after reading a thread a bit back i tried a rough test on my 3b with MRC remap,stainless from cat and standard air filter.note-the maf has the second mesh missing and the front of the bottom of the airbox is cut away.i only used a standard vacuum gauge.i checked the vacuum above the air filter and after the MAF(in all gears).there wasn't any movement on the needle when i checked above the air filter(i didn't check below because i guessed it flowed ok)but when i checked after the MAF there was roughly 2 ins.hg.which i could copy myself by sucking on the tube of the gauge.so i concluded the the standard filter is good for at least 290 bhp.which is amazing when you think it's the same as a 1.8 8v golf,i think.so when my mate with a 2.0 primera sport plus said 'i've put a right good foam filter on and it's made it loads faster',i said 'good for you.now take that **** and shove up your ****,put your standard box back together and put that cold air feed into the box'.which he did and sure enough he said 'that's actually better'.i was trying to get hold of a dwyers magnehelic gauge,but i thought that if there was a lot of vacuum it would show up on anyway.
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You must also remember Bernoulli's principle.
In short, air pressure will reduce as velocity increases .
So if you were to put a gauge directly in front of the turbo , where the diameter of the intake (before boost) is smallest you would see quite a drop in pressure.
This is part of the laws of physics and aero dynamics and is what makes planes fly, the air over an aircraft wing is speeded up because of the curve and thus reduces it's pressure relative to underneath the wing and that's how you get lift.
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but at least i can sleep at night knowing that i'm getting as much air as possible.i'll keep checking it and only change the filter when the it goes up.
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