Texas Two Step Taco

[Texasstar]

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.

Well I learned what MSV was and the flame propagation speed of Methanol. Lol.

I was looking for why we had a cool head reading but melted a piston. So why does Methanol melt a piston at a lower temperature than race fuel? How did we have a cool head reading but 1000 degrees on the EGT?


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[Texasstar]

Wouldn’t your MSV mass squish velocity have to be higher than the flame propagation speed of Methanol?


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[teazer]

... what happens when the flame propagation speed of methanol is greater than your squish velocity? So methanols flame propagation speed is 44 m/s.

They are not related. Squish is what happens as the piston rises forcing (squishing) the air/fuel mixture inwards and creating turbulence. Although the ignition event is initiated in that time, flame doesn't reach that part of the combustion chamber until the piston is traveling downwards and arguably it doesn't burn in that quench are at all.

It's not really like a wave of fresh mixture traveling in is met with a wave of flame traveling in the opposite direction. Look at the burn sequence over time to get that sequence of events a little clearer. Peak pressure is around 15-17 degrees ATDC. MOTA shows the temperature and pressure rise over the cycle and EngMod2T probably does as well.
I wouldn't worry about that.

 

[teazer]

..
I was looking for why we had a cool head reading but melted a piston. So why does Methanol melt a piston at a lower temperature than race fuel? How did we have a cool head reading but 1000 degrees on the EGT?

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Uh oh. Did you melt a piston recently?

Good question but presupposes that high EGT equates to high CHT. Detonation is simply a circumstance where the combustion temperature and pressure are high enough to promote detonation which is basically an explosion. EGT and CGT are not directly variable.

Factors including too much spark advance or lean (hot burning) mixture can easily generate the right conditions for detonation but don't necessarily generate high EGT. They say that you need to watch CHT and EGT and if EGT drops and CHT spikes, it is starting to detonate.

We tend to think that conditions that cause high EGT cause high CHT but life is more complicated than that. Simple example is a rich mixture that burns cool in the combustion chamber but continues to burn in the pipe causing low CHT and high EGT. Retard ignition timing at the top end to get a similar effect. Changes in pipe temperature will often be much larger than CHT temperature changes.

Too much spark advance will often create detonation but not necessarily a hotter pipe. At least not proportionately.

An example of this issue is disproportionality was earlier this week, we had a bike on the dyno where the engine temps came up OK but the pipe didn't get as hot as predicted or as needed to make peak power. Pipes were too hot to touch of course but not nearly hot enough to be working as designed.

And methanol has a very high latent heat of vaporization, so it cools the intake and allows for a more dense mixture with more oxygen to burn which also leans it out a little. Two strokes don't seem to respond exactly the same way that 4 strokes do to a fuel change.

Aren't you glad you are working with a nice simple two stroke?

 

[Texasstar]

Uh oh. Did you melt a piston recently?

Good question but presupposes that high EGT equates to high CHT. Detonation is simply a circumstance where the combustion temperature and pressure are high enough to promote detonation which is basically an explosion. EGT and CGT are not directly variable.

Factors including too much spark advance or lean (hot burning) mixture can easily generate the right conditions for detonation but don't necessarily generate high EGT. They say that you need to watch CHT and EGT and if EGT drops and CHT spikes, it is starting to detonate.

We tend to think that conditions that cause high EGT cause high CHT but life is more complicated than that. Simple example is a rich mixture that burns cool in the combustion chamber but continues to burn in the pipe causing low CHT and high EGT. Retard ignition timing at the top end to get a similar effect. Changes in pipe temperature will often be much larger than CHT temperature changes.

Too much spark advance will often create detonation but not necessarily a hotter pipe. At least not proportionately.

An example of this issue is disproportionality was earlier this week, we had a bike on the dyno where the engine temps came up OK but the pipe didn't get as hot as predicted or as needed to make peak power. Pipes were too hot to touch of course but not nearly hot enough to be working as designed.

And methanol has a very high latent heat of vaporization, so it cools the intake and allows for a more dense mixture with more oxygen to burn which also leans it out a little. Two strokes don't seem to respond exactly the same way that 4 strokes do to a fuel change.

Aren't you glad you are working with a nice simple two stroke?

