Revolutions per minute - 2 stroke

[troybilt]

What limits a 2 stroke on revolutions per minute? I've been thinkign about this, is it rod bearings, rod itself, flywheel, spark control, reeds...??? Any ideas? On a four poker you can develop a lot of power by going to a desmodromic valve configuration or air control valves, (think Formula One, i.e. 20,000+ rpm...) typically the valve springs limit your rpm. But 2 stroke don't have that "achilles heel"....

So if guy was to build a mega horsepower 265cc motor to compete with the displacement handicap against the 4 strokes, I see RPM as one avenue to get there. Any idea how you'd build a 265 to reach 60+hp? and 38+ torque?? I've been researching, (seems about all I do these days), but the old kart racers used to crank up the 125cc motors to well over 20,000 rpm... food for thought.

[8686]

I think it would be awful tough to race a setup like that, at least in motocross. Where small displacement engines are concerned, it seems that the more peak horsepower you get from the engine, the narrower the powerband is. Power is shifted to the upper rpm's, and that's awful tough to race with, unless you're drag racing or flat track racing.

[troybilt]

Originally posted by 8686
I think it would be awful tough to race a setup like that, at least in motocross. Where small displacement engines are concerned, it seems that the more peak horsepower you get from the engine, the narrower the powerband is. Power is shifted to the upper rpm's, and that's awful tough to race with, unless you're drag racing or flat track racing.

Yeah that's very good point.

[troybilt]

now you get the idea...

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:eek2:

[wilkin250r]

I think it's a combination of limiting factors.

8686 hit the first one, narrow powerband. On many engines (let's take the 400EX for example), the engine simply can't breath enough to rev really high. Even if you put a desmo valve system on a 400EX, you'll never get more than about 13k out of it, it just can't breath enough. You'll barely make enough power to keep it running that speed, and nothing left to put to the ground.

On a 2-stroke, this translates to port size. But you can't make the exhaust significantly wider without restricting the intake, or vice versa. So the only direction you can go is UP, which shifts the RPM higher, and narrows the powerband. The more power you make, the more narrow the power. Pretty soon, it really is just a lightswitch.

Then there is the pipe effect to consider. You can't make a pipe that works at ALL power ranges. The pressure waves inside the pipe that help your engine work have two main factors to consider, there's the STRENGTH of the pressure wave, and then the DURATION of the pressure wave. Strong pressure wave gives good boost, good performance. But a longer duration is effective over a larger RPM range. But you generally can't increase one without decreasing the other, so you can either build lots of power in one spot, or a little power over a wide range.

[atvmxr]

the old honda road race bikes from the 60s (i think thats the time frame) had a 6 cylinder 250 that would turn 14000 rpm or so. oval shaped pistons. custom stuff and way ahead of its time

[cataway]

Originally posted by troybilt
What limits a 2 stroke on revolutions per minute? I've been thinkign about this, is it rod bearings, rod itself, flywheel, spark control, reeds...??? Any ideas? On a four poker you can develop a lot of power by going to a desmodromic valve configuration or air control valves, (think Formula One, i.e. 20,000+ rpm...) typically the valve springs limit your rpm. But 2 stroke don't have that "achilles heel"....

So if guy was to build a mega horsepower 265cc motor to compete with the displacement handicap against the 4 strokes, I see RPM as one avenue to get there. Any idea how you'd build a 265 to reach 60+hp? and 38+ torque?? I've been researching, (seems about all I do these days), but the old kart racers used to crank up the 125cc motors to well over 20,000 rpm... food for thought. the CDI limets the rpm ,cant say this has always been the case

[wilkin250r]

Originally posted by atvmxr
the old honda road race bikes from the 60s (i think thats the time frame) had a 6 cylinder 250 that would turn 14000 rpm or so. oval shaped pistons. custom stuff and way ahead of its time

You're close.

It was 1979, so early 80s was the era, and they were 4-cylinder.

The reason was because Grand Prix rules limited the size of the engine depending on class (500cc, 750cc, ect ect), and also stipulated a maximum of 4 cylinders, but the rules to NOT specify 2-stroke or 4-stroke.

