Texas Two Step Taco

John Murray

Been Around the Block
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
You can, but not with one of those, it'd be way too small. Automotive dynos sometimes use a turbine style flow meter to to measure air flow into the engine, but these are way too big for our little Buls. You could easily adapt a mass air flow sensor to do the job though. But that brings us back to the same old thing - are we working towards a good reading on the flow meter or a good reading on the torque gauge? Ultimately we want the big numbers on the torque gauge (and tach) so why not just focus on that?

Having said that, flow bench testing can be a useful and much quicker way of testing ideas. I spent some time a couple of years ago playing with the intake tract of the Rotax. It's weird at first watching the pressure and flow when testing with a reed valve - the depression rises to a certain point before any flow at all occurs (the reed opening pressure) then it stays relatively constant as the flow is increased, up until the point that the port itself starts to have some influence. You can see the effects of using different reed thicknesses. Several carbs and throttle bodies were tested; a good carb will have no effect at all on flow yet will still have a good signal. The "stuffer" or fairing piece in the reed intake has a significant effect and is worth spending some time on. I didn't spend any time on flow testing transfers or exhaust ports, but that could be interesting too. You'd really want a junk barrel to do this though - not all changes will turn out to be good ones so you wouldn't want to trash your good barrel. With a junk barrel material removed can be easily replaced with body filler so you can start over.
 
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John Murray

Been Around the Block
How about transfers in your pistons?


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No, for one it'd be much too heavy, especially if it's an 80mm+ piston and you're trying to run 9k or so. The other thing is getting the flow from the transfer aimed where you want it and not so turbulent that it's just spraying all over the place. This why the shape of the transfer passages is important in directing the flow - you can't just cut a roof angle into a shallow port and expect it to work. Take a look at this cross section of a transfer duct and see how it sweeps smoothly, directing the flow and avoiding tumble and turbulence. We'll never get our old Buls to look like this - at least not without a lot of welding - but the closer we get to it the better.

FOS A-transfer duct geometry.png
 

Williansh

New Member
Page 3

Here's another trap that catches some, especially those that tune on an inertia dyno. Here's how it goes: you've been tuning your bike on the inertia dyno and it's making good power, more than it has ever made. All the signs show the mixture and timing are both on the safe side so the next weekend you head out to the track full of confidence. It leaves the line hard and pulls like a train - or at least it does until you click it into top and feel that heartbreaking sensation of a piston sticking. What the hell happened, it was on the safe side on the dyno? Why did it grab?
What happened is this: methanol likes a rich mixture to start from cold, requiring more enrichment than petrol. No problem, you've learned to give it plenty of fuel and it fires up instantly. You sit on it and back the mixture off as you warm it up on the dyno for half a minute or so, the engine sounding clean and crisp as you gently blip the throttle. After the brief warmup you click it into gear and commence the dyno run, which being on an inertia dyno is over after maybe 4 or 5 seconds. Power is good, there's no sign of leanness and everyone is happy.
Why then did it seize at the track? While you were starting and warming up the bike only a very small percentage of the fuel was vaporising and being burnt. The rest was collecting in the crankcase and forming an ever larger puddle at the bottom. Because you like to be a bit gentle on a cold engine you weren't revving it hard and the puddle remained in place. Once the dyno run commenced though and revs increased the puddle was stirred up into a mist that passed into the cylinder and was burnt. That mainjet that you carefully selected after multiple dyno pulls wasn't the only source of fuel - a significant proportion was being drawn from the crankcase pool. And because the dyno run was so brief it was all over by the time the puddle had been completely consumed.
Out on the track though it wasn't so good. It started out well but after a short time the puddle was all gone and then the only fuel supply available was from the mainjet - which the engine clearly showed wasn't enough on its own. How do you manage this? I think the main thing is to be aware of the possibility of this happening and being the cause of unexpected lean-outs. Try to make your tests long enough to eliminate the supplementary fuel effect. Don't let the engine idle for too long and lean the idle mixture out as much as you can and as soon as you can after startup. Don't be surprised to find that the idle fuel can be shut off completely for quite a while before the engine starts to lean out - the crankcase fuel builds up quickly and takes a while to be used up. And be wary of an engine wanting a suspiciously small mainjet - it might suggest it's getting fuel from somewhere else.
Another factor to consider is temperature. Methanol's cooling effect is well known and that makes it a very good choice for engines like the big air-cooled Buls that are normally badly undercooled when the horsepower level is increased significantly. But there's more to it - methanol is also very sensitive to temperature and needs large adjustments to the A/F as temps change. It's much more sensitive than petrol in this respect. It also requires a different way of thinking - with petrol we focus on keeping the engine as cool as possible to maximise VE and power. Methanol on the other hand likes some heat and a heavy load to really burn well and of course we have to think of the pipe too. It's not unusual to be in the situation where the challenge is not in cooling the engine but in getting enough heat into it. We need to get some heat into the motor (especially the head) quickly.


