Closing date for April issue -> February 17
expansion exhausts massage
Pass the pipe to Maarten. That saying is outdated. You pass your pipe to M, of course.Athijs. That's Mathijs Neijensteen from far-off Overijssel. Mathijs is one of those classic loners who specializes in classic (competitive) two-strokes. And those machines get their power from their expansion exhausts.
A well-designed expansion chamber provides significant power gains on a two-stroke engine compared to conventional mufflers. The basic idea behind an expansion chamber is to use the momentum and pressure of the exhaust gases to create a pump that forces more air and fuel into the cylinder during the intake stroke. It essentially works the same as a turbocharger, but without any moving parts. This is because the back-and-forth motion of the exhaust pulsation contributes to the most effective breathing possible.
Whatever you do to the rest of the bike, the exhaust makes or breaks it. It can be tuned to significantly alter the engine's characteristics to match the desired power and torque. There aren't many NOS or aftermarket parts available for these older machines. For more recent parts, there's more choice: well-known aftermarket exhaust manufacturers pull out all the stops to make your motocross bike run as smoothly as possible, preferably in the widest possible rpm range. This range is called the "powerband."
This powerband is therefore partly created by the exhaust's shape. Unfortunately, the specific exhaust shape is only perfect for one rpm range. In other words, the exhaust shape is always a compromise. This means that the manufacturer of a two-stroke exhaust always designs an exhaust specifically tailored to a particular engine type and displacement.
And Mathijs has learned to restore the often-dented and broken, wonderfully sculpted parts to their optical and therefore mechanical perfection. It's a matter of patience, heating, tapping, and pulling.
A while ago, the incomparable Frits Overmars wrote a series of very informative, understandable, and sometimes hilarious columns in – I believe – Moto73 about his area of expertise: the operation and dimensions of expansion exhausts. Here's a piece about the discovery of expansion pipes, a phenomenon often attributed to MZ gurus. Kaadenis attributed to:
In 1956, expansion pipes were installed on an MZ racer for the first time. Yes, but in 1951, DKW engineer Erich Wolf already such a pipe under the 125 cc bike of the factory rider Ewald Kluge The two-stroke MZ still had megaphones at the time, and it took another five years before Kaaden saw the light. And that was quite a disappointment, because the 158° exhaust timing of the MZs was far too tame for a proper expansion pipe. It didn't work at all, and Kaaden couldn't get his megaphones out of the closet fast enough. It wasn't until 1960 that he realized his exhaust ports needed to be at least 30° up, and this time the expansion pipes did what they were supposed to do and were allowed to stay. The gas flows and calculation models continued to improve after that.
Oh yes: Mathijs' skills don't stop at just the rebirth of expansion exhausts.
Mathijs can be reached at 06-29467107.
I'm a true two-stroke enthusiast and regularly attend trade shows. At 62, I'm one of the younger crowd. That's perfectly normal, my son rode a "plastic" scooter, while I built mopeds myself as a teenager. The group that grew up with this is getting older and falling apart... Lost knowledge and demand... Now we can still enjoy ourselves and keep the small businesses, often the true craftsmen, alive.
Unfortunately, demand will decline in the coming years….
I also tried this once, ended up with a pipe with a length of 9,8 meters🤣 that didn't seem right.
Always that hassle with commas!
Yes, I find those stories about the (first) expansion pipes fascinating. What a discovery! With those cow horns (Greeves, Royal Nord, Itom…) and megaphones, those fast two-strokes certainly didn't run optimally.
Rumi, with his 125cc twin racer, was also among the first to use them. Suddenly, the competition discovered that the Rumi racers had become much faster when accelerating out of a corner. The exhaust's appearance gave no indication! What had those rascals done? They'd hidden the new expansion mufflers in the old exhausts!
Best regards, Ötzi
People with a brilliant craft are scarce thanks to government policies on all fronts. But look what brilliant hands can do. When I see an expansion pipe like that, my mouth waters. Simply brilliant. And then knowing what needs to be done. That even makes me speechless. Under the motto "we will keep going," I cherish these brilliant people. Awesome!!
I'm quite curious: how did government policies on all fronts make brilliant craftspeople so scarce? Just to be clear: I completely agree with you that they are (seemingly) in increasingly short supply. Could we perhaps explore how we can address this without immediately blaming someone (or entire cabinets)?
We are a "knowledge-based nation," and (the government) doesn't want us Dutch people getting our hands dirty, resulting in, among other things, more and more immigrants, some of whom don't want to do the work. The result is poorer public health, too much sitting, lower cognitive capacity due to, for example, the recent rise of "AI," etc.
Brilliant people, keep them alive! I've read and saved Frits Overmars' articles about tuning the Suzuki GT500 twins with great pleasure and admiration!
Well... It started with the closure of vocational schools. But times have changed, too. Back in the day, as a boy, you wanted to be a firefighter or a police officer. Recently, the ambitions of 12-14 year-olds have changed: They want to become software geniuses or imitators. We're still quite a knowledge-driven nation, you know. Only the craftsmanship has disappeared.