This obviously gives the 2-strokes the advantage, because you get twice as many combustion cycles. In other areas, engineers would just create more cylinders to get higher revs (a 4-cylinder 750cc engine will rev much higher, and produce more power than a single cylinder 750cc engine). But since Grand Prix rules limited the number of cylinders, the engineers at Honda had to find another strategy.

The oval piston design gives you enough room to put 8 valves per cylinder, basically allowing the engine to breath like an 8-cylinder engine, but only have 4 cylinders. THIS is the key to the engine's power, not necessarily the piston shape. More valves, breath better.

And they would rev to upwards of 20,000.

[machwon]

Reliability is the key, stay under 4000 fpm. I'm sure the 4 strokers newer technology can push that number a little further and you can push it more just not for long periods of time. Different bore/stroke and rod combinations can help with the speed too.

[hondamancbr03]

I came across this article several years back and it kinda stuck with me....Very interesting but it speaks in Alien tongue at a couple points.

It will describe the limits of the two stroke RPM's and why.....


http://www.macdizzy.com/bore.htm

[troybilt]

Originally posted by wilkin250r
You're close.

It was 1979, so early 80s was the era, and they were 4-cylinder.

The reason was because Grand Prix rules limited the size of the engine depending on class (500cc, 750cc, ect ect), and also stipulated a maximum of 4 cylinders, but the rules to NOT specify 2-stroke or 4-stroke.

This obviously gives the 2-strokes the advantage, because you get twice as many combustion cycles. In other areas, engineers would just create more cylinders to get higher revs (a 4-cylinder 750cc engine will rev much higher, and produce more power than a single cylinder 750cc engine). But since Grand Prix rules limited the number of cylinders, the engineers at Honda had to find another strategy.

The oval piston design gives you enough room to put 8 valves per cylinder, basically allowing the engine to breath like an 8-cylinder engine, but only have 4 cylinders. THIS is the key to the engine's power, not necessarily the piston shape. More valves, breath better.

And they would rev to upwards of 20,000.

I'm reading a book "Two-Stroke Performance Tuning" by A. Graham Bell

anyway he claims you're exactly right except is was Suzuki and Yamaha leading the charge for rpms on 2 strokes, and this was during the 1960's... :)

Anyway, here was their thought process back then:

"So effective was this method of cylinder scavenging that the fuel/air 'wedge' was actually being partly lost out of the exhaust before the port closed. ....[they] overcame this problem by opening the transfer ports later and closing them earlier..." the article goes on to say ..."Horsepower rose, instilling in Japanese engineers the idea that dominance in Grand Prix racing would depend on them reducing transfer duration to contain charge loss our of the exhaust and increasing crankcase compression to ensure efficient pumping of the fuel/air mixture from the crankcase into cylinder..."

..."Japanese engineers embarked on a scheme of cylinder size reduction to enable very high rpm to be attained reliably. Again, power levels increased, providing further stimulus to reduce cylinder displacement. This led to the development of such machines as the three-cylinder 50cc Suzuki and the four-cylinder Yamaha 125 which produced 40hp at 18,000 rpm."

Interesting to say the least. There is alot more to the book, and I suggest anybody interested in 2-stroke performance should read this book. I'm only on page 54 and I've learned a lot so far.

[wilkin250r]

Originally posted by troybilt
So effective was this method of cylinder scavenging that the fuel/air 'wedge' was actually being partly lost out of the exhaust before the port closed.

GREAT book, I would reccomend it to anyone looking at building performance 2-strokes. It doesn't go into super detail, like complex equations for port/time area or anything, but it covers the basics and then some.

If I'm correct, the "wedge" that Bell is talking about has to do with the evolution of the port placement. The first set of transfers to open are the ones at the sides of the exhaust, right? This causes the new fuel/air charge to wrap around the sides if the cylinder, and collide near the back (away from the exhaust). This collission, now almost directly opposite the exhaust port, pushes forward toward the exhaust, pushing the spent exhaust gasses out and giving you the most efficient cylinder filling.


