Upgrades & Enhancements
As much as I hate to admit it, the 20 valve I5 could use some help in the power department. It's got plenty of pull in the high RPMs, but it has very little low end grunt. From what I understand, this is because the engine was one of the first multi-valve designs (based on the 16V VW I4 and 32V Audi V8), so it lacks variable valve timing. It's still a fun powerplant, but if you're looking to do some stoplight drags, look elsewhere.
One of the first things to do when looking for more power is to make sure that the stock components are working correctly. These cars are getting to the age where worn-out parts start to affect performance. Check out the Engine Problems & Diagnostics page for some info on common problems. Make sure the car has the proper set of spark plugs and good wires. Change the air and fuel filters, and please make sure the timing belt is in good shape! You should also check the operation of the idle switch and throttle potentiometer. Once everything is running as the Audi engineers intended, then you can start to look for improvements.
Since this is a normally aspirated engine, a simple chip upgrade won't give the dramatic increases seen in the turbocharged cars. Basically, all the chip can do is mess with the ignition timing to squeeze out more power. The only chip I've heard people use is the one from TAP, although SuperChips markets one in the UK which is apparently not available in the US. The TAP chip claims a mild horsepower increase, but the biggest benefit owners report is more torque in the lower RPM range.
More power can be squeezed out of the engine, but it's expensive. You can bore out the cylinders and get oversized pistons; TAP sells some pricey Piper cams; the intake and exhaust can all be reworked. You will have to decide for yourself if the end result is worth all of the money and effort.
Possibly the best option is to drop in a 20V turbo motor out of a '91 200, early S4 or S6. The biggest obstacle here is that the supply of these engines is low and demand is high. Anyone who is considering an engine transplant simply MUST visit Brandon Hull's Ersatz S2 site. It is a very detailed description of the evolution of his car.
Air Intake Mods
K&N sells a drop-in panel filter replacement for these cars, but it doesn't really help performance much. I've got it in my car simply for the maintenance aspect. They say that you only have to take it out to clean it every 50,000 miles (I'll at least visually check mine more frequently). This may sound like a trivial improvement, but the air filter on these cars is a PITA to get to! Wetterauer sells a similar free-flow panel replacement from RamAir that is claimed to provide better filtering.
K&N also sells a cone filter replacement that is more likely to give the car a little extra power. If nothing else, it makes the car sound cool. The picture to the right is Christian Long's setup. There are concerns that hot air from the engine bay will counteract any positive benefits that the cone provides, so it's a good idea to build a heat shield like Christian did.
If you don't want to go through the trouble and expense of fitting the cone and fabricating a heat shield, or you don't want to increase the intake noise much, there is an alternative. You can improve the stock air path by simply replacing the 9" puffy hose that connects the air intake panel (behind the headlight) to the air box with something larger and smoother for better flow. For an investment of about $10 and a couple of hours you can improve one of the intake system's weak spots.
It just so happens that 3" black ABS pipe and fittings are nearly a perfect fit for replacing the intake air hose. Go to the plumbing section of your local hardware store and get one straight piece (you only need a few inches at most, but a 1 foot piece will make it easier to cut) and a 1/8 yard elbow (1/8 is the amount of bend in the elbow joint). The yard elbow should have a male end and a female end, not two females. You'll also need some epoxy, and some ABS glue.
Take the skinny end of the yard elbow and cut off it an an angle, removing about 5/8" more from the pipe on the inside of the curve. This allows that end of the elbow to go into the panel behind the headlight at a slightly increased angle. Now cut about 1" off the female coupling on the elbow, then cut a roughly 1" piece of the straight pipe, to fit into the female end of the elbow. This should leave about 3/4" of the straight pipe sticking out. You can remove one of the mating rings from the puffy hose by unscrewing it, and use it to mate the straight pipe to the airbox. Put everything together and mount it on the car to see if you need to make any adjustments. You may need to adjust the cut at the small end of the elbow to fit the panel.
Once you get this far, you can set the intake panel in place with the pipe resting in the air intake, and glue the joint between the pipe and the elbow, making sure everything fits (use ABS glue - it's less messy and sets faster). Finally, remove everything, place the mating ring on the lower end of the pipe, put everything in place again (just to be REALLY sure it fits) and epoxy the mating ring on the end of the pipe. When the epoxy sets, remove the whole thing, and file and sand the mating ring and epoxy so it's flush and smooth with the inside of the pipe.
You can open up the intake port behind the headlight by cutting off the plastic ring on the panel and a portion of the incoming elbow so that the opening is flush with the outside of panel. This opens up the fishmouth considerably, and gives a desirable diagonal opening into the pipe (cuts potential resonances).
You will be able to remove the panel just as before, except the pipe may be more difficult to move out of the way. Spray some silicone on the mating ring at the bottom to ease future removal.
See also: Engine Problems and Diagnostics
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