Power valves / reeds?

[sandman1976]

Hey all need a technical head....
Would it be possible to manufacture a power reed?
And fit it to a none reed block engine?
I was just figuring out the way these things work and to be honest they look like quite a genius but basic system... Just seems to be a reed that holds back some fuel in its narrowest point then splurts it all out in one go at W.O.T.. Is that right?

Opinions pleeeeease!

Ta.

[Frost]

Not sure i understand. So a servo operated reed? As the engine takes the same amount of gas into the cylinder in each rotation there is no need to adjust the reed it's just a one way valve.

[sandman1976]

[Kickstart]

Hi

Reed valve just opens on demand. When the pressure increases on the "after" side of the reed valves they shut.

Means you can have stupidly advanced intake timing for peak power without it pumping most of the intake air straight back out at low revs.

Putting a reed valve straight onto a non reed valve engine would probably just lose you power.

All the best

Keith
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[Pete.]

No, you've got it just about all wrong.

A reed valve is in the induction system. It's simply a one-way valve for fuel-air mixture to pass into the engine but not back out.

A Powervalve is a servo-operated valve that partially restricts the exhaust to provide torque at low revs.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[sandman1976]

Ahhhh! Right i get it now!

So i could probably fabricate a crude kind of power valve on one of my workshop engines and increase low end torque?

[Kickstart]

[Frost]

Yes you can stick a powervalve on anything, but unless the timing of the engine has been designed to make use of it you'll just restrict gas flow and lose power as a result. You;d get better results doing the opposite and trying to help the gas flow more easily by removing restrictions like the silencing in the exhaust or smoothing out the insides of pipes and such.

[MarJay]

The MZ racers convert MZ's to run on RD350 reed blocks, when they are normally just piston ported...
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British beauty: Triumph Street Triple R; Loony stroker: KR1S; Track fun: GSXR750 L1; Commuter Missile: GSX-S1000F; Cheap project: CBR900RR FireBlade
Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

[Teflon-Mike]

Reed valving piston ported two stroke engines is VERY 'done' in some circles.... I have competed against an AWFUL lot of supposedly 'classic' twin shock trials bikes, like Ossas and Bultaco's, that are more Yamaha TY than they are anything else!
They work too... and damnably well, curse them.... just a bit against the grain of the 'ethos' to my mind.... though T-Shock isn't time bound strictly for 'Classic' bikes.

The 'Pre-65' class had originally be concieved to let enthusiasts run 'old thumpers' from the days when to go trials riding, you took a standard road bike, and chopped it about the way you wanted, to make something that did the job... that were made rather obsolete by the later 'factory' two strokes, pioneered by a certain Irishman, Sammy Miller.....

But by the mid 1980's twenty years on, it was no longer the preserve of teh enthusiastic 'Clubman' running a cheap, home built bike, for 'fun', people were taking it deadly seriousely, and two camps were fermenting.

On one side, were the purists, insisting that pre-65 meant pre-65 and anything made later than 1965 shouldn't be allowed, and that bikes should be authentic museum pieces... On the other were folk bulding ever more sophisticated 'modern' bikes, using 'old' engine cases... the cranks custom machines, modern forged pistons, electronic ignition systems, Japanese carburettors, Italian gas shock absorbers, plastic mudguards and fibreglass tanks in 501 tube, brazed frames.....

The idea of a 'cheap' class for 'old' machinary, enthusiasts could have a bit of fun with was lost, and so for the NEXT generation of classic clubman class, they decided to bound it by suspension technology, hence Twin-shock, rather than a simple year cut off, so people could run, the by then obsolete two strokes, and or make thier own specials.

Twenty years LATER, th same scism has re-emmerged in the T-Shock class, as did in the pre-65's twenty years ago.

All irrelevent to you, BUT..... precedent is there.

Piston ported two strokes rely on long induction tracts with small diameters to get teh charge sucked through the carburettor traveling fast and give it as much momentum as possible when the piston is rising, sucking charge into the crank case, so that when the piston gets to TDC and starts to fall again, the charge in the induction tract is going too fast to be that eager to turn around and be blown back through the carburettor..... SOME of it get's crammed in, and stays there.

Idea of the reed valve is that by sticking a simple one way valve in the inlet tract, when the piston starts to fall, valve closes trapping whatever has been sucked into the crank case, stopping it being pushed back out again.

