Yep, but even in turbo setups the exhaust gasses must be routed the best way out. That means: the bigger the exhaust diameter gets the more youll have to push the gasses out with more gasses...
To create velosity in the gasses you step up in diameter...
This way the expantion off the gasses forces itself out off the exhaust...

 

Uhh...so? Sell it to Eric.


Damn stagnation and flow eddys. haha.

Paul's is 3" "bell-mouth"; 3" pipe smooshed oval right at the turbine flange. I don't know how much power or spool he'd make with a nice divorced WG DP using a smooth trumpet-horn-like transition at the turbine outlet. Whatever increases are there, they aren't worth the extra time or cost, IMHO. Unless you want to be badass and impress everyone with fancy words like stagnation and flow eddys.

EDIT: Any exhaust will "work". Me saying I built this XX times and it works great is fine and all, doesn't mean another setup won't work better. Christ I'm sure a 2.25" crush bent pipe would work fine for most people.

 

Or twin turbos

 

HA! Yes!

But there ya go, 1 turbo is better, until I do a sequential. But that may never happen as I finally sold the last bit on that kit (horray!)

 

The velocity of the gas as it heads down the exhaust path decreases as it cools, negating the need to maintain the same diameter the further back you get. But I'm a computer guy not an injuneer. Sounds good when I type it.

Frank

 

Actually it's the other way around. As the gas slows down you want to increase size to create more internal volume.

 

yep, the gasses expand when they cool, so you wanna go bigger so they don't fill the volume and start to create backpressue.

 

4" ftmfw!!1!!!

flow>volume, however, best results are if you have both.
ex: If your DP flange is 2.5" diameter, then the ideal thing if you must use an exhaust would be a transition from 2.5" to whatever your exhaust is (3" more than likely).
The length of the transition will be whatever you have room for, but it should be right off the turbine housing for the best results. This way you have a smooth expanding exit right off the turbine wheel that promotes the gases to accelerate as they come out. This will help spool and efficiency.

 

Sounds Good Zabac If only we had the room to do this nice and close. It is tight on 2000 with FMII installed.

 

Yes, that is why I am noy buying anything off the shelf and making my own ****, for this very reason. I have no AC or PS and can place my turbo anywhere i want, within reason, so Im gonna place the turbo where i want it and mock the mani/DP according to my location, simple enough. Plus I found this perfect pipe at DH Griffin, it is 3" with a 45* mandrel bend which turns into a reducer about 4" long, should be perfect for what I want.

 

rly.

if you've ever been toobing or canoeing or kayaking on a flowing river, you'd know that the river stops dead behind rocks or other large objects in the water. you can sit there all day because the flow of the river is pushing you back up against the rock.

the situation with gases is slightly different but similar. you generate backflow turbulence.

If you dig through engineering books on pipe flow, there's actually an end correction factor for transitional flow like that. Because the gas is going directly from a high pressure to a low pressure, it stagnates.

it's basically aerodynamics, only the flow is on the inside instead of the outside. that's why the backs of streamlined objects aren't flat blocks but smooth down to a point. the inverse of that is your ideal downpipe.

I was trying to find a nice graphic but I can't... I'll have to dig out the fluids book at home.

 

Gas expands when it cools eh...

Remind me to not take that hot air balloon ride.

I also better bolt my refrigerator door shut.

 

It doesn't expand due to cooling, but because gasses expand to fill their container. As the gas velocity slows, you also have it expand to reduce the amount of energy it takes to push it out of the exhaust.

 

Huh? When I ran nitrous we increased tank pressure by heating the bottle. That would indicated to me that gas expands when it's heated. Or is there another chemical reaction going on here?

When I was getting my diff mount welded in the Miata, gas starting coming out of one of my disconnected fuel lines because of the heat from the welder.

What am I missing here?

Frank

 

haha. whoops. what i meant to say was the velocity decreases as it cools. you wanna go bigger volume, so when the exhaust starts to slow it's easy for it to find it's way to the low pressure zone. if not, the exhaust will backup in the smaller pipes and start to create backpressure.....less energy wasted pushing against the exhaust, is more energy to push down pistons.

sorry bout that.

 

Wasn't there a recent thread about scott hitting his head on a rollbar?

 

yeah yeah yeah, my fingers weren't following what i trying to say...

 

Explain again why I have this?
https://www.miataturbo.net/forum/showthread.php?t=10096

Admittedly, I do enjoy that.

 

Because you were willing to pay more for the (probably) little gain over a 3" bellmouth. Maybe your downpipe does make a lot more than a bellmouth would, I don't know; you'd have to compare them back to back on the same car/same setup otherwise.