Texas Two Step Taco

View attachment 230588
So Frits Overmars commented on this picture of an expandable two stroke chamber on Facebook but it and it needs to be saved and archived on a forum so I am adding it here. The following is his comment: I've posted the following text before, but judging by the above reactions not many have read it, so here it is once more:

There are several options in lengthening an exhaust pipe. You can move the end cone, or you can lengthen the header, like a trombone.
The gas pressure generates a force that is proportional to the cross section area of the moving part and proportional to the pressure difference at either side of that area. For a moving end cone this force can be up to 4 times larger than for a sliding header. That is one reason to go for the trombone system rather than the moving cone system.

The second reason: sealing. The circumferential gap that has to be sealed, is three times shorter for the trombone system. That means three times less leakage and three times less friction.

The third reason: say you wish to lengthen the total length of the pipe by 10 %. If you do it by moving the end cone, you will also enlarge the pipe volume by a little over 10 %.
But in a good pipe configuration the header length is about 1/3 of total pipe length, so in the trombone system, lengthening the pipe by 10 % will be done by lengthening the header by about 30 %. That gives a far greater variation in the pipe's Helmholtz frequency than a 10 % volume change.

It is true that the length percentages of all pipe components should be in a rather fixed relation to each other. Varying the lengths of all components by the same percentage would be the theoretical optimum, but that is not feasible.
Lengthening the belly will disturb the optimum relations, as will lengthening the header. So the pipe in its lengthened version will not be the optimum for the low resonance rpm dictated by the length. But it will be a hell of a lot better than using an exhaust power valve that spoils the 180° effective exhaust timing, necessary for true resonance.
And a pipe shortened beyond its optimum may not show the optimum length relations between its components either, but it will be a lot more effective in overrev than artificially raising the exhaust gas temperature by retarding the ignition, or by weakening the mixture strenght through closing a power jet, which has the disadvantage that not all inhaled air is used for combustion.




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I've seen some engines using variations of the variable length chamber - mainly karts and outboards. Which kinda makes sense with applications like these that lack a sufficient spread of gear ratios, or any at all. It's not something I'd want to do unless there were no better alternatives.

It reminds me of something I discovered by accident a few years ago. I was running the 370 Pursang engine in the Metralla on the street and was trying to quieten it down - it was obnoxiously loud with a garden variety packed silencer. I'd made quite a few different types of silencers and was testing them to see if I could find something better. Anyhow, one that I built - a baffled design - was a total failure at noise reduction, but what got my interest was the effect it had on the performance below the powerband. The engine was very noticeably stronger and smoother below the powerband, but a bit weaker when on the pipe. It turned out that the baffled muffler flowed better than the packed ones - I later discovered that the packed mufflers can flow very poorly with the pulsating flow, much worse than the straight-through design might suggest. The reduction in backpressure slightly reduced the top end but improved the bottom end by impeding the pipes ability to function out-of-phase when below the powerband. I was never able to make the baffled muffler both quiet and free flowing but it made me wonder whether a dual outlet, dual muffler pipe might work. At speeds below the powerband both would be open, and the lack of gas density in the pipe would make it relatively ineffective at reducing power, while once on the pipe the larger primary outlet would close off, allowing full power. I've haven't tried it though..
 
Morning guys. Sorry I haven’t posted in a while. Didn’t buy any motorcycle parts is 2021. Won’t go into it but it is has been a long hard road but I am coming out of my cave and feeling better.

Fired the Taco up only once in 2021 and one more time last night. I need a goal. Let’s see if we can break the 50 whp on Alky! Hope y’all are well.


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Page 1 of John Murray’s tutorial on methanol.

