Vijay

Will a low restriction exhaust decrease low-end torque?

RM

It should help low end torque on a turbocharged car and also overall power. I'm assuming that you are talking about exhaust leaving the turbocharger and not exhaust coming from the head to the turbo.

Vijay

I was referring to a "cat-back" type system. I know that on normally aspirated engines, they can increase peak high rpm horsepower, but at the expense of some low-end torque. Since low-end torque is one of the things I love most about the S4, I didn't want to install an aftermarket exhaust if performance would be impaired in this rpm range. Any suggestions for an exhaust system? I do not want it to be loud. What I find particularly annoying is a heavy low pitched sound when the engine is under load.

RM

On a turbocharged car, the less restriction past the turbo, the better the car runs. Personally, their is not a whole lot about this car that bothers me. If I were going to change cat-back exhaust for better sound I would try to stick with a German part. I think they have been working with these cars longer than anyone else and would have a better quality part. If I were you I would wait and see what others do and see how they like what they did. This car is so new here that there seem to be few people who have done any mods. I haven't seen under the car to see what would be involved, but from my experience with the 944 turbos, a cat-bypass would be the way to go if one is offerered and emissions can be passed. It would add a more throaty sound and also do more for the power band than a cat-back system.

Audiboy

While low restriction exhaust will make the turbos spool faster, the S4 already has fast spooling tiny turbos to make it the king or torque at a measly 1800 rpm. And changing the exhaust to very low restriction may cause the turbos to overspin at lower rpms, thus confusing the ECU as it plays with the air control valve and fuel and timing to compensate for this overspin and overboost at low rpms.
Well, of course that depends on the exhaust design in general, but IMO, if putting a low restriction exhaust, probably a good idea to increase turbo size and chip it (then again, intercooler space is tight) but 350-375 hp is possible.

Probably belongs in the Performance Forum now, but comments are welcome

RM

I really think the ECU is smart enough to control the engine parameters for maximum torque at even lower than 1850 rpm. It would an interesting experiment if a cat-bypass were available for this car. I'm sure the main restriction is in the converters and not the catback system which is probably very effiecient already.

Audiboy

True, if you keep the cats, no problem as it is not really a true low restriction exhaust