Dual Exhaust- are 2 pipes better than one? 3

[drwebb]

I have noticed several family cars today sporting dual tailpipes- i.e. one on each side of the back of the car. Yesterday I saw an older Camry with two tailpipes exiting the muffler next to a newer one with separate pipes on either side.

Now true dual exhaust was the trick setup in the days of 57 Chevy's and muscle car V-8's, but seems like today virtually all have a crossover to improve scavenging and other mysteries of fluid dynamics.

So is this merely a stylistic feature, or is there a technical reason? I am a little skeptical that the target market for Camry's would pay a premium for an extra muffler and tailpipe (and maybe even 2 cats?), but considering Chrysler's successful rebranding of their Hemi maybe I don't know my auto marketing from a hole in the ground?

 

[globi5]

Actually if you look closer you notice that these new cars with dual exhaust usually have a single pipe leading to the back which then ends in 2 mufflers.
As long as the target market doesn't look underneath the car, it won't notice the dual pipe which is really a single pipe and be satisfied to pay a premium for some useless extra weight. And the manufacturer is happy to sell 2 new mufflers when they are rusty.

 

[GregLocock]

globi5's cynicism is right on the money. Styling and model differentiation are the main reason. More subtly, a V8 benefits from two parallel systems, with a cross-over. Other engine arrangements don't, so much.







Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Marquis]

I've found that it had a much larger benefit on a V6 -particularly the low speed torque. This was probably helped with the variable cam phasing and increase in overlap at low speeds. An "H" connection is probably beneficial to fine tune the low speed curve, while I found the farther back you made this balance pipe the more lower you could tune the torque curve.
I found it made some difference on a cruciform crank type V8 but not to the same degree as the V6/inline six.

For NVH and sound quality, having two pipes exiting at either side of the car is like having speakers in your room at different ends, and having the twin pipes close together is like having the speakers mounted close together-makes little sense.
Also, seperating the pipes- allows you to increase your silencer volume- witness the E39 BMW M5- 4 tail pipes and MASSIVE silencer volume. This is the correct way to silence- and achieve low back pressure (250-300 mbar at peak power) and not do what the "Big 3" do- by having no package space and thus ramping up back pressure to achieve noise target levels- but then, they typically think 600 mbars is an acceptable backpressure level!

Some papers have also been written in having very slightly different path lengths bank to bank- to get a slightly uneven sound order modulation for added sound character. BMW did a paper on this. The uneven path lengths are quite far back in the exhaust system so don't adverse effect even cylinder to cylinder distribution significantly but make a big difference to sound quality.

 

[oab]

In today’s cars, the 6 cylinder has more to benefit from two pipes. With the almost mandatory 240* exhaust duration, it is easy to set the exhaust up with two 3 cylinder headers, each with a tuned length pipe to enhance the peak torque speed, filling, and long enough to allow the low speed inertia flow, to assist the scavenging at lower speeds.

With the 240* duration, only one cylinder uses the pipe at a time. This allows the system to be set up, as if it were three, single cylinders, taking turns to use a tuned exhaust system.

The two separate headers then enter the front open section of a converter or resonator, to expand the high pressure gas plug, and reflect the sonic wave. From there on, it is a matter of just reducing the sound level.

Harvey.

 

[globi5]

Here are a few photos of the pretty popular Duratec 3.0 V6 engine found in a Mazda 6 (and a variety of other Fords, Jaguars and what not) with a dual rear muffler set up:

http://forum.mazda6tech.com/viewtopic.php?t=2508

Before the exhaust gases actually reach the rear mufflers, they're forced through unequal length pipes (reduced scavenging), 2 cats per cylinder bank, a variety of bends, diameters, dents and other restrictions. At least in this case it appears that pressure drop plays a significant role in the manifolds, pre-cats etc. (where pressure is higher and gases are hotter and faster as well) and pressure drop in the rear mufflers is insignificant in comparison. (If Ford and Mazda wanted to reduce backpressure, they would have worked on other areas of the exhaust system in the first place).
I have no problems to believe that it is different in the case of BMW and its M5, but I doubt that there was much more than a simple design intend in the dual tailpipe set up that can be found in most of todays new(er) V6 family sedans. Keep in mind that nowadays many I4 family sedans even come with a dual tailpipe set up.