Two strokes: Power from the exhaust

Auto Motor Klassiek » Practice and technology » Two strokes: Power from the exhaust
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Dream away from an exhaust system. How romantic can it get? Although: you are talking about a piece of traditional craftsmanship with a very serious batch of calculations behind it. About a sculpture in perfectly modeled, spotlessly welded thin-walled plate. About one of the most important parts of a two-stroke engine where the interplay of the exhaust elbow and the expansion chamber perform the actual function of the intake and exhaust valves in a four-stroke.

Thanks to MZ

These expansion exhausts are an invention of MZ constructor Dr. Walter Kaaden. The pipe runs from a front cone through a cylindrical section to a counter cone and an end pipe or tail pipe, creating a pressure wave that creates additional counter pressure in the cylinder. The angles of the cones, the cylindrical part and the tailpipe and the combination (s) thereof are of vital importance. Before Kaaden's time, two-strokes ran with straight, open pipes or even with megaphones.
Back pressure is essential for a two-stroke engine to keep the filling and flushing of mixture and exhaust gases under control. The correct dimensions of the expansion pipe ensure a considerable increase in power. This increase is caused by the fact that, at the correct measurements for one particular speed (the power band), the pressure wave has exactly the right timing to push freshly drawn-in mixture that has already partially entered the exhaust back into the cylinder. This causes a greater combustion volume and more complete combustion.

'Timing' is precision work

The 'timing' of the pressure pulses is high school calculations. And the timing, as mentioned, only works optimally within a limited speed range. You notice this very well when a tuned, snotty running two-stroke machine clumsily climbs up until it reaches its calculated working area. That's where Hannibal Jones from the A-team must have gotten his slogan 'I love it when a plan comes together'.

An expansion system for a road racer can therefore be designed very differently than that for a trial bike or a cross sidecar combination. In addition, the matter is so complex that swarms of 'experts' or self-proclaimed gurus swarmed around it. Now that is less in connection with the less attention to two-strokes. But on Kreidler and Zündapp forums the discussions tend to run out considerably.

But with two-strokes, expansions stood and stand for more power

At the time there were also a lot of providers who supplied them for the most common 'Japs'. In addition, the two-in-one and three-in-one usually provided less top speed, but higher torque. Reimo was a standing name at the time, but Ack Bants 'Bullet' exhausts were also very serious things. Currently there are also very good expansions for sale of the now famous Jolly Moto. But in Haaren Fedor Copal makes very nice sets. With all those exhausts it was also a matter of keeping a close eye on the ignition, the nozzle occupation and in the inlet side. Just putting a bunch of expanders underneath was a guarantee for piston damage or at least a bad running engine that had lost a lot of its power.

In the glory days of two-stroke racing, the Netherlands was quite a leader. And when it came to the show, Frits Overmars - a gifted calculator and an extremely amiable person - was an international top player. Already quite a long time ago he wrote pleasantly readable columns in Moto 73 based on his craftsmanship.

In the meantime, two-strokes are actually history. But there are still manufacturers who can make very nice expansion exhausts 'with the knowledge of today'. Then you have (considerably) more power. And more noise. A moderately to poorly muffled two-stroke exhaust note is a thing these days.


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  1. Still have one of the last 2-stroke motorcycles produced
    An Aprilia RS250 from 1993
    250 cc 2 cylinder 2 stroke
    With 2-stage exhaust valves
    Just bought a set of Arrow exhausts, it has been on the wishlist for quite some time.
    Great that 2tact cry.

    And for one of the mopeds I have a Homoet exhaust.
    Well-known name from earlier times, but does not seem to be top.

  2. In the glory years of the racing 2strokes, the late 50's to the mid-70's, they often only tried something by trial and error, also by factory teams.
    Theories were difficult to measure, but it was often plausible on paper.
    Power dyno was a rarity for motorcycles and racing teams.
    Of course there is much more to it than just the exhaust; compression, mixture and ignition are just as important.
    If all this is correct, good results can be achieved.
    Oh yes, just like racehorses, the smaller the jockey, the faster they go! (50cc's)
    Most guys who became a guy after their moped know all about that 😉

  3. @ Dirk: The Italian DEMM released a 50cc four-stroke moped in the sixties. Racing model a la Flandria .. Once parked at my home in the sixties, I was 7 at the time and my older brother explained to me what the difference was with a two-stroke, with the valves and so….

  4. @ Andre de Heus, Bennie v Andel from Dussen / Vanand did indeed create many beautiful things in the two-stroke and two-wheeler field at the time.

  5. In the period 70/73, unlike the usual Kreidlers and Zundapp, I drove an Italian Garelli, which was expertly performed by Vanand engine tuning and had a beautiful expansion exhaust manufactured. With excellent features, I managed to hit a whopping 115 km / h before the police bus left.

