Two-stroke performance tuning?

[bucketracer]

also, while the final figure of 1.40:1 is correct for a vtwin TZ250, I can't work out how you arrived at these primary compression ratio figures (any of them, just using this as an example)

Primary compression is calculated by dividing the case volume at TDC by the case volume at BDC.......

From the figures you posted, I can't calculate that (for any engine you list)

Can you explain?

Your correct, primary compression is calculated by dividing the case volume at TDC by the case volume at BDC.

For the TZ250 try (313 + 125) / 313 = 1.399 or simply 1.4:1

313cc + 125cc is case volume at TDC and 313cc is case volume at BDC if you read Jennings you will find it explained there.

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[SS90]

Your correct, primary compression is calculated by dividing the case volume at TDC by the case volume at BDC.

For the TZ250 try (313 + 125) / 313 = 1.399 or simply 1.4:1

313cc + 125cc is case volume at TDC and 313cc is case volume at BDC if you read Jennings you will find it explained there.

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Er, yes.

But, "your figures" (correctly) quote the piston displacement of a TZ250 (for one cylinder) at 123.67cc

so,

313cc case volume at BDC
123.67cc piston displacement

would be, (313+123.67)/313= 1.395

so, using "your figures" the primary compression ratio of a TZ250 is 1.395
not 1.40:1 (as you state)

Also, you have not explained why these engines you listed run lower primary compression ratios that 1.5:1, when you are advocating that "the higher the RPM, the closer to 1.5:1"

Based on the SAE paper.

The figures you listed (then later said they where "standard" (unmodified)) are ALL lower than 1.5:1 (in fact, much like I have said, the higher the RPM,the lower the primary compression ratio)....the lowest being a 100cc Kart sport engine (which rev to 15,000), which runs at 1.31:1 (standard), followed by a Honda RS125, which runs at 1.33:1, (and revs to 13,000)


Can you please clarify for us all?

Are you advocating that all these engines should have their primary compression ratios INCREASED?

And, can you quote your "scientific" reason for suddenly increasing the piston displacement of this example from 123.67cc to exactly 125cc?

[speedpro]

I've been sitting back watching numbers being quoted and the variations being suggested for various RPMs. I would have thought the differences could easily be put down to errors when measuring. The latest quibble is over .001cc or .005cc. That is either 1cubic mm or 5 cubic mm. Does anyone actually think that small amount would measurably affect performance or even be measureable?

[Buckets4Me]

[Yow Ling]

I've been sitting back watching numbers being quoted and the variations being suggested for various RPMs. I would have thought the differences could easily be put down to errors when measuring. The latest quibble is over .001cc or .005cc. That is either 1cubic mm or 5 cubic mm. Does anyone actually think that small amount would measurably affect performance or even be measureable?

well obviously SS90 does, if it was a real problem you could slightly over torque the base gasket to correct the fault, or use a little extra goo when assembling the cases so it oozed into the crankcase a bit

[bucketracer]

so, using "your figures" the primary compression ratio of a TZ250 is 1.395
not 1.40:1 (as you state)

Can you please clarify for us all?

And, can you quote your "scientific" reason for suddenly increasing the piston displacement of this example from 123.67cc to exactly 125cc?

Just for you SS90

I was working in the proper enginering manner of using 3 significan figures, which is appropreate for discussing these compression ratios. I thought you would appreciate it.

I thought you would be familiar with this methodolagy, most Technical people are. But I will explain.

Since virtually all numbers used in engineering are based on measurements, they have inherent uncertainty. The number of significant figures implies the magnitude of that uncertainty.

The figure of 123.67 implies an error or uncertainty of measurement of +- 0.005, or 5/1000 of a cc that sort of accuracy is not likely but 124 implies an accuracy of +- 0.5cc which is much more likely.

In its simplist form, results of calculations are reported to 3 significant figures.

So reporting my results to 3 significant figures 1.395 with rounding becomes 1.40 and 123.67 would become 124 as well as the more realistic implied accuracy of measurement using 3 significant figures it also does away with unnessary numerical clutter.

As for using 125 instead of 124, nothing scientific about it. I used 125 to help you quickly see where the value came from.