This is crucial, and is also another limit on the 2-stroke design. If you try to radically change the port layout to get more port area, you run the risk of altering this beautiful ballet of fresh fuel/air mixture and spent exhaust gasses. The intakes have to be aimed in a specific direction, and opened in a specific order to be able to push the exhaust gasses out effectively. Ruin this process, and you run the risk of leaving exhaust in the cylinder, and losing power.

[troybilt]

Originally posted by wilkin250r
GREAT book, I would reccomend it to anyone looking at building performance 2-strokes. It doesn't go into super detail, like complex equations for port/time area or anything, but it covers the basics and then some.

If I'm correct, the "wedge" that Bell is talking about has to do with the evolution of the port placement. The first set of transfers to open are the ones at the sides of the exhaust, right? This causes the new fuel/air charge to wrap around the sides if the cylinder, and collide near the back (away from the exhaust). This collission, now almost directly opposite the exhaust port, pushes forward toward the exhaust, pushing the spent exhaust gasses out and giving you the most efficient cylinder filling.


This is crucial, and is also another limit on the 2-stroke design. If you try to radically change the port layout to get more port area, you run the risk of altering this beautiful ballet of fresh fuel/air mixture and spent exhaust gasses. The intakes have to be aimed in a specific direction, and opened in a specific order to be able to push the exhaust gasses out effectively. Ruin this process, and you run the risk of leaving exhaust in the cylinder, and losing power.

Yes once again you're exactly correct. According to Bell, the old 2 strokes used to direct the intake mixture from each side of the exhaust so they collided in the center, but in an upwards direction, like 60 degrees or something. I forget what problems this caused, anyway, currrent 2 strokes direct the mixture towards each other so they basically collide at 90 degrees to the piston axis or parallel to the crown of the piston.

I was interesting to see all the things manufacturers tried with ports and timing. They really left no stone unturned.

I'm going to expirement with my cylinders, not actually porting them but I want to draw port maps and compare, 89 stock to B+B 310 XC motor, and Sparks MX motor to see what each builder's thought process was/were/is.... I've thought of a unique way of inputing these maps into a 3D CAD file and might help to see the "whole" picture sort of speak of the duration and timing of the ports... I'm afraid as the 2 strokes die off this art work that these builders did may die with them. I want to try and keep a record of all the port works that I come across for a reference. Maybe someday I can do some of my own porting and expirementing.

What other books do you recommend may I ask? I've got along ways to go in fully understanding the information in this book, but I'd like to have a small library of information. I just did some googling and came across good reviews of this book and picked it up from Amazon for 24 bucks. I'm probably going to be asking more questions as I go along... Thanks Troy

[wilkin250r]

Originally posted by troybilt
What other books do you recommend may I ask? I've got along ways to go in fully understanding the information in this book,

I haven't found any better than Bell's book, but I haven't got a huge library myself. Honestly, I think you're better off re-reading Bell's book down the line (because you'll always find details you missed or didn't understand the first time) than reading another.

[ARC_89TRX250R]

I believe there are two marjor basic limiting factors to 2 stoke engine rpms,

1. How fast you can move the air

at some point your going to hit a point where you cant suck in enough air for the rpm your after. A 265cc at 8000rpm with a volumetric efficiency of 80% needs 28 Liters of air a second, that's about 1 cubic foot of air.

isn't that crazy but cool.

so think of a 265cc at even 60% volumetric efficiency at 20,000 rpm needs 52 Liters a second of air or 1.8 cubic foot.

at some point you just cant pull that in with the crank vacuum, so its going to need a supercharger or something.



2. Piston velocity

This is the final word in maximum rpm,

eventually at some rpm the piston speed is so high the forces on the piston are so high they are above the material strength capability and that's it

but the Macdizzy link covers that...

anyways just my thoughts....

[Lasher]

I have that book in the "throne room" and have read it about 5-6 times....just open to a page and start reading.

But one thing he touches on is how the 2-stroke, 4 stroke cycle keeps going around. 4 strokes were king...then the 2 stroke...now back to 4 stroke.

