I was going through the two catalytic converter posts I'm participating in, and I came up with this question. It is reported that removing a cat. converter causes low end power loss. Here'* some of what I would like to ask:
Is it because of the way that the 3800 makes power that it needs back pressure, or laminar flow out of the exhaust ports, to get better low end power? I would like to start with a Radomir classic: A hypothetical. If I were to install 5" exhaust piping after 2.5" header pipes coming from each exhaust port and port matching, with of course a 5" in/out catalytic converter and muffler, what would it take in my engine to get either the same low end power as I have now or more of that?

Now moving away from that to a bigger picture, what element of the 3800'* design causes loss of low end power when a loss of back pressure occurs?

Dig around with the search. There is a lot of info here.

The over simplified version is "gas velocity". With properly sized piping the high velocity exhaust from one cylinder "pulls" the exhaust from the next. If the piping is too large the exhaust "stalls". Think of a fast moving on ramp merging into a traffic jam.

In talking with an exhaust expert this is tough one to nail down and requires much trial and error to get it right.
As an example Nascar teams have an exhaust guy that continualy tests and tunes.
If it were a formula or a hard set of rules then crew chief would/could do it.

Mike has it covered though, the exhaust of one pull the other

How to offset this? Make more boost (power) than needed but it would still benifit from a good exhaust tune.

There is also the issue of cylinder scavenging. With proper exhaust velocity, the exiting exhaust helps "pull" intake charge into the cylinder during the cam overlap period.

In your hypothetical example (2.5" header pipes), you would have so little exhaust velocity that you would get almost no scavenging effect.

This is a quote from Scott Stutler(engineer) of DynoMax
" For any given power output and displacement, a generic sizing chart can get you in the ballpark when choosing exhaust-pipe diameter. However, engines equipped with power adders have a different set of needs. "If you're running a turbo or supercharger, there'* no such thing as an exhaust that'* to big," Scott explains. For forced induction, the bigger the better. "In a race application, on a nitrous motor, the exhaust should be bulit in respect to the max horsepower it makes on the bottle. Conversely, in a street application with nitrous, the exhaust system should be built in respect to the maximum horsepower the motor makes off the bottle. That way, you'll optimize the exhaust system for the operating conditinos that the engine sees the most"

A motor that has positive intake pressure(boost) does not need the scavenging effect from the exhaust that an n/a motor needs to make power. And don't confuse scavenging with backpressure, you want exhaust "scavenging" in an n/a motor not backpressure.

Let me quote Scott Stutler(engineer) of DynoMax again,

"Some people say a motor needs a bit of backpressure to preserve low-end torque, while others argue that the less backpressure you have the better. "The fact is, an engine does not need backpressure", Scott says. What it needs is scavenging, which is directly related to exhaust velocity. A common misconception is that you'll lose some low-end torque by stepping up to a larger after-cat system. What'* really happening is that the larger-diameter piping is moving the rpm range where the scavenging takes place higher up the powerband. Since exhaust velocity increases with rpm, you're not losing power but moving it higher up in the rpm band instead"


ROD

On an engine designed and built for low-end torque, that'* the same as 'losing'.

From the cat back, we typically see no difference other than several of us who have noticed a remarkable loss in low-end for short periods of time without a Cat. From there back, you're not going to affect performance much.

Messing with the EM runners, collectors, and DP will dramatically affect performance.

Wow, a lot of information here! Thanks for all the input so far. I'll read up and get back to you. There'* some very good info here, and now I feel a lot better about exhaust details.

I was told by 1337ssei'* dad that in diagrams that show how to maximize traffic flow, cars should slow down to let others merge in front of them, and that keeps everyone moving the whole time. Is this practically how exhaust gases should flow as well?

While I'm on really weird hypotheses, what if I were to find a way to attach independent vacuum hoses to the exhaust ports? Would engine efficiency increase? Even though I may never install vacuum hoses on my exhaust ports, I'd like to know if there'* a limit to how well exhaust gases can flow, and what they'd do for the engine at peak flowing efficiency.

Vacuum is created by the manifold. What are you going to do under boost?

Even if you didn't have boost, you're going to suck in burnt up exhaust fumes (carbon monoxide) that doesn't combust and has no oxygen in it, and try to combust it again?

Oh, I meant running an electric vacuum cleaner or something. Like a Shop Vac. Probably not going to work, but it'* just a thought considering how we've found it'* important to keep moving gases evenly through the pipes.