Disclaimer: Please do not perform this mod if you are not comfortable working on your car or working with electricity/electronics. You perform this mod at your own risk. I provide this information as a reference only. What works on my car may not work on yours. Please use common sense when working on your car. In fact, I recommend you do not perform this mod. Please, please don't do it. :-p
The original inspiration for this mod came from Mance (UrS4 on AudiWorld). Thanks, Mance!
The Audi 2.8 12V engine features a dual-path Intake Manifold (IM). This design uses longer IM runners at low RPM and shorter IM runners at high RPM. This allows the Intake Manifold to optimize power throughout the power band. Cars with a single path manifold have to compromise between long and short runners. From the factory the IM switchover occurs at 4100 RPM. This is fine for a stock or near stock car but once you start to add mods you will notice when the switchover happens more and more by a kick in power. This is because you are actually losing power a few hundred RPM before the switchover occurs. Now, wouldn't it be nice if the IM switchover occurred a few hundred RPM sooner?
More information about the dual-path IM and switchover can be found here (compliments of 12v.org, thanks Elliott!).
Parts:
Necessary Tools:
Let's get started....
First you have to decide where to mount everything. I choose to put the RPM Switch right inside of the fuse box so I can get to it for easy adjustments. Leave enough slack in the wires so you can pull it out several inches.
I decided to mount the relay with the existing relays behind the driver's side knee bolster. I plan on getting a stock relay socket and wire everything cleaner for a stock appearance.
An optional LED can be added so you can visually see when the switchover occurs. Plus for only a few dollars it's a great way to make sure the RPM Switch and Relay are working.
Now, for the optional Solenoid. The solenoid is just a valve that opens to allow vacuum to be applied to the switchover actuator. You can absolutely use the existing solenoid and just disconnect it from the ECU but this will cause a CEL. If you can live with a CEL save yourself some money and use the existing solenoid. If you don't like the idea of constantly driving with a CEL (like me) you will need to pick up a 2nd identical solenoid. I got mine with the stock wiring harness for $40 from Force 5. Since I'm running Mance's S3 Hose Kit, I also got a 3/16" aluminum metal plate from Onlinemetals.com (thanks for the link, Scott) to mount the solenoids on. A 12"x12" H32 Aluminum plate was about $12. If you have the stock throttle body plenum, there should be space to mount a 3rd solenoid.
Now that we know where everything will be mounted let's remove the driver's side knee bolster. There are 4 8mm screws which hold the knee bolster on.
First, you need to remove the fuse cover which is on the end of the dash. It just prys off.
Second, use your small flathead screwdriver to remove the two small covers to either side of the steering column being careful not to scratch the dash. There are two screws behind these covers. Remove them. The 3rd screw is inside the fuse cover at the bottom. The forth screw is on the opposite side of the knee bolster. You have to stick your head under there to see it. The entire knee bolster should slide forward. Note that there are two clips that hold the bottom of the knee bolster on. One is white plastic and the other is metal and is located behind the 4th screw. If you have a 97 A4 the OBD2 connector is attached to the knee bolster. You will need to unclip it from the inside.
Now that the knee bolster is off you will need to wire everything. Before working with any wiring disconnect the negative terminal on your battery!!! For positive switched power you can attach to the 75x post below the relays. There is a 10mm nut on the 75x post.
A good ground is located in the fuse box. There are two screws which hold the fuse box in. You can use either of these screws.
Here is the wiring diagram I followed. It is absolutely important to not hook any accessories or LED's directly to the white wire on the RPM switch!!! You will fry it. The only thing that should be attached to the white wire on the RPM Switch is the Relay.
The solenoid will have a '+' sign on top of the connector and a '-' sign on the bottom. As you see in the pic, attach the wires opposite of the signs. When doing some probing with my multimeter I found the voltage from the stock ECU to be opposite of these signs. It may work either way but I played it safe and hooked it up the way it was from the factory.
Originally, I had purchased a 30 amp Radio Shack Auto Relay. It blew after being activated twice (and I only had the LED hooked to it at the time). I have no idea why. The Audi relay is a 40 amp relay and works great. You should be able to use any auto relay. Maybe 30 amps is not enough so look for a 40 amp relay to play it safe.
I'm sure you've noticed by now that there is a Green wire from the RPM switch which is the lead to the RPM signal. I hooked this to the RPM signal in the ECU box. First you need to remove the ECU cover from under the hood. There are 5 8mm screws which hold it down. One is underneath the "water gutter". There is a round "plug" in the rain gutter you can remove to access this screw (the wipers must be stopped vertically to get to it). Be careful not to drop it back there! You need to remove the clip holding the ECU down. Gently pry it up with your medium flathead screwdriver. Now remove the ECU and push it to the side. There is a row of connectors below the ECU. Here is the wire you hook the RPM lead to (you can see the 3M T-Tap connector I used). You will need your needle nosed pliers to attach the wire tap.
While the ECU cover is off, this a good place to route the positive power lead to the solenoid. I used a wire snake (coat hanger will work, too) to feed the wire up through the ECU box. I then ran the wire through a hole I had drilled in the BOTTOM of the ECU box. There is a rubber nipple on the secondary firewall you can pull off to get the wire to the engine compartment. I grounded the solenoid to the ground location under the coolant reservoir. There are 3 phillips head screws which hold the coolant reservoir down. Just push the reservoir aside.
If you have opted for the extra solenoid you will need to move the vacuum hoses from the old solenoid to the new one. Make sure you keep them oriented correctly. When mounting the solenoids, I had to adjust the sizes of some of the vacuum hoses. You may need some extra vacuum hose for this.
Now that everything is installed you need to program the RPM Switch for the number of cylinders and RPM you want the switch to activate. 3800 seems just about perfect for my car. Play around with it and see what works for you. Read the RPM Switch instructions thoroughly so you understand how it works and how to program it.
I also found this article from howstuffworks.com helpful since I've never worked with Relays before.