The Future of 2 Strokes Once And For All

[mtk]

The ATKs and XRs were and are, turds.

The new 4-strokes are not some magical achievement by any stretch of the imagination. They've simply applied what they know from superbike racing and built high specific-output race engines to take advantage of the rules. Rules that were written to try and make the aforementioned turds competitive. The rules give an unfair advantage to the four-strokes, which is why the 450s all make more power than the 250s. It's even worse in the 125 class. But peak power is only a small part of it. Not only do they make more power, but they have a larger spread of power, making them easier to ride. It's the same reason that 1000cc twins have always dominated 750cc fours in AMA and World Superbike racing.

The 2-1 displacement relationship ignores the increased thermodynamic efficiency the four-strokes get from their higher compression ratios.

For the record, I hate fart bikes, I mean four-strokes.

Camshafts belong on the street.

 

[SoCalMatt]

What confuses me is that I thought the original reason for high HP 4 strokes being pushed on us consumers so hard was to lower emissions, right? I say I am confused because my buddy just bought a 2004 YZF450 and he received a red registration sticker, which means he cannot ride in certain riding areas during the summer in California. :coocoo: Maybe the sound is too loud for a summer flea?? :laugh:

I just don't get it. The 4 strokes are great, hell any dirtbike is great, but I'll stick with my 2 stroke for as long as possible. Whatever happens, I'm riding for life!

 

[Green Horn]

2 stroks r faster then 4.

 

[XRpredator]

. . . Rules that were written to try and make the aforementioned turds competitive. The rules give an unfair advantage to the four-strokes, which is why the 450s all make more power than the 250s. It's even worse in the 125 class. But peak power is only a small part of it. Not only do they make more power, but they have a larger spread of power, making them easier to ride.

yes, the rules were written that way, but a long time ago when the 2 strokes began dominating. Hell, you could have a 540 cc 4-stroke in the 250 class up til just a few years ago. When the 125 class rules were written, nobody was worried about the hottest bike out there, an XR250. And up til the YZ250F, the most technologically advanced 4 stroke 250 was the KLX, but it was still a pig.

Besides, I think we've seen, in both classes, it's more rider than bike.

and I think we've also seen that guys who ride 2 strokes are whiny crybabies. :nener:

 

[Rooster]

and I think we've also seen that guys who ride 2 strokes are whiny crybabies. :nener:

You don't hear me crying big guy! :cool:

 

[Rich Rohrich]

The 2-1 displacement relationship ignores the increased thermodynamic efficiency the four-strokes get from their higher compression ratios.

Thermodynamics has little if anything to do with the power differences. It's just a matter of BMEP averaged over a spread of RPM. A 2:1 ratio was probably fairly logical at a time, but recent results have shown that it's probably time to re-factor things in the 125 class.

 

[noddy barber]

Most 250f Riders In Uk, Are Switching Back To 125 . The General Feeling Is That Although 4strokes Are Easier To Ride, They Dont Have The Same Buzz As A 2stroke And They Cost Way Too Much Too Rebuild. The Classified Adds In Tmx News Are Full Of Second Hand 4strokes. They Are Very Difficult To Sell.

 

[Green Horn]

The General Feeling Is That Although 4strokes Are Easier To Ride,

:think:

 

[CaptainObvious]

The General Feeling Is That Although 4strokes Are Easier To Ride

Stop right there. I'm all about easy.

That's why I like the 4 strokes. The power delivery of the 4 stroke gives me greater confidence.

Long live the easy-to-ride dirtbike.

 

[rzracer]

I agree the displacement needs to be adjusted. Strokers make more HP per stroke although not per cc as they only have half the power strokes of a pinger. In reality the ratio is about 1.7 - 1 which would put a 4-stroke in the 125 class at 210 -215cc. The 4-stroke would still have a fatter power band but offset with a slight weight disadvantage and it would be a pretty fair game.

