Spannies

[Le_Fromage_Grande]

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.

Sounds (and looks) like Microns, they were pressed from sheet steel and the halves welded together by Codnor Light Fabrications, worked well though.

[Yorick]

The spannies created a sound wave, which bounced back and pushed unburnt fuel into the cylinder. They have no effect on the intake gases.

[Mr. Dazzle]

I wonder if anyone ever did variable geometry spannies.

As Cheesy says, some give you top end and others give you mid range. That's because changing the shape/size of them changes the resonance frequencies of the pipes (think of a trombone) so you'll change the engine speed at which they're most effective. So you could get around that by making variable geometry ones - somehow!

I suspect alot of the V4 GP bikes actually had multiple designs on the same bike for similar reasons?

[Le_Fromage_Grande]

I'm feeling all inspired to get my YPVS running and play with two strokes again, looks like the old power valve is going drag racing this year.

[Yorick]

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, .

Complete bollox.

[Taipan]

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.

I put Allspeeds on my RD250e. Looked uber cool (to me) but you lost the centre stand and the ground clearance was even worse. Sounded good though...

[Le_Fromage_Grande]

I wonder if anyone ever did variable geometry spannies.

As Cheesy says, some give you top end and others give you mid range. That's because changing the shape/size of them changes the resonance frequencies of the pipes (think of a trombone) so you'll change the engine speed at which they're most effective. So you could get around that by making variable geometry ones - somehow!

I suspect alot of the V4 GP bikes actually had multiple designs on the same bike for similar reasons?

Teams used differently tuned too ends and exhausts at different tracks, in the 80s the rules were a lot looser than they are now.

[Mr. Dazzle]

I wonder if anyone ever did variable geometry spannies.

As Cheesy says, some give you top end and others give you mid range. That's because changing the shape/size of them changes the resonance frequencies of the pipes (think of a trombone) so you'll change the engine speed at which they're most effective. So you could get around that by making variable geometry ones - somehow!

I suspect alot of the V4 GP bikes actually had multiple designs on the same bike for similar reasons?

Teams used differently tuned too ends and exhausts at different tracks, in the 80s the rules were a lot looser than they are now.

Yeah that would make sense. I would be surprised if they didn't do it still even with 4 strokes (although not spannies obviously) unless it's banned. I know they mess around with F1 exhausts alot 'cause we used to make 'em.

[Rockburner]

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?

Further. (like most things)

[Rockburner]

I wonder if anyone ever did variable geometry spannies.

As Cheesy says, some give you top end and others give you mid range. That's because changing the shape/size of them changes the resonance frequencies of the pipes (think of a trombone) so you'll change the engine speed at which they're most effective. So you could get around that by making variable geometry ones - somehow!

I suspect alot of the V4 GP bikes actually had multiple designs on the same bike for similar reasons?

Teams used differently tuned too ends and exhausts at different tracks, in the 80s the rules were a lot looser than they are now.

Yeah that would make sense. I would be surprised if they didn't do it still even with 4 strokes (although not spannies obviously) unless it's banned. I know they mess around with F1 exhausts alot 'cause we used to make 'em.

Someone's done variable geometry inlet manifolds (can't remember who).

[Mr. Dazzle]

They're really common, you even get them on a couple of road cars. They're banned in lots of motorsport now though. I suppose an exhaust gets a lot hotter and is hence much harder to do. Come to think of it though, I'm 99% sure variable geometry exhausts are banned in F1 too.

[KungFooBob]

Someone's done variable geometry inlet manifolds (can't remember who).

MV have them on the F4 and maybe F3 iirc.

[Mr. Dazzle]

And of course, there are any number of bikes with flappy exhausts aren't there...'course there are! I forgot about those.

[KungFooBob]

And of course, there are any number of bikes with flappy exhausts aren't there...'course there are! I forgot about those.

EXUP!

Quite a few have flaps in the airbox too.

[Mr. Dazzle]

Yeah I turned those off in my CBR

I wonder if anyone's done it on a 2T though.

[Count Steer]

Thanks chaps.

It all makes sense now. Looking at it all it should, oddly enough, make them more fuel efficient in mpg terms, but somehow I don't think that was uppermost in the designers' minds.

Was it the combustion efficiency or the fact that they burned oil that got 2Ts out of favour with the environmental legislators? It's a shame, because the mechanical simplicity is

[Mr. Dazzle]

Its the oil burning which knackers them from a road vehcile POV.

As you say, they actually have the potential to be more efficient than 4 strokes and its an area of research. The problem now is the ticking clock...there's less and less incentive to develop the tech when EVs are looking to be the future.

[Tricky]

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, .

Complete bollox.

Err, no, not complete bollox.
@Mr. Dazzle 's post is factually correct, as I read it anyway- -when the exhaust gases flow out they expand , and in that create a vacuum at the exhaust port which pulls more charge into the cylinder

Maybe you'd like to share the reasoning that makes you say his post (and I guess mine too now ) is complete bollox?

[Tricky]

I'm feeling all inspired to get my YPVS running and play with two strokes again, looks like the old power valve is going drag racing this year.

I might join you on mine if you do

[Count Steer]

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, .

Complete bollox.

Err, no, not complete bollox.
@Mr. Dazzle 's post is factually correct, as I read it anyway- -when the exhaust gases flow out they expand , and in that create a vacuum at the exhaust port which pulls more charge into the cylinder

Maybe you'd like to share the reasoning that makes you say his post (and I guess mine too now ) is complete bollox?

That's how I saw it. If the pressure difference at the exhaust port ie the suction from the falling piston was greater than that created by the escaping pressurised exhaust gases, the exhaust gases would flow backwards ergo there must be a - relative - vacuum at the exhaust port. The expansion chamber will increase the suction as a given volume of gas can expand/reduce pressure more than if the chamber wasn't there. That's got to add to the suction on the incoming mixture.