Texas Two Step Taco

[teazer]

On measuring the exhaust port diameter won’t a port map give you the chordal port diameter by measuring it flat on a table?

Arc or chord width doesn't matter as long as you know which you have. Software converts that to the appropriate dimension. Some use Chord and others use Arc. We have the technology.

Most software uses Blair formulae or versions of it and few of them seem to create an exhaust that works for some odd reason. One reason is that they all make assumptions as to exhaust gas temperature and they may be way off for a particular motor. MOTA calculates exhaust temperature all down the pipe and shows it as a moving wave which is really neat.

You could also use EngMod2T which is even more complex.

Just a thought but if that motor is reaqsonably similar in some respects to say an H2 big cc motor or say another 750 triple or close to say an RM or CR250, that might give you some ideas as to what to aim for and could give you a place to approximate the pipe at least as a starting point.


Making a pipe is maybe the easiest part of this exercise as long as you can think in 3D

 

[Texasstar]

Most software uses Blair formulae or versions of it and few of them seem to create an exhaust that works for some odd reason. One reason is that they all make assumptions as to exhaust gas temperature and they may be way off for a particular motor. MOTA calculates exhaust temperature all down the pipe and shows it as a moving wave which is really neat.

wow what a huge difference based upon exhaust temperature!

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[Texasstar]

My wife says that heat would also be impacted by the pressure so how would we measure the pressure in the pipe? My wife is so damn smart.

“Do'nt believe Dr. Blair, he never made a GP-winning engine!
Better keep your 86X86 bore and stroke.
You should easily be able to achieve 120-130 HP with it.
If you do everything right!
Achieving the same MEP as the Aprilia 125 you should even achieve 140HP/8500
at the gearbox output shaft.
But you will not find a big-enough carburetor for it, should be around 65mm!
So fuel injection is the way to go!” Jan Thiel

So we are looking for our MEP?

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[idahofrank]

I am learning stuff. I like to learn. Thanks

 

[teazer]

Your wife is correct that increasing pressure increases temperature and vice versa. See Boyle's and Charles' law for more details.

That is part of the beauty of MOTA simulation - and probably EngMod2T as well. It calculates both over a full cycle at given RPM. That way you can see the temperature and pressure waves through the exhaust or any other duct. For us mere mortals one of teh useful things is we can run the Simulation at our target peak RPM and see where the different waves are out of synch. For example we can see if the pressure wave coming back up the pipe arrives and peaks just before the port closes. Often it will arrive too early or too late. Too early raises cylinder temperatures as it loses power and too late just costs us power.

Another interesting thing to see is when a street pipe with parallel header doesn't pull gas out of the cyclinder fast enough, the cylinder pressure is higher than crankcase pressure when the transfers open, so the gas flows down the transfers before it comes back up. In that case, you need more blowdown time-area or change the exhaust to provide a stronger suction wave with say 2 or three stages so that it essentially sends a wave on top of a wave back to empty the cylinder.

All this and more is well understood by those in the know.

On that website, check out the x-rays of an Aprilia 125 cyinder to see how intricate the transfers are. Or check out say an Aprilia RS250 street cylinder and you can see how a street 250 makes 65hp with a few tweaks. Those cylinders are similar to say a Honda RS125R race motor or a 90hp TZ250 V twin. For something simple, look at say an RM250 Suzuki cylinder or any similar capacity sled cylinder. They make our old clunkers look like they are 50 or 60 years old. Funny how that works.

We can't always just copy and paste new tech onto an old design, but we can at least observe the development path and sometimes can adapt some of it. For example we can add reed cages and that lets us add boost ports to provide additional transfer time-are as well as offering opportunities to change port flow direction. Doesn't mean we'll get it right but that opportunity to try exists.

 

[teazer]

For MEP or BMEP you are probably at around 6 at the moment and fully developed should be closer to 12. Aim for say 10 and see where that takes you. See what time-area you need in each port and work out the restrictions and what can be done with them. If 10 is easy, see what changes are needed to get to 11 or 12 without losing all the bottom end.

ANother thing is compression. I like a lot of compression because it helps on a street motor as long as the pipe isn't too effective. With a really effective pipe the effective compression when on the pipe may be too high, so once again it's about optimizing for your application. With a slider clutch, low speed power is less of an issue. But with manual clutch it's harder for most of us to leave the line at high enough revs. If you are Alex Hughes or Ricky Gadsden it's probably not an issue, but for us old guys, we need a power curve that works for our ability level.

