"Actually what they've done by oversizing the exhaust is lost velocity. You need exhaust gas velocity so the gasses are drawn away from the combustion chamber. Ergo what is required is a pipe diameter that optimises the velocity of the exhaust gasses exiting the engine whilst minimising the back pressure."

I wouldn't necessarily be surprised if simply increasing the exhaust system bore on a highly-tuned performance car resulted in reduced power output instead of the anticipated increase, but I think this would be down to a side effect such as it upsetting the delicate fuel/air ratio or sommat. Because from a purely engineering/scientific point of view, the explanation at the top of my post just doesn't make any sense to me.

What on earth is 'lost velocity'? Yes, the 'velocity' of the exhaust gasses will be reduced if the bore size is increased, because the same volume of gas is being shifted. So what?! It's still a damn sight easier to force gas through a large bore pipe than a narrow one. At no stage whatsoever (unless I'm overlooking something like, ooh, turbulence) will an increase in bore size suddenly make the gas find it harder to escape - that just doesn't make sense. Calling it 'lost velocity' is just bad science as far as I can see...

(But I am getting ready to apologise chust in case...)

you would be right if it were simply a case of moving gas down a pipe

however its complicated by the fact there is a valve opening and closing

gas as with all mass has velocity when moving

get that column of gas going down your tube and suddenly shut one end and the gas keeps going by doing so the pressure of that gas reduces as the column is stretched

now open the valve and the reduced pressure is acting like a vaccum on the air the other side of the valve, sucking it through( or presenting less of an obsticle if the next stuff is being forced out)

a smaller bore exhaust forces the exhaust gas to move faster causing it to suck the next charge out and /or helping to get the inlet charge in to the chamber

now think of the heads on your car you have a colum of air moving at less than atmosphere ( inlet) the inlet valve, the chamber the exhaust valve and the column of escaping exhaust. There is a lot of fluctuating pressures, opening closing of valves at some point both are open at the same time, then there are two cylinders going at different times which could cancel each other out or amplify the effect. Then the opening and shutting of the valves sets up pressure pulses in the gases( run an engine with no heat exchangers in the dark its really cool to see what the gases look like)

there is a formulae for working out the best diameter pipe for a given volume at a given rev range, and un surprisingly the peak diameter is just slightly wider than a standard exhaust, the boffins at citroen worked it out, they didnt just guess the size and lenght of the main exhaust tubes,

once its been thrugh the cross box exhaust length and diameter really dont matter that much

DA,
cut 'n paste time.
Where's your hat, might be needing it later...

"When the exhaust valve opens, a high-pressure pulse of hot, expanding exhaust gas travels down the exhaust port at approximately 300 feet per second. This wave of hot, moving, high pressure gas has mass and inertia of its own which pulls a suction or a low pressure rarefaction behind the pulse.

Depending on the engine, the pulse can have a positive pressure of anywhere from 5 to 15 psi with the low pressure rarefaction behind the pulse being anywhere from 1 to 5 psi of negative pressure. As this low-pressure rarefaction is several milliseconds behind the initial high pressure pulse, it can be exploited to help suck residual exhaust gases out of the cylinder toward the end of the exhaust stroke as the piston approaches TDC. The build up of this negative pressure and its timing in the exhaust stroke is closely associated with the primary pipe's length and diameter, just like an organ or other musical instrument."

ken.
( Luckily, Citroen did all the hard work for us, so unless you're keen enough to start chucking a motorcycle engine into your A series vehicle, the standard system is _very_ hard to beat for all round performance and economy.)

Exhaust gas doesn't come out of a multiple cylinder engine in a single smooth stream. There is an overlap as the cylinders fire.

The point about having gas velocity is that you get a scavenging effect. Since the exiting exhaust gas has both velocity and mass it also has momentum. The vacuum created by the momentum of the exhaust gas from one cylinder helps draw out the gases from the next cylinder to open its exhaust valve.

The engine still has to expend energy pushing the exhaust gas out of the cylinder. However if the exhaust is appropriately sized the scavenging makes the process more efficient. Clearly there is a trade off here. A larger pipe will be better at high RPM, a smaller one will be better at lower RPM. Hence changing the exhaust can alter both the power output and the torque of an engine.

Sean, Ken and twofifty...

Bastards!









I apologise

Fascinating stuff. My thoughts (not necessarily proven in any way, and not particularly technical);

Put very simply(for the yoof), the designers knew what they were doing, and an engine must be considered as an entire system, all the components are carefully matched to each other in order for it to work well, changing any one component can put the system out of balance, and other components will need tweaking to return the engine working to working at it's best again.

However there are some areas where the engineers don't have a free hand, and the accountants or 'noise police' like to have a say.
There are some cases on other engines where cost or space limitations may have dictated an awkward cast iron exhaust manifold, and potentially a pretty tubular job could be better. But remember, the carburation was set up to work with the stock item, and changing the manifold alone could potentially cause the engine to run worse.

