HOT AIR: What is the Best Exhaust Diameter?

Generally speaking, the bigger the exhaust pipe diameter, the more effective the power gains will be at higher r.p.m. and conversely, the narrower the diameter the more effective the power gains will be at lower r.p.m.

This, however is a general rule and there are many exceptions to this rule appearing almost daily.

The previously mentioned VR6 for example:

When the exhaust pipe diameter was reduced from 63,5mm to 57,15mm, the general expectation was that there would be an increase in bottom-end and mid-range power with a resulting power drop on top end.

The results were almost the opposite: There were major gains in top-end power and mid-range power!

Another good example is the Golf Gti MKI, with a gas-flowed head, 286° cam and racing branch manifold.

When we decided to increase the exhaust pipe diameter from 57.15mm to 63.5 mm we were expecting a power drop at midrange with significant improvement in top end power. Instead we achieved significant power gains at mid-range and hardly any gains on top-end.

This was truly amazing, since the 4-1 merge collector had already been modified to increase gas velocity at the converging pipe to maximize midrange. (We had effectively reduced the diameter of the flex joint intermediate piece from 63mm to 54mm with the intention of aiding the scavenging ability of the big bore branch)

When the exhaust pipe diameter is increased, two things will influence the determination of the correct diameter:

  1. 1. Noise: the bigger the bore, the more problems we will have to minimize the exhaust noise to acceptable limits. In a good free-flow exhaust conversion, very often I encounter difficulty in obtaining the correct silencer box combination to achieve a sporty, yet unobtrusive sound, with acceptable resonance levels inside the car, without resorting to the use of specially fabricated silencer boxes (which are normally not obtainable as off the shelf universal items). It is sometimes possible to achieve noise levels very similar to the stock exhaust levels and, at the same time reducing back pressure to the best “free-flow” standards.
  2. 2. Exhaust Gas Velocity: Reducing the exhaust pipe diameter increases exhaust gas velocity in that exhaust pipe. However, too small a diameter also induces back-pressure, which is an undesirable bi-product in the search for the optimum exhaust gas velocity. Increase the exhaust pipe diameter too much and the exhaust gas velocity is drastically reduced, thereby losing beneficial scavenging effects.

Whether the exhaust in question is for the road or for the track, size does matter.

A happy medium has to be found and, the only way is by testing and re-testing on the dyno.


Free-flow exhaust construction

Besides the different types of silencer boxes and the appropriate pipe diameter, there is an additional aspect of free-flow exhaust fabrication that can be crucial to the required optimum gas velocity, namely, the quality of the pipe bending and finish on that system.

The number of instances over the years where I have come across so-called free-flow exhaust systems which have restricted exhaust piping due to misaligned welding, kinked or angled welds on connecting pipes and squashed bends causing a bottleneck, is staggering. Each bend along the length of the exhaust pipe increases back-pressure and, if the bend is badly flattened or misaligned or whatever the restriction may be, the back-pressure is increased even more. If larger diameter tubing is used in areas of tight bends, restriction will be minimized, solving the problem, but to date, I have not seen this done; on the contrary, with few exceptions, the quality of work relative to free-flow performance exhaust fabrication, has definitely deteriorated over the years, specially as uninformed ‘enthusiasts’ opt for cheap (read ‘rubbish’..) fabrications..

That’s all for now!

Abel dos Santos