Texas Two Step Taco

[John Murray]

Been thinking about limited-blowdown engines and ways of reducing cylinder pressure in that first 30 odd degrees after EO. I'm imagining a diffuser slid right up to the cylinder (or in other words a header pipe with a greater than usual taper) in conjunction with a longer parallel center section to maintain the length. Or possibly a short initial diverging taper that's active during blowdown followed by a parallel or gently diverging section before flaring into the center section. My 370 Pursang engine responded well to tapered header tubes but I doubt that I went far enough with it, and it probably has less blowdown capacity than the Montadero cylinder.

Edit: that engine also responded to filling the floor of the exhaust duct, something I did in an attempt to improve flow in general. But considering the lower part of the port is relatively unimportant, a raised floor may help in making the duct more effective with a partially open port.

 

[Texasstar]

Been thinking about limited-blowdown engines and ways of reducing cylinder pressure in that first 30 odd degrees after EO. I'm imagining a diffuser slid right up to the cylinder (or in other words a header pipe with a greater than usual taper) in conjunction with a longer parallel center section to maintain the length. Or possibly a short initial diverging taper that's active during blowdown followed by a parallel or gently diverging section before flaring into the center section. My 370 Pursang engine responded well to tapered header tubes but I doubt that I went far enough with it, and it probably has less blowdown capacity than the Montadero cylinder.

Edit: that engine also responded to filling the floor of the exhaust duct, something I did in an attempt to improve flow in general. But considering the lower part of the port is relatively unimportant, a raised floor may help in making the duct more effective with a partially open port.

I am trying to picture this


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[John Murray]

Let's assume that we want to improve blowdown as much as possible without increasing port area, and we're gonna try to do this by increasing the suction effect of the pipe. Seeing as we want this to happen as soon as possible we could do this by having a fairly steeply diverging cone that commences right at the port window. It would be quite short - if we assume the pressure pulse peaks somewhere around 10 degrees after EO then we'd only have about 20 degrees left to pull hard on that initial flow, before the transfers open. The barrel duct - the section from the window to the pipe flange - would be a significant portion of this so would have to be shaped accordingly. After this short initial cone the header pipe would have a section of parallel pipe before resuming normal proportions. I know this makes no sense lol. I haven't done any calcs on this yet so could well be talking out of my arse.. But I'll do the sums and if it looks feasible I'll do a rough sketch so you can see what I'm trying to say.

Edit: thinking about this while mowing the lawn (with a 2stroke Victa!) - a horizontal divider set at transfer height in the exhaust duct could possibly be a big help in getting the cylinder pressure down quickly..

 

[teazer]

With a tapered header and say three divergent cones, the pressure drop is really fast and it drops a long way as the pressure wave passes each transition, so you get one long negative pressure event. That should help with cylinder filling but it will not necessarily overcome the restriction of insufficient initial port-time area.

having a more aggressive header taper would generate a stronger pressure drop and a very fat pipe. I'd try that and see what a short fat high volume pipe looks like. With a 360cc cylinder there's a lot of gas volume to extract. Maybe look at some sled pipes for inspiration. They tend to be large diameter and volume but they run over a narrow rev range which might work for a drag pipe.

 

[John Murray]

I've had positive results from more aggressive header tapers, though I didn't have blowdown in mind and I doubt they had any effect on it. If blowdown is the objective then we are only really looking at the first 100mm or so of the system (at a guess) - probably just the duct and a very short part of the header. The initial pressure wave wouldn't have time to reflect back after travelling any further down the pipe if its going to do anything useful before transfer opening.

Quite possibly not going to achieve anything at all but I'm keen to test the idea and see what happens. It's rare to not learn something from experiments like this, even if the results aren't what was hoped for..

 

[Texasstar]

With a tapered header and say three divergent cones, the pressure drop is really fast and it drops a long way as the pressure wave passes each transition, so you get one long negative pressure event. That should help with cylinder filling but it will not necessarily overcome the restriction of insufficient initial port-time area.

having a more aggressive header taper would generate a stronger pressure drop and a very fat pipe. I'd try that and see what a short fat high volume pipe looks like. With a 360cc cylinder there's a lot of gas volume to extract. Maybe look at some sled pipes for inspiration. They tend to be large diameter and volume but they run over a narrow rev range which might work for a drag pipe.

How about this 370cc


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[teazer]

Now that's a pipe to be proud of. Size matters.....

I wonder if he's over compensating for something.

That is like some of the sled pipes that Olav Aaen made over the years.

 

[Texasstar]

Now that's a pipe to be proud of. Size matters.....

I wonder if he's over compensating for something.

That is like some of the sled pipes that Olav Aaen made over the years.

That makes almost twice what we are making at the present but it is all modern cylinder tech. As John says, “the engine is ancillary to the pipe”.


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[Texasstar]

I've had positive results from more aggressive header tapers, though I didn't have blowdown in mind and I doubt they had any effect on it. If blowdown is the objective then we are only really looking at the first 100mm or so of the system (at a guess) - probably just the duct and a very short part of the header. The initial pressure wave wouldn't have time to reflect back after travelling any further down the pipe if its going to do anything useful before transfer opening.

