Boyesen reeds.

[evo550]

At BDC the exhaust port is wide open so there is a bloody big volume above the piston. Case volume is normaly bigger than the cylinder volume with transfer ducts included.
As above, not argueing, just asking. Can you include exhaust system and transfer ports as part of volume, An engines "displacement" is measured through stroke and bore size. If they helped to increase volume of fuel to the combustion chamber, why not just make the ports bigger?

As Chris said a 2T engine is force fed in scavenging and plugging. generaly a 250 would displace 325-330cc per stroke. A fuel particle takes 20 cycles to go through the engine. Is their any research published on this, might be an interesting read.

If the ultimate aim is to get more fuel into the combustion chamber, why do people stuff cranks, therefore decreasing the crank volume?
........now back to Boyseen reeds

[Kane Mcguire]

gotta love the boyesen discussion! comes around often. many magazines over years and years have tested items on a dyno then on the track. a popular quote used for years from one mag when they test engine components  "a dyno produces cold, hard, statistical facts - not always the same as seat of the pants facts".  i do agree with you lozza that examining boyesen reeds certainly does make me wonder why i have success with them, as the smaller set are always so stiff and the upper set too flexible. this seems so for every set i have seen?

[ChrisACT]

Gas compresses.  Gas has momentum.

OK not argueing, just asking. My understanding of vacum is that its a case of atmospheric equalization, as the piston travels, up a vacum (negative air pressure) is created in the crank, the area in the crank then tries to equalize itself with the air pressure outside the crank, by drawing air in, once equalized the vacum no longer exists, air stops flowing in.

If you were talking about a liquid then yes.  But the fuel air mix is not a liquid, it is a gas.  The piston stopping at TDC does not immediately stop the inlet flow to the crankcase.  The momentum of the inlet charge flowing into the crankcase means that it wants to keep flowing into the crankcase even after the piston has hit TDC.  It is not until the pressure it is flowing against is high enough to reverse the gas flow that it is no longer charging the crankcase.  That's when the reed valve closes to reduce the inlet charge being forced back through the carby.  Although that takes time too and so inevitably, some of it does make it back past the reed valve, which affects air/fuel ratios on the next inlet stroke.

As the fuel air mixture is drawn into the crankcase, there is momentum involved from the previous cycles.  Basically, the fuel air mix is still flowing into the crankcase well after the piston hits TDC.
At 8000rpm, there are 133 revolutions a second, so if my maths are any good the fuel has 266th of a second, before piston goes from tdc to bdc

Yep, it happens real fast.

Then there is also the scavenging effect of the pipe which draws the mixture through the combustion chamber, including drawing the mixture through the transfers which in turn draws more mixture into the crankcase.

I thought the fuel mixture was forced up through the transfers through positive atmosphere pressure (compresion) created by the piston traveling down.

No, it is sucked up through the transfers by the scavenging effect of the expansion chamber.  Think how fast that exhaust gas is leaving the chamber.  Much faster than the piston is travelling.  The combustion chamber volume is about a third of the crankcase volume.  The scavenging effect is much bigger than the compression in the crankcase.

It actually draws fresh fuel air mix right through into the exhaust pipe but the returning shockwave from the expansion chamber then forces it back into the combustion chamber before the exhaust port closes.  So you have fuel air mix coming from both directions helping the charge the combustion chamber (at least when it's on pipe anyway).

I thought the returning shockwaves where timed to travel back up the pipe to reach the exhaust port just as new fuel charge potentially made it's way out, before the piston closed off the exhaust port.

Yes but it only does that perfectly at one particular RPM depending on the pipe.  The rest of the time it is off to some degree.  If the shockwave reaches the exhaust port before the fuel air mix does then the fuel air mix won't enter the pipe much.  If it reaches the port after the fuel air mix does then some will enter the pipe and then be forced back into the combustion chamber.  It's all in the timing.

That's why expansion chambers are so critical to get right.

[LWC82PE]

I have recently found out that Boyesen and Carbon Tech are the same company! who would have thought that. Anyway the Carbon Tech people said they actually have the patterns for single stage PE400 carbon reeds and can do them for you on special order. I cant remember the price but from memory it was $30-40 US i think. You can order directly from Carbon Tech USA and saves a lot of time hand cutting them out of a sheet which can be time consuming and tricky especially if you want to do a professional quality job.

Plenty of people have used Boyesens in PE400's with no probs, but you cant expect to drop them in and away you go. You should expect to do some jetting adjustments. Each bike is different.

[ChrisACT]

Here's a readout of what is happening to the crankcase pressure in a DT3 engine at 9,000RPM (yes, I know they don't normally rev that hard but bear with me).  You will notice that the change in pressure doesn't exactly correspond to the change in direction of the piston.  1.0 represents 1 atmostphere.  Below one is a vacuum.  Above one is compression.  The issue is, what is causing the vacuum and compression in the crankcase.


Now this is a readout of exactly the same engine at the same RPM showing the velocity of flow in the main transfer port.  Look at what the gas flow is doing between the exhaust port opening and closing.  The gas is still being drawn into the chamber well after the piston starts it's upward stroke.  That's the scavenging effect of the expansion chamber.


And here is the exhaust flow.  The dark blue line is the flow at the port.  See how it flows back into the combustion chamber just before the port closes (EPC).

[Lozza]

The spinning crank is a big source of intake inertia.Crank rotation is not constant nor is piston velocity, as at TDC and BDC the piston even comes to a complete stop.Crank accelerates faster after TDC and is slower from BDC to TDC.The piston has to also work against combustion pressure.

