4 Into 1 Classic Motorcycle Exhaust Systems

Dresda Honda
Dresda Honda 500/4/Flickr CC 2.0

 As Japanese large capacity motorcycles began to dominate the marketplace in the 1970s, many tuning and performance shops changed focus from British bikes to the new machines.

For the tuning shops, the early Japanese machines were a gift; they were easy to tune, they had poor handling and customers were eager to copy the looks (and sounds) of the many racers who were now using them.

Of the many performance-enhancing items to be made for the Japanese bikes, none were more popular than the 4 into 1 exhaust system (reputably first introduced by Dave Degens of Dresda fame).

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4 into 1 Classic Motorcycle Exhaust Systems

According to Degens, who had been commissioned to build frames for the French Honda importers (Japauto), he built a special set of pipes for the French team to use at the Bol d'Or Endurance race. Degens had correctly surmised that the French circuit would suit a motorcycle that could be leaned over to the maximum before any parts touched down, even if it meant losing some performance from the engine. (It was reported that the Honda factory had tried a similar system and deduced that it did not work!).

The Dresda Honda’s built by Degens went on to win the Bol d'Or classic back-to-back in 1972/3.

As more race teams began to use the 4 into 1 systems, street riders wanted similar setups for their machines—in many cases for the sound the mufflers produced. Before long most of the Japanese four-cylinder machines had 4 into 1 systems available from many different manufacturers including:

  • Yoshimura
  • D and D
  • SuperTrap
  • Vance and Hines
  • Two Brothers

In the early days of 4 into 1 exhaust systems for the aftermarket, many of the companies would offer a system that was a catch-all. That is, a single muffler (and occasionally lightly modified pipes) would be offered for many different makes and models. Needless to say, the systems would work well on some bikes (gaining a large following of customers who claimed they had the best) but poorly on others. Later, many of the aftermarket 4 into 1 system manufacturers would specialize in one brand—Yoshimura with Suzuki for instance.

Again in the early days, it was not uncommon to find that the carburation (jetting) needed changing to complement the characteristics of the 4 into 1’s being used. From a purely practical standpoint, the performance of the early systems was typically good only in certain rev ranges (ok in racing, not so good for street use, but they did give much better ground clearance.

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More Stringent Rules

 In fairness to the manufacturers, their exhaust design teams were faced with much more stringent rules to follow in various forms –because of the many countries where their motorcycles were likely to be sold. In addition, the stock system was expected to perform well in all climates using all fuel types by riders of varying skill levels.

Today, many of the more popular 4 into 1 system manufacturers still offer systems for early classic Japanese motorcycles. All of the systems have been developed over the years to give much better performance than when they were first introduced. This is directly related to the sophistication of later dynamometers used by the performance industry when testing their systems.

For classic riders considering the purchase of a 4 into 1 system, he/she is well advised to further investigate systems from the main suppliers—particularly the ones that have stood the test of time (see above list).

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Fitting an Aftermarket 4 Into 1

Fitting a new exhaust system is relatively simple, requiring only a few basic hand tools. The sequence for fitting a system is as follows:

  • Removed old exhaust system (taking care not to get any dirt inside open ports)
  • Wear latex gloves (fingerprints on chrome pipes during assembly can spoil the finish
  • Following the manufacturer’s instruction (if available), the new system should be loosely fitted to the bike
  • Fully tighten the system after all parts have been loosely fitted, starting with the header to cylinder head bolts/nuts (using the correct torque wrench)
  • Recheck tightness of all hardware after approximately the first 100 miles—only check the tightness when the system has cooled
  • If the system instructions require a jet change, this must be accomplished at the same time as fitting the 4 into 1 (some systems cause the engine to run lean which can cause serious internal damage)

Note: A word of caution. It is reported that Devil exhausts in France have declared Bankruptcy. However, some of their systems are still advertised. Customers should approach any adverts. with this in mind.