From what I just found on my truck you can run a 20 inch exhaust, if you have a stock size turbo it don't matter, your still runnin the stock exhaust size. I just pulled the stock elbow off the back side of the turbo to replace it with the Megamouth Elbow and what a shocker. The hole on the elbow is even smaller in diameter then the stock exhaust was. Now going under the rule that we use for fuel, that your flow is restricted by the smallest passage way in the flow, then the size of your exhaust means absolutley nothing if you are running the stock exhaust elbow off your turbo.

I will get some pictures posted later in the evening on this to show what I'm talking about. It's scary when you see the restriction.

 

This photo shows the end that connects to the back of the turbo. The turbo outlet itself is the same size and shape as the whole plate that you see there. So as exhaust is exiting the turbo, as you can see from the black coating half of the exiting gasses are hitting a dead end wall and having to recirculate before escaping. If the porting and polishing we do to the exhaust manifolds make that big a difference in performance, I can't imagine what this is doing to air flow.



This opening measures 2 1/2 inches across for inside diameter. So no matter how big of an exhaust you are running, if you have the stock turbo and elbow you are running a 2 1/2" inch exhaust.

 

I suspect this also applies to the 1st and 3rd gen trucks also.

 

how much did you end up paying for that megamouth thing

 

I got it out of the classifieds on here, used. Think it was $150. They are $199 off the site new but it's a 6-8 week wait. He makes em up for each order and then they have to ship from Canada.

 

The elbow on the 3rd gens is nothing like that one. It has no "flat" area, it's a completely round unit that mounts with a band clamp, no bolt holes like I see in the one you show.

 

I'm glad to hear that they did improve on it cause this is crap.

 

And the 3rd gens are 4" now...

 

This is a fairly major issues, gotta be some strong opinions out there someplace about it. Somebody tell me I'm wrong in how I'm lookin at this and explain why. I'm havin a really hard time beleiving that with all the advancements we have made on these trucks that something this critical could go mostly unnoticed.

 

Thats not compleeeeetely true Dennis. Imagine having a pipe that is 20' long...the surface area on the ID of that pipe = X amount of restriction to flow. The longer the pipe the more restriction. The same ID pipe at 5' will have far less restriction. Now factor in the scavenging affect. When you try to force a liquid or gas from a large pipe, through an orifice, then to a larger pipe again you get a scavenging affect which increased the velocity. SO, this is why you see a 200-300* temperature drop in EGT's just by changing to a turbo back exhaust. BUT, yes...making the flange or orifice larger will move more liquid or gas and decrease velocity at the expense of lagging the turbo because of reduced backpressure. The restriction at the flange for a mildly modified truck is actually a good thing.