I let the 3port just vent to the engine bay). This was the best spot since it was the point where the two intakes connect to the turbo inlet. I also wanted a point that wasn't being disturbed from boost being recirculated back into the intake. This manometer also had a record function so I was able to strap it into the engine bay and not have to worry about running a bunch of vacuum lines.
This test would be on my 05 LGT with a BNR HTA68 on E85 tuned for 22psi peak. This car makes approximately 330-360 whp at 5280 feet (I never dynoed because this is a very common setup so the power is predictable). I did two tests. One with the K&N typhoon and one with the stock airbox with a K&N panel filter. The first was a reading at idle and the second was recording the maximum restriction after a WOT pull to redline in second and then third gear.
Both intakes at idle had zero restriction regardless of the intake. However, the results under load were quite different. The Typhoon had 16.58 inches of water under load and the stock airbox had 50.52 inches of water under load. Basically the typhoon creates a restriction of .60 psi, while the stock airbox creates a restriction of 1.8 psi.
stock airbox with snorkus removed showed 38.62 inches of water.
Typhoon with only 16lbs of boost was 13.24 inches of water.
added MAF voltages and the last datalog I took for the e85/typhoon/22psi