Got a few questions reg. exhaust gas and turbo's

[bu-bye]

Ok guys I have some questions. How fast do exhaust gasses move in the pipe? What CFM is needed to power a small turbo. What CFM is needed for XXX PSI of boost? How much does the size of header pipe slow or decrease full CFM at WOT? How does the displacement of the engine increase CFM? So what is the CFM for saaaaaaaay a 2.0 vs a 3.8? thanks for the help.

[Josh R.]

first in computing the best turbo for an engine you must know a couple of things.

engine size- 231 CID
engine type- 4 stroke
maximun speed- 5600 rpm
maximun boost pressure - well just say 10 psi
ambient temp- 70*F is a fair #
barometer- 29 in Hg. is also a fair #


1.)start by converting displacement to cubic feet:
231 cu in.
------------------ = .134 cu ft.
1728 cu in./cu ft.


2.)next calculate the ideal volume flow through the engine:
.134 cu ft. 5600 rpm
------------ X ------------ = 375.2 cfm
revolution 2


3.)but that's at a volumetric efficiency of 100%, so reduce the VE to 80% for at least a closer guess:
375.2 X .8 = 300.16


4.)assuming a 65% compressor efficiency, calculate the density of the intake-manifold air(multipling by 2.03 converts psi to in. Hg.
manifold pressure abs
pressure ratio = ---------------------------
inlet pressure abs

10 psi + barometer
= ------------------------
barometer

(10 x 2.03) + 29.0
= ----------------------
29.0

= 1.7

5.)use a table to find Y for normal air and diatomic gases, which turns out to be .162


6.)ideal temp rise-AT(ideal): where A means delta, also note that 460*F is added to convert to degrees Rankine)
AT(ideal) = Y x T
because inlet temp is 70*F
AT(ideal) = .162 x (70*F + 460*F)

= 85.9*F


7.)Actual temp rise-AT(actual):
AT(ideal)
AT(actual) = --------------------------
compressor efficiency

85.9
= --------
.65

= 132*F

8.)then calculate the intake manifold temp
= compressor-inlet temp + AT(actual)
= 70*F + 132*F
= 202*F
air will not actually be this hot due to the cooling effect of fuel, plus it doesn't take into effect intercoolers


9.)Then calculate density ratio-DR:
inlet temp outlet pressure
DR = ------------- X ------------------
oulet temp inlet pressure

70 + 460 20.3 + 29.0
= ------------- X ----------------
202 + 460 29.0

520 49.3
= ----- X ------
662 29.0

= 1.36

10.)Under these conditions the:
compressor-inlet flow is = compressor-outlet flow X density ratio

= 300.16 cfm X 1.36

= 408.2 cfm


11.)now convert cfm to lb/min
lb/min = cfm x .070318

= 408.2 x .070318

= 28.70 lb/min

Now all you have to do is go back to step #2 and replace the rpm with a couple of different numbers. After you map 2 or 3 different rpms, take those figures and compare them to turbo compressor maps. Make sure that that all of the figures you mapped are in a good efficiency range on the compressor map you choose. Please, take into account these are some very rough calculations on my part. I'm sure glad that those Chem 1/2 classes came in handy somewhere

Also, here's a link to some compressor maps to help track down a good match.
http://www.turboneticsinc.com/comp_maps/fig5.html

[ January 28, 2003: Message edited by: Josh R. ]</p>

[black34v6]

damn. id have to say this is one of the best rookie posts ive ever read.

all i can say is: wow, thats a ****load of info rite there LOL.

-R

[Arctc Wolf]

good job josh. i'm glad somebody here is also into theoretical racing formula's as much as I am.

[93v6firebird]

wow now i know where all the nerdy kids hang out... you know theres a math joke in the lounge, right?

[HAZ-Matt]

There are only a few things...

The inlet pressure should be less than ambient, since it is under vacuum... this will affect the density ratio.

The intercooler is a very important part of the calculation, so that we can get the correct lb/min flow...

Without an intercooler, and ~ 1lb of vacuum at the inlet, I got a density ratio of 1.83 and an outlet temperature of 224.6F... and then 28.7lb/min of flow. Not too different from your numbers, really. The intercooler will change the lb/min a lot though.

[bu-bye]

WOW I'm almost sorry I asked. Thats a lot of info Josh, thanks. I will paly around with it a little.