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Lets talk 2T's

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I was sent this link by someone the other day...

http://www.pit-lane.biz/t117-gp125-caracte...ues-aprilia-rsa

If you are into two strokes do yourself a favour and read it!! It is probably the single best 2T read i have found yet... it starts in French but if you persist through to about page 10 or 15 it pretty much becomes English (or read it through Google Translations and be prepared to laugh)

Reading this excellent thread has got me thinking about a lot of things...

Cylinder heads...

I am developing my own thoughts but it was interesting to hear Jan Thiels observations that a sharp inner edge to the squish band was best for combustion. Most tuners insist that a hemisphere is best but i am not so sure. I can definitely see the benefits of angular sections rather than hemispheres and radii. What are your thoughts on cylinder head design?

Ports...

I keep swinging between boost port opening first or boost port opening last... there is some interesting info in that thread that has changed my mind again. I have been given strong advice by tuners who swear by one or the other, but im sitting on the fence a little now... i need more thinking time

Thoughts??

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Depending on the exhaust port type largely dictates the "ideal" transfer openings...

Most t ports ex side trans open first is best.

Most 3 port the ex side trans opens later/second...

Thinking about what you can do with the aux ex port if the transfer is lower...and your limited in exhaust area previously...

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That's an excellent link, Dave! I ran out of steam today around page 30, but not before seeing tons of beautiful alloy and reading some really interesting stuff. I'll start back where I left off tonight.

It was great to see that the rotary valve is not dead (the Aprilia rear-mounted valve seems ideal to me), and also to hear that I'm not the only one who thinks reeds have strange throttle-response characteristics.

My own thought on 2t combustion chambers is that they should be designed for better scavenging rather than improved turbulence/ignition characteristics, but perhaps that's just not realistic with modern cranking pressures.

So far engmod hasn't shown any discernable power differences with variations in relative TP/BP timings that couldn't be explained by variations in port flows alone (obviously variations in overall TP/BP timing is a different story).

Ray

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Depending on the exhaust port type largely dictates the "ideal" transfer openings...

Most t ports ex side trans open first is best.

Most 3 port the ex side trans opens later/second...

Thinking about what you can do with the aux ex port if the transfer is lower...and your limited in exhaust area previously...

What are your reasonings for this? I can see an equal number of scenarios when each would work and also when the opposite would be true. Hasve you come to these conclusions from testing or are they more your theories from knowledge you have gained?

I can see that in the example you mention if you need more exh area in a 3 port exh setup then openign the main transfers last allows you to drop the sub exh floor to gain more mean area, but what if there was already enough area... you would then design primarily for scavenging?? And if you did this i think there is more weight to the idea of opening the transfers in the order main/aux/boost in order to scavenge with minimal mixing of fresh/burnt charge... i feel that using the opposite order boost/aux/main will lead to more mixing of the gasses in most cases (even though this is the current setup on my CR250... about to be changed though!). Of course openign the boost first also means that it is closed last and offers better opportunity for the pipe to suck up more mixture from the cases without short circuiting, this may extend mid range a bit but i am wondering if its at the expense of scavenging.

I find it very interesting that Jan Theil says they did not change port timings in 15 years... instead focussed solely on transfer angles and how they collide with each other... in other words scavenging. I think the average tuner pays very little attention to this and instead worries more about timing... probably due to a lack of understanding of what is actually happening in the cylinder (I am included here... but hoping to change this soon!).

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What are your reasonings for this? I can see an equal number of scenarios when each would work and also when the opposite would be true. Hasve you come to these conclusions from testing or are they more your theories from knowledge you have gained?

I can see that in the example you mention if you need more exh area in a 3 port exh setup then openign the main transfers last allows you to drop the sub exh floor to gain more mean area, but what if there was already enough area... you would then design primarily for scavenging?? And if you did this i think there is more weight to the idea of opening the transfers in the order main/aux/boost in order to scavenge with minimal mixing of fresh/burnt charge... i feel that using the opposite order boost/aux/main will lead to more mixing of the gasses in most cases (even though this is the current setup on my CR250... about to be changed though!). Of course openign the boost first also means that it is closed last and offers better opportunity for the pipe to suck up more mixture from the cases without short circuiting, this may extend mid range a bit but i am wondering if its at the expense of scavenging.

