Exhaust pipe diameter

Regarading the exhaust piping size, which of the following makes more sense-

1. Use a pipe of slightly larger than stock diameter between the headers and the mid muffler, and stock diameter between mid and rear muffler- the idea is that exhaust cools down and decreases in volume along the way, so we need a somewhat smaller diameter piping near the end to maintain the flow speed;

2. Use the reverse of the above and actually using a larger diameter piping towards the muffler, as I've read being done on a 400bhp drag Integra with B18C in a car modding mag. This is also a feature in one of APEXi's exhaust system design, the reason being something like slowing down the flow to match that of the boundary layer (the exhaust flows slower close to the pipe's internal surface) to reduce turbulence due to the speed difference.

Generally speaking the theory behind #1 is correct, and is actually used in many production cars.

Very true,

A lot depends on your application, and i'm sure Apexi has done some very in depth exhaust tuning, but i would stick with theory #1. If you are custom fabbing an exhaust though, theaory #1 sounds more like a collector, which is usually integrated into the header design. Collector shape and length both have efects on exhaust tuning. I can't remenber formula of the top of my head, but i know there are for both. Remember, inuction tuning goes from intake to tailpipe, don't leave anything out of the equation.

I like #1. Any time you ask the exhaust to change velocity, it won't like it. That's why so many exhausts make so much noise at the tailpipe, and its partly why some exhausts sound "blatty" and some rumble. Most of that is in the muffler, but tail pipe design has a lot to do with it.

In general, if you had a straight pipe that was a constant diameter, the cooling (and therefore slowing) would not cause any turbulence and the design would be fine. You might notice tiny little torque gains by tapering the pipe from its original diameter down by about 1/4" to the tip, but since we're dealing with bends, mufflers, and too many unknowns, your idea is sound.

Placing the diameter change before or after a muffler is smart since the exhaust is already turbulent there. Randomly placing a step down somewhere in the piping will piss it off.

The exhaust actually don't cool down as much as one might think, and if the car is fitted with a catalyst the temperature is actually higher after the cat than before.

This, together with that a high speed flow doesn't like diameter changes, typically one diameter is recommended.

2. There is always a difference in velocity in a tube, the boundary layer does move slower due to the friction. To get the lowest friction, and thereby pressure loss, you want the flow to be slow (large pipes).

hmm. never though of it that way. but exaust isnt exactly cold. in a friends '83 mudd truck with straight pipes we measured the exaust tempature at the manifold colector and at the tail pipe. if i recall correctly it was ~1020 in the colector and ~750 4" inside the end of the exaust. if exaust stayed equaly hot through out a continuesly larger diameter would be best. wouldnt it? im not 100% sure but it would create a vaccume makingthe newly emited exaust want to move there.

Hmmm.... do I see satin-finished low internal friction exhaust pipe coming?? ^_^

Or maybe heat-wrapping the rear end of the exhaust pipe?

Hmmm.... do I see satin-finished low internal friction exhaust pipe coming?? ^_^

Or maybe heat-wrapping the rear end of the exhaust pipe?
Heat wrapping does infact increase friction, the faster the exhaust cools the slower its velocity will be. Heat wrapping can be good to help keep the exhaust temperature up to the way to the turbo or catalyst, or to protect other parts from the exhaust heat but that's basicly it.

For protection against heat radiation car manufacturers typically used heat shields, sometimes double wall exhaust systems is used on the way to the catalyst to get it to warm up more quickly.

Wouldn't slower exhaust flow create more backpressure?

Wouldn't slower exhaust flow create more backpressure?
It depends on what is making it slow. If its slow due to a restriction, then the answer is yes.
If its slow because the exhaust diameter is large, then no.