Wouldn't that mean you might have to vary the exhaust closing point depending on rpm and load (throttle opening)?

The GM 4200 I6 (Trailblazer, etc) varies the exhaust cam phasing up to 50 crank degrees. Hmmm...

 

Joining the fray late, but I thought I might be able to add something. Bear with me, I'm going to do it all in one great big lump:

this is something that I've wondered about lately. I've designed/built a few exhaust systems that basically got smaller as they went thinking exactly the same thing (ex, 3.5" collectors into the X, 3" out of the X and into the mufflers with 2.5" tailpipes). In a nutshell I wasn't able to document any power loss over running open collectors/extensions (didn't dyno though, track testing only) or running a more conventional, single sized system (say 3" all the way back), just less noise.

But what the OEM's have been doing lately, especially on some of their efficiency leaders (the diesel trucks) is just the opposite. A great example is the Cummins, most of which have a 2.5 or 2.75" OD tube coming off the turbine, and people are seeing power increases by eventually opening it up to 4" downstream. In trying to explain this the only thing I can come up with is as the exhaust cools its energy drops and it needs a less restrictive exhaust to keep flowing???

I said this here before and it fell on deaf ears… you guys could learn tons about this if you gave ported/chambered loud speaker design a shot (I've been doing it semi professionally since I was a kid). The nice thing about using it as a learning tool is that all the equations are easily available (most of it is just the helmholtz equation anyway) to describe what is happening with the assorted chambers and ports/tubes. Everything is very well documented. Take what you learn there, add some cam timing numbers and you get some rather interesting results.

These published and accepted numbers are highly overrated and as far as I can tell are just used by flowmaster to sell mufflers.

A great example of this is my '97 WS6. As an experiment I cut and flanged the stock exhaust and made a matching 3" straight pipe to replace it. In a few minutes I could change between 2 exhausts with very similar total lengths where one should be much less restrictive then the first. I found something pretty interesting with this setup. Both running 13.0@107 (basically the stock car) and 120mph with spray, no matter what I did I was unable to run a best time with the straight pipe. I always ran about .1 and .5-1mph faster with the STOCK fully muffled, 2.75" I-pipe + 2.25" tailpipes. (do the math and figure out how many hp it takes to run 120mph with almost 4000#)

I'm not the only one either. I know of a gn drivetrain living in a 4th gen formula that runs high 10's through a stock 4th gen exhaust.

This really isn't that hard to explain. Think of shorties as the exhaust equivalent to an LT1 intake. Basically untuned but flow well. Long tubes are something like a superram or a decent tunnel ram, but most have lengths that tune for midrange and feel much better on the street then comparable sorties. That being said, the tuning that you're getting isn't doing any real good for most of your dragstrip pass. Because of this, as long as you don't cause a restriction somewhere else you won't gain anything (and quite often you'll see a car run the same times with short and long tubes).

Probably the most useful thing said in this thread. If you put an exhaust (or for that matter intake, heads…) on a car that should be better matched to what you're doing and go slower then you need to rethink your cam timing. Adding a restriction to the exhaust is somewhat like kicking yourself in the nuts because you got some chick pregnant… They (your nuts and the exhaust) did what they were supposed to do. It's not their fault that the brain (cam) is doing something wrong.

 

 

I'm not a big diesel guy, but I think the Banks, et al. chips or programs are adding lots of fuel to get more power, and they need more exhaust flow capacity because they are drastically increasing the flow. However, "down pipes" from the turbo are very expensive and maybe aren't worth all the extra cost. They are probably high-end stainless or maybe inconel if they are expected to run many 100 of thousands of miles.

The "cat-back" (diesels don't have cats (?) but are getting cat-like particulate filters added) 4 inch systems are effective and relatively cheap. They also look cool,of course.

If the OEM muffler was the restriction (good chance), it only needed the smaller tailpipe because the restrictive muffler absorbed more of the heat, so output flow was less. Less restrictive muffler and more flow (also because of added fuel) needs the larger tailpipe.

That's my take.