BMW twin tailpipe Valve - GGH
I have noticed whilst driving behind twin tailpipe BMW's that some have a butterfly valve in the end of one of the two pipes.
On a 530D (I think ) the valve was open at idle and closed on pulling away, a 330CI did the opposite. Does anybody know the purpose of the valve as it intrigues me.
BMW twin tailpipe Valve - Roly93
I've noticed this on my travels also, and would be interested in knowing the answer.
BMW twin tailpipe Valve - storme
to increase or decrease back pressure
BMW twin tailpipe Valve - L'escargot
I vaguely remember that the length (and possibly also the bore size) of an exhaust system has an effect on power output. It's something to do with the wavelength of the exhaust pressure pulses travelling along the pipe, and the interaction between the pulse from one cylinder and the pulse from another. At idle the only requirement is for minimum back pressure.

If I'm wrong please feel free to put me straight!
BMW twin tailpipe Valve - L'escargot
Aren't the lengths of exhausts on very high performance cars like F1 and dragsters "tuned" for maximum performance?
BMW twin tailpipe Valve - L'escargot
It may even give better silencing at speeds above idle. Google for "engine exhaust pressure pulses".

But I'm still only guessing!
BMW twin tailpipe Valve - Andy P
Like this you mean?
BMW twin tailpipe Valve - Number_Cruncher
Hmmm. There's lots of twaddle spoken and written about exhaust back pressure. It is always a BAD thing.

What people confuse with back pressure is, in fact, exhaust pulse tuning as described by L'escargot. Some two strokes will barely run without the correct exhaust in place.

An engine will always produce more output with a lower **average** pressure in the exhaust. Think about it - we deliberately reduce the power output of engines by putting a throttle in the inlet manifold, until we floor the throttle, allowing the engine to produce its rated output. Why should throttling the exhaust simply to produce pressure become magically helpful?

Now, there are wave effects, as L'escargot correctly describes, but these only work over relatively narrow speed ranges.

The desirable effect that you are after to increase power is that;

when a pressure pulse reaches a change in impedance (change in section of the pipe for example) part of the wave gets reflected back towards the engine - but, the wave gets reflected as a negative of the original wave - i.e., as a relative vacuum.

This pulse of relative low pressure should arrive at the exhaust valve just before it closes, thus emptying the cylinder.

The upshot of this is that while there are some engine speeds where this effect is helpful, there are also some speeds where it is detrimental to engine performance. One way to avoid the complication is to simply use a very short stub exhaust, and ignore the swings and roundabouts of tuning, and take the beneift from avoiding all of the frictional losses incurred by pumping the gas down a long tube.

It is possible that the valve will change the effective length of the exhaust, thus allowing the engine to be tuned in two different speed regimes.

The scale of this phenomenon can be understood by thinking about the organ pipes in a church. At low engine speeds, longer pipe runs are important, while at higher engine speeds, probably only the manifold and downpipe are of any real significance.

Having said all of this, the wave effect devices fitted to inlet manifolds, (such as Dual Ram on Vauxhalls) which vary the effective length of the manifold at different engine speeds offer much more scope for increasing engine output. Obviously, for an intake system, you want a wave of positive pressure to arrive at the intake valve, just before it shuts, thus trapping more charge in the cylinder.

As witnessed by the difference in size between intake and exhaust valves, it's much more difficult to get the air into an engine than to get it out again afterwards.

The valve in the rear pipes of some modern cars is AFAIK simply an acoustic device, designed to produce a more pleasing exhaust note; I think the performance change is minimal.

BMW twin tailpipe Valve - cheddar
back pressure. It is always a BAD thing.

Not sure, my ZRX 1100 is tuned for a full race spec titanium Akrapovic race system (as was my previous ZX7R) I have a race can that is road legal with removable baffles, sure it feels quicker at high revs with no baffle though equally with the baffle in place the response at low revs is better and it feels more torquey, same for the 750 ZX7R.

Yamaha have been fitting a valve within the exhaust system on some of their bikes for 17 years now, this is know as EXUP and changes the internal diameter of the system effecting back pressure Quote: " And the 20-valve engine features EXUP exhaust control for excellent midrange and improved intake/exhaust efficiency".

>>(such as Dual Ram on Vauxhalls)>>

The 95 on 2.0 16v Ecotec in the Vectra was so much more gutsy in the mid range than other 2.0 4cyl at the time and much better than the same engine in the Astra and Calibra which at the time did not have the variable length manifold.

BMW twin tailpipe Valve - Number_Cruncher
>>back pressure. It is always a BAD thing.

Having tested an engine on an engine dynamometer with throttling valves in the exhaust. I stand by my statement.

The results you get from your motorbikes are, while interesting, not conclusive. If you have tuned your bike for maximum power (at high engine speed) with one system, then the comparison with another system, at another engine speed doesn't prove anything.

There is a lot of twaddle talked about engines and their mythical need for back pressure.

The analogy with the throttle in the intake is right - those who say that engines need back pressure are in effect saying that engines need to be throttled to produce full power - what nonsense!

Yes, manufacturers fit valves in the exhaust, but, just increasing back pressure will always make an engine produce less power. If, however, the change made to the exhaust *also* changes the wave tuning, then, it's possible for there to be a net benefit. This is how I understand EXUP works.

Again, having said all of this, intake tuning is much, much more important.


BMW twin tailpipe Valve - Number_Cruncher
These delightful publications might also be of interest in answering this question;


BMW twin tailpipe Valve - Group B
I was only half paying attention to Top Gear last week, but I'm sure that Corvette had valves in the exhaust tailpipes didnt it? I assumed they were for acoustic reasons only, and didnt realise they were supposed to serve a real purpose.