  6. It reads nicely and I don't want to deprive anyone of any fun, but hey: I'm so glad that the 2-stroke history is! But I admit: when my 11y. older brother around '70, an old Vespa 125cc started… mm, that was great. As long as you didn't crash with it, it was great. But in my mind I still love my third-hand Honda Amigo from back then. What a blissful roaring 4-twig and handling for such a 'ladies moped'. Yesterday I watched an electric crosser on the internet versus one with petrol and I don't know what to think. Even less noise and stench, but what a pathetic affair! In my mind I am already with the Honda Amigo and Novio. The Chinese do not succeed in bringing the same quality, about 40 years. later. I know that all too well with that *** Jing Cheng. The healthiest moped of all: I was constantly missing it. By the way: which European brand has ever put a 50 cc - 4 stroke on the market?

    • There were certainly brands in Europe that brought 50cc four-strokes to the market. Eg Demm and Alpino. I have had those two myself, but there have been more. I also thought eg Royal Nord. And just for the record, that was well before Honda came out! Something completely different was the diesel moped from Lohmann.
      All a very long time ago ..

      • The point with those old Italian 4-strokes was that sometimes you might also want to refuel with mixed lubrication, because the lubrication of the valve train was "moderate" at best.

  7. I drove a Suzuki 1978 T myself between 1981 and 500 and even drove it to France. In our town Bolsward there were no fewer than 7 people who rode the Suzuki two-stroke engines of 250 cc, 500 cc and 750 cc three cylinders. It was always tinkering and a lot of noise gave these race monsters. On the Afsluitdijk I achieved my personal record of 225 km per hour, above that everything started to vibrate. You could accelerate to 100 km per hour in a few seconds and only then switch to 3….
    Tough stuff but not suitable for touring. I also had a JAWA 250 from 1951, the robust two-stroke from Czechoslovakia.

    • Maybe I was also with that 7.
      Then rode a 380 with 3 in 1.
      I can still remember that I went to the barracks in Nijmegen at 5.00:XNUMX am on Monday morning during my duty.
      When the wind was favorable, my mother heard me switch back for Sneek with the bedroom window open.
      That were the days.

  8. I think the very first idea of ​​an “expansion exhaust” came from Garelli. I read that several times (unfortunately I cannot find the story behind it on my laptop).

  9. Nice article. And don't forget the freetech50. There is still 2-stroke racing.
    I myself have to explain to the youth what 2-stroke is. In the video see stainless steel exhausts that are 5 kg lighter.

    Greetings Marcel

  10. Yes Fine Kreet I have refueled ACDC, as above revive times of Tomos / Puch.

    DKW 3 = 6 far / high in the revs at that time there was a BP racing team that raced those DKWs at Zandvoort.

    Great story


  11. KTM has (about a year ago?) Released a motocrosser with a two-stroke and injection that meets environmental requirements.
    So there is hope.
    A hobbyist I've been following for a while (2stroke stuffing) on ​​yubtube is designing a 50 cc and making it about 26 !!!! Pkaatjes has to deliver. Show his developments (and failures😜) step by step. Very interesting. Does not run on gasoline but nitromethane (?). Is a swede. The construction of the cylinder and flushing port is seriously interesting. Last year already made a speed attempt on the salt flats in US, which sadly (literally) fell into the water.
    Since my Kreidler of 60 km / h I have kept a curiosity for 2 strokes ..
    Not much to do with classics is interesting

  12. In 77 I saw for the first time such a water buffalo with 3 in 3 expansion pipes near school (Mts) and waited half an hour for the third grader to go home, what a sound, as if he had tanked ACDC.
    I shook up my lambretta with an English PM expansion pipe….

  13. We imported All Speed ​​exhausts from England when I was still working at a motorcycle shop (Leen Post Motoren, Naaldwijk) what a cool sound.

  14. yes, the Netherlands was leading in racing in the 50cc class with the Van Veen Kreidlers, 2-stroke exhausts were an interesting subject at the time, but it was not only in the exhaust, there was also the intake length, among other things (after that with 4-stroke car engines), the necessary measuring equipment, based on rpm. and pressure measurements inside the crankcase, etc., were dead and for a measurement with the engine running at full throttle the fast printer spewed 4 meters of paper at one revolution! However, it was too late to convert the results won and put them into practice, the end of 2-stroke racing was already announced!

  15. We used to make our own exhaust system for two and four strokes in the past
    With the molds we made ourselves.
    Outside we had a barrel of water that we filled with carbide & we autogenous the whole thing
    to each other, it was more melting together of the 0mm plate we used to make welding wire
    times to save have changed

    • Nice article!,

      I have raced two-strokes and I know renowned tuners.
      The one who is leading in the Netherlands and even in the world is Sam Balder. (SAMBA Racing)
      At the time, he constructed many exhausts for and together with Hans Spaan when they rode WK125cc
      Sam has made many exhausts for Aprilia and Honda HRC after that career.
      A lot of empirical (test bench & circuit) test work ultimately results in the calculation modules

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