smaller exaust pipe for mpg

[gone-ot]

...yes, there are really two distinct "movements" within the exhaust gas: (1) the actual mass movement of the exhaust-gas "slug," at about 300 fps, and (2) the acoustic speed-of-sound wavefront/shockwave within the gas mass, at about 1700 fps.

...for HP, pipe diameter is set by mass velocity, while pipe length is set by acoustic velocity (Helmholtz tuning).

...with Helmholtz tuning, the goal is to have the "reflected" rarefacation wave arrive back at the just closing exhaust value, so that it can literally "suck" the last vestiges of exhaust gas from the cylinder as the exhaust valve closes, which, if done correctly, will then actually create a slight vacuum within the cylinder that helps it "suck-in" more air when the intake value opens.

...the speed of sound in normal air is about 1,100 fps, but in hot exhaust it's about 1,700 fps (there's an approximation equation available if you're interested).

[Ryland]

Want to find out the correct size of pipe for the RPM that you are running at?
Autolounge.net | Calculators | Exhaust Pipe Sizing

[mwebb]

..."(1) the actual mass movement of the exhaust-gas "slug," at about 300 fps,"...

i always assumed the speed of the exhaust "slug" column would change based on RPM and load ......

are you saying the speed is a constant ? 300fps more or less ?

i have a box filled with under reporting MAF sensor s which could be used to provide a waveform ,
which will no doubt be different than the pressure differential sensor and the actual pressure transducer
i am using now

sometimes
i use a venturi adapter connected to the measuring devices and sometimes
just a tube in the tail pipe

sync is all over the place ,
it is a huge problem .
sometimes i need to create a problem to measure the delay before continuing ..... to build in a little kentucky windage to the test results

using a MAF sensor the speed of the column would have an effect on where the waveform occurs in relation to whatever engine event it's synced on
if the speed is constant
it would make a difference ....it would be easier

=================
also assumed the OP was seeking optimum efficiency at cruise as opposed to MAX horse power at WOT
so exhaust pipe ID requirements should be very different ....

[mwebb]

Originally Posted by Ryland

Want to find out the correct size of pipe for the RPM that you are running at?
Autolounge.net | Calculators | Exhaust Pipe Sizing

i will look at this - thanks

[moonmonkey]

the calculator says i need a pipe around 1.10" for my 1.5 litre engine @ 2400 rpm, if im using it right, i dont know what size my stock pipe is but its way bigger than that.

[gone-ot]

Originally Posted by mwebb

..."(1) the actual mass movement of the exhaust-gas "slug," at about 300 fps,"...

i always assumed the speed of the exhaust "slug" column would change based on RPM and load ...... are you saying the speed is a constant ? 300fps more or less ?....

...no, the optimum velocity is ~300 fps(*) at the engine rpm (or just slightly beyond) where you want the HP to peak.

...and, yes, you are correct, cylinder displacement and engine RPM are the two parameters that most affect which pipe diameter to use to achieve the desired mass slug velocity of 300 fps(*) at HP RPM.

...basically, you're exchanging cylinder volume and piston velocity for pipe volume and pipe velocity. Piston velocity is a function of engine RPM while pipe velocity that you want is basically a constant 300 fps(*) at that same engine RPM:

Vg/Vc = Ag/Ac, or: Vg = Vp*Ag/Ac

where:
Vg = gas-slug velocity, fps
Vc = piston velocity, fps
Ag = area of pipe, sq-in.
Ac = area of piston, sq-in.

...hence, obviously, an engine RPM of 6,000 (for HP) will dictate a larger diameter than an engine RPM of 2,000 (for FE)...which is the point being made.


(*) ~300 fps for race engine, 80% of that (~240 fps) for street vehicles.

[Crazyrabbit]

Moonmonkey,

Smaller is not a good way to go in the exhaust system. Back pressure is not good for FE. The engine performs useless additional work forcing the exhaust out on the exhaust stroke. Also, the increased combustion cylinder pressure at exhaust valve closing, will cause dilution of the incoming charge and will upset the tuning without a closed loop system. That adds exhaust gas recirculation and lowers the maximum combustion temperature, which lowers the efficiency. So both power and efficiency drop.

On the flip side, increasing the exhaust system size may not neccessarily reduce back-pressure on the engine. The transition from the original flowpath to the new larger one must be handled carefully because a sudden stepchange without a well designed diffusion section, could result in a Vena Contracta, that can cause increased flow resistance. Conical diffusers are typically limited to less than 7 degrees included angle. Swirling flow can stand a little more before flow separation occurs and a smaller angle works better. Gases have mass, and smooth attached flow in a diffusing or expanding situation requires very smooth bends, and expansions free of steps.

[gone-ot]

...just as there's too small a diameter, likewise, there's a too large a diameter.

...the goal is to optimize the diameter for the engine RPM range you want...and, many people forget that Helmholtz tuning, is harmonic repeating, thus something designed for 6,000 RPM will tune again at 3,000 RPM, but not as efficiently (only about 3-5% loss).

[mwebb]

cylinder displacement and engine RPM are the two parameters that most affect which pipe diameter to use to achieve the desired mass slug velocity of 300 fps (at HP RPM).

ok
so why then the magic 300fps -could be at lower rpm for efficiency or higher rpm for power
either way ----
does that somehow add or subtract from the
sound waveforms at 1700 fps ?

it can not be the same for an 8 cylinder engine with an optimum of 8 pulses in 720 degrees 90 degrees apart
with exhaust valve open times overlaying them selves even if completely separate dual exhaust s are used

as for a 3 cylinder with 3 pulses in 720 degrees 240 degrees apart
==========

[gone-ot]

Originally Posted by mwebb

ok, so why then the magic 300fps

...that's the 'critical' gas velocity where gas-slug "friction" with the pipe walls becomes excessive. For race engines (full-race hemi) that's about 300 fps, but for street engines (non-hemi, wedge, and pent-heads) the value is typically 80% of this, or ~240 fps.

...camshaft valve timing and engine RPM affect header length (valve to end-of-pipe/atmosphere):

L(intake) ~ 85*Vs/RPM, where: Vs ~ 1050-1100 fps in air.

L(exhaust) ~ 120*Vs/RPM, where: Vs ~ 1600-1800 fps in hot exhaust.

...here, 85 and 120 are typical values representing, respectively, a percentage of the intake- and exhaust valve durations, in camshaft degrees. If you know the actual values for your engine, use them instead. Header lengths (L) are in inches.