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                                                                                                                                                       This page updated on 05/08/2020

Two-stroke motorcycle history.

All About Ports.                                    

Loop scavenging and the very significant boost port.

In 1892 the Englishman Joseph Day produced the final version of his crankcase compression two-stroke engine, a 3 port type device comprising of a single inlet port, single exhaust port and 2 transfer ports. Ports are holes or cavities in a cylinder barrel wall and can come in many sizes and shapes, the object of these ports is to move the gasses in or out and around the engine for induction, combustion and extraction.

The following is my attempt to produce the fullest and definitive account of the development of loop scavenging and it’s bedfellow the boost port, this is something that has always fascinated and interested me. An Englishman invented the two-stroke engine, wouldn’t it be great if it could be said that an Englishman also invented the boost port, however it was mostly the German’s who Initially got the most out of Joseph’s invention, with their Schnurle’s loop scavenging, boost port and more.

Which is the most significant development, loop scavenging or the boost port?
Production motorcycles with loop scavenged (flat top piston) engines appeared in the early 1930’s, followed shortly in the mid 1930’s by engines with boost ports, and since then boost port development has advanced dramatically.
Choosing the most significant of these two developments goes to the boost port, because not only is it used in loop scavenged engines it is also used in the modern crossflow (deflector piston) engines.

Loop scavenging.
In two-stroke terminology scavenging is the action of getting the gasses in and around and out of the engine, in the early years scavenging was known as rinsing or flushing.

The early two-stroke engines used deflector pistons utilizing a system known as cross scavenging, this scavenging method was eventually replaced with loop scavenging, an improved method of getting a fresh charge of mixture into the combustion area and the burnt gasses out of the exhaust.

A milestone in two-stroke development was in 1932 when Jorgen Skafte Rasmussen, the Danish boss of DKW purchased Adolf Schnürle’s 1925 general patent license, thus possessing all of the rights to Schnürle reverse scavenging (loop scavenging). This came about after Rasmussen had read Schnürle’s doctorate on thermal function for two-stroke engines and also after reading Dr Herbert J. Venediger thesis on the improvement of two-stroke engines, as well as obtaining the Schnürle’s patent, Rasmussen also employed Venediger for continued loop scavenging development.

Schnürle’s scavenging system was originally developed for large slow-running stationary two-stroke diesel engines as used by the Deutz Company, however development now lay in the hands of DKW as Deutz had no interest in the perfection of small, fast running two-stroke engines.
The result being that around 1933, DKW released motorcycles to the market with engines equipped with flat top pistons and Schnurle loop scavenging.

It is important to understand that Schnürle's patents referred to a certain symmetry based on the shape and location of the scavenging ports which were always aimed at the back wall of the cylinder. The porting arrangement consists of two angled transfer ports located either side of the exhaust port, this provides a circular looping gas flow, flowing in and across the piston crown from the transfer ports, up and around the combustion chamber and then out through the exhaust port, improved scavenging, a cooler piston and more power.

Schnürle piston crowns can be flat, slightly domed or even bowl shaped, a contrast to the deflector piston of the old cross flow scavenging system, from then on deflector pistons engines gradually declined, however rather surprisingly deflector pistons are still in use, not dead yet, more about this at another time.

Loop scavenging was so significant that many manufacturers attempted to invent their own scavenging systems for motorcycle use, some successful and some not.
The concept of flat top piston scavenging, although pretty well dominated by the Schnurle patent is by no means new as around the time of the Schnürle patent doubts had been raised about his system.
A German patent dated 1908 was obtained by the Italian engineer named Kind and used by Zundapp, there was also another inventor in 1911 who had already outlined a similar system. Are these two very early scavenging systems in fact the same one?

When introduced by DKW, the design proved efficient enough to cause serious consternation among other manufacturers. Either they would have to pay costly fees to DKW to use the design, which DKW didn't seem interested in doing anyway, or new designs would have to be developed to avoid patent infringement. This was the course taken by Ardie in 1935 with its 'x-cross scavenging' as well as similar developments by Villiers, Sachs and Zundapp.


