Home arrow Technical Articles arrow Hot Air 15/01/2003 (Other Forms of Branch Manifolds)
Hot Air 15/01/2003 (Other Forms of Branch Manifolds) PDF Print E-mail
We have dealt with various aspects of branch manifolds, but only 4 cylinder manifolds.

Let us take a look at 5 cylinder, 6 cylinder and 8 cylinder branch manifolds:


5 Cylinder:

The only 5 cylinder branch in S.A that I know about was developed for the 2.5 VW Microbus, and later the 2.3 and 2.6. Initially a 5 – 1 was developed which worked reasonably well. However the first batch of 2.5 Microbuses made approxamily 10% more horsepower on the dyno, and the 5 – 1 suited this set up. Dealers denied that there was a more powerful motor, but the dyno said otherwise! To support our observation, the 5 – 1 did not work too well on the newer batches of 2.5 Microbuses.

We set up development on a 5-2-1 with the idea of improving mid-range power specifically and to try and widen the power band as much as possible.

This branch eventually replaced the 5 – 1 and all the manufactures of branch manifolds converted to the 5-2-1 format.

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 57mm through-flow exhaust system. This system routed the exhaust gases towards the front of the vehicle, made a u-turn and exited on the left, same side as the branch. 63 mm systems were tested 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.

Some fitment centers utilized an “in and out same side” box instead of turning the pipe around through 180°. This was effective in silencing the system but 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.

Amazingly local manufacturers of branch manifolds used different firing order combinations, e.g. cylinders 1,2 and 5 together into a 3-1 collector and 3 and 4 cylinders into a 2-1 y-piece, or

1, 2 and 3 joined together and 4 and 5 together, or 1,4 and 5 joined together and 2 and 3 together.

They all perform reasonably well with very similar results on the dyno.


6 Cylinder:

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 BM 3 series is a good example of this. The exhaust headers must share the space provided for the R.H.D. steering components. What 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 down-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.

I would like to dwell for a bit on another 6 cylinder animal, the VW VR6 (NEXT ISSUE)


Abel dos Santos
 
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