I'm somewhat amused on all this 'this one or that one is faster or better'. I grew up on 2-strokes and they will always be my true love but, I now own both and can say that on certain tracks and track conditions each has an advantage over the other. But the truth is that it boils down to the rider. At least 90% comes from rider skill, at least in the novice and intermediate class. If you can't go out and consistently place in the top 5 in your class on a bone stock bike, that extra 1.5 hp for that new fancy pipe just isn't gonna make the difference. It tickles me to hear all the " I blow away 4-strokes/2strokes every week" talk. If you are great, but 90% of it is your skill level and if you were on the opposite you'd most likely still beat them. Bikes are like women, blondes, and brunettes...most people have a preference....but it's nice to have a choice. Nuf said...

 

[mtk]

Thermodynamics has little if anything to do with the power differences. It's just a matter of BMEP averaged over a spread of RPM. A 2:1 ratio was probably fairly logical at a time, but recent results have shown that it's probably time to re-factor things in the 125 class.

Sorry, Rich, but you're wrong. They're not called "Thermodynamic Laws" for nothing. They don't take a day off, ever, and this is no exception. Thermodynamics IS the study of engines (among other things); it most certainly is relevant to this discussion.

What you've said is simply the practical, measured application of the aforementioned laws of thermodynamics.

Brake Mean Effective Pressure is derived from the operation of the engine, it's not some magical number you pull out of your butt. Drop the compression ratio of ANY engine and BMEP WILL drop. More importantly, BMEP is an idealized concept in itself and has no actual relation to the actual workings of an engine.

The simple, idealized models place upper limits on what you can get out of an engine. Thermal efficiency increases as compression ratio increases. There's no two ways around it.

 

[Chili]

[banner]Chili grabs a coke and some popcorn and pulls up a chair[/banner]

 

[Studboy]

[banner] Studboy sits back and waits for the fireworks to begin. [/banner]

 

[wornknobby]

4-strokes may be nice, BUT 2-smokes still RIP!!! :aj: :aj:

all my friends bought 250f's and want me to go over there,i aways tell them 2-strokes for life and no one gets it. but when i show up to the track and maybe not win, but certainly don't loose to all the 4-strokes. :) :)

You know what,
its not the bike its the rider :aj:

 

[CaptainObvious]

[banner]Junk just wonders why it took so long for this thread to go to hell[/banner]

 

[keithb7]

I have a CR250, my budy has an XR250. I can kick his but any day. :laugh: So that prooves it. Two strokes are more better. Ever try to ride an XR250 in the sand dunes? Again, I prove my point. 2 strokes rock dude. :nener: LOL.... Just kidding. Just wanted to add some fuel to the fire.
Keith (snicker snicker)

 

[YZ165]

:laugh:

 

[MXSparx]

This thread is NOT going to turn into another my bike is better thread. Sorry. Lets try to keep away from that.

 

[Rich Rohrich]

Sorry, Rich, but you're wrong. They're not called "Thermodynamic Laws" for nothing. They don't take a day off, ever, and this is no exception. Thermodynamics IS the study of engines (among other things); it most certainly is relevant to this discussion.

What you've said is simply the practical, measured application of the aforementioned laws of thermodynamics.

Brake Mean Effective Pressure is derived from the operation of the engine, it's not some magical number you pull out of your butt. Drop the compression ratio of ANY engine and BMEP WILL drop. More importantly, BMEP is an idealized concept in itself and has no actual relation to the actual workings of an engine.

The simple, idealized models place upper limits on what you can get out of an engine. Thermal efficiency increases as compression ratio increases. There's no two ways around it.

While that certainly sounds good in Shop Class 101 when you are discussing theoretical thermal efficiencies and perfect gas cycles, actual real world combustion efficiency in a running engine isn't so cut and dried. Here's a few real world numbers to mull over.

If you compare a 125 two-stroke with an 11.47:1 mechanical CR and an exhaust port timing of 85 BTDC you get a trapped CR of 6.46:1 and a thermal efficiency from the CR of 52.60 % .

Now compare that to a 250F Yamaha with a 12.0 mechanical CR and an IVC timing of 78 ABDC . You get a trapped CR of 8.49 and a thermal efficiency from the CR of 57.50%. That 5% increase in theoretical perfect gas cycle efficiency sure looks good doesn't it?