               Loop scavenging - flat top piston                                  Cross scavenging - deflector piston                                                                             

Boost ports.
In 1933 the boost port was another milestone in two-stroke development, and understandably as Zundapp was DKW's main competitor in Germany they had to keep up with or better the competition. With the result that Zundapp’s Managing Director, Hans-Friedrich Neumeyer demanded that Chief Engineer Richard Kuchen must immediately design a new engine, it had to incorporate reverse scavenging (loop scavenging) and at the same time it must be an original design. 
Kuchen did what he was told and designed a new engine featuring a three transfer port design, the third port Zundapp called the ‘assisted scavenging control port’, this new control port was in fact a boost port. It was located opposite the exhaust port and directed upwards at a very steep angle to improve the control of the flow from the main transfer ports. This was a bold but controversial attempt by Zundapp to get round DKW’s Schnurle patents.
Zundapp’s boost port scavenging system was patented in 1933, which so far I have been unable to locate a copy of, however I do have details of the 1934 French patent No.769.158.

The auxiliary transfer port, or now more commonly known as a boost port is a cul-de-sac, a blind port in the cylinder wall, it’s function is to trap a portion of additional fresh charge for release into the combustion area, these boost ports are big enough to hold a few cc's of mixture.
How does it work?
When the piston descends it compresses the gas in the crankcase, the piston continues down its stroke to the point of maximum crankcase pressure, just before the transfer ports open the piston skirt then covers the boost port trapping a few cc’s of compressed gas. As the piston nears the bottom of the stroke, the main charge of mixture transferring into the barrel through the transfer ports has almost stopped, the boost ports are then uncovered releasing the compressed gas into the base of the barrel. This extra gas helps to eject the exhaust gases and also to fill the barrel with ‘good’ mixture thus boosting the existing charge.

As well as an improvement in power, another benefit of the boost port is the elimination of the so called 'diesel operation', also known as dieseling or engine run-on, a condition that can occur in sparked petrol powered internal combustion engines, when the engine keeps running for a short period after being turned off, due to fuel igniting without a spark. This dieseling effect is caused by pre-ignition of the new mixture by combustion gasses, which due to insufficient scavenging still remain within the combustion chamber.
In 1934 Zundapp introduced their Derby 200 with their new Kuchen five port two-stroke engine, the first production motorcycle with a boost port engine, immediately creating a distinct advantage for Zundapp over the competition.
Unfortunately for Zundapp, their new system with an extra port opposite the exhaust port was deemed to have infringed the DKW (Auto-Union) Schnurle patent. The decision was based on the following grounds: The amount of new air entering through the rear transfer port (the boost port) is too small to be relevant for the scavenging (little did they know). Incidentally, today this decision is now seen as incorrect.
A countersuit Zundapp patent was submitted with a structural modification, the result being that a patent was granted.
Between the time of Zundapp’s failed patent and their successful new patent they used the ‘Signor Kind’ scavenging patent mentioned earlier, a scavenging method originally intended for larger cylinders but was equally effective in engines of 200cc, the Kind patent had expired long ago so there was no risk of patent infringement. Derby 200 production continued up until 1951. After World War Two all patents on reverse scavenging (loop scavenging) were lifted, giving all manufacturers free use of the design.

It is not clear if Zündapp actually ‘invented’ the Boost port, maybe not, the only other known production engine featuring a boost port was the Scott PA stationary unit of 1939, patent GB512980 and credited to Scott Motors Ltd Chief Engineer, William Cull.
It is said that Scott concern had supposedly been using boost ports in earlier Scott racing two-stroke engines back in the 1920's, if so was it Alfred Scott himself who initially came up with the idea, but Scott had left the company in 1918 and died in 1923, so maybe it was his engineers Harry Shackleton or the already mentioned patentee William Cull?
Conjecture therefore suggests that the invention of the boost port might have come from within the Scott concern, the only way we would know for sure I suppose would be to dismantle one of their old racing engines, if one still exists and carefully of course!