Running Methanol with Two-Stroke Engines
What follows is based on what I've learned from running methanol with two strokes, and it's turned out to be much longer than I expected so my apologies for the wall of text. It's probably more detailed than many want or need, and that's because it's aimed at those who want to extract the absolute most from their engine. For those that don't want to go to so much trouble don't stress, just run 'er rich and ignore all this. But I hope those of you who are obsessives (like myself) find something of interest.
The use of alky with four stroke engines is well understood and well documented but it's a little harder to find information on dealing with alky's quirks when used in a two stroke. Methanol has two very well known advantages: one is a boost to torque right across the rpm range and the other is a very significant cooling effect. Various percentages have been quoted over the years for the power increase (15% seems a common one) but it really depends on what you're comparing it to - whether it's low grade pump fuel, Avgas, oxygenated race fuel etc. Realistically the increase over a good race fuel can be quite small, and if you don't need the extra cooling it may well be simpler to stick with the race petrol.
Methanol can sometimes be a pain in the arse and many have reverted back to petrol after trying alky, but then again many use it very successfully. It has some special requirements though, so if you're new to it you should be well prepared in order to get consistent performance and avoid burnt pistons. One thing I really like about it is the absence of petrol stench - I can work on the fuel system in my downstairs garage without making the house stink like a service station and I can carry the bike in my van without it making it smell like a lawnmower..
Most of you will already know that an engine will need a much larger volume of alky than it did petrol - around 2.3 times as much. Another way of estimating fuel flow requirements is to use this rough rule of thumb - (flow in ml/min)=(peak hp x 16). When you check this it's important to make the test as close to real operating conditions as possible, so collect the fuel from the drain of the float bowl so the needle and seat are part of the flow path. Don't fill the tank to the brim, just to the lowest level you expect to see under racing conditions. Test long enough to get a couple of litres from the tank to confirm that the tank vent is big enough, and try to run the vent line to a high pressure zone if the speeds are going to be high.
Be aware that fibreglass tanks and alcohols don't get along. It also doesn't like Viton (sometimes used on the tip of float needles and some shaft seals) but is fine with silicone sealants. The plain old black nitrile o rings and seals will last a reasonably long time. Keep in mind that the tip of the slide needle in the needle jet may become the ultimate flow limiter, and when this happens increasing the mainjet beyond a certain size has little effect. What most people do in this case is hand-grind the end of the needle down to give control back to the mainjet, or alternatively use a different needle/NJ combo that gives a larger annulus at WOT while still providing a usable mid- throttle mixture. Resist the temptation to simply cut the end of the needle off unless you hanker for the kind of excitement you get when the throttle you just opened wide coming onto the straight refuses to shut again.


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Page 2 John Murray’s tutorial

Compression ratio is one of those areas where the two-stroke has some special requirements. Methanol is quite resistant to detonation so in the four stroke world it's very common to run a CR of 16:1 or more. This works well in four-strokes but two-strokes are a bit more fussy. And although there are some two-stroke snowmobile racers running up to 17:1 they have completely different operating conditions to us. We have to balance the gains from improved thermal efficiency against the losses from a lower EGT and the resulting longer virtual pipe length. It's long been said that too much compression "kills the top end" of a two stroke, and the reason for this is that the high CR lowers the EGT. So the pipe that previously worked perfectly is now too long and is signing off much earlier. I tend to think of the pipe as being the boss of the whole show and to me keeping it happy is the prime priority. What I'm trying to say is increasing the CR is a game of diminishing returns with a two stroke so be conservative, keep the pipe hot and make power. I've had good results from ratios between 15 and 16, and decreases in power after dropping to 14:1. Remember that as you raise the CR you need to be increasingly on the ball as far as the tune goes, and that your "safety zone" will be shrinking. For an air cooled engine I'd be starting off at around 14:1.
Methanol is well known for its wide usable mixture range - it's regarded as being very tolerant of over-rich mixtures and will still make good power while running very rich. But this applies more to a four-stroke than a two. A four stroke will lose very little power even if it's so rich it's spraying raw fuel out the exhaust. But again, we have to keep the pipe happy with a two-stroke. Too much fuel cools the EGT so torque and rpms both drop. With a two-stroke the maximum torque mixture isn't a great deal richer than the one that burns holes in the piston.
Tuning the WOT mixture can actually be challenging with methanol because it doesn't show the same landmarks petrol does and you can easily get into a situation where you don't really know where you are. So the wide useable A/F range that should make tuning easier can in fact make it harder, and if you're the type of person who can't live with the slightest imperfection in tune this vagueness can be frustrating. Petrol on the other hand is much more clear-cut with a two-stroke - if you're too rich the engine will start to stutter in an unmistakeable show of the too-rich flag. Lean it back from the onset of stutter much more than 5-10% and you know you're heading towards piston toasties. The sweet spot might be narrow with petrol but at least it's easy to find and the markers are obvious. I wish I could give you a foolproof method for quickly finding the optimum setting for methanol but so far I haven't found one that works better than leaning it out til the piston starts to overheat then fattening it back up 10%. Or just be guided by the torque readings keeping it just a little on the rich side of the peak. Not very scientific I know (and it might cost you a piston or two) but it will get you very close to the maximum possible torque and power and it provides a definitive reference point for the future.
Going leaner than the melty-piston zone the torque drops off significantly, and even leaner than that it just won't fire. I don't fully trust instrumentation for guidance here, it only gives an opinion; the only indicators that are truly dependable are the torque gauge and the piston crown. Bottom line: yes, alky does have a very wide range of A/Fs over which it will run cleanly. But with a two stroke, the range that is both safe and will produce maximum power is much narrower.