I remember the Dodge Stealth (US version of the Mitsubishi 3000GT) had a manually controlled solenoid valve to divert flow around one of the silencers. So you could have it on quiet setting to keep the neighbours happy, then let rip when you get down the road, (or vice versa?!).
BMW twin tailpipe Valve - DP
Number Cruncher, much as your explanation makes sense to me, Yamaha would seem to disagree with you.

They have used their "EXUP" valve on their four cylinder sportsbikes for nearly two decades now. It's a servomotor controlled "flap" which reduces or increases the flow of exhaust gas through the exhaust dependent on revs and throttle opening. What difference it makes is the subject of huge debate, but it is a fact that one of these bikes with a sticking EXUP valve loses a massive chunk of power.

In any case I've been into bikes a long time and I know that even four strokes are very "fussy" about exhausts. Just swapping pipes over can make 10 bhp difference (+ or -) on a modern Japanese litre sportsbike.
BMW twin tailpipe Valve - Number_Cruncher
I'm sure that EXUP must do more than simply increase back pressure - it may act as a reflecting surface for pressure pulses, it may change the path through the exhaust so that the way pulses overlap and interfere is different. If it simply increases back pressure, then I can't see how engine output is increased by it.
In any case I've been into bikes a long time and
I know that even four strokes are very "fussy" about exhausts.
Just swapping pipes over can make 10 bhp difference (+ or
-) on a modern Japanese litre sportsbike.

I think that this is true for any engine that has been tuned to take advantage of the resonant effects in its manifolds. These resonant effects are functions of the whole engine system, from the length of the intake, through the valve timing, and the exhaust.

Typical aftermarket attempts to change a single component without considering the whole system tend to be short sighted and a little naive IMO. The very least that should be done on changing over an exhaust is for the bike to be re-tuned on a dynamometer to make sure that fuelling and ignition are set OK. Of course, it's possible that valve timing changes might also be required to get the best out of a particular coinfiguration, but this is going way beyond the realm of the interested amateur.


BMW twin tailpipe Valve - cheddar
Hi NC, whilst I agree that ulimately inlet tuning is more important I think you are underplaying the importance of the exhaust, my experiences with bikes might not be conclusive however they are not unique, by way of clarification, the bike is set up for the free flowing race pipe and in such configuation it feels as though it breaths more freely at high revs however the addition of the baffle, which makes it strictly road legal, has a marked benefit in low speed response and torque.

I am picking at the depths of my memory here and stand to be corrected however I think the key benefit of back pressure is that, as long as the cylinder scavenges efficiently, some back pressure will increase the effective compression ratio by a small degree because the pressure in the cylinder is slightly higher than atmospheric at the point that the exhaust valve closes, I beleive that it is this along with pulse wave tuning that EXUP delivers.

With regard to inlet it is true to say that optimum inlet gas flows are always a compromise between air flow velocity and volume, if you have a large throttle body then ultimately more power can be produced however the air flow velocity falls to an unacceptably low rate at low engine speeds, hence the need for twin choke carbs etc. Take a Dolly Sprint and replace the twin 1&3/4 SU's with 2 inchers and you get a better top end though to the detriment of low speed flexibility and mid range torque.

I guess this gas velocity v volume is a factor in exhaust design as well as inlet.
BMW twin tailpipe Valve - Altea Ego
NC is right of course. The valves in the exhaust are there to make nice sporty noises.

TourVanMan TM < Ex RF >
BMW twin tailpipe Valve - Number_Cruncher
>>I think you are underplaying the importance of the exhaust...

Before posting this, I've just checked in my copy of Heywood. The full formula is too tedious to post, but the gist is that changes in inlet manifold pressure give approximately the compression ratio (i.e., approximately 9 or ten) times the effect on volumetric efficiency that changes in exhaust pressure give. Obviously these changes have opposite signs! positive for increasing inlet manifold pressure, and negative for increasing exhaust back pressure. For those with access to Heywood, I'm on about equation 6.2.

Incidentally, this equation gives a hint of the result that the dynamometer tests I mentioned were looking at - throttle valves in the exhaust aren't a particularly good way of controlling the output of an engine - a throttle valve in the inlet is much more sensitive.

The requirement for a high inlet manifold flow velocity isn't as important with fuel injection - there is less need to keep fuel atomised and in the flow rather than pooling on the floor of the manifold. High speed engines have the bare minimum of inlet manifold to avoid these resonance - anti resonance effects, but, of course, stub exhaust pipes would lead to obvious objections on the grounds of noise.


BMW twin tailpipe Valve - cheddar
The following is from Riccardo Engineering and describes the Ferrari 550 Marenello

" The exhaust system is also characterized by its variable geometry. Two by-pass valves, located on the rear silencers, are governed by an electro-pneumatic servo mechanism governed by the engine management system based on sensor inputs for engine speed and throttle position. The possibility of modifying the back pressure of the exhaust system makes it possible to optimize engine efficiency in the various engine operating conditions. Greater back pressure, with the valve closed, allows the torque to be improved at light loads, while a smaller back pressure achieved by opening the by-pass valves enhances engine efficiency at high speeds and full load."
BMW twin tailpipe Valve - cheddar
I could have sworn that I had taken that extra "c" out, it is of course "Ricardo".
BMW twin tailpipe Valve - Andy P
This is what Chevrolet have to say:

"The Corvette Z06?s powertrain and drivetrain systems are matched to the LS7?s performance capability. The light, four-into-one headers discharge in to new, close-coupled catalytic converters and through to new ?bi-modal? mufflers. The mufflers each feature a vacuum-actuated outlet valve, which controls exhaust noise during low-load operation but opens for maximum power"

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