Texas Two Step Taco

John Murray

Been Around the Block
BTW a late model two stroke race bike will have a cranking compression in the range of 200PSI as do many sleds and banshee quads.

What I find interesting about this is how it is often made possible without a high CR number. Modern cylinders with wide exhaust ports and auxiliaries can achieve sufficient blowdown without resorting to extra-long exhaust duration, hence the cranking pressure.
 

teazer

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200 degrees ATDC? I think that might be the duration, so 360-200=160 or 80 degrees ATDC EPO and 80 BTDC EPC. That's quite wild and will have a deleterious effect on effective compression unless/until it's on the pipe.
 

teazer

Over 10,000 Posts
DTT BOTM WINNER
I think that the Last model TZ250 kit EPO was 83 degrees, so not much different to our clunkers but much more area at the top to promote effective blowdown for sure
 

Texasstar

Can't is a four letter dirty word
200 degrees ATDC? I think that might be the duration, so 360-200=160 or 80 degrees ATDC EPO and 80 BTDC EPC. That's quite wild and will have a deleterious effect on effective compression unless/until it's on the pipe.

That is the duration


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John Murray

Been Around the Block
200 degrees ATDC? I think that might be the duration, so 360-200=160 or 80 degrees ATDC EPO and 80 BTDC EPC. That's quite wild and will have a deleterious effect on effective compression unless/until it's on the pipe.
200 or even a bit more isn't unusual at all for these old girls, it's just what you have to do to get any top end. So with 200 duration it opens at 100 deg bbdc and closes 100 abdc. Modern road racers seldom run over 190 duration.
 

Texasstar

Can't is a four letter dirty word
200 degrees ATDC? I think that might be the duration, so 360-200=160 or 80 degrees ATDC EPO and 80 BTDC EPC. That's quite wild and will have a deleterious effect on effective compression unless/until it's on the pipe.

Here is when we checked he exhaust duration with BB

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teazer

Over 10,000 Posts
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200 or even a bit more isn't unusual at all for these old girls, it's just what you have to do to get any top end. So with 200 duration it opens at 100 deg bbdc and closes 100 abdc. Modern road racers seldom run over 190 duration.
Or 80 ATDC/BTDC since that's the same point but expressed from a different point of reference. :)
 
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John Murray

Been Around the Block
Ha ha yes your TDC is my BDC. It's not as bad as it sounds, the con rod swing through BDC means the piston dwells much longer at BDC than at TDC. So even with a long duration exhaust you don't lose as much stroke as you might think.
 

Texasstar

Can't is a four letter dirty word
200 degrees ATDC? I think that might be the duration, so 360-200=160 or 80 degrees ATDC EPO and 80 BTDC EPC. That's quite wild and will have a deleterious effect on effective compression unless/until it's on the pipe.

IMG_0355.png



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John Murray

Been Around the Block
Seems OK to me, assuming you've accounted for piston dome and clearance volume. Back in the day trapped CRs of around 6 to 7:1 were common. Modern engines, because they have shorter duration might be a bit higher.

It does seem a little ridiculous that we don't close the door until after the piston is over halfway back up the cylinder. But the fact is that with these old exhaust-limited engines horsepower continues to rise as the duration is increased even above 200 degrees. It's a good illustration of just how important blowdown is when we can sacrifice so much working stroke yet still make more power. Such long duration also makes the effectiveness of the pipe even more critical - you can imagine how much of a dog an engine with such long duration and a weak pipe would be.

Incidentally Snr Bulto was one of the first to discover what could be done by combining long durations with effective pipes. Kevin Cameron wrote in Classic Motorcycle Race Engines "..through repetitive testing engineer FX Bulto discovered that Ing Wolf's pioneering resonant exhaust pipe could be used to make much longer port timings - and much higher power - practicable. Bulto was a sophisticated and well-travelled man."

Should also mention that late "trapping" isn't just a two-stroke thing - many four-stroke racing cams have intake timing (seat-to-seat) of well over 300 degrees and an intake closing figure not too dissimilar to our exhaust closing.
 