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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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Page 4

How do we deal with this? For a start you can try to make your testing and jetting procedure closely duplicate the temps you're likely to see on the track. Or you might work out a warmup routine that has the engine up to temp before it even leaves the start line (not forgetting the puddling issue mentioned earlier). Of course you can just do what most people do - simply jet the thing good and rich and live with the fact that you'll be leaving some horsepower on the table. Another option might be to have a mixture control that you can adjust on the fly, guided by the EGT gauge (and this works well with LSR racing). At any rate be aware of the temp vs A/F curve - it can be something that frustrates tuners with unexpected inconsistencies.
Ignition timing is critical with chamber-equipped 2 strokes burning petrol and it's the same with methanol. Alky generally likes a little more advance right across the rpm range, about 3 or 4 degrees more would be a good starting point. This might seem odd given methanol's faster flame speed, but the faster pressure rise produced by earlier ignition gives the heat and pressure that methanol loves. A retarded spark reduces the output very drastically, but given the higher octane level of alky you may get away with a little too much advance without problems (but be careful)!
One of the more difficult and critical tuning tasks with forms of racing involving high speeds and tall gearing is setting the timing within the rpms ranging from 1000 or below the start of the powerband up to the beginning of the powerband. Timing errors in this range can mean the difference between outstanding performance and not even being able to get onto the pipe in high gear. So if your form of racing is like this give the just-under-the-powerband rev range as much attention as the powerband rpm range. As with petrol, methanol will want a relatively large amount of advance below the powerband, rapidly stepping back as the engine gets on the pipe and then further tapering off at the very top end. Obviously some sort of programmable ignition is ideal, but it isn't always essential. Most magnetically triggered ignitions will have some amount of retard with rpms built in, and often this inherent retard curve is close enough for practical purposes. With dirt bikes a wide powerband is normally more important than sheer horsepower, and with these styles of engines/pipes an ignition with 5 or 6 degrees of inherent retard is often sufficient. But if you have good traction, high speeds and tall gears you may need something with more programmability.
Spark plug heat range doesn't need to be much different to petrol, the improved cooling balancing out the increased power. But keep in mind that although alky is more resistant to detonation it can be quite prone to preignition. For this reason projected nose plugs (with their long ground straps) are out and you should start with something coldish like an 8. Back in the old days when ignitions were weak it wasn't uncommon for engines to be started on a hot plug (to reduce the risk of fouling) and then once warm the cold racing plugs would be fitted. With modern ignition boxes we don't have to put up with this nonsense and can run a super cold plug anytime. A sufficiently strong ignition can fire a plug no matter how wet or oily it is. I like surface gap plugs for their absence of any potential preignition sources, but they do require a fairly high energy ignition. It's possible to run alky with a mag or an old school CD ignition but a fouled plug with these can and probably will stop the engine. With methanol the plug nose will very often be wet, and be aware that many "high performance" aftermarket ignitions are anything but high performance.