Too bad it won't necessarily be realized in practice. A 125 cc two-stroke with it's 54mm bore has a very compact efficient combustion chamber with a low surface to volume ratio, while the 250F has a much larger 77mm bore and very high surface to volume ratio. The more compact and efficient you can make the combustion chamber the fewer crank angle degrees it takes to complete the burn. Modern two-strokes have excellent chamber designs with very low surface to volume ratio. Our YZ250F has deep valve pockets and an irregular piston and combustion chamber shape that significantly extends the burn angle to reach complete combustion and diminishes a lot of the theoretical thermal advantages it should have due to heat loss during the extended combustion period. There is also the matter of pumping losses that come from early ignition timing as well.

Even if we assume for the sake of discussion that the combustion efficiency of both engines is the same what would that 5% thermal difference mean in terms of indicated mean effective pressure (IMEP) effective pressure ? About 7 psi, which in this example is good for about a bit over 1 horsepower at peak rpm.

So where do the differences really come from and why did the AMA choose the 2:1 displacement ratio?

I think the 2:1 ratio came from some overly simple BMEP math on the part of the AMA. If you look at a 32 horsepower production 125 two-stroke and a 32 horsepower production 250F you'll see they have nearly identical BMEP at the power peaks. For those unfamiliar with BMEP or (Brake Mean Effective Pressure) it's nothing more than the average cylinder pressures in a running engine. It’s commonly used to compare engines of differing types because working pressure and rpm are what ultimately make torque and by association horsepower.

The problem with the AMA’s simple math is it ignores the fact that in real race applications the BMEP of a four-stroke can be higher at the peak than a two-stroke, and it can carry a higher BMEP over a much broader range of rpm, and that’s the key. High BMEP over an extended rpm range is a HUGE advantage.

Basically, BMEP peaks and torque peaks are inter-related. So if you can sustain a high BMEP from a moderate rpm to a high RPM you’ll make lots of torque , and if you can have a high BMEP that drops off slowly as rpm increases past the peak (this is primarily a function of engine breathing) you’ll have a very wide powerband that continues to be strong well after the torque peak. Magazine wanks like to call this overrev. This characteristic is what allows a 250F to carry a gear a few thousand rpm higher than a comparable 125 two-stroke.


Here’s a couple of graphs that show a YZ125 and some other good two-strokes and one that shows a well developed YZ250F and some serious automotive four-strokes. You’ll see two points for each engine, the lower RPM point is the torque peak and the upper RPM point represents the HP peak. As you can see the BMEP spread and peak is very high for the 4 valve four-strokes which tend to breath really well even at high rpm.

Basically what all this means in the end is, averaged over time (RPM) the “fires twice as often” advantage of a 125 two-stroke is diminished to a large extent by the lower working pressures and the lower rpm it has to run at compared to a 250F, and thermal efficiency differences account for only a very small part of this. The same holds true but to a lesser extent with the 250 class bikes.

Hopefully this makes some sense, if not I at least got to practice typing . :cool:

 

[CR3999]

Can you put all that in lay-men terms............Oh that is in lay-men terms, my bad. I just know I prefer two-strokes. The rest can ride four-strokes for all I care. I will stop racing them when they pry my cold dead hands off the grips. Sounds like NRA. hehehe

 

[biglou]

So, MTK (right? I don't wanna scroll back) hates fart bikes. Good for you. I'll throw out one of my favorite sayings from work: "It is what it is." Meaning, the rules were fine, and they still exist. But they are what they are. Kinda like test results at work here. I supply results that aren't always what people were wanting or expecting. They look at me as if I can magically make things different. Unfortunately, I get to tell them, "It is what it is". Not to be callous, but it is reality. If people have a beef, they need to change the rules. Whether its how much force I apply to a component based on industry standards or how many cc's are allowed in what class.

Oh, and one more "For the record". Me changing from my 450 to a 250 2stroke won't make my race results differ significantly one way or the other. And I don't really care if it would or not. I'm in this game for fun and that's all.

 

[mtk]

Yeah Rich, the real world isn't so cut and dried, that's obvious. But those theoretical models define upper limits of what you can get out of the engine. It goes down from there, based upon frictional losses and all the other stuff you identified.