Means you get HUGELY increased trapping efficiency, and a much better cylinder fill. Also means that you can exploit the fact that teh one way valve is stopping the charge back flowing, and use much bigger diameter, shorter inlet tracts, as they don't have to be deliberately restrictive to get inertial charging and hamper back flow.... so you can get a lot more top end power from better breathing and opening the induction tract up.... not so good for part throttle carburation, but thats another matter.... throttle 'response' though from being able to have a much shorter inlet tract, is greatly increased.

So theory is sound enough, just a case of putting it into practice.

Starting with a 1974 OSSA Mar, or 1976 Bultaco Sherpa.... pretty simple, you take the barel, and machine off the carburettor stub.

You then get a reed-block from a Yamaha TY175 or TY250, depending on what size motor you have.... and you open up the galery in the barel to accept it.

I think that on most of the Spanish piston ported motors, there isn't actually enough metal in the base of the barel to put the reed block actually IN the barel casting, so they are machined to teh stud patern of a TY reed-block, then an aluminium spacer, milled to actually provide the volume for the reeds to open into, over the actual port in the barel.

Mikuni Carburettor... couple of gasket, and you have a reed-valved trials special.....

Which wont do BUGGER ALL better than the original piston ported motor!

And Probably wont even run AS WELL as it did piston ported, becouse the piston ports are still choking teh flow as though it's piston ported, so all the reed valve does is provide an impediment to flow, and induce a lag in the induction tract from when the piston starts sucking to when the reeds open and let the carburettor 'feel' that suck.

Consequently, to make it worth while, takes rather a lot more injunuity....

To exploit the reed valve, the barel and piston need completely re porting....

These 'tricks of the trade' are oft kept secrets, and aren't divulged too readily...

BUT, it would SEEM that to make an OSSA MAR 'Competative' in National Level T-shock these days, it basically needs to be a well fettled last of the line, air cooled, Yamaha TY250 'Pinky' monoshock, in disguise!

Hiding behind the generator cover will not be a Yamaha CDi ignition module, nor even a Boyer Brandsen Electronic ignition, but a very expensive 'custom' built, and mappable electronic ignition made for you by a chap in his garage in Burbage, just outside Hinkley.....

The carburettor will be a Mikuni item, from a YZ400, cleverly re-jetted....

The reed block will have originally been from a TY250, but it will have been fitted with composite reeds, but 'split' so that the bottom leafs are a different tension and on a different stop gap to the uppers, to aid throttle response.

The barel will have been re sleeved, and as likely as not, have had the original twon transfer port barel recut to a four, or possibly even six transfer design..... piston will similarly have been selected from catalogue, and matched to the barel, which will have been 'ported' to the sort of timings used on much later motors.....

Result, instead of a 12bhp 250 single, reving to about 6000rpm, you get a 300cc single, with around 28bhp, reving to around 9-10K, with a big increase in low down stomp and an ENORMOUSE amount more instant trhottle response.....

My old Cota, is the last of the old 'plodders', heading into a climb, you turn the bars, and open the throttle, anticipating coing around teh corner and starting to go up, because as you open the taps it bogs, the engine note drops, and then it starts to suck, and c lear its throat, for the climb to come.....

One of these hybrids, rides much more like a modern banana bike, and they can dribble them into an obsticle, aply a little throttle where needed to haul the front end round, then line them up for the climb, and when they are ready, open the throttle and it instantly goes on demand.....

Which takes an awful lot of the 'fun' out of it, to my mind, because half the challenge for me, is in that riding ahead of yourself, predicting what's coming up and compensating for the lag effects and picking your line to suit what you CAN do riding around the inadequacies of the machine.... and getting the most out of it, in reward.

Hence in twenty odd years, doing it the hard way, I have not taken many trophies.... but then again, neither have I spent a huge amount of money creating a bike not 'quite' as good as I could buy off the shelf, for a LOT less money, JUST to beat blokes like me, in the dinosaur classes, rather than face the annoying brats in the contemporary classes, that can seem to ride on thin air, and point their bike in any direction they like, more easily than I can point my finger!

But... that's the beauty of trials, end of the day, its however you choose to have your fun, and scored solely on your own ride, makes no odds what other people do on their bikes through the same section, essentially its you, your bike, and a bit of ground, and can you beat the terrain, and get through the sections without sticking your feet down.... and if you cant do it on the first lap, can you beat it on the second, or the third or the forth or the fifth... or bugger it... NEXT WEEK (when the CoC lays the sections the OTHER side of that fucking tree! )!

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