I find it very interesting that Jan Theil says they did not change port timings in 15 years... instead focussed solely on transfer angles and how they collide with each other... in other words scavenging. I think the average tuner pays very little attention to this and instead worries more about timing... probably due to a lack of understanding of what is actually happening in the cylinder (I am included here... but hoping to change this soon!).

I have seperated each post to somewhat highlight each thought you posted as I saw it.

If you really look at motocross race engines over the past 10 years - say the 125 which represents the pinnacle of development for mx - what has changed? Not a heck of a lot in timings/and or areas.

What about pipes? Not much there either - though minor changes have happened.

A great example of the improvement in SCAVENGING is the honda cr125 from 00-05. A clear progression can be seen (from total shit to a yamaha copy in 05). And guess what - go ride an 05 (or up) and you will be amazed at the engine. It's good!

I have found that opening the main trans first MAY help MOST when short circuiting is a big issue. This (short circuiting) will be determined by the tunnel shape, volume, etc etc etc as well as the exhaust pipe design...If short circuiting occurs - which I define as intake charge leaving its INTENDED path - and going out the exhaust in a non controlled manner THEN allowing a small amount of spent charge INTO the main transfer that then will go OUT the exhaust during the short circuiting period thus wasting no precious intake charge...

But when searching for all out power - blow down is the main issue. Thus lowering the main trans can help gain this area.

We are constantly compromising to meet our goals on the two stroke...and a good design minimizes these shortcomings or compromises or comes up with unique ways around them (thus the engine posted in the forum you posted - with perfectly symmetrical transfers and full radial exhaust ports).

I do not think there is one ideal porting arrangement that fits all situations - as you said. I think, rather, there is often one similar porting arrangement that suits one end goal...

Jan Thiel went a direction that he saw fit his needs best...and ran with it. Honda went a T port direction they felt best...which did in fact compete for many years with the RS design...

Honda forever stuck with a short rod design - and if you look at the math for port time areas - a short rod makes a lot of sense.

But the aprilia - be it design, or the brain power (people behind the scenes) to develop the design best, clearly reigned supreme in the end.

I can not comment on the exact numbers or details - but even aprilia had major compromises in their design from "ideal"...and despite these it ran like a scalded ape.

What amazes me is the fact that often one of two guys find ALL the solutions to the problems found. IE optimizing cooling for each location and optimizing casting design...optimizing pipe design...piston design, crank design etc etc etc...

Some people are just truly gifted...

We merely dabble around...and make improvements. Those guys often make revolutions in concept.

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I do not think there is one ideal porting arrangement that fits all situations - as you said. I think, rather, there is often one similar porting arrangement that suits one end goal...

See this is the thing that i'm now very confident of... for a while i was searching for that one golden rule, but now that my understanding is developing further im confident in assessing each engine in its own right, and that requires an open mind and many different rules.

With regard to transfer angles... my thoughts have changed significantly since i first did any porting. What im now interested in is port roof angles, not so much wall angles... wall angles to me seem much more straight forward, whereas roof angles i think can have a huge impact and are a little harder to predict. Im now of the opinion that shallow roof angles (0-10 degrees) are better suited to lower rpm power or broad power, and steeper roof angles (10-25 degress) are probably better suited to higher rpm power or peak power. When i say better suited i mean 'provide better scavenging' for the given purpose. I think that the fresh charge enters and moves around the cylinder slower than I initially thought. I also think it hugs the piston and walls quite a bit more than i initially thought. Any thoughts?

Harris, you mentioned the 05 CR125, can you share what the transfer roof angles are on that cylinder if you have them handy??

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The 05 plus honda is 30 main 15 sec 60 boost.

This is about "ideal"

That port arrangement was pioneered from another measurement method mentioned in the forum you posted via yamaha R&D.

Roof angles are not the entire picture - transfer tunnel shape is extremely critical...

For instance if the short side is not well developed with a large radius even with a flat roof bulk flow may be at an upward angle anyways - but in this instance very little flow would be coming out of the short side...where as in a better developed transfer the roof angle would dictate bulk flow there - but no "dead" spot would exist on short side. Piston top shape plays a role too...

Further than that even if the roof angles are right and transfer tunnel shapes are right...if the volume of the tunnel is much too large there is not enough velocity with the intake charge to keep it on track.

Now that you have the software - look at mass flow during an intake cycle and when this occurs...you will learn a lot.

The 05 up honda cr250 is basically a bigger 05 cr125...steep roof angles on main, 15ish on secondary - t port etc etc. and fairly small tunnel volumes..

