Spannies

[Count Steer]

I another thread, the Venerable Tiepin mentioned 'spannies'.

Now I know they are 2T expansion chambers but realised that's all I know.

How do they work and why weren't they more common?

Ta.

[Mr. Dazzle]

why weren't they more common?

Because most bikes these days are 4 stroke.

[slowsider]

The tapers on the pipe helped to empty the chamber more efficiently, resulting in performance improvements.

[KungFooBob]

Most lates 80's and 90's 'sports' two strokes had them.

It's only the earlier 70's bikes up to and including the LC's that didn't. I suppose even the LC cigars were kind of spanny-ish.

Its all about science and gas flows. The pluses bouncing back from the cones helps scavenge exhaust gasses and also prevents the 'charge' leaking out the exhaust ports because two strokes don't got valves!

[slowsider]

Reed valves, disc valves ?

[KungFooBob]

Reed valves, disc valves ?

Exhaust valves... unless you mention power valves

[Mr. Dazzle]

Its all about science and gas flows.

Many things in life are.

I reckon this little gif is worth about a million words of explanation.



Essentially all you're doing it making a pipe which works in combination with the fact the engine doesn't suck/blow constantly. You see how the grey exhaust gases at first go rushing down the exhaust, then they sort of 'bounce' off the end push the green fresh petrol/air back into the cylinder? That's what the spanny is tuned to do.

You can also see that when the grey gases first start moving down the pipe they 'suck' the green up into the cylinder. The 'expanding' part of the spanny is the biggest contributor to that, the 'converging' bit is more about making the bounce come back.

[Count Steer]

Most lates 80's and 90's 'sports' two strokes had them.

It's only the earlier 70's bikes up to and including the LC's that didn't. I suppose even the LC cigars were kind of spanny-ish.

Its all about science and gas flows. The pluses bouncing back from the cones helps scavenge exhaust gasses and also prevents the 'charge' leaking out the exhaust ports because two strokes don't got valves!

Ah. Tiepin's comment on the 380 made them sound rather more rare.

So it's all part of the dark art of 2T zorst tuning? I only ever got into that with Lambrettas and, TBH that was more about buying an Ancillotti and bolting it on than understanding anything.

[Rockburner]

Its all about science and gas flows.

Many things in life are.

I reckon this little gif is worth about a million words of explanation.



Essentially all you're doing it making a pipe which works in combination with the fact the engine doesn't suck/blow constantly. You see how the grey exhaust gases at first go rushing down the exhaust, then they sort of 'bounce' off the end push the green fresh petrol/air back into the cylinder? That's what the spanny is tuned to do.

You can also see that when the grey gases first start moving down the pipe they 'suck' the green up into the cylinder. The 'expanding' part of the spanny is the biggest contributor to that, the 'converging' bit is more about making the bounce come back.

If you're genuinely interested - look up 'Walter Kaaden' - he was the one who a) realised that this was possible, and b) did hours and hours (years and years) of meticulous practical experimentation with race bikes by riding them up and down public roads testing various different exhaust lengths and shapes while working for MZ. It's why MZ won a lot of races in the early 60s (despite being a very poorly funded East German manufacturer), and it's the technology that Ernst Degner stole when he defected and went to race for Suzuki.

[Le_Fromage_Grande]

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.

[Mr. Dazzle]

Yeah I always wondered about that "stole" bit - surely it would have been obvious to anyone watching the MZs that their exhausts were a bit funny!

I suppose it would take everyone else ages to figure out why they were a bit funny. The idea of exhaust tuning goes back to at least the second world war though doesn't it?

[Le_Fromage_Grande]

Oops forgot the reverse pulse

[Le_Fromage_Grande]

Yeah I always wondered about that "stole" bit - surely it would have been obvious to anyone watching the MZs that their exhausts were a bit funny!

I suppose it would take everyone else ages to figure out why they were a bit funny. The idea of exhaust tuning goes back to at least the second world war though doesn't it?

MZ, Yamaha and Suzuki were the only ones running two strokes, Yamaha and Suzuki both had expansion chambers before Degner defected, what Degner really took to Suzuki was disc valve induction.

[Taipan]

Race cans for 2 strokes, but actually did quite a bit more. More importantly, they made crackling, spitting, screaming ring-a-ding noises!

http://2strokeengine.net/2strokeenginet ... mation.php

[KungFooBob]

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.

[slowsider]

Reed valves, disc valves ?

Exhaust valves... unless you mention power valves

Old tech
https://riders.drivemag.com/news/kawasa ... ur-engine/

[Le_Fromage_Grande]

This is interesting https://en.m.wikipedia.org/wiki/Ernst_Degner

Degner defected earlier than I thought, and also ride for Joe Ehrlich before riding for Suzuki, in the 80s Joe Ehrlich claimed to have invented expansion chambers.

[Taipan]

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?

[KungFooBob]

They weren't like different sized cones welded together, it was almost like the two halfs were beaten from a panel and then seam welded.

[Le_Fromage_Grande]

A lot of the aftermarket ones were junk, no better or worse than standard exhausts, even on 70s bikes, I had a set on a GT185 that were hideously loud but knocked 5mph off the top speed.