Bill Verburg

You'd lose that bet
954, muffler #8 is used on the street, megaphone #5 for off road rally use


964 C4 l/w rally car






For the normal older street cars they use the single transverse muffler as a packaging convenience, where they care about performance they are separate

current GT3, separate

Bill Verburg

To work well the pipes don't have to be absolutely equal in length an 1" discrepancy is acceptable, the merge geometry and size is much more important

Bill Verburg

yes, mea culpa

trophy

Awesome thread everyone, I love this stuff.....

vincer77

As I said, "in general" they work to improve midrange torque. But even fire engines can also benefit from cross-overs. Here is a dynograph of a Ferrari V-12 engine. Essentially two inline six cylinder engines. As our Porsches, it is an even fired engine. The headers are 2 x 3-1 on each bank. The factory Daytona had a muffler that connected the two 3-1 tailpipes together, and it was a frequent mod "in-the-day" to remove the muffler and replace it with straight pipes. The result was to hurt the midrange torque. While at Carobu, we decided to replace the straight pipes with a Burns X-pipe. The result was a significant increase in midrange torque.

The same mechanisms apply with muffled systems. The mufflers just add to the complexity. Using merge collectors, most of the tuning is in the headers and collectors.

Not from what have seen in practice. Most applications benefit from the crossover - including even-fired engines. The crossovers work best the closer they are to the engine. Some cross-overs placed in the rear of a front engined car are for sound purposes only - good and bad. Another way to think about the cross-over is that it minimizes the communication between the engine and the atmosphere. The smaller cross section can minimize reversion (i.e. reverse exhaust flow) as the cross sectional area of the cross over is smaller than for individual pipes.

vincer77

You really amaze me Bill at the wealth of pictures and schematics that you have at your disposal!

Since the merge collector actually broadens the rarefaction signal, yes, it makes the equal length aspect less important. But equal length is stil best IMHO.

Bill Verburg

The acoustic signal is reflected from the termination of the exhaust, on an unmuffled system this reflection is clean and can be tuned to where in the rev range you want it, The reflection in a muffled system is muddied up so much that it is next to useless, unless and extension into the muffler cavity is used

like this, muffler inlet at SA outlet at SC

vincer77

QED, the reflections become complicated. The further back in the system that the muffler is, the less effect the wave reflections have. The primary tuning is occuring in the header. Especially using a merge collector with an expanding section exit.

Not sure what point you are making with the diagram of a reflective muffler. Could you elaborate?.

poorb0yw

Camlob - Cool looking pieces and especially for the price. Well done.

I'm at work so I'll have to read through this thread in more depth later (looks fun) but here is an old crappy pic of the "equal length" pieces that I built for my car a while back.

If I remember correctly the runners were all within 0.25" of each other with a total length somewhere around 28".

Burns collectors, 321 stainless and custom flanges.

MarkD

+1

great info I hadn't seen before on some of the early race exh. Thx Bill

MarkD

I never get tired at looking at pix of the underside that car... awesome.

Bill Verburg

Adding a muffler (even one with zero backpressure) to a system with already optimized lengths can alter the pressure wave response such that the tuning is now out of phase with what is required and as a result, power drops. The trick here is to install mufflers such that they don't alter the tuned lengths of the system. Let us assume that the test muffler is attached directly to the end of the collector. A pressure wave is reflected either at the end of the exhaust pipe or when a sizable increase in cross-sectional area occurs. Open chambered mufflers such as Flowmasters often appear to the pressure wave much the same as the end of the pipe. This means the pressure waves see no change in length and reflection occurs largely as it did prior to the fitment of the muffler.

A glass pack muffler can act significantly different. It does not appear as a pipe end but as a substantial increase in collector length. Result: a reduction of power even though there is no measurable backpressure involved. From this we can see that many comparative muffler tests were in fact "pseudo pipe-length" tests. Although many invalid conclusions were drawn, these tests still demonstrated some important facts. The most important is that the engine's needs in terms of flow and pressure wave length tuning must be isolated, one from the other. This is easy to do by means of the pressure wave termination box (resonator box) mentioned earlier. Incorporating a resonator box into a system produces a layout along the lines seen in Fig. 9. With enough volume, the resonator box makes everything down stream appear invisible to the header's primary- and secondary-tuned lengths. With a flow capability of 2.2 cfm or more, the muffler appears virtually invisible from a flow standpoint. As a result, we have a muffled system that produces virtually the same power as an open exhaust.

Using an extended inlet creates a resonator box which the header sees as atmosphere
another example

Bill Verburg

Nice but pretty much irrelevant to n/a exhaust, though sans turbos it would work for n/a

silverboy

I would love to have Bill in the 997 GT2/GT3 forum...You are lucky guys.

vincer77

Well said Bill. Yes, I love seeing different tests for mufflers where they claim increases in power. You are correct that most of the effect is from differing tuned lengths. Absorption mufflers do not act completely as extended collectors for the pressure waves. The open area of the core is seen by the pressure waves as area changes. Different muffler styles have different transmission (and reflective) losses based on the design.

The open flowmaster type mufflers work as tailpipe terminations, I like to call them atmospheric boxes and we have used those to terminate tailpipes on dynos yet having an exhaust system. From a practical standpoint, it is difficult to put the termination box where you want it on a car. The tailpipe may want to be one length, but the only place that the muffler can go is in another place. I am working with a customer now that is restoring a 993 powered mid engined racer (GT1 maybe) and wants to package an atmospheric chamber. But the tailpipe wants to be longer than the calculated length in order to get to the location where the box can be packaged on the chamber. It is difficult to predict if the longer tailpipe is worth the trouble of packaging the box. Of course there is no dyno data to support the decision.

So I again say that getting the header manifold right is the best way to maximize power. A lot of the other bits are secondary. But if you have a good budget and can do dyno work to substantiate the tailpipe lengths and can package the mufflers where they are beneficial then these secondary effects can be taken advantage of.

However, when you look at professional racing cars, they do not incorporate these devices. At least I have not seen them.