After having a look at the stock plastic air box that came on my car and seeing the metal air box that came on the early model 325e, I decided I wanted to do some flow testing and a little experimenting. I went to the wrecking yard and got a metal air box and another AFM trumpet (the one that goes inside the plastic air box at the inlet to the AFM). I also got a new Mahle air filter for both the plastic and metal air box. The filter in the metal box is larger than that of the plastic box, which is why I thought it might be better. I also thought that the AFM trumpet in the plastic box was too close the end wall, so I wanted to see if it could be improved upon. I did my testing at two points, 12" of water restriction for a flow result, and then with 300CFM flow to get a restriction result. 27.7" of water is equal to 1 PSI for reference. 12" is a pretty low restriction for WOT, and 300CFM because I believe that is enough air for something around 200 HP.
I did my first test with the plastic air box, the standard AFM trumpet, the AFM, and a new Mahle filter. The results were 213 CFM @ 12" water, and 22" water @ 300 CFM.
The next test was the same as previous except with out the AFM trumpet. The results were 196 CFM @ 12" water, and 23.8" water @ 300 CFM. As expected this had a negative impact.
Set up same as the first test but with ~3/4" cut off the end of the trumpet. The results were 212 CFM @ 12" water, and 22.5" water @ 300 CFM. The 3/4" cut off the trumpet included the radius, so the sharp edge should reduce flow. The fact that the flow did not drop appreciably means that the trumpet was too close to the end wall for optimal air flow.
Set up same as last test but with an additional 1/4" cut off the end of the trumpet and the radius section taped back on. The result were 218 CFM @ 12" water, and 21.5" water @ 300CFM. This is a small improvement, ~2.3% better flow. The taping of the radius section back on the trupet at the lower height could definitely be improved on a permanent part, but probably still would not be worth the effort.
Onto the metal air box. The metal airbox does not have an AFM trumpet , it has a round pipe in the air box and then a short and rather abrupt cast metal adadpter to the square inlet on the AFM which is not as nice as the plastic box. The inlet pipe into the lower half is about 2.5" in diameter, where as the plastic box has an 3.15" diameter inlet.
The first test setup was the metal air box, the cast adapter, the AFM, and a new Mahle filter. The results from this were 204 CFM @12" water, and 22.6" water @ 300 CFM. This is obviously down from the plastic box despite the larger filter. It seems that the smaller inlet and the not so good AFM inlet piece hold this set up back. As the filters gets dirty this could change, but you can always change your filter more often if that is an issue.
The possible gains that I thought might be there don't exist or are not compelling, but I thought that some one might find the numbers interesting.
I did my first test with the plastic air box, the standard AFM trumpet, the AFM, and a new Mahle filter. The results were 213 CFM @ 12" water, and 22" water @ 300 CFM.
The next test was the same as previous except with out the AFM trumpet. The results were 196 CFM @ 12" water, and 23.8" water @ 300 CFM. As expected this had a negative impact.
Set up same as the first test but with ~3/4" cut off the end of the trumpet. The results were 212 CFM @ 12" water, and 22.5" water @ 300 CFM. The 3/4" cut off the trumpet included the radius, so the sharp edge should reduce flow. The fact that the flow did not drop appreciably means that the trumpet was too close to the end wall for optimal air flow.
Set up same as last test but with an additional 1/4" cut off the end of the trumpet and the radius section taped back on. The result were 218 CFM @ 12" water, and 21.5" water @ 300CFM. This is a small improvement, ~2.3% better flow. The taping of the radius section back on the trupet at the lower height could definitely be improved on a permanent part, but probably still would not be worth the effort.
Onto the metal air box. The metal airbox does not have an AFM trumpet , it has a round pipe in the air box and then a short and rather abrupt cast metal adadpter to the square inlet on the AFM which is not as nice as the plastic box. The inlet pipe into the lower half is about 2.5" in diameter, where as the plastic box has an 3.15" diameter inlet.
The first test setup was the metal air box, the cast adapter, the AFM, and a new Mahle filter. The results from this were 204 CFM @12" water, and 22.6" water @ 300 CFM. This is obviously down from the plastic box despite the larger filter. It seems that the smaller inlet and the not so good AFM inlet piece hold this set up back. As the filters gets dirty this could change, but you can always change your filter more often if that is an issue.
The possible gains that I thought might be there don't exist or are not compelling, but I thought that some one might find the numbers interesting.
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