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Cool
 

Texasstar

Can't is a four letter dirty word
No, for one it'd be much too heavy, especially if it's an 80mm+ piston and you're trying to run 9k or so. The other thing is getting the flow from the transfer aimed where you want it and not so turbulent that it's just spraying all over the place. This why the shape of the transfer passages is important in directing the flow - you can't just cut a roof angle into a shallow port and expect it to work. Take a look at this cross section of a transfer duct and see how it sweeps smoothly, directing the flow and avoiding tumble and turbulence. We'll never get our old Buls to look like this - at least not without a lot of welding - but the closer we get to it the better.

View attachment 235632

The M70 360 Bultaco is upside down from this diagram. The case side of the transfer is smaller than the top of the transfer. No room to swoop.
IMG_0142.jpg

IMG_0143.jpg



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Texasstar

Can't is a four letter dirty word
No, for one it'd be much too heavy, especially if it's an 80mm+ piston and you're trying to run 9k or so. The other thing is getting the flow from the transfer aimed where you want it and not so turbulent that it's just spraying all over the place. This why the shape of the transfer passages is important in directing the flow - you can't just cut a roof angle into a shallow port and expect it to work. Take a look at this cross section of a transfer duct and see how it sweeps smoothly, directing the flow and avoiding tumble and turbulence. We'll never get our old Buls to look like this - at least not without a lot of welding - but the closer we get to it the better.

View attachment 235632

Now in comparison to the M70 take a look at the M219 250 cc cylinder and we see more swooping taking place and the case side transfers are larger than the top of the transfers following your diagram more.
IMG_0144.jpg

IMG_0145.jpg



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Texasstar

Can't is a four letter dirty word
Here is a side by side comparison of the m70 Bultaco Montadero 360 and the M219 250cc Pursang
IMG_0146.jpg



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John Murray

Been Around the Block
The M70 and M219 are pretty typical for the era, and better than some others. I had a CZ that had no material at all on the short side, just the outside of the liner. Even today many multi-cylinder engines are forced to use fairly narrow passages simply so the cylinders can fit together closely. Suzuki used to get around this problem with some engines by rotating the cylinders slightly relative to the crank axis, thereby staggering the passages of neighboring cylinders.

The M219 is a good example of more-than-enough transfer area and not enough exhaust. The Monty barrel looks like there is plenty of meat to reshape the outer wall and add filler to the inner wall. The most critical part is the final curve into the port window, much like a four stroke where the critical part is the approach to the valve seat. Of course this is the hardest part to access but the big bore engines are nowhere near as difficult as the little ones.

If you need to add crankcase volume the transfer passages are a good place to put it. As the piston descends and the case pressure rises you can have a significant part of the fresh charge stored right there under the transfer port.

Still, I'd be focusing my attention on the hot side, that's where you'll get the best returns. Is there enough material in the sides of the exhaust duct to add a couple of small auxiliaries? Only when you've got the exhaust as good as it can possible be should you start looking at the transfers.
 

Texasstar

Can't is a four letter dirty word
John you have me dreaming about being inside of an engine examining the exhaust port.


