VAP
03-17-2003, 01:00 PM
<center><img src="http://pictureposter.audiworld.com/711/a4mafstack1.jpg"></center><p>
Amazing all the little snippets of interest one can come up with when R&Ding throttle bodies then looking at upstream components.
This is one I found 4-5 months ago and disclosed on the 80/90 forum at the time but until very recently didn't do any flow-testing on. And while flow alone isn't very telling, the modification still proves to be far greater and feelable than the sum total of it's parts and numbers would indicate.
Essentially what I've done here is take an early UrQuattro airbox apart and remove it's internal airhorn. A 4" long by 76mm (2.99") O.D. length of black heat-resistant injection molded ABS that was only indiginous to UrQuattros and had an integral airhorn molded into its inlet and has a 71mm I.D. I have to break em to get em out of the UrQ's as they are a one-way snap-in arrangement using locking tabs. Then I cut them down to about 1.5" long and on the 90 series cars I wrap a few times around with electrical tape so they're a tight interference fit. On the A4 2.8 airbox they are perfectly sized without any wrap. Just cut them to length and epoxy into the airbox top and flsuh up against the MAF screen. It's an identical I.D. match to the MAF screen's outer ring O. D. (I love it when that happens!).
Most people believe the screens in the MAF are there to protect it's internal hot wire or other componentry from flying debris. And while the screen does serve that purpose, it's true reason for being there is for laminar air stabilization. It has a smoothing affect on air flowing past it for an inch or two and across the MAF's sensor area. This serves to prevent the MAF from getting irratic, turbulent air with sudden spikes, lulls, eddies and vortex's and allows the MAF to output averaged, consistent fuel/air ratio signals better than without the screen. It's drawback is it does indeed cost some power in the form of diminished flow.
Placing an airhorn/velocity stack at the MAF inlet does a far superior job compared to the screen at stabilizing and smoothing the air and downstream flow is markedly improved. And it's a perceptably feelable difference. As to whether or not you leave the MAF screen in doesn't seem to matter as much any longer. I think thats further supported by some who are running cone setups with an integrated airhorn/velocity stack within the filter element and feel a definate improvement. But a cone setup unless sealed tightly against engine compartment heat has a more vague, arbitrary and ambiguous benefit than one drawing cool/dense air as the stock airbox which in that regard is very very good temp-wise. Davtron it sometime and you'll see.
The next caveat thats born out in testing is that the further away from the MAF any airhorn is located the less it's benefit. Any further away than 2" from the MAF inlet and the apparent benefit falls off at approx 20% per inch. At any more than 4-5" away it's no longer working or at least is no longer perceptable... you can't tell you did it.
The way I did it allows the stock airbox be kept without modification on the A4 and the 90. The stock airbox will flow just at 320 CFM of air with a stock filter and 370 CFM with a K&N panel element before it becomes restrictive. Thats plenty up to about the 225/270HP respectively on the A4. The 90 series airboxes are a little less efficient at 270CFM/295CFM. But find a way to open up the 54mm airhorn inlet behind the headlight to 75mm or greater and you'll flow the same numbers as a stock A4 airbox.
Another way to do this mod on either a 90 or A4 is to find a cone setup that can be adapted very closely to the MAF and boxed in to keep the intake charge cool. A simpler solution if you're running a stock airbox but still want most, if not all, the gain of a cone system is to find a K&N (or similar/cheaper) cone setup that utilizes the integral velocity stack and cut it apart to get the velocity stack/airhorn portion out of the bottom of the filter. The cheapest one you can find with a 2 7/8" or 3" flange... even used is fine, in fact prefferable as all cept the inlet & flange is going in the trash. Then have a muffler shop cut you a 2" length of 3" O.D. diameter tubing. Then deburr it's inside O.D. at both ends. On an A4 this can then be epoxied in as-is into the top of the airbox and up against the MAF screen once it's had it's cut-down v-stack K&N attached. On a 90 you can wrap a few layers of electrical tape around one end and test for fit. Soon as you have a good, snug fit just push it in and up against the MAF screen... again, don't install it permanently until the K&N V-stack is on it's inlet end. Don't worry, it apparently never falls out even on the bumpiest of washboard roads. We tried hard and couldn't make it fall out or even budge .001 of an inch.
Once you've gotten a K&N or similar fully dissected down to it's internal velocity stack inlet you can then cut all of it's flange off except for 1/4"-1/2". This is then fastened to the small length of 3" tubing with a tie/wrap (cable tie). Don't worry that it's not "Hose-clamped" on as again I couldn't make the thing come off on a bet using just cable ties. Once the pipe is attached to the filter flange you can then fit it to the inside airbox MAF inlet area. Make a few trial fitments first to make sure everythig fits and on the 90 make sure it's inlet end doesnt stick into the panel filter element when closing the box up (less important on an A4 as there's plenty room). Once everything fits with airbox closed and fastened you're ready to mix epoxy on the A4. On the 90 you're done.
If any part of this is unclear just ask questions. I'm sure many here already understand the concept.
Test drive and feel the difference! It's really quite significant for such a simple mod.
Pics show the UrQ airhorns as at one time I had four of them either sitting around or scavenged from salvage yards. But these are a LOT more work even if you could find one (they are rarer than hen orthodontists) than a simple K&N with VS inlet.