Quite possibly not going to achieve anything at all but I'm keen to test the idea and see what happens. It's rare to not learn something from experiments like this, even if the results aren't what was hoped for..

Let’s make another chamber with your specifications. This will be fun!


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[John Murray]

View attachment 235694
How about this 370cc


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To me that looks "right" for a 370, unfortunately for us though we just don't have that much room on our two-wheelers..

 

[Texasstar]

Checked cylinder compression

So that puts us at 13:1 compression

193 divided by 14.69 atmospheric.


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[Texasstar]

Checking everything. Spade connector loose on the coil. Was tight at one time.

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[John Murray]

Checked cylinder compression View attachment 235707
So that puts us at 13:1 compression

193 divided by 14.69 atmospheric.


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You have to take the exhaust duration into account, actual CR will be higher
Actual CR with exh. duration of 200 degrees is around 17.6:1. Trapped or "Japanese" CR is 9:1. Quite likely you'd pick up some top end with a reduction.

 

[Texasstar]

You have to take the exhaust duration into account, actual CR will be higher
Actual CR with exh. duration of 200 degrees is around 17.6:1. Trapped or "Japanese" CR is 9:1. Quite likely you'd pick up some top end with a reduction.

I have two compression gauges. Both read roughly the same if you check them with an air compressor and plug them into the tank. One is a cheap Chinese Harbor Freight. It read 165. We are suppose to have 14:1 compression. How do you calculate with the duration? 165/14.69=11.29

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[John Murray]

There are different types or definitions of compression ratio. The most commonly used number is the theoretical figure derived from:

CR= (swept volume+chamber volume)/chamber volume.

This has some value but doesn't take into account the exhaust port closing (in a two stroke) or the intake valve closing (in a four stroke). Delayed closure of either results in pressure being bled off, so cranking and low speed pressure is much lower than you'd expect from the theoretical figure. With our Buls for example cranking compression doesn't begin until the piston is roughly halfway up the bore. This why long exhaust durations with a 2T, or long cam durations with a 4T usually dictate a higher CR.

By adjusting our swept volume to only include volume above the top of the exhaust port we get a figure more closely tied to actual cranking pressure. Japanese bike manufacturers have traditionally used this figure in their published specifications while others commonly call it "trapped" CR. Four stroke guys call it dynamic compression ratio. It's a better guide to how the engine will behave as it takes into account port or cam timing.

Of course once the engine gets on the pipe (or cam) more mixture is trapped and cylinder pressure goes up quite a bit. But at higher speeds the risk of det is much lower so the increased pressure isn't so much of an issue.

Most two stroke tuners just work with the theoretical figure (four stroke guys call this static compression) and this is fine with exhaust durations in the "normal" range. Unusually short or long durations (and our Buls fall into this category) would indicate a little extra thought and adjustment of the CR may be required.

 

[John Murray]

I have two compression gauges. Both read roughly the same if you check them with an air compressor and plug them into the tank. One is a cheap Chinese Harbor Freight. It read 165. We are suppose to have 14:1 compression. How do you calculate with the duration?

That's a big difference between gauges. I'd be getting a 3rd opinion.. and suspect the lower pressure might be more correct.

If you remember your high school trigonometry and you know the stroke and rod length you can calculate the piston position when the exhaust port closes. Or you could simply measure the stroke above the port and use that to work out the swept volume. Or you could use one of the online calculators like http://www.wallaceracing.com/dynamic-cr.php and work backwards.

 

[Texasstar]

That's a big difference between gauges. I'd be getting a 3rd opinion.. and suspect the lower pressure might be more correct.

If you remember your high school trigonometry and you know the stroke and rod length you can calculate the piston position when the exhaust port closes. Or you could simply measure the stroke above the port and use that to work out the swept volume. Or you could use one of the online calculators like http://www.wallaceracing.com/dynamic-cr.php and work backwards.

How did you calculate that so quickly? I don’t think you used a calculator.


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[John Murray]

How did you calculate that so quickly? I don’t think you used a calculator.


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I used the Wallace racing calculator, estimating the CR and adjusting it til the pressure matched your measured figure. But that was a dumb way of doing it, I should've used this http://www.torqsoft.net/piston-position.html to get the piston position and worked out the CR from that.

Link text (for me at least) comes out in a color that's damn near invisible - is there a way to change it?
Edit: changed to Dark Mode, much better

 

[teazer]

Tex, how far down from TDC is the roof of your exhaust port? If you didn't measure it, but know the EPO time in degrees use that latest link to work it out.

Then do the conventional CR calculation with full stroke to get geometric or static CR. Repeat the calculation with EPO distance as the stroke to get "corrected" CR.

CR=(swept volume+clearance Volume)/clearance volume

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.

 

[Texasstar]

Tex, how far down from TDC is the roof of your exhaust port? If you didn't measure it, but know the EPO time in degrees use that latest link to work it out.

Then do the conventional CR calculation with full stroke to get geometric or static CR. Repeat the calculation with EPO distance as the stroke to get "corrected" CR.

CR=(swept volume+clearance Volume)/clearance volume

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.

Opens at 200 degrees


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