And on the dyno it ran great mid and up - but never won over test riders.

But if you look at 4 thousand rpm to 6k it makes poor power compared to the yamaha and I think that is what makes if poor as compared to great. Power at those non optimized RPM's is very hard to find...because nothing is really happenng in sync yet with the engine. This is probably why the yamaha has such low intake transfer timings but if those are raised it still maintains good bottom - so other factors are playing a role...

I actually think the 05 honda would gain from more case volume but not necessarily more transfer volume.

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Wow, good thread! I've never looked into the 125 GP bikes before, but I knew they get some crazy power out of them, but I had always heard that it was all top end and wouldn't be good for MX. Looking at the dyno from that thread:

power_10.png

The hp numbers from 9000 RPM to 11000 RPM are pretty much the same as a 125 designed for MX, just the GP bike revs to the moon and keeps climbing all the way to 54hp! I think it would be really interesting to try an engine like that on dirt.

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The 05 plus honda is 30 main 15 sec 60 boost.

This is about "ideal"

That port arrangement was pioneered from another measurement method mentioned in the forum you posted via yamaha R&D.

Hmm... I need to keep reading... Better go visit SAE again... My CR measures at about 23/15/55 give or take a bit, so very similar to the 125. Been reading up on Coanda and flow last night... Takes me back to my engineering days, I really should remember this stuff but somehow it's slipped away over the last 12 years... hopefully it'll come back to me quickly though. Building a better picture still. I was worried that those steeper angles would mix rather than push gasses on the piston crown... I used to see the fresh charge as an arrow shooting across the cylinder sepersting the gasses above and below it, but now im seeing it more as a slower moving wall that bends around the piston and walls as it enters, pushing the exh gasses ahead of it... Of course this only works properly if everything else is designed well... as you said inner transfer duct radius, roof angle, piston dome radius, duct volume, crank volume, etc they all have to work together to get optimum results

I can still see a place for shallower roof angles but just need to adjust the other design parameters in my head that would match them.

Now that you have the software - look at mass flow during an intake cycle and when this occurs...

Noticed some patterns the other day but need to consider it further... Still thinking about scavenging... It's crazy when I get an idea in my head it consumes me 24/7 until I've got an answer, my wife hates it at the moment cause when I'm not at work I'm reading and there's no time left for her! I'll add this one to the list as well.

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Wow, good thread! I've never looked into the 125 GP bikes before, but I knew they get some crazy power out of them, but I had always heard that it was all top end and wouldn't be good for MX. Looking at the dyno from that thread:

power_10.png

The hp numbers from 9000 RPM to 11000 RPM are pretty much the same as a 125 designed for MX, just the GP bike revs to the moon and keeps climbing all the way to 54hp! I think it would be really interesting to try an engine like that on dirt.

Agreed.

A moto bike however is limited with peak pipe diameter (Im sure creative chassis rearrangement would help this). A larger pipe diameter could help pull more intake charge through existing cylinders...and stuff it in there better. But as of now it just wont fit.

More so though - is cooling. How do you dissapate the heat that a 55 hp motor produces on an mx bike!??!?!? Not easy when the bike is WOT going 10 mph around a slow corner or pinned in the mud with poor cooling due to blocked rads...

The porting in those GP bikes is truly nothing all that special - rather the pipe and cooling and piston and supporting elements to KEEP 55 hp through the entire race IS impressive...

They also have no intake tract limitations - take a honda 125 and put a straight intake on it - bam 1 horse no changes... As power continues to be developed it becomes more and more important to have a straight intake tract - that ends in a resonant character at the correct length. Yes - a tuned length intake tract. This too is worth about a horse. KTM - perfect intake tract. KTM - lots of power stock!

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good info,, more to come I am sure, never dabbled in the GP stuff,, just woods and mx..

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I have a fair bit of 125 GP experience as a rider, and some KTM 125SX experience too. I can say without a doubt that you wouldn't want to ride a dirtbike with a gplike motor. In fact, I'm pretty sure most people couldn't do much with that 52-55hp GP motor on a GP bike without 5-10 years of practice.

These motors fall on there face so hard under 10k rpms that you go nowhere. If you have the nut to keep up the revs while leaned over 55 degrees, then open the throttle wide, it is indeed an exhilarating ride up the power/torque curve. You stand the bike up as power/toque rises. It takes a pretty talented rider just to use what the motor has to offer. They are in no way flexible powerplants.

t2jpg.jpg

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