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Texasstar

Can't is a four letter dirty word
I dunno what you've been smoking Pat, but................... could you send me some?

Seriously I have gained an understanding of why the Porter did what he did with my M51 and I appreciate it because of the limitations he had. I also know why he didn’t increase the upper transfer size because it was to big to begin with. He did make the lower transfer a lot bigger.

My left side case does not have any meat in it to hog out for another transfer. I would have to do major welding to increase the size of the transfers for the lower case. I do know how to weld aluminum ;) The right side has plenty of meat to add the case transfers. Somewhere I saw pictures of the surgery that you did on your lower cases.

After we test the methanol I want to go back and attack the exhaust port as you have suggested. I hear you loud and clear. It is not the transfers that is limited power.


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John Murray

Been Around the Block
Seriously I have gained an understanding of why the Porter did what he did with my M51 and I appreciate it because of the limitations he had. I also know why he didn’t increase the upper transfer size because it was to big to begin with.
Yes, he did very well within the constraints he had and it showed in the dyno results.
 

John Murray

Been Around the Block
Yes, like that. But if that isn't practical there are some things you can do that would increase the flow of the single port. A small radius on the vertical edges of the divider and the side edges of the port would help. Re-contouring the sides of the duct where they blend into the port window would also help (if there's enough material). The top portion of the port could pick up a little area by making the sides vertical down to the top of the transfers, and possibly even adding a little more width. What's the exhaust timing at now?


radial exhaust flow.png
 

Texasstar

Can't is a four letter dirty word
That picture
IMG_0164.jpg

is our other stock Montadero cylinder. Here is what mine looks like now. The stock Bultaco Montadero exhaust duration is 181, the Bandido is 192,
And we are north of that.


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pidjones

Over 1,000 Posts
This all gives me a headache. Sure glad I kept my RD400c stock with stock jetting. Back in the '70s I had a 125 Penton ISDT that we decided to flat track. Although I ordered a commercial low pipe that was touted to be the best, and a friend gave me another low pipe that he had around, I decided that a spare cut from the original high pipe would be good to have. Made one cut, rotated, and had a friend weld it back up. Perfect fit and the dyno said that it gave the best HP! Pipe science is neat, but sometimes "even a blind hog will find an acorn occasionally".
 

Texasstar

Can't is a four letter dirty word
This all gives me a headache. Sure glad I kept my RD400c stock with stock jetting. Back in the '70s I had a 125 Penton ISDT that we decided to flat track. Although I ordered a commercial low pipe that was touted to be the best, and a friend gave me another low pipe that he had around, I decided that a spare cut from the original high pipe would be good to have. Made one cut, rotated, and had a friend weld it back up. Perfect fit and the dyno said that it gave the best HP! Pipe science is neat, but sometimes "even a blind hog will find an acorn occasionally".

That is awesome! Not the headache but you make an interesting point with your chamber: I wonder how many penton chambers they went through to find the perfect one for your bike.

So the stock Montadero made 21whp with no chamber, 26 whp with an aftermarket 70’s chamber, and then for grins we put on a H2 chamber made for an H2 by Magee and it made 28whp.

This chamber that I built is not built for this engine but for a bike that flows way better than mine but we are making 47 whp with it. So I agree with you that it is not totally pipe science but more trial and error.

I have a piston to prove that you don’t want to fly blind on the dyno with no tach, EGT, 02. When we put the Magee pipe on it after we had it ported. That Magee pipe wanted to Rev to the moon and it did until I stuck the piston. It was fun though. Lo


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John Murray

Been Around the Block
So how do the dimensions of the pipe that's on the bike now compare to numbers of a pipe designed for your cylinder as it is at present?
 

Texasstar

Can't is a four letter dirty word
So how do the dimensions of the pipe that's on the bike now compare to numbers of a pipe designed for your cylinder as it is at present?

That is a very good question and I have sheet metal waiting to make another chamber that Teazer designed for it. We have been making incremental improvements with this chamber and it has a very nice curve so we have stuck with it for now.


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