One thing that amazes me is the simplicity of the 2 stroke engine. And that will play into the 4 stroke downfall I think. Costs too much to maintain at a high level of performance compared to 2 stroke.

[C-LEIGH RACING]

When theres a slipper pipe built, that will reliably work, together with a PV to change the port timmings & then an ignition system to complement it all with it all working together as rpms go up, 2 stroke will rule again, but getting a slipper pipe to work right will be the task.
It may be more money involved than one would want to spend just to get a 2 stroke to that point.

The Yamaha RZ350 YSPV came close, but no pipes to match everything else.
Neil

[troybilt]

Originally posted by C-LEIGH RACING
When theres a slipper pipe built, that will relibably work, together with a PV to change the port timmings & then an ignition system to complement it all with it all working together as rpms go up, 2 stroke will rule again, but getting a slipper pipe to work right will be the task.
It may be more money involved than one would want to spend just to get a 2 stroke to that point.

The Yamaha RZ350 YSPV came close, but no pipes to match everything else.
Neil

Neil, what do you mean by slipper pipe? I think I've got an idea, but now I'm not sure. When I think of slipper I think of either the BTL Hinson clutch "to help a 4 stroker run like a 2 stroke..." or a "Slipper piston" in 4 strokes like the new 450rs... I guess I've never heard of slipper being used with 2 strokes... Thanks! Troy

[rsss396]

the pipe slides in and out, "slipper" to change the tuned length of the pipe kind of like LRD did on their pipe.
But Slipper pipes do it while the motor is running so the length of the pipe can match the motors needs
I think there has been a few crude and elaborate ways of doing it.
I believe some of Kart racers do or use to do this

[troybilt]

Originally posted by rsss396
the pipe slides in and out, "slipper" to change the tuned length of the pipe kind of like LRD did on their pipe.
But Slipper pipes do it while the motor is running so the length of the pipe can match the motors needs
I think there has been a few crude and elaborate ways of doing it.
I believe some of Kart racers do or use to do this

Got any pics or websites I can check this out? This gives me some ideas. That wasn't what I was thinking, but its a good idea.

[wilkin250r]

For an ATV, with a bend in the pipe, it would end up acting much like a trombone.

It's a great concept, but we'll never see it on an ATV, I'm sure.

[troybilt]

Originally posted by wilkin250r
For an ATV, with a bend in the pipe, it would end up acting much like a trombone.

It's a great concept, but we'll never see it on an ATV, I'm sure.

I'd like to see the concept... do you have any information?

[troybilt]

Originally posted by wilkin250r
For an ATV, with a bend in the pipe, it would end up acting much like a trombone.

It's a great concept, but we'll never see it on an ATV, I'm sure.

I also never thought we'd see a 2 stroke that didn't require oil mixture in fuel and but still didn't have valves like a 4 stroke either... but here it is... circa 2003

Never underestimate...

Here are the claims:

What is claimed is:

1. An improvement of two stroke engine mainly comprises a sleeve piston over wrapped on the main piston, wherein the sleeve piston controls the opening and closing of the intake way and the exhaust way while the piston moves up and down in the cylinder and block the engine oil from leaking out of the crank case, it is unnecessary to mix engine oil into gasoline for the two-stroke engine of the invention, it produce the clean exhaust, which meets the emission standard, and the main characteristics are: the sleeve piston comprises a main piston and a sleeve piston; the main piston has the piston ring serving the sealing function and a taper at the lower part; the sleeve piston has a chamber with a large size groove race on the top and provides two sealing rings; the taper of the main piston is inserted into the chamber of the sleeve piston to form an integral sleeve piston assembly.

2. The improvement of two-stroke engine of claim 1, wherein the cylinder is designed in two sections.

3. The improvement of two stroke engine of claim 1, wherein the second section of the cylinder provides a retaining ring to hold a spring which supports at the internal bottom the sleeve piston so as to lift the sleeve piston by its expansion force.

