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augidog
Joined: 03 Mar 2003 Posts: 1360 Location: New Jersey

Posted: Fri Feb 02, 2007 11:30 am Post subject: Exhaust size calculations 


This info was found on another forum and is one of the reasons I upgraded my exhaust pipe size to 2.25". I deleted the resonator after the Cat after my faulty engine mount destroyed the old system piping.
I originally planned only a race bullet replacing the Cat and a rear muffler but I'm in NJ and that’s a hefty fine especially for the repair shop. (I tried)
The high flow Cat is actually very close to the where the resonator placement should be according to the calculations. Its shape is also close. It was to be about 52" from the primary. I thought the calculations are interesting and made some sense for a non dyno tested application.
I went just over on the cfm flow requirements on a small turbo muffler. It does tend to drone while cruising.
I would go longer next time for a deeper tone but still I am pleased with the results.
Quote:  You may be interested or wish to comment on the following written by a fellow MG owner
Exhaust tuning theory is actually fairly simple; it’s all about getting the negative (and, hence, scavenging) pressure pulse to arrive at the exhaust valve as it is opening. To do this we have to set the pipe lengths and diameters correctly.
The formula for Primary pipe length is:
P = [(850 x ED) / RPM]  3
Where:
RPM is the engine speed to which the exhaust is being tuned.
ED = 180° plus the number of degrees the exhaust valve opens before BDC.
P = Primary pipe length (on a 41 manifold), or Primary pipe length plus Secondary pipe length (on a 421 manifold), in inches.
Generally road engines will require the manifold to be tuned to the max torque rpm whereas race engines will be tuned to work either at max bhp rpm or a speed midway between the max bhp rpm and max torque rpm.
4 1 manifolds restrict the power band whereas 421 manifolds give better midrange power but reduce top end power by as much as 57%.
Generally speaking with a 421 manifold the starting point for Primary pipe length is 15 inches, thus Secondary pipe length is P  15 inches. Changing the length of the Primary pipe tends to rock the power curve around the point of max torque. Shorter Primaries gives more top end power but less midrange, and viceversa. There is, however, little change in the peak torque or the rpm where this occurs.
Ideally the Primaries should come off the cylinder head in a straight line for around 4 inches before any turns occur.
Inside diameter of the pipe can be gained from:
ID = ? [cc / {25 x (P + 3)}] x 2.1
Where:
cc = cylinder volume in cc.
P = Primary length in inches.
In some engines it can be useful to have a 'step' between the exhaust port and the Primary (ie the Primary bore is greater than that of the exhaust port). This tends to be the case in engines with rectilinear exhaust ports.
For a 421 system then, Primary pipe diameter is calculated as above. Secondary pipe diameter is given by:
IDS = ?(ID x ID x 2) x 0.93
Where:
ID = calculated inside diameter of the primary pipes.
The pipe diameter can be used to change the peak torque rpm – a reduction in diameter of 0.125 inches will drop the peak torque rpm by 500600 rpm in engines over 2 liters and by 650800 rpm in smaller engines. Increasing the pipe diameter by 0.125 rpm has approximately the opposite effect.
The total length of the Collector and Tailpipe (to the front of the silencer) should be equal to P + 3 inches (or any full multiple of P + 3 for a road car).
Tailpipe internal diameter is given by:
IDT = ?[(cc x 2) / (P + 3) x 25] x 2
Where P is calculated as above.
Collector length is given by:
CL = [(ID2 – ID3) / 2] x CotA
Where:
ID2 = diameter of Collector inlet
ID3 = diameter of Collector outlet.
CotA = Cotangent of angle of Collector taper (which ideally should be around 78° (certainly less than 10°).
The design of the collector should be such that the inlet pipes terminate abruptly otherwise the tuned exhaust pressure wave will carry on into the tailpipe and the calculations done to get the negative scavenging wave back to the exhaust valve on time will all be wrong.

_________________ 1978 924 95 mile daily driver.
Audi TB/POR174M/High Flow Cat/2.25" exhaust
I knew that positive thinking thing wouldn't work. 

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Rasta Monsta
Joined: 12 Jul 2006 Posts: 11378 Location: PacNW

Posted: Sat Feb 03, 2007 3:11 am Post subject: 


Yikes! And did you apply these formulas to your setup? _________________ Toofah King Bad
 Red (1987 924S)  Pro44/Spec944


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Martijnus
Joined: 29 Dec 2006 Posts: 2019 Location: Netherlands

Posted: Mon Feb 05, 2007 5:42 am Post subject: 


Finally, some simple formulas which I can use.
Hope they're accurate... I've mentioned this stuff in another topic and got some useful answers.. _________________ "Rule: Turbo's make torque, and torque makes fun." (C. Bell)
924 "50jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment 

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augidog
Joined: 03 Mar 2003 Posts: 1360 Location: New Jersey


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