ABY Airbox Bodge!

[Rusty]

I menntioned this to people at Combe and the RR day in October. Only just had a chance to take a picture of the airbox as I was hunting a boost leak on the low pressure side after buying one of AndyP's boost test kits.

Because I'm tight and couldn't be botheretd to fork out over £200 for an RS2 airbox and because I really didn't like the noise of a cone filter, I decided to modify a standard S2 ABY airbox to get rid of the beautifully sculpted bell-mouthed noise reducing trumpet on the front. This was a 'suck it and see' modification and I have kept it for the last 6 months as I really quite liked it

I have no idea if it's improved flow but a strong performance on the rollers last October supports the idea that it hasn't done any harm. The inlet hole is now about 3" x 4" instead of 2" diameter giving over 3 times the inlet area. There are of course arguements about turbulence but the air turns through 180deg anyway so I believe that the additional turbulence caused by the rough edges is minimal compared to the change in flow direction.

The car now makes a lovely purr as opposed to being silent or the harsher, more mechanical sound of the cone filter, and off boost it sounds like a mk.2 Golf that I used to own, also with a heavily modified airbox.

Overall then, more inlet area and a nicer (IMHO) noise, all for the price of a secondhand air box and an hour's work. Job done !

[Jamo]

WTF? Is that what happened to my old airbox?

Andy you butcher

[blown_vr6]

Hmmm - surely the bellmouth is to increase air speed and creates a negative pressure in the airbox to pull the air in quicker?.
Did you not think about fitting a larger bellmouth, ive heard of people using ports from speakers and the like before.
For what youve done to that you might as well have stuck with the cone filter imho.

Pete.

[Rusty]

Should I have warned you Jamo!!!

If you read the post you'll notice that I didn't like the cone filter. This retains the standard heat shielding and cold air feed so is more resistant to the heat problems can be experienced with a cone.

Thought about using a 3" speaker port that I have but couldn't be bothered.

Don't see why the bellmouth would pull in air faster. The air speeds up due to the narrowing cross section but the mass flow rate remains the same. The air speeding up in the bellmouth does create a local drop in pressure (Bernoulli 's principle) but when it expands and slows down in the air box it would increase in pressure again. As the pressure drop/gain is not 100% efficient, I agree that there is a pressure change across the bellmouth but I see that as a restriction, not a method of increasing flow.

Why does the RS2 item have a larger, shorter bellmouth? That would create less pressure drop across the bellmouth and thus increase pressure in the airbox, not reduce it.

The bellmouth is more than likely there to reduce noise and make the flow into the airbox more laminar, something Audi obviously thought was necessary.

[Fat Audi 80]

Lots and:

The bellmouth is more than likely there to reduce noise and make the flow into the airbox more laminar, something Audi obviously thought was necessary.

I would agree with that assumption...

Cheers,

Steve

[Jamo]

The bellmouth size does change airflow speed. Gasses in a more confined space increase in pressure and flow quicker.

[blown_vr6]

The bellmouth size does change airflow speed. Gasses in a more confined space increase in pressure and flow quicker.

As i thought.

I would be very surprised if your airbox is flowing as well as a stock airbox but if your happy then thats all that matters.
The RS2 Airbox principle is still the same as the S2 one - yes it has a larger shorter bellmouth but the principle of velocity of the intake air and pressure drop remains the same.

Pete.

[Jamo]

There's a tradoff between airflow and airspeed dependant and the size the bellmouth hence the RS2 allowing more air in but at reduce airspeed.

Same principle with exhausts in a 3" air cools quicker reducing flowspeed as it's under less pressure within the exhaust space when compared to a 2.5" system but the 3" can remove more air volume. IMO

[Rusty]

I would agree with that assumption...

Cheers,

Steve

Fatty, I'm suprised that my blatant butchering of the beloved OEM hasn't isnited rage .

[Fat Audi 80]

Fatty, I'm suprised that my blatant butchering of the beloved OEM hasn't isnited rage .

I do keep *some* opinions to myself







Yes you have buchered it, buts its your car and you obviously like the new noise

Cheers,

Fatty

[newsh]

One thing the bellmouth does on a stock airbox is to direct the air at the back of the box at high speed, I think this helps any water droplets to hit the back wall and run down and out the drain hole, helping the filter to stay drier.

[jameswkr]

Good point. I ran my airbox with the trumpet removed (hole then larger than MAF restriction) and it seemed better but the bottom edge of the filter pleats were getting damp so I put it back. I run without any of the "box" behind the headlamp and the car is definitely better without it ( see prevoius post on RS2 airbox).

[Rusty]

Got a bit bored today so decided to further butcher my airbox and drill a few holes in the cold side. This is a time-honoured method with Golf and K-Jet metering boxes, certainly makes some interesting changes as anyone who saw that typ85 90q tha I used to have would testify

Put it back on and there's a little difference in noise, certainly not the sort of difference you get on a NA car with this mod. However, there's a little more induction noise here and there and if anything it sounds a little more like a cone filter but without the rediculous volume and the DV making itself heard at all points, even in reverse

So cutting the front off is a good plan in my book, drilling it into a black plastic Emmental wannabe... not so much but it doesn't seem to hurt.

Good news is that on the test drive the car was absolutely flying and there's a lovely roar at about 5k upwards as I found out overtaking in 2nd gear :mischeif:

[Acidburn]

I was tempted to add some speed holes in mine when I did it like yours, got any pictures of the finished article

also what filter you using a standard new one or a uprated?

[Rusty]

Speed holes are the reason for my naming of the car as The Emmental Express. I've just drilled down the cold side, 16mm holes btw . Thought about taking pics but decided that the abuse would be rather immense!!

Running a K&N panel, much to other peoples' distress