4. The improvement of two-stroke engine of claim 1, wherein the inner diameter of the sleeve piston is tantamount to the outer diameter of the main piston and the inner diameter of the chamber is equal to the outer diameter of the taper of the main piston, after the taper is inserted into the chamber of the sleeve piston, the main piston and the sleeve piston will move synchronically up and down.

[wilkin250r]

Originally posted by troybilt
I'd like to see the concept... do you have any information?

I don't have any information, because I've never read anything about them. I thought of the concept years ago, because it's not rocket science. And once you get to the chapter on pipes in Bell's book, YOU will probably think of the same exact concept.

The different lengths and angles in a pipe are what determine the power range it is effective for. The angles determine the strength of the pressure waves, the lengths determine the timing of them.

So if the piston is moving slowly (low RPMs), you want those pressure waves to take their time. More time means more length, longer sections.

High RPM, the piston is moving fast, so you need short lengths to get those pressure waves to arrive in time.

In theory, you could design a pipe that would adjust automatically, that would be long at low RPMs, and shorten at higher RPMs, so that you're ALWAYS on the pipe, you're always in the powerband.

[C-LEIGH RACING]

Wilkins,
You have a way of writing something down that is so understandable, is to me anyways, thanks.

I can type something in & its kinda jumbled up & it gets the point across, but probably some that it confuses just trying to read it.

Back in somebodys post where they mentioned the piston couldnt suck enough in to support, well with a slipper pipe constantly changing the tuned lenght, there would be the suction wave action at different rpms to pull the intake up through the ports.

I would think through the years, at some point a slipper pipe was perfected, but because of the cost & all the headakes of keeping it operating to perfection & just the mere fact of trying to keep it on the bike & not fall off, it was way more easier for a rider to adjust to a single output of a pipe than a constant changing one.

I know people have had turbos on 2 strokes, but I cant for the life of me know why one would want to build it that way.
The whole concept of the two stroke, working on it to produce more power, cutting this, angleing this, pipe shape & lenght, reeds, when you add in the turbo all that is out the window, of what use is the porter or tuner after that.

The marvel of a 2 stroke & what one can learn to do to it & you realy dive into it with both feet & hands, that turbo is like somebody kicked the stool out from under you, everything you've busted you backside to learn & perfect in your mind & in test is right down the drain.
Neil

[hondamancbr03]

Originally posted by C-LEIGH RACING
that turbo is like somebody kicked the stool out from under you, everything you've busted you backside to learn & perfect in your mind & in test is right down the drain.
Neil

Neil,
allow me to kick that stool out from under you one more time! Check out this website, this is an engine builder i have known for too manys years....They build some insane Banshee and Honda bikes but i wouldn't let them touch anything i own!

Not that it's honda but go looking through his website and you will see what he's done with Honda's.....Not long ago he put two CR500 cylinders on a Banshee and pulled insane HP from it.

I seen this crew install turbo's 10yrs ago on Banshee's, he was never able to make it run right from what i watched.

http://www.trickracing1.com/Blown_Banshee.html

[machwon]

There is a shop just south of me in Michigan here that has done turbo Banshee's before. I've seen theirs and it kicks 180 hp on a stock 350, stock carbs and a 2 into1 pipe modded with a turbo. They are also into snowmobile racing (at least they have newer 2 stroke technology) and their 2 stroke motor sizes have grown enormous just to compete with a newer turbo charged 4 stroke sled. Never say never on the turbo, but I see lots of potential issues with the slipper pipe. I'd rather have one pipe and a turbo than a pipe that changes length and needs different cones and angles for each length.

[wilkin250r]

Neil, is the porting significantly different with a turbo than with a naturally aspirated engine?

[troybilt]

I guess it all boils down to what you want to do with it. I brought up this whole topic for one reason...

For me the magic # is 60. 60hp from a 265cc 2 stroke (current AMA rules on displacement) without adding stuff to make the motor as complicated or more complicated than a 4 stroke and still have rideable range of power delivery throughout the rpm range. That is the pie in the sky for me.

