High-tech, well thought-out: DTW gas flue systems

DTW exhaust systems – developed in cooperation with Meisterwerke Autosport

Nothing can replace displacement – except even more displacement!

Who does not know this often used quote? And how true it is – but more about that elsewhere!

Unfortunately, the performance optimization of naturally aspirated engines cannot be realized via a simple data set or simply by replacing the “downpipe” with flow-optimized catalytic converters, as is the case in our turbo times today. A little more boost pressure here, a little fewer cells in the catalytic converter – and we have a simple recipe for more power. No, unfortunately it’s not that simple with air-cooled six-cylinder engines – and that’s precisely where our challenge lies.

Interpretation

In recent years, we have taken up the challenge of getting to the bottom of the technical mechanisms of this “legendary matter”.

The basis of our development is a small, self-written calculation software that precisely determines the target variables of the charge exchange optimization on the basis of empirically determined formulas. In concrete terms, this means: pipe lengths, diameters, type of header geometry, silencer design, et cetera.

For optimum function, a manifold sport exhaust system must necessarily take into account engine-specific characteristics such as displacement, timing – in this case, above all, the timing of the valve overlap – and a defined operating point. Secondary, but no less important, are the pipe routing, the geometry of the headers and small special features such as the diameter jumps (so-called “steps”): more on this later!

There are fluid mechanical formulas for this in theory, which can be used to design the pipe dimensions. In addition, empirically determined parameters, i.e. formulas validated on the test bench, can be found after a thorough literature search, which allow the flow mechanics to be optimally determined.

To develop the exhaust system, we took a close look at all this partly “historical” documentation and transferred it to a calculation tool. For us, this calculation method forms the basis for a design optimized in terms of power and torque.

We were already able to carry out performance measurements on the first prototypes in 2015, which resulted in a torque peak of 365 Newton meters and over 318 hp. This was measured on a 3.6-liter engine with modified Motronic and a 304° schrick camshaft at a compression ratio of 11.5 : 1.

In addition to determining the correct pipe diameters and lengths, we implemented another fluidic optimization. After a defined length, our manifolds still have a wall thickness step in the primary tube. This involves a little more effort in terms of production engineering, but pays off in an overall increase in the torque and power band. In theory, this step has the same effect as a correctly selected primary tube length – namely, it reduces the pressure in the exhaust gas by means of an induced pressure wave. This is reflected at the “open pipe end” and helps the engine to achieve more efficient filling with fresh gas through the so-called “pulse tuning” effect.

Thus, in a nutshell, we create a charge exchange optimization. According to the ideal design, the “returning” vacuum wave should be in contact with the exhaust valve at the time of the valve overlap in order to bring additional suction/vacuum into the combustion chamber when the exhaust is opened (AÖ), thereby ensuring the charge change optimization described above through more efficient scavenging and faster flow of fresh gas. Even with a very sound theoretical design, this theory must be validated in practice by some test bench runs.

Performance

The two 200-cell catalytic converters come from HJS. We deliberately decided against 100-cells – based on the recommendation by HJS to go for 200-cells. We can thus ensure maximum longevity.

The difference between 100 cells and 200 cells in the catalytic converter is measurable in peak power – but not across the board. This is engine-specific, but an order of magnitude here is between three and five hp as a rule of thumb from our own experience.

Also because of the “Euro 2” emission requirements, which are not easy to meet after all, we decided on this compromise in favor of higher quality and maximum longevity in the interests of the customer.

According to many confirmed dynamometer measurements, the system is capable of delivering outputs ranging from 315 hp to over 400 hp for 3.6-/3.8- and 4.1-liter engines. It must be mentioned here that each engine does not allow a serious blanket statement at what “final power you will come out with later” due to specific engine/part tolerances and/or changes in camshafts, compression, throttle body and other components. However, we can assume that, in conjunction with a lambda map adjustment, additional output of between 35 and 45 hp is realistic. We have measured these values (and also much higher ones) several times in real life – below a dyno protocol of Falco’s 1991 Carrera 2 3.6 liter. The hardware changes are noted on the measurement protocol.

In the meantime, our exhaust systems have also been tested in practice several times in sports use on various circuits.

The specific challenge is to permanently withstand the extreme requirements of high full-load proportions.

Some race tracks do not allow the operation of vehicles that are too loud. One example of this is the BILSTER BERG, where we were able to stay below the permissible decibel limit during a video shoot in 2021 with two vehicles equipped with our exhaust systems. The BILSTER BERG is considered one of the most restrictive routes in terms of noise limits and their constant monitoring.

The crowning glory and accolade of our test drives was undoubtedly the video shoot, during which Walter Röhrl – personally at the helm – praised the balanced sound and the good workmanship of the exhaust system.