Being too lazy to hit the "quote" button, I cut this from your post:

"BMEP or (Brake Mean Effective Pressure) it's nothing more than the average cylinder pressures in a running engine. It’s commonly used to compare engines of differing types because working pressure and rpm are what ultimately make torque and by association horsepower."

BMEP is an idealized construction, used for comparison purposes. It assumes a constant pressure on the piston for the entire stroke length. That doesn't happen. The pressures in the cylinder are highest at TDC and drop off rapidly as the piston moves down. That's why the first 15 degrees of crank rotation past TDC are responsible for the lion's share of the power produced on that cycle. That's why the exhaust valves on a four stroke open when the piston is only half way through it's stroke; you don't lose much by sacrificing half of the power stroke because the combustion pressures have dropped off so drastically due to the increase in chamber volume.

Also, a CR125 has an 8.6:1 compression ratio according to Honda's website. The CRF250 has a 12.5:1 compression ratio, again from Honda's website. That's a pretty significant difference and a long way from the 11.4:1 number you quoted.

Yet another "Cut and Paste:"

"Basically what all this means in the end is, averaged over time (RPM) the “fires twice as often” advantage of a 125 two-stroke is diminished to a large extent by the lower working pressures and the lower rpm it has to run at compared to a 250F, and thermal efficiency differences account for only a very small part of this."

The working pressures of the engine are part of that thermal efficiency calculation.

But I've got to give you credit, that's one excellent post you put together. You make excellent points about the functional differences between the twos and fours. It's also a pretty good treatise on real-world applications of thermodynamics. Throw in a couple dozen equations and you could put that in a textbook. :)

BigLou, yes, I hate fart bikes, for no other reason than the awful noises they make. There's a proper natural order of things. Playboy playmates should be blonde. Football should be played on grass and outdoors, not on carpet in a dome. Dirt bikes should have expansion chambers. To do otherwise is an abomination, except maybe on the "playmates" part. :)

Sorry to disappoint everyone with their buckets of popcorn. :laugh:

 

[biglou]

BigLou, yes, I hate fart bikes, for no other reason than the awful noises they make.

Well, then you'd REALLY hate me! :rotfl:

 

[Micahdawg]

I just read in some MotoX mag a good proposal for an AMA rule change. They basically stated common knowledge stuff. First, the two stroke is a more efficient motor that is capable to delivering more power/volume when compared to a 4 stroke. If two strokes weren't handicapped in displacement, they would still be beating 4 strokes. You won't find many people that can disagree with this statement.

Back in the day when AMA ruled to allow 4 strokes up to 550cc displacement in competition against 2 strokes of 250 cc displacement, it was simply to encourage 4 stroke participation and development. There is no way someone will race with a less efficient motor unless they have some kind of possible advantage.

Now that the factories have jumped on this and poured tons of money into R&D to develop these bikes, it is obvious that the AMA rules allow more than just fair participation, but a straight up advantage for 4 strokes.

To deal with this, it is better to allow 4 strokes to continue participation in current 250/450 form. No sense in outlawing bikes that the big 5 have spent years developing. The last thing we need to do is "punish" the big 5 for their hard work. So rather, change the 125/250 class to a new 250/450 class. To encourage further participation of 2 strokes, they propose that you allow 125's to bore and increase displacment up to 250cc. Likewise, allow 250's to bore up to 450cc.

They suggest that you have to use a factory 125/250 bike and modify a factory cylinder. So in reality, you'll only be able to modify a 125 to a 175 and a 250 to a 300 or so. But even that relatively small increase in displacement is probably enough to even the playing field.

As for the KTM 300 (or is it 330?), that bike would not be allowed because it is not based on a 125/250 platform.

................
I think this ruling is perfect. It would allow better competition and encourage further R&D on the two stroke. The two stroke is a cheaper, more maintenance friendly bike so it could entice new riders to get into the sport more than some of the four strokes today.

And the best part, there is already a precident for this new rule change. They did it for four strokes!

Micah

 

[Chili]

The last thing we need to do is "punish" the big 5 for their hard work.

Is KTM part of that "big 5"? If so they've already been punished so to speak when their 520SX was killed for AMA competition when the 450 Class displacement was changed from 550cc (I thought it was 540) to 450.