Texas Two Step Taco

Texasstar

Can't is a four letter dirty word
This is the inside of the Arctic Cat
IMG_0084.jpg

M1000

Look at the size of the exhaust and the power valve

I think this is 90mm bore by 70mm stroke.


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teazer

Over 10,000 Posts
DTT BOTM WINNER
Bb was telling me this story about Thiel and that there was a heated discussion on the two stroke FB page about that one day. I think Fritz was on that page. I also asked BB how that would work in a case reed engine.


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Crankcase reed motors have all the transfer ports open to the intake so they don't need additional cross drilling. It's redundant.

That barrel looks similar to an XCR800 Polaris triple or even an RS125R or TZ250 V twin in terms of massive transfer area plus boost port above the intake. Aprilia RS250 motors also look like that but with different exhaust valve design and they use a bridged exhaust with eyebrows rather than one plus 2 arrangement.

Now show us the bottom end compared to one of our clunkers. Huge crankcase area with low primary compression because wave activity is more effective and less restrictive. They no longer need 14 speed transmissions to cope with the narrow powerband. (RK67)
 

John Murray

Been Around the Block
If there's a worse place to find reliable information on two-strokes (or anything else for that matter) than FB I haven't found it yet. AFAIK no-one has yet equaled or exceeded the outputs Thiel (with help from Overmar) achieved. I'd need a really really good reason to not follow their guidelines.
 

John Murray

Been Around the Block
Now show us the bottom end compared to one of our clunkers. Huge crankcase area with low primary compression because wave activity is more effective and less restrictive. They no longer need 14 speed transmissions to cope with the narrow powerband. (RK67)
Makes me wonder if the main advantage of Boyesen ports is the additional crankcase volume..
 

Texasstar

Can't is a four letter dirty word
If there's a worse place to find reliable information on two-strokes (or anything else for that matter) than FB I haven't found it yet. AFAIK no-one has yet equaled or exceeded the outputs Thiel (with help from Overmar) achieved. I'd need a really really good reason to not follow their guidelines.

I found Thiel’s post on Facebook concerning boyenson ports lol. It is fun to watch Fritz and Thiel on Facebook
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Texasstar

Can't is a four letter dirty word
Makes me wonder if the main advantage of Boyesen ports is the additional crankcase volume..

If I remember correctly you said the bulls were limited in this respect? The boyenson ports were huge in the triple Arctic cat.


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John Murray

Been Around the Block
Yes, the bigger Buls (350, 360, 370) have quite high crankcase compression, well over 1.5:1. This works well with the low rpm, non-piped engines like the Sherpa and Alpina but not so much with the others. An easy solution with most of these engines is a longer rod and a spacer plate under the barrel. I use a SeaDoo rod that is 9mm longer; it lowers the PCR as well as being stronger and more durable with high rpms.

The Rotax's also use big Boyesens; the only real advantage I can see is they eliminate the need for the windows in the piston's intake skirt that tend to weaken them a lot. I find the lightweight forged pistons tend to shrink a lot with high rpms even without any windows.
 
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teazer

Over 10,000 Posts
DTT BOTM WINNER
It could well be that the key is larger crankcase volume. Kevin Cameron wrote about that years ago.

I don't recall what a Bul crank looks like but if it's full circle, could it be pork chopped without throwing the balance out completely or could the OD be reduced to add some volume? I suspect the latter would result in increased windage offsetting any PCR improvements. I also doubt that holes the crank wheels would help either. They would increase the static volume but unlikely to make a real difference when running.

As a side note Kawasaki triples and Polaris and other sleds have pork chop cranks and my GT750 has high primary CR with full circle crank webs. Next year I should try a pork chop crank and extra ports.
 

John Murray

Been Around the Block
I found out about crankshafts the hard way as usual; if I'd a had half a brain I just would have copied current practice but no...

With my air cooled 370 engine the original crank/rod/piston brings the transfer cutouts in the piston down below the top of the liner cutouts at BDC, so that at BDC the window height is reduced by about 9mm. Does that make sense? There isn't enough meat in the piston to just raise the cutout, and anyway if you did the transfers and exhaust would short-circuit at TDC.

Still with me? Anyhow, the bottom of this window is formed by the outside diameter of the crank wheels, so I figured that by pork-chopping the crank and making the curve of the remaining crank webs slightly eccentric I could pick up that 9mm I was losing at BDC, as well as lowering the PCR a bit.

So I modified a crank, having to add some tungsten to maintain balance. Once on the dyno I found that the power had indeed changed, but not in the right direction. Torque was down at higher rpms, everywhere else there was little change.

I made two stupid mistakes, the first being a lack of research. I would have found that the effect of odd shaped cranks to cause extreme turbulence and disrupt breathing at high rpms was known of more than 50 years ago. The second mistake was to not take a close look at the current designs - if I had I would've seen smooth full wheel cranks either encased in sheetmetal shells (a la Honda) or with the holes and other iregularities filled and faired with plastic packers.

So, back to square one. The lumpy crank was obviously a dud, but I still wanted to see if a larger window had anything to offer. I ended up using a 9mm longer rod and a barrel spacer, these giving both window area and crankcase volume. I filled all the balance holes in the crank with pressed in aluminum plugs and radiused the inside corner of the crank wheels. I had to use a fair bit of tungsten to get balance with the crank mods and the longer, heavier rod.

Back to the dyno and the horsepower lost was back, plus a small increase. I know a lot of engines still use pork chop cranks and I think if rpms are moderate they are fine. But if rpms are highish (even as low as 8.5 or 9k) I really think there are measurable horsepower benefits to a streamlined crankshaft.
 

