Yeah, yeah, bigger is better an' all that :)
Honest question here.. I'm looking at playing with das turbo, and in the past in a car I've always just done a 3" exhaust on anything turbo with an off-the-shelf solution. Now that I'm looking to fab my own setup, it got me to thinking about it.
If the turbo outlet is, say, 1.75" ID at the v-band, how much benefit is there to going with a significantly larger pipe?
Well, I would say “bigger than the outlet is better”. It’s supposed to flow as freely as possible, so anything larger than the turbo outlet should suffice.
Someone smarter than me will eventually chime in with a bunch of mathematical equations and diagrams and charts.
Unless I'm wrong, and someone will correct me if I am, turbos get their energy from the pressure differential across the turbine. To recover the most energy you want the largest differential, assuming everything is within design parameters. So that means you capture more energy, and can create more boost, by removing as much exhaust as you possibly can right up to where you run screamer pipes through the hood, which is what I suggest. 
Failing that, big short pipes.
You want to minimize resistance to flow downstream of the turbo. Resistance to flow is dependent upon the velocity of gases in the pipe, and changes in direction of the gases in the pipe. BIgger pipe, lower velocity, lower resistance to flow.
So, bigger IS better, but to tell whether what is to be gained by going to 4" pipe over 3" pipe is significant, I'd need more info. Like how many cfm are you moving through the engine?
Another advantage of "bigger" pipes is the flow coming out of the turbo is rotational, I.E. it's spinning around. What you want is linear flow and to do that you have to cause the spinning gases to break up their motion. A big pipe does that.
Wouldn't the lower velocity of dumping a 1.75" pipe (off the turbo) into a 3" chamber (downpipe) cause its own sort of issues? I wasn't trying to calculate exactly for this situation at the moment, more of trying to get a general understanding of "X% larger area during this transition has the best tradeoff of velocity vs. pressure," "exducer * N" or something along those lines..
Everything that I've ever seen searching (and I've looked quite a bit) basically just repeats "Bigger is better, bro! I'd go 3" at least!" I guess there's a much deeper understanding of the fluid dynamics at play to understand how a 3" flange mated up to a 1.75" hole doesn't hurt due to turbulence?
A step-up in diameter will decelerate the exhaust gases which will create some backpressure. I don't see how having an exhaust bigger than the outlet would be beneficial. Most turbo exhausts have a step-up after the turbine because the wastegate exhaust re-joins there (which is the worst thing to do with wastegate exhaust, performance-wise).
Transitions should be gradual to minimize turbulence. Pretty good discussion here, albeit in an unusual place:
http://www.tercelreference.com/tercel_info/turbo_exhaust_theory/turbo_exhaust_theory.html
"Downstream of the turbine (aka the turboback exhaust), you want the least backpressure possible. No ifs, ands, or buts. Stick a Hoover on the tailpipe if you can. The general rule of "larger is better" (to the point of diminishing returns) of turboback exhausts is valid. Here, the idea is to minimize the pressure downstream of the turbine in order to make the most effective use of the pressure that is being generated upstream of the turbine. Remember, a turbine operates via a pressure ratio. For a given turbine inlet pressure, you will get the highest pressure ratio across the turbine when you have the lowest possible discharge pressure. This means the turbine is able to do the most amount of work possible (i.e. drive the compressor and make boost) with the available inlet pressure.
Again, less pressure downstream of the turbine is goodness. This approach minimizes the time-to-boost (maximizes boost response) and will improve engine VE throughout the rev range.
As for 2.5" vs. 3.0", the "best" turboback exhaust depends on the amount of flow, or horsepower. At 250 hp, 2.5" is fine. Going to 3" at this power level won't get you much, if anything, other than a louder exhaust note. 300 hp and you're definitely suboptimal with 2.5". For 400-450 hp, even 3" is on the small side.”
^ That's a good link, thanks T2! It does match with my goals of this project of < 250 RWHP, too, which is nice to see them say 2.5" should be fine.
Gameboy - You bring up an interesting point.. I wonder if it would be beneficial to actually create a better flange-to-Vband "manifold" get get me an outlet from 2.5" from the get-go.
I can probably optimize the outlet location a bit better for the integrated wastegate while I'm there, per T2's link.
FWIW, my experience was that even at 250 hp going from a 2.5" exhaust to a 3.0" netted 10 hp and a couple hundred RPM of spool.
Keep in mind that the turbine operates off a pressure ratio which is typically going to be slightly higher than the pressure ratio of the compressor. Say you're running 15 psi of boost, that's a compressor pressure ratio of at least 2:1 (at sea level), and probably more like 2.2:1 after accounting for the pressure drop in the intercooler. If your turbine ratio is slightly higher at 2.4:1, then every psi of backpressure in the muffler, cat, and other exhaust components gets turned into 2.4 psi in the exhaust manifold.
You'll need to log in to post.