Pic 1 is UrQ mod on a 2.8 12V A4 airbox
Amazing all the little snippets of interest one can come up with when R&Ding throttle bodies then looking at upstream components.
This is one I found 4-5 months ago and disclosed on the 80/90 forum at the time but until very recently didn't do any flow-testing on. And while flow alone isn't very telling, the modification still proves to be far greater and feelable than the sum total of it's parts and numbers would indicate.
Essentially what I've done here is take an early UrQuattro airbox apart and remove it's internal airhorn. A 4" long by 76mm (2.99") O.D. length of black heat-resistant injection molded ABS that was only indiginous to UrQuattros and had an integral airhorn molded into its inlet and has a 71mm I.D. I have to break em to get em out of the UrQ's as they are a one-way snap-in arrangement using locking tabs. Then I cut them down to about 1.5" long and on the 90 series cars I wrap a few times around with electrical tape so they're a tight interference fit. On the A4 2.8 airbox they are perfectly sized without any wrap. Just cut them to length and epoxy into the airbox top and flsuh up against the MAF screen. It's an identical I.D. match to the MAF screen's outer ring O. D. (I love it when that happens!).
Most people believe the screens in the MAF are there to protect it's internal hot wire or other componentry from flying debris. And while the screen does serve that purpose, it's true reason for being there is for laminar air stabilization. It has a smoothing affect on air flowing past it for an inch or two and across the MAF's sensor area. This serves to prevent the MAF from getting irratic, turbulent air with sudden spikes, lulls, eddies and vortex's and allows the MAF to output averaged, consistent fuel/air ratio signals better than without the screen. It's drawback is it does indeed cost some power in the form of diminished flow.
Placing an airhorn/velocity stack at the MAF inlet does a far superior job compared to the screen at stabilizing and smoothing the air and downstream flow is markedly improved. And it's a perceptably feelable difference. As to whether or not you leave the MAF screen in doesn't seem to matter as much any longer. I think thats further supported by some who are running cone setups with an integrated airhorn/velocity stack within the filter element and feel a definate improvement. But a cone setup unless sealed tightly against engine compartment heat has a more vague, arbitrary and ambiguous benefit than one drawing cool/dense air as the stock airbox which in that regard is very very good temp-wise. Davtron it sometime and you'll see.
The next caveat thats born out in testing is that the further away from the MAF any airhorn is located the less it's benefit. Any further away than 2" from the MAF inlet and the apparent benefit falls off at approx 20% per inch. At any more than 4-5" away it's no longer working or at least is no longer perceptable... you can't tell you did it.
The way I did it allows the stock airbox be kept without modification on the A4 and the 90. The stock airbox will flow just at 320 CFM of air with a stock filter and 370 CFM with a K&N panel element before it becomes restrictive. Thats plenty up to about the 225/270HP respectively on the A4. The 90 series airboxes are a little less efficient at 270CFM/295CFM. But find a way to open up the 54mm airhorn inlet behind the headlight to 75mm or greater and you'll flow the same numbers as a stock A4 airbox.
Another way to do this mod on either a 90 or A4 is to find a cone setup that can be adapted very closely to the MAF and boxed in to keep the intake charge cool. A simpler solution if you're running a stock airbox but still want most, if not all, the gain of a cone system is to find a K&N (or similar/cheaper) cone setup that utilizes the integral velocity stack and cut it apart to get the velocity stack/airhorn portion out of the bottom of the filter. The cheapest one you can find with a 2 7/8" or 3" flange... even used is fine, in fact prefferable as all cept the inlet & flange is going in the trash. Then have a muffler shop cut you a 2" length of 3" O.D. diameter tubing. Then deburr it's inside O.D. at both ends. On an A4 this can then be epoxied in as-is into the top of the airbox and up against the MAF screen once it's had it's cut-down v-stack K&N attached. On a 90 you can wrap a few layers of electrical tape around one end and test for fit. Soon as you have a good, snug fit just push it in and up against the MAF screen... again, don't install it permanently until the K&N V-stack is on it's inlet end. Don't worry, it apparently never falls out even on the bumpiest of washboard roads. We tried hard and couldn't make it fall out or even budge .001 of an inch.
Once you've gotten a K&N or similar fully dissected down to it's internal velocity stack inlet you can then cut all of it's flange off except for 1/4"-1/2". This is then fastened to the small length of 3" tubing with a tie/wrap (cable tie). Don't worry that it's not "Hose-clamped" on as again I couldn't make the thing come off on a bet using just cable ties. Once the pipe is attached to the filter flange you can then fit it to the inside airbox MAF inlet area. Make a few trial fitments first to make sure everythig fits and on the 90 make sure it's inlet end doesnt stick into the panel filter element when closing the box up (less important on an A4 as there's plenty room). Once everything fits with airbox closed and fastened you're ready to mix epoxy on the A4. On the 90 you're done.
If any part of this is unclear just ask questions. I'm sure many here already understand the concept.
Test drive and feel the difference! It's really quite significant for such a simple mod.
Pics show the UrQ airhorns as at one time I had four of them either sitting around or scavenged from salvage yards. But these are a LOT more work even if you could find one (they are rarer than hen orthodontists) than a simple K&N with VS inlet.
Pic 1 is UrQ mod on a 2.8 12V A4 airbox