The bold and underlined part are the most important point made in this entire thread, and frankly the only thing posted in this thread with actual science behind it (note that I am not discounting the truthfullness of what some here have said, only that there is a lot of bench racing, and some anecdotal evidence thrown in).

Without a cone dischange (I've always gone with 7 degree from the centerline, or 14 total conical degrees), you will get turbulence. Turbulence creates a vena contracta, and will actually increase the back pressure. So having said that, I will completely acknowledge that it is impossible to put a cone discharge on the back of a turbo (at least on a street car). So without the ability to do that, the next best thing is to have the downpipe start out at the same ID as the turbine's minor diameter, and get larger from there. In theory, until either infinity, or until you'll have laminar flow at the exhaust's exit. As that isn't possible, you eventually get back to the fact that you want it laminar at exit, and that the longer the pipe (of constant diameter), the greater the restriction.

The guy's post is really good, but he does contradict himself when he says this:
"There's more to it, though-- if a larger bellmouth is excessively large right at the turbine discharge (a large step diameter increase), there will be an unrecoverable dump loss that will contribute to backpressure. This is why a gradual increase in diameter, like the conical diffuser mentioned earlier, is desirable at the turbine discharge."

It isn't just a bellmouth that will cause that dump loss that contributes to back pressure. Any big pipe. Dumping it to the atmosphere will be worse than having a slightly undersized pipe. Not trying to pick nits, but the point of brining that up is that, even though the general rule of thumb that "bigger is better" usually holds true, indeed that is not always the case, as the phenomenon he mentions above will come into play.

In other words, there is some truth to both sides being argued, but you might have to go to extremes to prove it out. I'm inclined to think that with my puny little 26/8, anything bigger than a 3" exhaust would probably actually hurt me more than it helped (unless, of course, the downpipe gradually got bigger). But if I had the Stage III Garrett turbine on my car, I'd be inclined to think that I might benefit from something bigger than the 3" exhaust. Just a guess, though, I would run the numbers before I dropped the coin.

As for bends, the tighter the bend, the greater the restriction. It becomes linear when the radius of curvature = 2*ID of pipe, meaning getting a larger bend doesn't help you much. I don't have all of my old notes in front of me, and if I did, no one would go through said calculations anyway, but if you really want to make power, you can't ignore the the fluid dynamics behind it, nor can you dismiss it as merely "theory."

Anyway, I gotta run. Taking the track car to Road Atlanta tomorrow, and it still isn't together from a lot of work I started last month (con rod bearings, bigger brakes, wheel bearings and longer studs, 26/8 turbo, etc, etc, etc.). I'll be lucky if I get to sleep tonight!

 

If you are confusing the inventor of the telephone with a guy named "Corky," Corky Bell wrote a lot of stuff that is completely wrong in his book. Great for newbies, not so great for people who no what they are doing.

 

Done before I owned the car, I'll check if there's any identification on the chips.

 

That applies to NA cars.

 

20 years ago I worked on the chassis of the Nissan group C cars that raced at Le mans, they used twin turbo V8's and I remember being struck by the turbo's, they had huge compressor housings and relatively tiny exhaust housings, someone knew what they were doing though as one of the cars started from pole and finished 6th.

 

i agree with the smaller turbo opinion.

goodluck with the track car. i hope you make it to your event

 

Just had to add that to my list of favorite Rennlist quotations

Joking aside...just to be absolutely clear, the ideal exhaust setup on a turbocharged car would be a simple hyperbolic velocity stack bolted directly to the turbocharger exhaust outlet (probably with a mesh screen on it just to be safe), yes?

Of course it would be very loud, probably be blowing hot exhaust gases at something that responds poorly to hot exhaust gases, and would have no provisions for any sort of emissions controls...but the turbocharger itself would operate at the highest efficiency possible, correct?

 

No mate! Funny statement but I do know the difference and I have both books by both authors Corky Bells book is a little dated....

Search on amazon and you will find!

 

Lol - yep:

 

I guess that means that the famous first telephone call quote was actually “Mr. Watson, come here – I want you to tension the supercharger belt”….

 

This year, I saw two 951 race cars setup with side exhausts...something very much like the photo below. They had wrapped the fuel lines in heat tape, there was a heat blanket between the exhaust system and the floorboard and the routing from the down pipe to this exhaust was very straight.

They were both certain that they were seeing measurable improvement in spool rate...based on the way they pulled away from me on several occasions, I'm inclined to think they may be right.

Rick

 

Yep, thats a 'boom tube' exhaust made popular by Dr Gas (!). I have used them and I like them. They are quite ‘thin’ and don’t take up much space. When I use them for track cars the plastic side skirt gets trimmed (if it still exists!). O the really serious track cars the exhaust ends up above the floor pan boxed in with a lot of furnace insulation.

 

So my theory was right???

 

917

http://farm1.static.flickr.com/92/21...9aa6dd5b_b.jpg

 

Mre important.. LOOK AT THAT OIL COOLER!