Why 60hp? I guess because that's what I was told the 450r Pro class motors are pushing with 450cc's. I don't know if that's true or not but they were in the 62hp range.

I think one can be a little creative there and argue horsepower to weight ratio. Since the 2 stroker will be much lighter of a bike in the long run, maybe 55 hp should be the magic #?, IDK. Also rider plays a very important role there too...

I don't want a turbo'd drag bike, what I'd want is to build the best MX bike money can by with today's latest 2 stroke technology. I feel I'm on the right track with the chassis and suspension, I'm up in the air on the motor.

I simply brought up increasing RPM cause that is a ligitimate way to build more hp without displacement increase, Formula one cars are an excellent example of that. However, the downfall with a 2 stroke is the powerband would be razor thin and produce lightswitch type power.

So how would you do it? That's my question. I loved my 450r i could ride it faster than any bike I've owned... I want to build a trx250r motor that has a similar broad range of usable power but still have the 2 stroke's simplicity. I think that's where the focus should be.

I absolutely refuse to give up on this quest.

I despise the phase "don't reinvent the wheel"... to me that is BS! If the wheel was that good we'd all be riding around in our trucks/cars with rock wheels like the flintstones.

[C-LEIGH RACING]

Originally posted by wilkin250r
Neil, is the porting significantly different with a turbo than with a naturally aspirated engine?

I couldnt say, cause I've never built one & I dought I ever will.

Honestly, in my mind if I ever did, I would feel like I had disrespected the life works of every 2 stroke builder that has ever lived right down to Bell.
Neil

[machwon]

troybilt, you are right the magic number is close to 60. That's probably a touch high but a good rider on a reliable 56-8 hp will do it in the pro's I would think.

It is almost a dream on a 2 stroke and you will need nothing but the best of everything and many hours of tuning on a dyno. Then the biggest problem is the torque curve of 4,000 rpms of peak tq on a yfz450 and only 2,000 rpms of peak torque on a 250R makes it very difficult to ride.

I think your base is obviously the 265cc pv, cr ignition, 40.5mm carb and a hand coned pipe. Use some oxygenated fuel and you likely can get in th lower 50's. Add in lot of tuning trying 100's of tweaks and changes and it could be possible. Cyroed and nickle platted tranny gears removing friction from every possible angle, Knife rods, ceramic bearings, pvl could be more help. I know some people claim 60 HP from a stock cylinder but I've yet to see those numbers on my dyno. Alky would get you much closer but I'm not sure that would be an option.

Off to Red Bud for me to watch some ATV racing.

[mcboomport]

Just a thought, and not trying to get off the rpm tangent. What about fuel injection? I've seen that Gigot racing completed one for a banshee guy in another forum. The evinrudes had it. I'm sure you could see a couple of horse gain by losing carburetors.

[wilkin250r]

Indeed, the direct fuel injection is nothing new, the outboard boat motors have been doing it for quite some time.

I don't know about serious horsepower gain, but you'd certainly see an increase in throttle response, and (I believe) you could dramatically increase the carburetor size and pick up a couple ponies on the top end.

To be technical, it wouldn't be a "carburetor" anymore, it would be a throttle body, but you get my meaning.

[troybilt]

Originally posted by mcboomport
Just a thought, and not trying to get off the rpm tangent. What about fuel injection? I've seen that Gigot racing completed one for a banshee guy in another forum. The evinrudes had it. I'm sure you could see a couple of horse gain by losing carburetors.

My old college roommate is close friends with the Gigots family. I have Kevin's number I've been meaning to call him and pick his brain, just haven't yet.

One thing I'll mention on the EFI, there are pros and cons with that setup as well. Several LTR450's are switching back to the carbs cause its easier for the lay-man to tune. I personally prefer tuning with a laptop but I grew up in this generation.

Orbital injection would be the next step in EFI for 2 strokes, low emission setup. I read about it, but I don't remember the specifics.

Machwon, yes thats what I remember hearing was the Pro's were in that 60 range. I'm hoping to come up with some ideas to reach that goal... without any exotic turbos or slipper pipes, but those are all good ideas too don't get me wrong.