I had a 1948 Scott 600cc Flying Squirrel in my youth and I still have couple of seized pistons tucked away, heavy old lumps they are as well. If you look down the bore of an old two-stoke barrel, a Scott engine for instance and then compare it to a modern performance cylinder bore, there seems to be more holes than metal!

In the 1950’s the boost port became a bit of a hot topic because of it’s very successful development by the East German engineer Walter Kaaden and the successful results for the East German MZ racing team. Up till then not much was known about boost ports but following Ernst Degner’s defection to Suzuki complete with MZ’s secrets the two-stroke world woke up.
The impression given is that boost ports didn’t exist before Kaaden but that’s not the case, for we know that Scott had a boost port stationary engine and Zundapp had been producing boost port motorcycles for several years. Following Degner’s defection to Suzuki, they were not surprisingly the first Japanese firm to capitalise on the new technology, when in 1960 we had the appearance of their 125cc racing team at the Isle of Man TT races.

I was at the I.O.M in 1960 at the debut of Suzuki’s race team and luckily I had a press pass courtesy of cousin Rod, this meant we could go anywhere we wanted and as a result I was in the paddock to see the new Suzuki racers, neatly displayed in a row and draped in Suzuki’s blue covers.
In the same week Rod and I we were having a ride round the TT course when we saw one of the new Suzuki racers parked up, it wasn’t unusual in those days to see a real racer on the roads, anyway Rod and I stopped and approached the Japanese chappy looking down at his dead motorcycle and I "asked could we help", his only reply was, "won’t start, won’t start", we left him to it!

Although Suzuki had acquired MZ’s secrets and were successful because of that, it was Yamaha however who in the later years had the most success from this new technology. Yamaha achieved five World Championships in the 125cc and 250cc classes and as a result they learnt an awful lot about two-stroke engineering. The Yamaha 125 and 250 V4s were most definitely the best two-strokes of their time.

The impression given is that boost ports didn’t exist before Kaaden but as already mentioned we know that Zundapp had produced boost port motorcycles much earlier, and following Degner’s defection to Suzuki they eventually appeared in other Japanese motorcycles, mostly for racing at first and then for road use.
Author and journalist Matt Oxley wrote in his Stealing Speed book that Kaaden claims "he found the answer in a dusty old textbook and there discovered a 1930 Zündapp two-stroke design incorporating an extra transfer port". Following this so called ‘discovery’ MZ racers were to become very successful in the mid 1950’s.
Ponder this folks, it was only 17 years earlier in 1933 that Zundapp had patented their boost port and in the following year they were selling the boost ported Derby 200 range. The Derby range of motorcycles with engines of various boost port designs were manufactured between 1934 to 1954.
One has to question Mr Kaaden or is it Mr Oxley's version of events.
My Suzuki contact, previously mentioned in this websites 'The Japanese' page doesn't hold much credence to some of the material written about Degner and Kaaden. 

Another boost port chapter involves a Dr Joe Ehrlich, a wealthy Austrian who fled from the Nazis in 1937 and settled in England, he formed the British EMC company to manufacture a split single road motorcycle and to develop his own boost port system, as used very successfully in his EMC 125cc racer.
Kaaden copied Zundapp, whatever the truth is, it’s Kaaden who must be credited with early post war boost port development and in 1955 he had his first podium with a boost ported MZ125.
Just one year later in 1956 Dr Joe Ehrlich patented his version of the boost port, further development continued within the Bristol Siddley Engines company.

The time line for boost port development is Zundapp and Scott (pre-war) MZ and EMC (post-war).

