ESE's works engine tuner

[peewee]

didnt wob say one time the piston skirts fell off after using amsoil

[Grumph]

Here is me thinking it would be thermodynamics or physics.

Nah, the only limitation is money....The laws of thermodynamics and physics can both be at least bent a little given sufficient money....

[F5 Dave]

I hereby recant my sins and only prescribe apricot oil.

At 70:1

Continue.

[Frits Overmars]

Angle areas are not the limiting factor in a two stroke. Friction is.

Care to enlighten us Jonny?

[Haufen]

That's right. Piston velocity is not a limiting factor in a two-stroke; angle.areas are. And oversquare, or short-stroke engines are at a disadvantage.
Imagine an engine with a bore and stroke of 40 x 40 mm. Make the bore twice as big, and all port widths can be doubled. But doubling the bore means that the stroke has to be made 4 times smaller in order to keep the cylinder capacity unchanged. And dividing the stroke by 4 means that for a given timing all ports will become 4 times lower. So all port areas and angle.areas will be halved! That low piston velocity won't do you any good...

Would you be so kind and share your thoughts regarding long-stroke racing engines with us, Frits? Wobbly already did, and I would be very interested to hear your opinion on this subject, too.

Personally, I think a longer stroke RSA would have had the potential for even more hp, and the increased piston velocity would not have been much of an issue.

[jonny quest]

How much angle area is lost with a longer rod? How much power is gained by less side loading of longer rod?

[Frits Overmars]

Would you be so kind and share your thoughts regarding long-stroke racing engines with us, Frits? Wobbly already did, and I would be very interested to hear your opinion on this subject, too. Personally, I think a longer stroke RSA would have had the potential for even more hp, and the increased piston velocity would not have been much of an issue.

Increased piston velocity would have been no issue at all in the RSA; the crankshaft was bulletproof.
There have been experiments with a smaller bore for the RSA, but only with 52,8 mm instead of 54 mm, because the special small-bore piston had to be made out of a standard forging. Making a special piston forging was considered too expensive (in a period where millions of euros were wasted on development of the three-cylinder foul-stroke MotoGP engine).
Changing something in a highly-developed engine will almost invariably lead to a power loss and it will take quite some development to establish whether the change was worthwhile, but Jan Thiel did not get the opportunity to carry out this small-bore development because it was aborted by the Great Leader when there was no immediate improvement.
I expect that at the very least the axial scavenging angles would have to be adapted to the altered bore/stroke-ratio.

[seattle smitty]

With the port contours and entry angles optimized for the "square" engine, wouldn't you expect to have to revisit those contours and angles to get the most out of the smaller bore? In other words, maybe the engine that is somewhat undersquare would ultimately work even better than the square version but only IF there was the same painstaking development program specifically for it. This is something that makes me wonder whether all the folks who are trying their best to reproduce Mr. Thiel's ports in engines with different specifications might not quite get the results they hope for. To reproduce his results in a non-identical engine, I'd think unless you got real lucky you'd have to reproduce his development program: long hard labor by someone who knows what he's doing. No free lunch, alas.

[Frits Overmars]

Angle areas are not the limiting factor in a two stroke. Friction is.

Care to enlighten us Jonny?

How much angle area is lost with a longer rod? How much power is gained by less side loading of longer rod?

I was really hoping you'd give us an answer Jonny, instead of reverberating a question that I already answered: http://www.kiwibiker.co.nz/forums/sh...post1130874798

[jonny quest]

I mentioned that rod length because you gave up angle area. If a 2 stroke is angle area limited. .. why give any of it up?

Piston skirt friction is in the area of 20 to 30%, around 10% for rings, around 15% for crank bearings.

Will you gain 50% more power with ultimate ports?

[wobbly]

I think we have two streams of thought going concurrently.
There would appear to be NO reason at all to not run a long rod, well above 2:1 ratio, as its been proven that the tradeoffs between friction/case vol/angle area are
biased in favour of the longer version.
Aprilia did it for years, then Honda decided they agreed and went longer on the final champ winning RS250 going from 105 to 109.
The other issue is that there "could" possibly be an argument that a slightly undersquare engine might ultimately prove to be superior to the current accepted practice
of "square is best".
But this argument really is moot for anyone on this forum, as no one has the time/money/need/or knowledge to prove this one way or the other in anything like the detail needed
to make a water tight conclusion.
First you would have to replicate ( or improve upon ) the result gained by Jan Thiel with a square engine ( yea dream on ) to prove that you actually knew what you were doing
and then go into the same micro details of the small bore engine with the resources that 100 R&D employees at Aprilia had.
Only then could anyone say with any conviction that the theoretical advantage of a slightly improved angle area Vs the reduced rpm capability of the longer stroke was a trade off worth pursuing.
As it stands it HAS been conclusively proven that with our current knowledge " square is best ".
Proving otherwise is simply not going to happen in our lifetime, so discussing it in any detail thinking this might have some relevance to buckets or any other minor racing class I think is a waste of energy.
Ive just spent 3 days on the dyno testing air filters and mufflers,and gained 2.4Hp in 40, changing nothing else - thats not a waste of energy, as no theoretical musing with no useful end use is involved.

[twotempi]

"There would appear to be NO reason at all to not run a long rod, well above 2:1 ratio"


Is there a reasonable upper limit for this.

A Yamaha TA125 has a stroke of 43mm and a rod of 98mm which gives a ratio of 2.28 although I think this was more of a dimensional "make things fit" decision rather than a technical design one.

What other engines have a high rod to stroke ratio ??

[breezy]

"There would appear to be NO reason at all to not run a long rod, well above 2:1 ratio"


Is there a reasonable upper limit for this.

A Yamaha TA125 has a stroke of 43mm and a rod of 98mm which gives a ratio of 2.28 although I think this was more of a dimensional "make things fit" decision rather than a technical design one.

What other engines have a high rod to stroke ratio ??

the mighty" BSA BANTAM 125" 58mm stroke 125mm conrod

[breezy]

Here is me thinking it would be thermodynamics or physics.

husaberg, have you come across any 2 stroke engine designs that in corporate reed controlled transfer ports, with the transfer ports high in the cylinder above the exhaust port/s. transfer ports connected directly to the inlet and not through a crankcase?

[husaberg]

the mighty" BSA BANTAM 125" 58mm stroke 125mm conrod

The villiers as well but to be fair the poms needed such long rods because the piston skirts were so long and the piston pins so low.
The cooking MZ's had very long rods as well.

husaberg, have you come across any 2 stroke engine designs that in corporate reed controlled transfer ports, with the transfer ports high in the cylinder above the exhaust port/s. transfer ports connected directly to the inlet and not through a crankcase?

I may have yes, but I am not at liberty to divulge where.