While you mention CR ignition, what are your's and everyone else thoughts of a 2 position ignition like the 2011 Husabergs? 2 position for different track or trail conditions? I think this wouldn't be too hard to do. Then take that a step further what about an ECM that controlled a variable iginition? How much does the ignition affect the range of power delivery is there anyting substantial there to research?

[troybilt]

More information on Orbital Injection:

http://www.orbeng.com.au/orbital/bsengdev/bedoverview.htm

Notice they say small non-automotive 2 stroke engines...

anyway, I'm pretty sure the prototype KTM 300 2 stroke utilizes Orbital injection.... FYI

[slamdak8782]

Obviusly one way is powervalve cylider to increase rideable powerband next would be an advanced port like the pumas have. Also a pipe could be made wih a bladder in it. Connected to vacuum control. To spin faster lighter rotating assembly parts. Other ways would be lighter frame and other part. Different ignition with a better timing map. Bigger bore. 300. High octane fuel possibly alcohol. It could be done. Easy route probably nos or boost. Iknow not an easy route but, basically we need a way to emulate variable valve timing, oh direct injection would be another avenue. N

[mxduner]

I believe on the di, they claim to get an increase in torque, correct me if i am wrong.

Idk much about 2 stroke pipes, but like above poster, the bladder idea is close to my thinking, as rpm's build what happens to exhaust psi? is there any kind of material out that 1 could run for exhaust, that could change shape and/or size? maybe some kind of a waste gate could be incorperated to release ex. psi, and shrink up the ex. expansion chamber. Am i way of base hear? lmao if so. i'm with troy about the reinventing the wheel thing.


While you mention CR ignition, what are your's and everyone else thoughts of a 2 position ignition like the 2011 Husabergs? 2 position for different track or trail conditions? I think this wouldn't be too hard to do. Then take that a step further what about an ECM that controlled a variable iginition? How much does the ignition affect the range of power delivery is there anyting substantial there to research? Sleds have been doing this for some time with the ignition, also dirt bikes have been altering ignition timing based off throttle position and rpm. so no it cannot be tough in the digital ignitions imo

[woodsracer144]

I didnt read through it all but I have a few questions.

they said some where about the stroke and the size of the piston limiting the RPM and something about FPS for the travle of the piston.

It was kinda hard for me to understand cause its alittle loud around here right now and i cant read it and understand with it being quiet.

so now to my question.

would a STOCK bore ( 66.4) piston get more RPMs with a +4 stroker then the standard crank lenth, short or long rod set ups.

to get a 78mm stroke you would need to have a +6mm stroker? with 3mm gained at BDC and 3 gained at TDC.

the reasons i ask this is i had a guy fight wigh a chick at CT that was really driving me nuts cause she was saying i was going to toast the piss out of my motor set up.

66.4 piston with the +4 stroker with a port lay out from FTZ for a mid range


another question i've found with my motor is that i have crazy power through out the ban but would a over rev make it bubble on the top end? I feel as if its a over rev in the motor or something cause it doesnt feel lean or right, it just bubbles. Im also running all stock ignition.

Sorry if i highjacked and ran off topic.

[wilkin250r]

Originally posted by woodsracer144
would a STOCK bore ( 66.4) piston get more RPMs with a +4 stroker then the standard crank lenth, short or long rod set ups.

Porting, pipe, compression, and ignition all have an effect on revs. But as a general rule, strokers don't rev as well, you'll generally get more RPMs from a shorter stroke.

Bubbling on the top end could be jetting. 2-stroke carbs generally run rich at the top end, just by nature. And they have to, because if you lean it out to be dead-on at max revs, you'll be dangerously lean at lower revs.

Some 2-stroke carbs have a powerjet to fix that problem, they have a secondary fuel jet that is open at midrange, but cuts off at high RPMs to lean out the mixture a little. But you can't just throw that carb on a 250r, they need an external trigger (I assume from the CDI) to tell the powerjet when to shut off. The 250r doesn't have the electronics to supply that signal.