Kaaden undoubtedly got his boost port info from Kuchen (Zundapp) dusty book or not. Did Kaaden and Ehrlich share information, unlikely, did Ehrlich 'borrow'  from Kaaden, possibly?
It has been suggested that Ehrlich got his boost port info by passing on high quality parts from the UK to Kaaden as East Germany lacked such items i.e. Norton forks, Girling suspension, Lucas ignition and Amal carbs, however another source says he got a stock of Norton forks from England's ace tuner Francis Beart, with the help of the Shell Mex/BP trade baron, Lew Ellis.
Unfortunately Ehrlich is regarded by some as a bit of charlatan, probably because of his supposed dealings with Kaaden and the possible similarity to Kaaden's MZ engine, in Joe's defence his boost port design is  totally different to Kaaden's. Another niggle might be concerns his EMC motorcycle split single engine and the alleged similarities to the Austrian Puch split single engine, personally however I can't see any visual similarity, maybe it's sour grapes, after all he wasn't British!
Whether any of the above aspersions are true I don't know, the man had to start somewhere, regardless of his beginnings Dr Joe Ehrlich went on for many years to be a successful and original two-stroke developer and builder
Over time the boost port has evolved somewhat, instead of just one boost port as in the Scott, Zundapp and MZ racers or two as in the EMC racer, in modern times a cylinder bore can now have a multitude of different ports with different sizes, volume, position and shapes.


3 boost port examples - exhaust port orange, transfer ports yellow and boost ports blue.

In 1967 Yamaha introduced their ground breaking boost port range, the 5 port AS1 road and the AS1C trail motorcycles, to be followed in 1971 with a 7 port range of motorcycles. A far cry from Joseph Day's early 2 port engines and his ground breaking 3 port engine that followed, the 3rd port being the invention of Frederick William Caswell Cock, this is where the piston controls the inlet ports opening and closing,  thus ending up with the classic 3 port Day/Cock engine, 1 exhaust port, 1 inlet port and 1 transfer port


Getting the fuel mixture into the engine efficiently was a challenge, first we had the simple inlet port in the cylinder barrel and later the adoption of a reed valve in the inlet tract of the inlet port. The next development was to ditch the simple cylinder reed valve inlet port altogether and instead mount the reed valve in the side of the crankcase, this is known as case-reed. Why muck about with inlet ports, just stick a carburettor on the crankcase.

Another way of getting the gasses into the engine was another crankcase method, this time it was a rotary disc valve, basically a disc on the end of the crankcase with cutaways to allow the gasses in at the appropriate moment, the carburettor being mounted forward of the disc. This crankcase rotary disc valve induction method for motorcycle engines was invented in 1922 by Sun, a British company, but it was only in the late '60's that this superior two-stroke induction development really took off when the racing crowd 'discovered' it, and to this day it is seen as the most efficient with regards to power, however constraints regarding carburettor vulnerability, styling and economy lessened it's appeal for generally motorcycling, particularly as the reed valve had reached new levels of efficiency.
Honda, Yamaha and Suzuki's grand prix racing motorcycles used rotary disc valves for maximum power in their early years, but interestingly all 3 manufacturers were using case-reed induction in their final years of the 500cc racing. The following year 500cc two-strokes were dropped to be replaced with four-strokes.


In 1968 Yamaha introduced 2 significant new 125cc motorcycles, the AS1C Trail Cat and it’s road version the AS1, the first the post-war volume manufacturer to produce motorcycles with boost port engines.

Road tests of these 2 motorcycles show them to be very lively indeed, much more than other comparatively sized two-stroke motorcycles.


1968 Yamaha AS1C 125 Trail Cat.

Specs:  Engine: 125cc two-stroke, 5 Port Parallel Twin. Max HP: 15.2 hp @ 8500rpm. 5 speed gearbox. Max Speed 75-80 mph.


The following is an extract from the website and the road test of the Yamaha AS1, Yamaha's road version of the AS1C Trail Cat, picture shown above.

