I did a search and came up empty, so I thought I would ask the engineers. Is there a straight forward calculation that will show flow rate over a given distance on a specific diameter tube?
Question relates to exhaust being expelled from any given turbine exhaust housing then travelling through a length of exhaust tubing.

 

I did a rear turbo setup not to long ago, and there was nothing but figuring calculations to get it to work right.

I would look around searching for rear mount turbo info/specs/sizing, thats how i found a site on flow amounts and pipe sizes.
I thought i had it in my favorites, but its not there anymore.

I remember when i did it, I figured out that I needed to use no bigger then 2.5", and no smaller then 2.25", with a total of 10.5' of pipe, and 7 bends.
But that was feeding the turbo.
I would think you would do the opposite, if your trying to get rid of the gasses. Go from 2.5 off the turbo back about 5', then increase to 3" the next 5'.
But then again it depends on turbo size, engine size, etc.

But like i said, Look around for rear turbo info, thats how I found all my info i pipe sizing, and how much air they flow.
I beleive it was on one of the chevy forums, they talked all about the calculations.
I look around again, try and find it.

 

when in doubt, put the biggest friggin peice of tubing you can under it. that way you know you wont be restricting it.

 

ok, I found some notes I had, all the info I used for my build to figure out some stuff.

MaxBoost uses .4 Mach (450 ft/sec) as their rule of thumb for upping tube size.

Another rule of thumb is each 90 degrees worth of smooth bend is good for a 1% flow loss.

One other thing to consider is boost lag. From Maximum Boost (p. 49),
Time = 2* Volume / Flow Rate

For a 2" pipe, 18' long, volume is 679 cubic inches. 2.25" has 26% more volume (859 cu inches).

For 150 cfm at cruise: 2 * 60 * 679in^3 / 1728 / 150 cfm = .15 sec
2.25" would be .2 seconds lag.

19' of 2.25" pipe: .15sec * (19/18) * (1.26) = .2 sec
The (19/18) is for you using 19' of pipe
The 1.26 is for you using 2.25" pipe instead of 2.0" pipe

Including 4 90* bends: .2 * 1.04 = .207 sec

Using the 450ft/sec comes out to 745cfm MAX. for 2.25" pipe. Using the generic 2.2 HP/cfm then it comes out to 338 HP.
That means these calcs also recommend 2.25" or 2.5" pipe.

I hope this helps a lil'.

 

LOL, yea thats true, good for gettin rid of it, but not if you wanna keep it.
Some times you need a lil' back pressure.
I just put straight pipes on my cobra yesturday.
And man, besides sounding like a pack of harleys, and being just plain outrageously loud, but kinda cool.
That 32valve has NO POWER down low,
but it screams at 6800 rpm LOL

The neighbors dont like it at all- > LOL, I dont like them either.

 

Temps have a hand in this also.
EX. Pipes near the heat source need to be larger due to higher pressure as compared to tail pipes where you can reduce the size due
to exhaust gases cooling (less psi).
Another thing to consider: If you knew where under your car a low pressure area exist at speed, you could exit the exhaust there and pull the exhaust out. (turbocharging you turbocharger)

 

Hello,

I design thermofluid piping networks for powerplants to transfer natural gas and saturated water.

Your pressure loss will be dependant on a few factors,

reynolds number
surface friction
momentum loss (due to sudden changes in direction of flow, or contractions and expansions)

minimizing all of these will promote less pressure drop.

You can:

Choose smooth surface piping to reduce friction losses which is very evident at lower reynolds number, which is not the case with us turbo turbulent flows, but still beneficial to have smooth pipe.

Increase pipe diameter, this is a VERY sensitive parameter to pressure drop. Pressure drop is proportianl to the inverse of the FIFTH power of pipe inside diameter.

Pipe diameter also has a large effect on pressure drop which shows up in the e/D term of the colebrook equation for friction factor. Bump up Diameter to reduce the relative roughness if you cant reduce the actual surface roughness.

The working fluid cannot change, therefore your reynolds number will have a single working viscosity (although this isnt entirely true, you can assume this)

Use as little bends as possible, if you have to bend (and this is unavoidable) use large r/D factors (radius of bend to pipe diameter).

Bigger is not always better as we all know, too big of a pipe can reduce velocity albeit also reducing pressure drop. Its a give and take game.

One way to exploit the fluid is by have large piping going to the turbos, and by about 8-12 diameters before the turbine inlet cone down to a gradual contraction which will accelerate the gas back up to desired velocity. This contraction will prove more pressure drop in the local vicinity but i will bet you the net pressure drop will be less.

I am really interested in rear mount turbo setups, as they offer one huge advantage, less under hood heat and better turbo cooling since they are in a direct stream of air.

The pressure drop may be adequate if DONE RIGHT. Rear mount turbos arent a bad idea in my book, although they have flaws, but I wont get into that.

Good luck


P.S. 570Cobra makes a good point, the larger the volume of air in the cold side piping the greater the fluid inertance and thus turbo lag. Again, this is another give and take game, too big of a pipe on the cold side will increase the fluid steady state mass which increases lag. Smaller pipe will decrease mass and help response. The smaller the pipe, the quicker the engine gets the pressure signal from the compressor, but the greater the pressure drop.

In the end its all about what you want, you make the compromises, and calculations. Design the setup before you build it and the rear mount will scream I gaurantee.

 

thats what i was gonna say......