NA Exhaust System
 

Hello!

Hoping for some advice, I've tried researching this but I've struggled to find the answers. My exhaust is completely shot, so I'm looking to get a replacement in the form of a complete stainless system, but due to finances can only afford a used one.

I've been offered one with the following width/bores:
Exhaust manifold is 2 45mm/1 3/4" ID pipes into 1 64mm/2.5" ID pipe then splits again to 51mm/2" OD pipes.
From something I remember reading at one point, if the pipes are too large there is a risk of power loss, which I of course do not want to risk, so could anybody advise me as to whether or not this would be ok my for stock NA FC?

Many Thanks,
Jon

Those pipe diameters seem sufficient. The problem on N/A S4's is that the auxiliary port actuators use exhaust pressure to rotate the auxiliary intake port valves open at around 4k rpm. The tube that facilitates this is attached to the factory catalytic converter air injection tube. Some aftermarket exhausts don't have this tube, and even if they did there isn't enough pressure in the system to move these actuators due to piping that is too large. My system has a Racing Beat "road race" header on it. This header has a pickup tube on it pointed directly into the front exhaust port. Racing Beat also sells a catalytic converter delete pipe (I am however not sure if it is stainless or not.) The cat delete pipe, also known as a test pipe, will attach to the exhaust where the main cat usually goes, and has a tube with a flange that will connect to the factory air injection tube. You could also opt for an aftermarket catalytic converter, but there are no guarantees it will have the tube on it. If you live in an emission control area, the cat delete pipe might get you in trouble if you install it on a street driven car. None of the above applies to S5 N/A cars, as their auxiliary ports are directly actuated by the air injection pump, not exhaust system pressure, or so I have been told.

Those sizes sound about right. Does this system have any sort of pre-silencer/resonator in it? On a n/a you'll want some sort of pre-silencer or muffler somewhere before the main mufflers at the rear to help cut the noise down. As to the tube for the aux port activation on a S4. The Racing Beat pre-silencer for the 86-88 cars have it, the universal ones do not. Also you could make your own tube but you can't just drill a hole and weld a tube to it. This is why I believe some after market exhaust doesn't work with the actuators. It's not backpressure it's the tube setup. Their( RB) tube bends 90 degrees once in the exhaust path so the opening of the tube is directly in the exhaust flow. Seems they are using the exhaust pressure to open them, not backpressure.

S5 uses the air pump as previously stated.

2 inch is okay for the back i think thats what the size is stock. i had a custom single straight pipe 2.5 from the cat back and i noticed a hell of a difference only costed $150 as well. only downfall i had RB downpipe and presilencer which made it extra loud.