There are 100's of articles on this type of subject only they are very dry technical reading in the form of SAE papers.

An exhaust or transfer ducts are not included on engine displacement, ports don't do the work the pipe does the work. What would be easier to move  fluid with a straw or a 100mm pipe?



The crankcase volume plays no part in the scavenging/plugging process. Case stuffing is a old school idea, as soon as pipes became efficient larger volumes were needed .The only engine that would benefit from this would be a SX only engine trials engines would have low case volumes to begin with.

There a no such thing as a 'shockwave' in a 2T engine it is an acoustic wave that has a particle and a pressure component. Particles travel at half the speed of the pressure.

For the umteeth time there is NO WAY pressure in the crankcase can equal residual pressure from combustion, so the decending piston wil never generate enough pressure to force anything anywhere.

  Lozza says dyno's don't lie and people's senses are easily fooled  Yes the top petal flaps uncontrolably as rpm rises(Ron Chinoy from RD Dreams strobed and filmed them) how will the bottom petal ever lift? Maybe you have yet to try other reeds?

You obvously paid attention at school Chris ?

[ChrisACT]

Yeah, this crazy dude explained it to me and eventually it made sense. 

[Nathan S]

My (limited) understanding of Boyesen Reeds is:

The thin reed helps bottom end power, like it says on the box.

The trouble is that the top reed flutters at high revs (like thin reeds always will), and there's never enough pressure differential exerted on the bottom reed for it to operate properly - so the promise of the best of both worlds cannot be fulfilled.

Ultimately they're basically the same as fitting conventional style reeds made from a thin material. Sometimes this will be a good thing, usually it won't.




 

[evo550]

Thanks Chris,
The graphs paint a better picture of what's going on in there.
On the graph that shows flow through transfer DUCT what creates that sharp drop (maybe reverse flow) between epo and bdc, if I'm reading it right the piston is on it's way down, shouldn't gases be on the up and up at this stage?

[Lozza]

Thanks Chris,
The graphs paint a better picture of what's going on in there.
On the graph that shows flow through transfer DUCT what creates that sharp drop (maybe reverse flow) between epo and bdc, if I'm reading it right the piston is on it's way down, shouldn't gases be on the up and up at this stage?

CORRECT. What happens roughly between EPO and BDC and what happens before EPO? This for the eleventy millionth time is why decending pistons have no real effect on case pressures.
CORRECT Nathan how can the bottom petal lift if the top petal blows open with a slight breeze.

As a experiment try hitting a reed petal(any type) with your compressed air gun nozzle and see how far it lifts.

[ChrisACT]

Thanks Chris,
The graphs paint a better picture of what's going on in there.
On the graph that shows flow through transfer DUCT what creates that sharp drop (maybe reverse flow) between epo and bdc, if I'm reading it right the piston is on it's way down, shouldn't gases be on the up and up at this stage?

That's the transfer port opening.  The pressure in the combustion chamber is still high enough to briefly force exhaust gasses into the transfer port.  But as the expansion chamber starts to scavenge the remainder of the exhaust gasses out of the chamber, it then also draws the inlet mix through the transfer port.  As you can see, the piston is already well and truly on the way down but the residual pressure in the combustion chamber is still much higher than the crankcase pressure.  In fact you can see on the first graph that the crankcase pressure never exceeds 1.5 atmostpheres.  The peak combustion chamber pressure on this engine is nearly 66 atmostpheres.

I suppose I could run the transfer porting a little later on this engine to get longer blowdown (the part of the cycle in between when the exhaust port opens and the transfer port opens is called blowdown) and take advantage of this remaining pressure but it would mean smaller transfer ports so it may not actually make for HP gains.  The other option is to make the exhaust port higher which would start the blowdown earlier.  But this one is already wild enough.

[Kane Mcguire]

lozza can you recommend a brand of reed and reed thickness(as some brands offer a choice) for a standard 77 rm125b. i will try a set. i prefer low end response. thanks.

[Lozza]

Is it a power reed ? If yes then a 0.38-0.42mm carbon mono petal is the go. I can supply one for $20 or you can get the carbon tech which you will get 2 for how ever much Serco charge or what you can get online somewhere.

[PE MAN]

Plenty of people have used Boyesens in PE400's with no probs, but you cant expect to drop them in and away you go. You should expect to do some jetting adjustments. Each bike is different.

yea plan on fiddeling around with the jets, got the pipe on it now. fiddled around with the needle setting last week with a few of the old boys at pt. gawler. it improved the top end but bottom end was running rich. an was fowling plugs if idled for too long. boys down there said best thing to do before changing jets in the carb was to make sure reeds were in good cond. as it could affect it. which is all good. i planned on checking them out anyways an wanted to fit something aftermarket in. im mechanically minded so pulling apart an re fitting isnt no drama. it just the tuning that i have to get around. there theory was to go down one on the pilot an one up on the main. sounded pretty good to me so will see how it will go next time i go out.

[LWC82PE]

PE man, i advise you to hop onto the Yahoo PE forum and read about the guys digging deep into PE400 jetting. The guys you want to speak to specifically are Bob in UK, Franois in Canada and Alex in Victoria. Also maybe oldfart on here could give you a few pointers too. I would be speaking to other owners of 400's who already have them set up right before listening to just any one who doesn't specifically know the 400's inside out unless they were a professional 2T tuner. I also would highly advise you speak to Ian Williams www.iwt.com.au in Adelaide. He's a 2T nut from way back and also is a Mikuni dealer/distributor. Yes  hsi site looks all liek karting stuff, but he used to and probably still does a lot of bike stuff, its just that the karts have always been his main thing as he races them