Rework the calculation with 125 and you will see the primary compression ratio for the TZ still points to 1.40:1

1/1000 did not change the big picture, amatuer people often get lost in small detail and then become confused, keeping it to 3 significant figures helps maintain clarity.

You can read more about significan figures and errors of calculation and measurement here, from the Stanford Engineering Dept no less:-

http://www.stanford.edu/class/engr1n/Precision_E1.pdf

So SS90 you learn't something new today.

Is there anything else you would like help with?

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[SS90]

I've been sitting back watching numbers being quoted and the variations being suggested for various RPMs. I would have thought the differences could easily be put down to errors when measuring. The latest quibble is over .001cc or .005cc. That is either 1cubic mm or 5 cubic mm. Does anyone actually think that small amount would measurably affect performance or even be measureable?

Well put speedpro,

Speedpro, I am still interested in your experience (rather than quoting web pages) on balance factors for high performance 125cc two strokes....... can you write a few lines for us all?

While we are on the subject, it would be now perhaps prudent to suggest that accuracy is quite important when measuring volumes.

The only reason I "quibbled" over Bucketracers figures, is because I seriously doubt his experience.

I have said this before, but does Bucketracer have anything to contribute to threads other than antagonism?

He, so far (in this thread) has advocated that "the higher the RPM, the closer to a primary compression ratio of 1.5:1 (as per the SAE paper from the 60's).....(which, by the way, I was the first person to mention in this series of threads)

Yet, Bucketracer himeself posted figures of Japanese performance two stroke engines, (most high revving) all of which had ratios closer to 1.4:1.........

Now, perhaps now it would be wise to remind you what I am trying to establish with this series of posts.

With modern two strokes, the trend has been (actually, from the Japanese, this has been from the mid eighties) to decrease primary compression (the higher revving, the higher the crankcase volume.

Contrary to the SAe paper from the 60's. (written by the Yamaha engineer Fujio Nagao)

Why is this?

Well, one telling piece of information lies in the SAE paper.

"any deficiency in air delivery due to a crankcase volume too great for a given speed is fairly well compensated for by a properly tuned intake and exhaust pipe"



Ahem....

so....

any deficiency in air delivery due to a crankcase volume too great for a given speed

This, (to me) suggests that the speed (RPM) that your engine is designed for has an effect on what primary compression ratio best suits that engine.

either that, or we could all follow bucketracer advice and increase the primary compression on all the TZ250' and RS125's racing in the world....(maybe we could polish the transfers while we are at it....because, it would really go then!)

Hell, screw it, Bucketracer, send an email to Yamaha, "tell them how it's done"......tell them to increase the primary compression on their two stroke GP bikes........You will be a national hero for sure!


Let's also remember that in the 60's, expansion chamber, ignition and cylinder head design was somewhat "poorer" than it is today.........

That said, Gordon Jennings (by far my favourite writer) seems quite prophetic in his writings...particularly in regards to exhaust design VS primary compression ratios (and the spread of power)......he actually mentions "much lower that a ratio 1.5:1" when looking for a long spread of power opposed to peak power" in his book "THE TWO STROKE TUNERS HANDBOOK"

(in the chapter "Crankcase pumping")

Also, see what he has to say in regards to exhaust design (when looking for this particular "long spread of power"), then take a look at an up to date expansion chamber (I posted some pics on the ESE thread of something I have played a part in the design of 18 months ago, and certainly it is not "cutting edge", but could be considered "modern thinking")

Then, factor in modern approaches to "scavenge patterns", and remember that a good scavenge pattern "improves cylinder filling at high RPM" (particularly when coupled with modern transfer port design. (transfer port dividers for one), along with ignitions that apply retarding (later) ignitions the higer the RPM.... (giving "overrev" in the high RPM's, but also, by passing heat from the combustion on to the exhaust (rather than the cylinder), thereby changing the exhaust frequency (and as we all know, taking advantage of these changes is one of the things a good 2 stroke tuner does.)

While it as old book, it seems to suggest that there are advantages to be made when you build an engine with a particular purpose in mind.

I'M OFF TO THE PUB!