Texasstar

Can't is a four letter dirty word
I found out about crankshafts the hard way as usual; if I'd a had half a brain I just would have copied current practice but no...

With my air cooled 370 engine the original crank/rod/piston brings the transfer cutouts in the piston down below the top of the liner cutouts at BDC, so that at BDC the window height is reduced by about 9mm. Does that make sense? There isn't enough meat in the piston to just raise the cutout, and anyway if you did the transfers and exhaust would short-circuit at TDC.

Still with me? Anyhow, the bottom of this window is formed by the outside diameter of the crank wheels, so I figured that by pork-chopping the crank and making the curve of the remaining crank webs slightly eccentric I could pick up that 9mm I was losing at BDC, as well as lowering the PCR a bit.

So I modified a crank, having to add some tungsten to maintain balance. Once on the dyno I found that the power had indeed changed, but not in the right direction. Torque was down at higher rpms, everywhere else there was little change.

I made two stupid mistakes, the first being a lack of research. I would have found that the effect of odd shaped cranks to cause extreme turbulence and disrupt breathing at high rpms was known of more than 50 years ago. The second mistake was to not take a close look at the current designs - if I had I would've seen smooth full wheel cranks either encased in sheetmetal shells (a la Honda) or with the holes and other iregularities filled and faired with plastic packers.

So, back to square one. The lumpy crank was obviously a dud, but I still wanted to see if a larger window had anything to offer. I ended up using a 9mm longer rod and a barrel spacer, these giving both window area and crankcase volume. I filled all the balance holes in the crank with pressed in aluminum plugs and radiused the inside corner of the crank wheels. I had to use a fair bit of tungsten to get balance with the crank mods and the longer, heavier rod.

Back to the dyno and the horsepower lost was back, plus a small increase. I know a lot of engines still use pork chop cranks and I think if rpms are moderate they are fine. But if rpms are highish (even as low as 8.5 or 9k) I really think there are measurable horsepower benefits to a streamlined crankshaft.

Now your typing! This is what I like about you sharing your experience! You tell us what you tried and why it didn’t or did work but more importantly you know why and share it. So it has been known for 50 years but I always feel like Columbus when I discover something new to me. What did Mark Twain say, “I knew a man that grabbed a cat by the tail and he knew 40% more about a cat than the man that didn’t.”

So Zeke is learning Solid Works at the moment. Solid works let’s you run simulations on how your part will perform. John can you imagine what you could have done if you had modeling software? Can you imagine what you could do if you had a 5 axis CNC like Ralf? Is there a way to measure the efficiency and volume of our pumps other than a dyno?

I look at our engines like singers. So M1000 is a Pavarotti who has huge belly and knows how to sing from their diaphragm but our Bulls are singing from the throat and not from their diaphragm. What intrigues me is someone like Jacob Collier or Celine Dion who know how to make power with a small package. I have been trying to make power with a small package all my life. Well I won’t go into those short comings. But I think you are on to something that the Boyenson port would allow our Bulls to breath better from their diaphragm. Don’t the Boyenson Reeds have multi layers that operated at different RPM?


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John Murray

Been Around the Block
Boyesen reeds are rubbish, don't waste your time on them.

I get what you're saying about modelling, and I do use modelling to some extent. But I find I can dyno test all sorts of things pretty quickly too. Plus dyno testing often brings to light things that I wouldn't be aware of when setting up a sim.

Kevin Cameron once compared a two stroke engine to a tiny bird; he said how does such a small bird make such a loud sound? By concentrating all its effort on producing a single note. Our old bikes are like that, they're loud but can only sing one note.
 

Texasstar

Can't is a four letter dirty word
Boyesen reeds are rubbish, don't waste your time on them.

I get what you're saying about modelling, and I do use modelling to some extent. But I find I can dyno test all sorts of things pretty quickly too. Plus dyno testing often brings to light things that I wouldn't be aware of when setting up a sim.

Kevin Cameron once compared a two stroke engine to a tiny bird; he said how does such a small bird make such a loud sound? By concentrating all its effort on producing a single note. Our old bikes are like that, they're loud but can only sing one note.

I can’t wait to here my bird sing on methanol! Jacob Collier just took the world by storm because he explored the undertones and overtones of a note. Like you he said about modeling that real life doesn’t sing each note perfectly.


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John Murray

Been Around the Block
I've never dug out a pair of Boyesen ports in a Bultaco barrel, but if I did it would only be to avoid having holes in the intake skirt. I wouldn't expect any power gain.

One thing that has always stood out with two strokes: they're relatively indifferent to any changes made to carburetion, intake porting, transfers etc. Which is not to say these things aren't important; they have to be right. But small changes there generally won't make a big difference to the output. But even minor changes to the pipe and the exhaust port can produce very marked changes to the output. I think of these two as the power producers and the rest of the engine as being in a support role. If you're looking for power focus on the hot side first.

Or to use your analogy - the pipe and exhaust port are kd lang, the rest of the engine are backup singers. Find power and you'll be singing Hallelujah :)
 
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John Murray

Been Around the Block
Not my work Frank, though I wish it was! That's a factory Kawasaki KX250 crank, the one above it with the tin shrouds is a Honda CR part. My own work is much more, um, agricultural....
 

Texasstar

Can't is a four letter dirty word
Can we measure how much air is being drawn into the cases. I have one of these.

Dellorto Weber genuine German synchrometer carb balancer + SU Stromberg adaptor
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