Let us take a look at 5 cylinder and 6 cylinder branch manifolds:
The only 5 cylinder branch in S.A that I know about, was developed specifically for the 2.5 VW Microbus, and later the 2.3 and 2.6. The motor was borrowed from the Audi 500 in those days for the development of the VW Caravelle Microbus. A turbo’ed double overhead cam derivative of that motor is still employed in the Audi RS3. Initially a five into one was developed which worked reasonably well. We achieved approxamily 10% more horsepower on the dyno, where the five into one suited this set up. Although VW dealers denied this, newer batches of the 2.5 Microbus had a less powerful motor and the 5 – 1 did not work too well on these newer batches of 2.5 Microbuses.
We decided to re-develop this branch and opted on a 5-2-1 format with the idea of improving mid-range power specifically and to try and widen the power band as much as possible.
This newly developed branch eventually replaced the five into one format branch.
Generally the power gains were significant. The 2.5 Microbus had a maximum power gain on the wheels of 7kw from 57kw to 64kw. (The first batch gained 7kw from 62kw to 69kw).
We used a 3 box 57.15 mm through-flow exhaust system. This system routed the exhaust gases towards the front of the vehicle, in similar fashion to the stock system, made a u-turn and exited on the left, same side as the branch. We tested 63.5 mm systems as well: The resulting 5 cylinder sporting sound was a beautiful noise, but a bit too “sporty” for the general family man, towing a caravan for long distances and we reverted to 57mm with a 63.5mm ‘U-turn’ section.
Some fitment centers utilized an “in and out same side” box instead of turning the pipe around through 180°. This was effective in terms of attenuation, but the system robbed power. I personally preferred the 57mm system with two boxes instead of three, simply because I love the sound of a revving 5 cylinder motor.
6 Cylinder branch manifolds are usually reasonably easy to develop. A straight six will have two 3-1 branches.
1, 2 and 3 cylinders are coupled and 4, 5 and 6 cylinders are coupled into two 3-1 collectors. These two collectors join the exhaust system by means of a 2-1 intermediate, or secondary pipe.
The main problem in developing a straight 6 branch is usually lack of space for adequate diameter header pipes: The BMW 3 series is a good example of this. The exhaust headers must share the provided space for the R.H.D. steering components. We can safely bet that L.H.D. headers develop more power?
In the case of a V6, the two banks of cylinder heads will have a 3-1 branch each, also joining up into a single outlet through joined intermediary pipes.
The appropriate diameter of a matching free-flow silencer system will depend on the capacity of the motor and, naturally the state of tune of that motor. Invariably the diameter will be above 57mm, sometimes as big as 76mm.
Abel dos Santos.