[Pudding]

Neil from the pm's that have been passed around I think Bucketracer is of the opinion your a bit of a tosser who is trying too hard. And if you can't back up what you say with a reliable reference or two he may be right.

In your last post you have continued to try and belittle Bucketracer.

Not many of us have forgotten your rant when Speedpro suggested the crankcase was just a place to store the air/fuel mix. With the larger crankcase volumes being advocated it looks like he is more right than wrong.

If you want serious input you need to stop belittling people who don't agree with you.

You asked for Bucketracers input, which he gave. You couldn’t understand it, and asked for clarification, which he also gave and interesting it was too.

Then to demand an explanation about the accuracy of his calculations because he confined his workings to only the significant figures was foolish, and in demanding an explanation, exposed your lack of knowledge in a way that invited the response you got.

In doing your own calculations you wrote down all the decimal places from your calculator, and thanks to Bucketracer you now know this approch hasn't any significance and is not the accuracy you imagin.

If you follow the link he provided you can educate yourself and be more informed about achiving significant accuracy in measurement and calculation.

You got what you deserved, learn the lesion and be-grateful that he took the time to explain accuracy in engineering measurement and reporting, to you.

Good luck with your efforts in getting your scotter sorted honey. LoL Pud.

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[speedpro]

One thing that has not been mentioned is that what was "high" revving for an engine 20 years ago is pretty tame nowadays, 16,000rpm 600s for a start. It makes comparing (20 year old) apples with (modern) apples a bit difficult.

Anyway . . . . crankshaft balancing. Why more people don't do it I'll never figure out. The first one I did was a Suzuki TS100. Originally it cracked the pipe, frame, number brackets, and anything else on the bike EVERY time I rode it. I talked to lots of people including engineers working for the railways designing locomotives. These guys have serious qualifications but anyway the problem is a fairly simple one - the engine has a piston going up and down. The forces are proportional to piston speed. It also has a crankshaft spinning. The crank forces are proportional to the square of the speed of rotation. The problem therefore is that the forces increase at different rates and can only more of less equal, and counteract, each other at one speed. The TS100 was designed so that they equalled at the speed the cockies used them at - slow. The crankshaft forces ramp up pretty quick so the start point had to be lowered - ie the amount of weight opposite the big end pin had to be reduced if it was going to all even out at the revs I was using. After speaking to some old guy who used to race singles I just pressed a couple of slugs of alloy into a handy hole in each web up next to the pin and gave it a run to see. It was all sweeeeeeeeet.

The Honda crank was a bit different as it had a balance shaft but the principle was the same though I made a lot of measurements and decided on the engineering changes based on my measurements.

[bucketracer]

While we are on the subject, it would be now perhaps prudent to suggest that accuracy is quite important when measuring volumes.

I have said this before, but does Bucketracer have anything to contribute?

Yes "accuracy is quite important" and now thanks to me SS90 you know how to achieve that, and produce a meaningful result by using only the significant figures in your calculations.

A positive contribution, a real gem, thanks Bucketracer.

That’s ok SS90, with practice you will get quite good at it and it will serve you well, as you will look so much more professional, inteligent and educated when you use it and less self tought and desperate to make a name for yourself.

In fact in that one tip about making accurate measurements, calculations and producing meaningfull results I have given you an insite into something far more valuable and usefull than anything you have given us in all of your posts to date.

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[Buckets4Me]

[saxet]

This is worse than watching kids argue.

[TerraRoot]

nah kid's are entertaining arguer's(ur a donkeys hed!). this is more like dick measuring teenagers.

[Buckets4Me]

just because mine is bigger than your's

[F5 Dave]

Well I've been away for a while using my considerable skills & experience where they are better employed (I've been wiping my own arse since I was a little boy so I suppose I am somewhat of an expert, though the hours these little guys keep is a bit of a struggle).

Anyway just to back the truck up a tickle. Skunk, first raise the barrel to the transfer height desired & adjust the exhaust up if required as it's easier to do that transfers. The inlet will need to go down, but be careful as if you make TS/TF barrels inlets too wide down low they will crack the barrel sleeve stub off.

That after market barrel SS posted looks quite good, but the transfer passages look pretty parallel to the bore rather than sweeping.