OK... in response to the actualy materila you posted Cog, what can we say?

if we look at your chart at the top, we see that recommended maximum exhaust "backpressure" (which is simply pressure evident immediately after the turbine, and, i suspect, in a 'static well' (i.e measure the pressure in a pocket of 'still' gas), shoudl be 3" mercury max.

lets say you want to go for half that, at 1.5"Hg.

now, we'll calculate a nominal CFM value (and so I can check how you've got yours).

we will assume the volumetric eff constant to be 2.5 (a VERY turbo'd diesel). this number actually comes from the "pressure ratio" x VE. if the actual VE is about 0.8, and pressure ratio is about 2.5 (boost about 22psi), then that ve number would be 2. But, we'll give it a bit more for when you turn up the boost. Displacement in CI is about 122. so we will say that at 5000rpm

CFM = 2.5 x 5000 x 122 / 3456 = 441CFM

to make that into exhaust flow, from the formula on the chart we have

Icfm x (1200 + 460)/540 = 1355CFM

The reason I have put 1200 in the calc instead of 1000, is because Cog has already measured his EGT's and it is fair to say that under high load this is a much more accurate value (I also see these tems in my diesel).

So there you go, we are flowing 1355CFM of exhaust gas at 1200F. What pipe size do we need for an ideal 1.5"Hg exhaust presure?
using
http://www.pressure-drop.com/Online-...tor/index.html

and plugging in some nominal values such as a predicted gas density of 0.4kg/m3, dynamic viscosity of about 0.34kgmsx10^-6, roughness of 0.3mm (pretty dirty) ,adn length of 7m (because we assume a 90degree bend adds roughly a meter, and there are a couple of them - hey, this is fluids people!)

a 3" (76mm) pipe will give a pressure drop of 0.09bar or 2.7"Hg
a 3.5" (89mm) pipe will give a pressure drop of just 1.2"Hg.

So there ya go Cog, ideally you want a little bigger than 3", but 3" will probably do.