Exhaust design: bore / diameter

[edc]

When designing or specifying an exhaust how do you determine the optimum bore?

If you are building an engine to make xyz bhp, be it normally aspirated or forced induction, what factors go into making the exhaust match?

[love machine]

This is a thing I've spent many a while thinking about. First stop should be a "Tuning a 2 stroke book" to get you up on your exhaust science. It isn't widely written about from a sizing point of view, the manufacturers choose a pipe for you and then aftermarket people deviate on that a bit. Take minis, which are one of the most widely tuned cars, 3 sizes of tubular manifold generally available.

Lots is written about reverse cones/controlled vortex/reflectors/etc and how they work with different cams, etc, etc. Pipe size is left to someone else. Obviously, you know about the pipe sizing arguments from deviating from the standard?!?! I would suggest a wise idea would be to use cross sectional area and deviate the capacity/ pseudo-capacity (boosted) from the manufacturers choice, or a standard performance reference.

Obviously pipe sizing from a mathematical point of view is immensely complex, takes into account of the characteristics of the flow relative to the engine spec, then you have pipe cooling, velocity reduction, the characteristics of the silencer, cat, etc. Since most performance exhausts on cars are generally pipes which are the same one end to the other, this tells you there is a compromise.

The 2 stroke book will tell you about as much as you can take on board about exhaust design, without resorting to journals and serious maths.

It seems across the board in motorsport/petrolheddism that an empirical approach is favoured, if you ask too many theoretical questions, you usually get your head chewed off.

If you fancy bouncing any ideas off me, whang me a PM.

Stu

[dilbert]

http://flow.netfirms.com/venturiflow/calculator.htm

[350matt]

A little something I prepared earlier... Matt
Exhaust and intake length tuning

You will also need to determine the capacity of your engine, by using the number of cylinders and the capacity of each. To discover the best length and diameter of pipe for your engine, you will need to know the exhaust valve timing; that is the number of degrees before bottom dead centre that the valve starts to open. with this knowledge you can then set about making a tuned exhaust system by working out the following formula and applying it to your own engine:
850 (180 + N)
L =(------------- )- P
RPM
where L = length of pipe in inches
N = degree before BDC exhaust valve opens
P = distance from exhaust valve to manifold
and RPM = desired RPM For peak power

The diameter of the pipe needs to be calculated so that the volume of the exhaust pipe attached to each cylinder is twice the volume of each cylinder. The exact diameter of the pipe, incidentally, is not critical and should only be used as a guide in determining which standard sized pipe diameter should be used.
As a guide, this is example calculation. If the exhaust valve opens 110 degrees before BDC. The engine capacity was 700 cc per cylinder and the desired peak power RPM is 6,500

850 (180 + 110)
L =(--------------) - 3 in = 887 cm = 34.9 inches.
6,500
To find the diameter of pipe, the following formula can be used:
π r^2 L = 2 * 700cc
π r^2 * 223 cm = 1400cc
r = 1.4 cm
Diameter = 2.8 cm or 1.1 in
The nearest standard stainless steel tube available was 1.5” Ø.
If a primary pipe is oversize the consequences are far less severe than being undersize and unless a pipe is massively oversized it will have little detrimental effect.
To reduce the weight and complexity of the finished system, the exhaust pipe can be of a two-into-one type, with the joint at half length. The single pipe then finishes the required length and then the two pipes can then be joined together into a single collector and the exhaust silencer is then fitted onto the end of this single pipe. Typically this will not produce as much power as a well designed 4 into 1 system but is often far more convenient to package


INTAKE RUNNER LENGTH ii
ECD = Effective Cam Duration
RV = Reflective Value = 2 for 2nd reflection 3 for 3rd etc – the earlier the reflection = the stronger the tuning
ECD = 720 - (Adv. duration - 30)
Speed of sound = 396m/sec
intake runner length = ((ECD/360) × (1/(tuned rpm /60))* speed of sound)/(2*RV)
( ((470/360) x (1/6000 /60))*396) /(2 *6) = 0.43mtrs




All of the above calculations will get you somewhere close to the perfect length ie within an inch on the exhaust, but this can only be confirmed by dyno testing, consequently these numbers are a rough guide only.

[chuntington101]

i thought the speed of sound (in air at 1 bar apsolute) was 330(or maybe 333)ms-1??????

Chris.

[edc]

Interesting replies I now need some time to read and digest.