LeonV

There is no such thing.

You will not lose torque by changing the exhaust pipe. The header and collector can change the powerband, but the rest of the system really doesn't. The post-collector pipe should be as big as required to route the gasses while minimizing losses. Any bigger and it's just a waste of space and material, but not torque.

blasphemy101

1) I believe an exhaust can be over-sized; too large of a diameter and the exhaust gas loses velocity, becoming a restriction for new exhaust gases entering the pipe.

2) In either case, I would have experienced a loss in gas mileage just as dramatic as the mileage I regained after my maintenance, instead of a slow, steady decline.

LeonV

Actually, the physics of this is opposite to what you think. Higher velocity equals more restriction, this is a fundamental principle in pipe flow dynamics.

It's a very common misconception to think of exhaust flow in terms of velocity (and wanting to keep it high), since it's something that can be visualized easily. However, this leads to confusion as it's not really what we're really after. It is better to look at it in terms of volumetric flow as it's more descriptive of what is actually going on. Basically we are saying that we want to know more than just how fast the exhaust is moving, since it's only part of the equation, but also how much of it is moving.

A given engine can only flow so much mass through it, and that amount depends on RPM, displacement and volumetric efficiency at that RPM. The more fuel mass you burn, the bigger the "boom" you get. We want to increase flow, which means we must increase one or more of those properties, without overly (if at all) decreasing the others. We can't use the exhaust pipe to increase engine displacement or directly increase RPM limits, so throw those out. All that is left is VE (volumetric efficiency). VE is what essentially determines your torque curve.

When keeping exhaust diameter small, frictional losses increase and thus exhaust pressures increase. The higher exhaust pressure increase the amount of unburned fuel that stays in the combustion chamber, which drops VE. Moreso, increased exhaust pressure increases pumping losses. Pumping losses are the sum of the power lost due to intake and exhaust cycles, i.e. how much power your engine has to use up in order to suck in and push out. A smaller pipe will make it harder for the engine to push that gas out. This is the reason for my comment, in a nutshell.

In the exhaust design field, it is desired to keep the pipe as straight as possible (minimizing "minor losses") and large enough diametrically as not to cause "major losses", while at the same time physically packaging it under the car and maximizing ground clearance. The header and collector are what influence the powerband's characteristics, but that is a whole different discussion! The job of the pipe, post-collector is to get rid of those exhaust gasses efficiently (also possibly quietly and without fumes ).