I recently picked up a 2001 a4 coupe and am looking to improve the acceleration for a great SOTP feel when blasting around town. The car has corsa cat back exhaust with 3:15 gears and 295/35/18 Toyo Proxes T1 sport tires in the back and no engine mods. I was thinking about going with 3:73 gears with a Yank 3200ss stall but was concerned that with street tires wont provide enough traction. I don't want to run DR tires because I will be driving this car daily during the summer and will run into rain on a regular basis. If that's the case I would just run 3:42 gears or just consider just adding a stall to help my 3:15 gears. Any input you guys could give me as far as drivability/traction and MPG with the 3:73 would be great I really want to go with them because I know it will provide the best SOTP improvement but it wont do any good if the tires just go up in smoke.

 

If you modulate your gas and don't just "stomp" on it, decent street tires should still hook up pretty good.

If you are wanting to feel more pick up, you're on the right track with the 3.73 gears and a 3000 or so stall. Your engine is going to determine the best stall for your torque converter. If the engine is basically stock, you probably want to be in the 2600-3000 rpm stall speed. Even keeping the 3.15 gears, you would notice a nice pick up in SOTP performance if you went to the same 2600-3000 stall mentioned earlier.

With a performance cam moving your horsepower higher in the engine rpm range you can go with a larger stall. There are two approaches when choosing the torque converter stall speed. The most common approach is to select a stall speed that is at least 500 rpm higher than the where the camshaft is entering its power band. The other approach is that your stall speed should equal where the engine is at or nearing max torque. While you are going to see some difference in the stall speeds between these two methods, both will get you into a good stall speed range for your car based upon engine mods, etc. All of the torque converter manufacturers I'm familiar with will spend time talking to you to determine what mods you have made, mods you are still planning on, street driveability versus performance, etc. Some will have spec sheets you can complete and either email to them, or at least fill it in and have the information handy when you speak with them by phone.

If you already have 3.15 diff gears I'm not sure you would notice too much gain going to the 3.42. I speak from experience because I did that recently, but only because I was going to a performance built 3 rib differential that already had the 3.42 gears. If I had already had the 3 rib diff with the 3.15 ratio, I would have looked at either the 3.73 or 3.90 gears.

If you haven't driven a higher stall car before, the thoughts of it can be a little intimidating but you quickly adjust to it. Most people assume the higher stall TC is going to be all or none on acceleration. You can actually still control your take offs easily - even in rainy weather. Of course, if you've never met a 3000 rpm launch you didn't like then you would have traction issues with wet roadways.

If you have a Corvette club in your area, you might try seeing if anyone there has an A4 with a higher rpm stall TC. With any luck you could find someone with a stock engine, 3.15 diff and a 2500 rpm stall and someone with a modded engine, 3.73 diff and 3500-3800 rpm stall to really see how the difference feels to you.

Best wishes with your trip down the performance pathway!