Linc's 200":22jz9ssc said:
xecute®™© he he":22jz9ssc said:
A stock, new cat will flow well over 400 cfm at 6 psi of backpressure..
Where did you obtain that data?? I have never, ever seen 6 psi in any portion of the exhaust unless there was blockage (usuallly chunks of cat plugging the muffler), and even then......only at full throttle.
Sources are from any Darcy-Weisbach formula in enginnering bookes, most heating engineering books, any university gradegas flow book like Sears and Zeminsky. My main bacground is materials testing things like emulsions and bitumen. I did my NZ Certificate in Engineering back in 1996, and found that most laboratory tests and engine books by air flow freaks like David Vizard were pretty easy to understand once I knew what the terms were.
Example, based on an excerpt from the Modifying Fords OHC 1.3/16/2.0 engines, released in 1988.
When a 1/8" brass union and a fuel pressure gage is hooked up, it is hard to get
less than 4 psi backpressure at maximum power on any factory exhast.
When flow figures are given in cfm, they are done at a pressure drop of 10, 20, 25 or 28 " Hg. That is 5, 10, 12.2 and 13.8 psi respectively. We know most muffers flow from 120 to 400 cfm at 10 psi from flow figures given in Car Craft articles, trade literature and books like David Vizards.
For a standard 2 inch internal diameter pipe, that is
Flow is Q = v.a
A 10" head of mercury is a gauge pressure of 5 psi.
Given a 400 cfm flow at 10" Hg catalytic convertor, that is fed by a 351 truck engine at 5000 rpm, there is less than 430 cfm of air getting into the engine, and a similar amount goes out the pipe as your adding fuel to create a 75% heat loss via cooling system, block convection, transmission losses and then the exhast.
The up shot is that
Q= 430 cfm exhast flow through system.
v= 330 feet per second average air speed (tops).
a= is 0.0218 square feet with a 2" pipe.
Pressure is bascially heated air at 1350 degress f at the header, down to 200 degrees at the exhast exit, and the headloss is calculated by
hl = 0.002083 l (100 / c)1.85 (q)1.85/ (dh)4.8655
where
hl = head loss (ft)
l = pipe or tube length (ft)
c = design coefficient determined for the type of pipe or tube
q = flow rate (gal/min)
dh = hydraulic diameter (inch)
This works where the gas flow is has kinematic viscosity of approximately 1.1 cSt. 1 cSt (centiStokes) = 10-6 m2/s
There is information on how to calculate it, but even with those figures, the exhast pressure is still above 6 psi at 5000 rpm.
The power increase by backpressure reduction only occurs by proportion. At full revs, it is greatest. If you cut back pressure by half, like going from 6 psi to 3 psi, you would gain 17 hp at 5000 rpm at the power peak for an engine giving 200 hp. You'd then get 217 by backpressure reduction. It's hard to drop it that much, and still reduce noise, but it is possible. Mid range torque would drop if that backpressure was done by bringing the gas flow speed below 200 feet per second.