I like to think that the front half of an exhaust is important to making it run well, the silencer on the back is to keep it quiet. When I built the exhaust for my bee-em engine I copied the dimensions of the original downpipes and collector as closely as possible, and when it came to the silencer I stuck with the standard bore of the secondary pipework but made the silencer as free flowing as possible(straight through), this is what I believe would give the best power from a stock engine. Since then however, I've added a second, noise orientated(more restrictive) silencer near the back of the car. This far away from the engine, the pulses in the exhaust gas no longer have any positive effect and need smoothing out in order to reduce the noise from the tailpipe as much as possible. This second silencer has had the desired effect on the noise levels, but doesn't seem to have restricted the power noticeably.

Considering the original question, assuming the R80 motor is in standard tune then I'd copy the dimensions of the front pipes that BMW spent a lot of time and effort developing, including the diameter of the secondary pipe, the choice of silencer is mostly a compromise between power and noise, a small step in the bore either up or down is unlikely to make any noticeably difference.
When I first had my 1150 engine running it had a 2" diameter swan neck pipe flowing into the original 1 3/8" torpedo silencer, that did restrict the power, although I would guess by less than 10%.

This is from STP, a German 2cv tuning guy who knows what he's talking about, and it answers a few questions.

http://www.citroentuning.de/doc/Exhaust ... essure.pdf

joolz
you will need some info on the bike engine and the cam duration

from the 2cv tuning manual :-

length of primary pipe(s) in inches( american formulae?) from the back of the exhaust valve=L
duration of the exhaust valve from opening to TDC in degrees=ET
anticipated revs at which max power is anticipated-(less)500rpm

so L=(5100 X ET) / (RPM X 6)

that gives a theoretical length of the primary pipe the actual exhaust pipe will need to be L less the length of the exhaust port in the head.

primary pipes can be multiples of L

diameter of pipe formula again in imperial
Cylinder cc X2
divide by 16.4 to get cubic in
divide by L from above
divide by 3.14
Sq root
multiply by 2 add 0.120
from that you add 10% or 15% due to the fact that its a contorted pipe not a sraight one.
for a standard 2cv thats:
301cc X 2=602cc 602/16.4= 36.7 cu in 36.7/L(34.8in)=1.05in 1.05/3.14=0.33 0.33 sq root= 0.57 57 x 2 + 0.12= 1.27 finally add 10% and you get 1.4in
right thats the 2 main pipes length and diameter to the point they both join together

so the primarys on a standard 2cv producing(ideally) peak power at 5750rpm would be 11/2 inch pipe 30.18 inches long or 64.8 (or keep adding 34.8)

seconary or tailpipe
is the above formula but its cylinder cc X 4(1204cc) at the start

does that help?
Sean

(and jameswallace you are banned from chipping in to this thread if your maths is anything like your grasp of English)

aye we've had that one as well no matter how many times i read it it kind of makes sence:lol:

back pressure is not good for peak power or stuffing a potato up the tail pipe would be a performance gain we can all agree on that

however an element of restriction in the exhaust is needed (or is inevitable) for the car to be driveble in everyday situations and that means peak torque down about 3.5krpm

his performance gains are all about cam profiles(and they are good when they come on song eh Ken? ) so unless you are going to change that you need to keep close to the standard setup that Citroen or BMW worked out was the best compromise between performance, economy and reliability for their particular engines. (its like the old bike frame makers addage:- lightness, strength, affordability...choose two!)

Sean
suppose going back to the original question "back pressure" not important as much as the design of the exhaust

Thanks Sean that's some valuable information there. By those calculations the secondary pipe diameter works out about 2", that's a lot bigger than the standard 2cv exhaust, I seem to remember reading elsewhere that secondary pipes should be about 1.4 times the size of the primaries (coz it's the sq root of 2?) for a 2-1 collector. I seem to remember reading a formula that based the pipe size on the size of the valve opening, but as that in turn should probably be based on cylinder size it boils down to the same thing.
Obviously it's actually the cross sectional areas that are important and first thoughts are that the secondary needs to be the sum of the primaries, but my further thoughts were, pulses of gas in the secondary pipe contain the same mass of gas as they do in the primaries, they just come along twice as often, so I'd have thought the secondary needn't be as much as twice the size.
Anyhow that was just my own musings, and not based on any evidence.

You didn't mention secondary length, I would assume it has negligible effect because any silencers will have distorted the pulses beyond predictability?

I usually end up reading through formulas like that until about 90% of the way through, and then deciding that life's to short. But if I can find cam info for my motor I may try measuring the primary lengths and working the formula backwards to see where it should make peak power, (definitely peak power, not torque?)

This thread's far too serious and hasn't got enough photos, so just to liven it up slightly;
http://i686.photobucket.com/albums/vv22 ... C00256.jpg
http://i686.photobucket.com/albums/vv22 ... C10515.jpg

Oh yeah, and where do balance pipes fit in to all this?