Texasstar

Can't is a four letter dirty word
So here are our specifications for the Bultaco Bandido/ Montadero Texas Two Step Bandera 360cc Bultaco

Bore 86.5 mm
Stroke 64
Rod length 116mm (thank you John)
Piston to Wall .0075”
Exhaust Duration 200 degrees atdc
Intake Duration 145 atdc
Head volume 34 cc
Head design Ralf “toroidal” Shippman
Squish .046”
Ring gap .025”
Plug NGK BR10ES .035” gap
Q16
Reed Box
Tm 38mm Mikuni Flatslide 290 jet
Gearing 13/45
Aprilia .4 ohm coil
Compression 14:1
Fuel Q16 40:1 Redline
47 whp
1/8th mile 8.08
215 lbs wet






IMG_0354.jpg



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Texasstar

Can't is a four letter dirty word
Opens at 200 degrees


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Teazer,
My measurement from TDC to the top of my exhaust port is 32mm on my port map but in my journal I have written 30mm. When BB and I did the duration it opened 200 atdc but with my 32mm measurement I am at 196 atdc. So I am inclined to go with BB’s measurement.


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teazer

Over 10,000 Posts
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And that raises the interesting point that we all measure port opening times slightly differently and that's partly because of the chamfer on the ports which makes it hard to determine actual opening.

When I am porting a barrel, I tend to use an upside down piston with square cut skirt. That's the easiest way with barrels off. With them on I use a rotary encoder to determine angle or I use a very long DTI and a flashlight. I have also used a feeler gauge on top of the piston going into the port and then adjust for feeler gauge thickness but that gets to be hard if the roof of a port is curved.

The hardest way is with a flashlight because the piston rarely has a sharp edge to the crown - the edge is usually a 1mm angle and the port has a 1-2mm chamfer and those combined give a different reading than other techniques.

And which is the true time at which the gasses start to enter or depart from a port?

Are we having fun yet?

BTW, that MSV value is fairly high- as in close to what is considered to be "normal range" :)
 

Texasstar

Can't is a four letter dirty word
Don't get me started on squish..

Will too much squish velocity cause auto ignition? Is there such a thing as too much squish? If your squish is designed for 35 m/s what happens when the flame propagation speed of methanol is greater than your squish velocity? So methanols flame propagation speed is 44 m/s.

Here is a fun read from some engineers. https://www.eng-tips.com/viewthread.cfm?qid=238238

Riddle me this. How can you have a cool head and still melt a piston on alcohol? I think I found the answer in that string.


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Texasstar

Can't is a four letter dirty word
And that raises the interesting point that we all measure port opening times slightly differently and that's partly because of the chamfer on the ports which makes it hard to determine actual opening.

When I am porting a barrel, I tend to use an upside down piston with square cut skirt. That's the easiest way with barrels off. With them on I use a rotary encoder to determine angle or I use a very long DTI and a flashlight. I have also used a feeler gauge on top of the piston going into the port and then adjust for feeler gauge thickness but that gets to be hard if the roof of a port is curved.

The hardest way is with a flashlight because the piston rarely has a sharp edge to the crown - the edge is usually a 1mm angle and the port has a 1-2mm chamfer and those combined give a different reading than other techniques.

And which is the true time at which the gasses start to enter or depart from a port?

Are we having fun yet?

BTW, that MSV value is fairly high- as in close to what is considered to be "normal range" :)

Oh...I have been trying to figure out what MSV stood for. It is Mass Squish Velocity! You saw that on Ralf’s print out. I read this yesterday trying to figure out why we melted a piston but still have a cool head. It made my brain spin with questions. Chamber was a 1000 degrees but the head reading was cool. Did you see the reluctor video I posted?

Back to the taco we are a little faster than that since our squish is .046”



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John Murray

Been Around the Block
I believe very very little of the popular wisdom on squish. The thread quoted above has much in the way of opinion but little in the way of concrete test results - as usual. The current fashion in two stroke cylinder heads in my opinion is based on shaky foundations if my limited testing is any indication.
 

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