The little Yamaha was a real head turner, the neat little twin-cylinder engine sits slightly inclined in the equally neat frame and you just can’t help liking the bike, however, the 125 wasn’t just a pretty face.
The most important element of the package was Yamaha’s use of 'boost ports' on a conventional two-stroke, without a reed or disc valve, a huge amount of fuel is wasted because of inefficient combustion. The Suzuki T500 I raced at Daytona managed a wallet-destroying 12 miles to each gallon of race fuel simply because so much gas in each combustion cycle was never used to its full effect.

Yamaha added two “boost ports” to the AS’ cast iron cylinder. These dramatically improved the scavenging of the engine and gave a significant increase in power and torque. Boost ports were nothing new in that Walter Kaaden had developed them fully with his MZ race bikes and a second form, designed by British engineer Joe Ehrlich, had been around for 10 years before the launch of the AS125. However, both of these two-stroke gurus employed boost ports as hand-crafted tool-room exercises, undertaken very slowly by skilled craftsmen.
What Yamaha did so cleverly was to mass produce the system so that race technology went straight into production machines. And the effects were dramatic. The little AS1 produced an astonishing, for a production road bike, 15 horsepower, and at only 8500 rpm.
As an indication of just how softly the road bike was tuned, Yamaha sold a GYT race kit which transformed the machine into a full-on track bike. The conversion included new cylinder heads, barrels and expansion chambers and, depending on how clever the tuner was, power now jumped to anything around 22 hp at 13,000 rpm. Just how good this power output was can be seen by comparing it to a 500cc Grand Prix bike of the day, like a Matchless G.50 or Manx Norton, both of which gave around 48 hp. This was double the power of the 125cc Yamahas – but four times the engine capacity.
In the real world, the AS1 road bike did have to be revved hard but it was capable of a genuine 70 mph and, better still, it was utterly unbreakable. The biggest weakness was the ignition timing which, in the days before electronic sparks, could wander and very easily lead to holed pistons. I had a Velocette Venom which was almost always in sick bay and the AS1 was my spare bike, which I rode ruthlessly and without mercy – and it never missed a beat.
A major contributory factor in reliability was the labyrinth oil seal which finally solved the problem of seal wear in multi-cylinder, two-stroke engines.
If the engine was good, the gearbox and clutch were its equals. The five-speed gearbox gave sweet and positive changes and the clutch was light and fuss free.
At a time when the European manufacturers still demanded that riders use a measuring cylinder built into the filler cap of the fuel tank – yes, really – to calculate the oil/gasoline ratio before filling up, Yamaha had fully automatic, and totally reliable, lubrication. Top up the oil tank every 500 miles and ride away: that was it.
Much of this technology was driven by the American market. How important American customers were at the time is sometimes now overlooked, as history is being increasingly re-written. It wasn’t only that there were a lot of potential motorcycle owners in America but the type of customer which the American market presented was radically different from traditional, hardcore motorcyclists. In England, every blue-blooded biker had been brought up with kick starts, manually adjusted ignition and all the associated black arts of coaxing a highly strung motorcycle into life.
In fact, anyone who wasn’t a master of these quasi mystical skills was considered to be rather effete and limp wristed.
The contrast with the USA was stark. American customers wanted, and demanded, in a motorcycle what they had from a car, a washing machine and a fridge. They wanted to use a bike – not treat it like some mechanical deity demanding worship.

It is interesting to note that although the AS1 was designed using Grand Prix technology and European engineering concepts, it was signed off for production neither in Japan nor Yamaha’s European headquarters in Amsterdam. No, the AS1 was going nowhere at all until Yamaha’s Los Angeles Office gave the green light.
If the engine was impressive, then the chassis was just as competent in its own way. Theoretically, the AS engine is a stressed member of the chassis because the frame does not continue beneath the engine. However, with only 15 hp, and a very modest amount of torque, the single spine steel frame is well up to the job.