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Page 5

Lubrication is simple - use castor oil. There are some esters that are alcohol compatible but they aren't even in the race. There are some common myths about methanol and castor oil, one being that the oil will drop out of the fuel over time, especially with low temperatures. I have three glass jars of premix that have been on a shelf in my garage for several years. One is petrol/Shell M, one is methanol/Shell M and one is methanol/Putoline Castor R. None have ever shown any signs of separating even after some nights of freezing temperatures. I have no problems using premix that has been stored for weeks and have never seen any sign of dropout with the any castor I've used.
Be aware that some oils that claim to be castor blends or even pure castor seem to contain very little castor in reality. I've tried a few but have gone back to Shell M despite it's higher price. Putoline in particular – supposedly pure castor with additives – performed very poorly. Another myth is that castor is dirty, filthy stuff that will make a mess of your engine and stick the rings in the grooves in no time. This is only true if castor is used inappropriately in low powered engines. They do get gummy when used like that but in a racing application where the power, rpms and temperature are all consistently high they run quite cleanly.
As far as the mixing ratio goes 20:1 seems to work well for me, though this might seem like a lot of oil given the fuel flow rate. Still, at 20:1 it doesn't smoke much once it warms up and there's always sufficient oil present when the engine is opened up. It's probably not a bad place to start from.
Unfortunately you can't just turn off the fuel tap and park the bike once you've finished riding, like you can with petrol. Alky is very aggressively hydroscopic and pulls moisture from the air at a surprising rate. A bike that's been simply turned off and parked might have half a cup of water sitting in the crankcase after just a couple of days and you can imagine what this does to bearings and iron liners. If the engine will only be shut down overnight it's enough to shut off the fuel and run the engine until it stops. But for periods any longer than that you should run the engine dry and then run it for a short while on a petrol premix. It'll run extremely rich on the petrol but that's OK, it'll help get rid of any traces of alky. Petrol left in the fuel bowl also helps prevent corrosion and the buildup of the white crap that methanol and water produces. If it's going to be parked for any length of time you should drain and flush the entire fuel system including the carb, then motor the engine while spraying a corrosion preventative (eg. Lanox) into the intake.
That'll do for now. Like I said at the beginning the casual racer could ignore most of this - just make sure you have a good fuel supply then bump up the timing a little, run it rich and enjoy. But if you want to get the maximum possible power you'll have to work at it and be prepared to burn a couple of pistons.
John Murray 2021


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Another thing to remember about methanol is that it is poisonous and will absorb through the skin. Not like arsenic, but still not something that you want long-term exposure to. We used it to clean and dehydrate parts at work, wore nitrile gloves and used it in a fume hood.
 
Welcome back Tex. Hope you're doin' OK.

And don't forget that methanol burns with a colorless flame. Yep. That's right it burns and all you can see if you're lucky is a heat haze.

We used to run methanol in fiberglass tanks, but by the end of a race weekend, they felt quite soft and sponge like.

I wish I could find the article I penned years ago about using Methanol. In theory IIRC the power increase is 17% which is the net effect of lower energy density and lower A:F ratio.
 
Another thing to remember about methanol is that it is poisonous and will absorb through the skin. Not like arsenic, but still not something that you want long-term exposure to. We used it to clean and dehydrate parts at work, wore nitrile gloves and used it in a fume hood.
Yep, if you don't have a fume hood, goggles too because it goes in through the eye pores. I have access to 95% Ethanol from a local distiller that is a lot safer to handle and it burns a nice blue flame in my ethanol fireplace. Wonder if I can make the bike run in it as I have 12 Gallons in the garage right now.
 
Yep, if you don't have a fume hood, goggles too because it goes in through the eye pores. I have access to 95% Ethanol from a local distiller that is a lot safer to handle and it burns a nice blue flame in my ethanol fireplace. Wonder if I can make the bike run in it as I have 12 Gallons in the garage right now.

Good thing we added another door
IMG_9797.jpg



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Welcome back Tex. Hope you're doin' OK.

And don't forget that methanol burns with a colorless flame. Yep. That's right it burns and all you can see if you're lucky is a heat haze.

We used to run methanol in fiberglass tanks, but by the end of a race weekend, they felt quite soft and sponge like.

I wish I could find the article I penned years ago about using Methanol. In theory IIRC the power increase is 17% which is the net effect of lower energy density and lower A:F ratio.

Thanks Teazer. Hope you find the article. Will keep an eye out for the haze...


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So here is a Teazer on what carby we are going to use for methanol


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Using John’s calculations with a target of 60 whp. I wonder if this calculation should use BHP or WHP?

Let’s say it’s WHP.

16 x 60 whp = 960 ml per minute of Methanol


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The rule of thumb is deliberately safe/rich. I think an increase of 17% is pretty optimistic, but again it'd be safe.
 
The rule of thumb is deliberately safe/rich. I think an increase of 17% is pretty optimistic, but again it'd be safe.

I watched a friend run a 6.7 in the 1/8th on alky with only one cylinder the other night because the other two cylinders were fat and wouldn’t fire. This was with a 1.7 60’ time. I was reading your tutorial while at the track and was experiencing it in real time. We learned you can’t let the bike sit and idle because it floods the case because there is no load on the engine. Teazer has mentioned an anti knock detection device...I am wondering if that is a good investment.


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