 

[Texasstar]

For MEP or BMEP you are probably at around 6 at the moment and fully developed should be closer to 12. Aim for say 10 and see where that takes you. See what time-area you need in each port and work out the restrictions and what can be done with them. If 10 is easy, see what changes are needed to get to 11 or 12 without losing all the bottom end.

ANother thing is compression. I like a lot of compression because it helps on a street motor as long as the pipe isn't too effective. With a really effective pipe the effective compression when on the pipe may be too high, so once again it's about optimizing for your application. With a slider clutch, low speed power is less of an issue. But with manual clutch it's harder for most of us to leave the line at high enough revs. If you are Alex Hughes or Ricky Gadsden it's probably not an issue, but for us old guys, we need a power curve that works for our ability level.

Fun! I hear you loud and clear on the slider. My forearms were screaming at me also! Tonight I will make an exhaust port map from the inside of the exhaust port and not a rub like I currently have.


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[Texasstar]

ANother thing is compression. I like a lot of compression because it helps on a street motor as long as the pipe isn't too effective. With a really effective pipe the effective compression when on the pipe may be too high, so once again it's about optimizing for your application. With a slider clutch, low speed power is less of an issue. But with manual clutch it's harder for most of us to leave the line at high enough revs. If you are Alex Hughes or Ricky Gadsden it's probably not an issue, but for us old guys, we need a power curve that works for our ability level.

We are 14:1 on compression.



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[teazer]

sounds about right and with a well designed squish band and torroidal head design the ignition advance should be closer to your original plan, but test it on the dyno to be sure.

 

[Texasstar]

sounds about right and with a well designed squish band and torroidal head design the ignition advance should be closer to your original plan, but test it on the dyno to be sure.

On the exhaust port length do we measure the longest (55mm) which is the bottom or the top which is the shortest (49.66mm)

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[Texasstar]

Exhaust port cylinder measurement

Left window pictured

Width is 32.56mm
Height is 30mm
Right bottom radius 4.8mm
Left bottom radius 12.76mm
Top left 8.09mm
Top right 6.12mm

Right window
Width is 32.44mm
Height is 30mm
Right bottom radius 12.76mm
Left bottom radius 4.8mm
Top left 6.12mm
Top right 8.09mm


I used different sockets to find the radius of each corner. Measured the diameter of the socket and divided by 2.

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[Texasstar]

Here is a video of AuMechanics Exhaust Port/ Diameter Calculator for two stroke tuning. Measured the ports and entered the data. Let me know if you thought the compromise was correct.

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[teazer]

Exhaust temp is way too high unless that's degrees F not C. How far from TDC is the roof of the exhaust port?

Now measure the transfer outlets and inlets and the intake plus

Bore:
Stroke:
Conrod length:
Piston pin offset if known:

 

[Texasstar]

Exhaust temp is way too high unless that's degrees F not C. How far from TDC is the roof of the exhaust port?

:

Oops it is suppose to be Celsius I have it as F





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[Texasstar]

[QUOTE="teazer, post: 932697, member: 5019"

Bore:
Stroke:
Conrod length:
Piston pin offset if known:[/QUOTE]


32.75 mm roof of exhaust to TDC (corrected measurement)slid the map back into cylinder to double check. Also checked the port map for the Bandido exhaust and the duration is almost identical. The Bultaco Bandido map is 33.07mm) something’s don’t change generationally.

What is the rule of thumb for exhaust port duration? I have read that if the exhaust port duration is raised it narrows the power band but raises the revs. Both Frits Overmar and Jan Thiel on that 500cc thumper string said max RPM should be 9k but target should be 8500 for longevity. They also said something interesting that it was a goal to shoot for a slightly oversquare engine design.
bore:86
stroke:64
Conrod length 116 Center to center I will call Hughes today to verify. 115.87mm from Hughes Bultaco




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[Texasstar]

Does the port window calculation look correct?

calculator window

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[Texasstar]

Used the online calculator to calculate the stock Bultaco Montadero exhaust duration and came up with 183.57 and when we did it with a degree wheel it was 185. View attachment 222910

 

[Texasstar]

The online calculator for the exhaust duration for the Bultaco Bandido and our current exhaust port is 193.24

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[irk miller]

Is this build hinting that you need to borrow $15,000?

 

[Texasstar]

Is this build hinting that you need to borrow $15,000?

Nope lol!! but I do need to sell some more parts to fund my endeavors. Zeke walked into the kitchen while I was measuring last night and he said, “watcha doing?” I said, “trying to beat your tail.” He said, “good luck with that.” However Racing gets expensive... lol.


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