“Did you melt a piston recently?”

A close friend did but only one of the three pistons but many spark plug electrodes.


I have been watching the methanol whoosh bottles on YouTube and feel like I have missed out on something in my childhood. I can picture how unspent methane could ignite in the pipe and raise EGT. I can also understand how a rich mixture cools the head. I have seen it on the datalogger.

Did you see the magnetized reluctor? It was causing major hiccups in the datalogger and magnified the problems with methanol over other race fuels.

He had this problem and still won the man cup. Amazing.

But methanol is a cantankerous mistress.


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[John Murray]

Spark plug cooling is important, especially with methanol which is easily preignited. Overhot plug will preignite which leads to det which leads to melted piston. Surface gap plug avoids all this as does a head that cools the plug.

Piston top will always run hotter than the head; it's exposed to combustion just like the head but also gets washed with hot exhaust gas while the port is open. As Teazer said low CHT and high EGT is indicator of late ignition. Keep in mind CHT is generally too slow to react to be of any use.

 

[Texasstar]

Spark plug cooling is important, especially with methanol which is easily preignited. Overhot plug will preignite which leads to det which leads to melted piston. Surface gap plug avoids all this as does a head that cools the plug.

Piston top will always run hotter than the head; it's exposed to combustion just like the head but also gets washed with hot exhaust gas while the port is open.

Going to try this plug NGK bur9eq for methanol

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[John Murray]

No no, thats not really a surface gap. Go for a BUE or similar. You'll only need one, they last forever.

 

[Texasstar]

No no, thats not really a surface gap. Go for a BUE or similar. You'll only need one, they last forever.

I have a bunch of resistor caps for the terminal stud but none for the solid terminal cap so back to Summit we go.


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[John Murray]

That's better! It won't make any more power but you can be absolutely positive it won't cause preignition. They do require a fair bit of energy to fire consistently; an inductive ignition won't do it but any decent CDI will handle it easily.

 

[Texasstar]

That's better! It won't make any more power but you can be absolutely positive it won't cause preignition. They do require a fair bit of energy to fire consistently; an inductive ignition won't do it but any decent CDI will handle it easily.

That's better! It won't make any more power but you can be absolutely positive it won't cause preignition. They do require a fair bit of energy to fire consistently; an inductive ignition won't do it but any decent CDI will handle it easily.

Like this?

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[John Murray]

Like this? View attachment 235732


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Ouch

 

[pidjones]

All that you have to do to turn a resistor cap into a non-resistor cap is replace the resistor with a piece of copper wire or tubing. Some NGK caps just screw apart, others require judicious pulling of the center electrode pin that goes into the wire to access it. Since so many non-resistor plugs are disappearing from the market (often replaced by Chinese knock-offs), it is what I am doing to my caps for all the bikes.

 

[Texasstar]

All that you have to do to turn a resistor cap into a non-resistor cap is replace the resistor with a piece of copper wire or tubing. Some NGK caps just screw apart, others require judicious pulling of the center electrode pin that goes into the wire to access it. Since so many non-resistor plugs are disappearing from the market (often replaced by Chinese knock-offs), it is what I am doing to my caps for all the bikes.

Sorry I wasn’t clear. This NGK BUE spark plug is a non resistor plug and has a solid terminal on top and caught me of guard when I tried to unscrew it. All of my resistor caps are the screw type for the screw terminal stud. Our Zeeltronic electronic ignition says to run a resistor plug. So do my neighbors When I was a kid it drove me nuts when my radio got interference and I never knew why. lol. So I am running a non resistor plug with a resistor cap but the cap has to fit a solid terminal stud and NGK only makes one.

NGK 8744 LB05EZ Spark Plug Cap

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[Texasstar]

They are not related. Squish is what happens as the piston rises forcing (squishing) the air/fuel mixture inwards and creating turbulence. Although the ignition event is initiated in that time, flame doesn't reach that part of the combustion chamber until the piston is traveling downwards and arguably it doesn't burn in that quench are at all.

It's not really like a wave of fresh mixture traveling in is met with a wave of flame traveling in the opposite direction. Look at the burn sequence over time to get that sequence of events a little clearer. Peak pressure is around 15-17 degrees ATDC. MOTA shows the temperature and pressure rise over the cycle and EngMod2T probably does as well.
I wouldn't worry about that.

