Spannies

[Mr. Dazzle]

I think that bike style two strokes are a different engine type to industrial two strokes.

Well yeah - deffo an argument you can make. They're certainly different branches of the same tree.

To me an industrial 2 stroke is arguably closer in concept to a bike 2 stroke engine than a bike 2 stroke is to a bike 4, if you see what I mean. Depends on whether you think the scale or the underlying concept is what matters.

Then you get Kawasaki confusing the issue with a 16v DOHC 2-stroke with no transfer ports at all

https://riders.drivemag.com/news/kawasa ... id=1&pid=1

[porter_jamie]

When I bought my RD the fella I got it off reckoned the spannies were a special job. Made by some bloke who was as well known expert, but retired. He had to go sit on his doorstep for a week before the bloke gave in and made some.

I just can't remember the name he said.

Tony Green?

Andy Bacon?

[Yorick]

(I knew about inlet port and transfer port from spending many 'happy' hours polishing them in an attempt to screw another 0.0005hp out of ageing Lambrettas ).

At school the physics teacher showed us how a 4 stroke engine worked, which was dull. Then he showed us how a 2stroke worked.
That looked more fun but he couldn't answer my questions. So I rode home and took barrel and piston off my Puch 50 and took it to show him the next day.
He dug out some books from the library and we saw how each each hole performed. And how they could be improved.

So before I rebuilt it, I went and bought some of those rotary drill thingies worked on the ports. It was shit so I sold it.
Next bike was a GT185 and I had a go on that. Buggered it again. It looked easy, but the theory was different.
Sold that and bought and RD250 and after reading articles in the MCN, I had a little bit more success with that.

Still 18, I then got a GT750. But before I could have a go at that, MCN did a weekly series on expansion pipes. Each week they built a set and put the RD250 on a dyno.
Was hit and miss each week, but slowly they got see gains all through the rev range. About 8BHP at peak power. About 30% IIRC
They explained what they did at each step and it was mesmorising to me.

So after they did a review of the Piper 3 into 3 spannies, a set of those went on. Lost about 20 kilos from the old 4 pipes and got a lovely sound. And it revved much freer.
That engine was the basis of what Sheene raced all over the world, so had plenty of potential.

My first race bike was a Beckett RD400. Was a flyer until it chucked a rod through the crankcase.
After new cases and barrel, I couldn't afford to get the barrel ported. I tried it standard, but mixed barrels didn't work.

So I got out my trusty drill thingies and had a go myself. Only this time I had an expertly tuned barrel to work from.

The most critical thing on a 2 stroke is the exhaust port height, so that was replicated perfectly. The transfer ports had masses of ally drilled out, but were left rough to get the gases swirling.
The later LC engines could have more taken out, due to not having the head tighten down the the crankcases allowwing the barrel to 'bow' and seize if overtightened.
The inlet port itself was not made any bigger, but the ports in the piston were made about 50% bigger. (Stan Stephens just chopped the back off. Was faster but more seizures.)

I managed it OK and the bike was as fast as the other RD400s. But soon got a 350LC

After that I never bothered tuning any more of my bikes as I needed the extras power from the professionals. But did help a few road pals get a bit more speed

[KungFooBob]

Now that's a spanny!



https://www.ebay.co.uk/itm/313852119218

[Yorick]

A beauty on mine

[The Spin Doctor]

LCs have expansion chambers that work, as do a lot of 1970s trail bikes.
They work by creating negative pressure which sucks the burnt mixture out of the combustion chamber, I could probably find the equations for making them with a bit of effort, I think I've got a two stroke tuning book in the loft.
What most people call spannies are the single skin poorly silenced pipes like Allspeeds and Microns, LC standard exhausts are double skinned and quiet, Allspeeds work no better than standard LC pipes, but are a lot lighter, I can weigh both if anyone is that interested.

Microns have more top end at the expense of mid range, and I had a set of Lomas F2 pipes on a YPVS that gave fantastic top end power at the expense of power everywhere else.

They also produce a reverse pressure wave - as the gif shows - to blow some of the scavenge charge backwards - it's effectively to 'supercharge' the cylinder before the compression stroke closes the exhaust port so that you not only get a fresher mixture going in with as little as possible of the burnt gases left behind, but you maximise the amount of power each 'bang' produces.

The shape of the expansion chamber can be tuned to change the timing / intensity of the scavenge / reverse pressure wave to match the port timing.

Back in the days of the Endurance Moped Racing series, my brother and I tackled an AR50 motor - one of the first restricted mopeds - and managed to get it up to a top speed of about 60 mph... on the 80 gearing, it was only about 5 mph slower than my AR80 and was just about the fastest bike out there. That was with a Micron pipe. Only the Kawasaki Staff AR50 was quicker, and that was a bike with engine number 04! Mind you, it wouldn't tick over, and had a powerband less than 2000 rpm wide. We probably overtuned it, even on the lowest gearing we could fit, it was a bugger to ride on the tight courses they laid out and we rarely made it to the end of the race without the clutch beginning to slip.

Moped Mayhem.jpg (1.41 MiB) Viewed 86 times

That was at Doncaster. My brother's going round the outside of what looks like an MZ Simpson.

Moped to Le Man 01.jpg (439.19 KiB) Viewed 86 times

And that was in France during the Phase One Endurance moped race to Le Mans. That's our other team regular Andy tucking in behind the screen, complete with camping kit on the rear. We finished second, after starting two hours behind everyone else due to losing a front sprocket before we got to the ferry. Fortunately, there was a Kawasaki dealer in town and the mechanic sold us the front sprocket off the bike that was in for a service and ordered a new one! Mind you, we did pass several of the teams pulled up at a bar and having a chilled out beer

[Mr. Dazzle]

Its probably worth remembering that the pressure waves and scavenging effect aren't unique to 2 stokes and spannies. They're probably at their most extreme, but the principals apply to nearly all piston engines.

In a 4 stroke you get a very similar effect at the top of exhaust stroke. Both valves are open and you get charge flowing in and exhaust flowing out simultaneously. The exhaust is tuned so that it helps "suck" on the exhaust valve just at the right moment which has the twin effect of pulling the exhaust out and "dragging" new chaege in from the open inlet valve.

This effect is tuned (via the exhaust design and the valve timing) to work best at certain engine speeds. You just get better control of it on a 4T cause you've got something other than the piston which can physically block the valves/ports, but the concept is very similar. Its pretty common in 4 strokes now to have variable cam phasing so that you can adjust the valve overlap "live" and reoptimise the timing for different engine speeds. This is variable valve timing, but not as extreme as the full on lightswitch VTEC stuff.

Indeed scavenging in general is a big part of any piston engine design.