What do you think of this picture (the same piston that had the pre detonation). It melted at the edge. So we have pre detonation on the top center and we melt at the edge. 1000 degrees Egt. Chamber Head was cool. Reluctor was somehow magnetized. Data recorder said the ignition was hiccuping. We were burning electrodes of spark plugs on two previous runs on different cylinders. Piston to wall is .0035” NGK BR10ES plug. Methanol.

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[Texasstar]

Will the Zeeltronic electronic programable ignition fire the NGK BUE surface gap plug?

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[teazer]

I see a tiny amount of detonation in the center of the piston crown but little to none around the edge of the crown. But the top land has seized very significantly, so we know that the piston crown temperature was too high. BR10 plug is fairly cold, but maybe still too hot for the application. I used to run D12s on methanol in a Honda CB77 back in the day and still had fuel boiling in the float bowls if I wasn't careful.

What does the plug in that cylinder look like? Is it showing signs of overheating and what does the underside of the crown look like? Just as another data point, top fuel dragsters are known for the ability to melt electrodes which is partly a function of high power magnetos and partly combustion temperatures.

In this case it looks like excessive combustion temperature and maybe just on the edge of detonation.

With the lack of further data, it suggests that the plug got too hot and that caused pre-ignition (when the plug tip lights the fire before the ignition is timed to) which led to excessive combustion temperatures.

I would try a surface disharge plug to see if that brings down combustion temperatures by reducing pre-ignition.

I am guessing that you meant the piston to wall clearance is 3.5 thou not 35 thou...needs another zero..

 

[Texasstar]

I see a tiny amount of detonation in the center of the piston crown but little to none around the edge of the crown. But the top land has seized very significantly, so we know that the piston crown temperature was too high. BR10 plug is fairly cold, but maybe still too hot for the application. I used to run D12s on methanol in a Honda CB77 back in the day and still had fuel boiling in the float bowls if I wasn't careful.

What does the plug in that cylinder look like? Is it showing signs of overheating and what does the underside of the crown look like? Just as another data point, top fuel dragsters are known for the ability to melt electrodes which is partly a function of high power magnetos and partly combustion temperatures.

In this case it looks like excessive combustion temperature and maybe just on the edge of detonation.

With the lack of further data, it suggests that the plug got too hot and that caused pre-ignition (when the plug tip lights the fire before the ignition is timed to) which led to excessive combustion temperatures.

I would try a surface disharge plug to see if that brings down combustion temperatures by reducing pre-ignition.

I am guessing that you meant the piston to wall clearance is 3.5 thou not 35 thou...needs another zero..

“I guess you meant .0035” yes and edit
Is complete.

They say a wise man learns from others and this is why we are going to the NGK BUE surface gap plug in the BULTACO. Our piston to wall in the Taco is .0075” so we are good for Methanol.


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[John Murray]

I think once the ignition is sorted you'll find it's still a little rich. The piston issues look to have been initiated by preignition which was a result of the ignition gremlin, not the plug overheating. I think the plug damage was a result of the preignition, not the other way around. Piston crowns normally run a lot hotter on the exhaust side from the wash they get when the port opens so damage is usually more severe there. I wouldn't pay any attention to the CHT figures, there's too much latency for them to have any relevance.

Two-strokes on methanol make best power when they're getting close to being dangerously lean. The one pictured here was caused by a lean condition (due to a stupid mistake on my part) yet it ran over 150mph before it seized, so it was obviously still making some power.

 

[Texasstar]

I think once the ignition is sorted you'll find it's still a little rich. The piston issues look to have been initiated by preignition which was a result of the ignition gremlin, not the plug overheating. I think the plug damage was a result of the preignition, not the other way around. Piston crowns normally run a lot hotter on the exhaust side from the wash they get when the port opens so damage is usually more severe there. I wouldn't pay any attention to the CHT figures, there's too much latency for them to have any relevance.

Two-strokes on methanol make best power when they're getting close to being dangerously lean. The one pictured here was caused by a lean condition (due to a stupid mistake on my part) yet it ran over 150mph before it seized, so it was obviously still making some power.


View attachment 235742

What was the mistake?


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