The Future of 2 Strokes Once And For All

[biglou]

I thought it was 540 also. That's what Coop rode a couple years back. That brings up another point I was thinking about-at some point I think you get beyond a usuable amount of power and get to retain any kind of handling or traction characteristics. But that would be in the bigger class. Not the 125/250f class, IMO.

 

[Rich Rohrich]

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."

I looked at it more based on what really gets built rather than stock OEM spec. FWIW, I've yet to measure a production japanese four-stroke MX bike that actually had the rated CR. It's always lower by a half point or more. :)

But back to the point, and something I've found interesting about high rpm four-strokes. As you raise the RPM you have to increase the time-area available for the engine to breathe. You can do this by adding more valve area or by increasing the valve open time. usually it's a bit of both. What's interesting is combustion efficiency runs inverse to breathing efficieciency once the rpm really starts getting up there. To make room for bigger valves , the bore size becomes ever larger, increasing surface to volume ratio and diminishing combustion efficiency. To get enough room for high lift cams and bigger valves the depth of the valve reliefs in the piston crown have to get deeper, which just complicates the combustion problems further. The reason I find this interesting is, there will always be a battle between breathing and combustion in high output high rpm four-strokes. At the end of the day you have to make a compromise , and from the engines I've studied good breathing is preferred over good combustion. It's funny to hear F1 and WSB engine guys talk about having to run 50 degrees of ignition lead just to get everything to burn as engine speeds reach above 15000 rpm. Problems both combustion related and otherwise tend to go up with rpm, but so does power so the temptation to push rpm limits higher are hard to ignore.

Interesting stuff maybe if four-strokes are your thing , regardless it's clear to me at least that the 2:1 ratio used by the AMA isn't working if the idea is to have a level playing field between two-strokes and four-strokes. Rough math says a 150 cc two-stroke vs a 250 four-stroke is a bit closer to level. I get the feeling that the AMA isn't trying to keep things level and this is a good way to ensure it.

 

[XRpredator]

2 strokes still suck. so there :nener:

 

[Rooster]

I have a headache now. I just want to twist the throttle and go. Don't care how many strokes it is :yeehaw:

 

[Rich Rohrich]

2 strokes still suck. so there :nener:

.... and you STILL smell like POTATOES :nener:

 

[Micahdawg]

It may be 540, I don't really know...just going by what I read.

If money was no factor, I would probably have a garage full of new 250cc two strokes and a host of 250F, 450F's in addition to enduro bikes and some straight up Motards......not to mention a bunch of vintage bikes.

Money is a serious factor for me though...so I have to choose a bike that mostly fits into my needs. I want a fast, light bike with the best suspension period. So that puts me into Motocross bikes. On top of that, I need something easy to work on that I understand. That puts me into 2 strokes.

4 strokes are fun bikes too, but when I'm shelling out money on rebuilds it's nice to never worry about timing chains/gears/tensioners, cams and lobe wear, lash adjustment, valves, valve grinds and port jobs, etc... Mechanical simplicity often equates into less $$$$ to spend. And that is priority #1.

Micah

 

[jimyz]

Well put Micahdawg, I am on the same page after having been on the four stroke band wagon for five years and now back to pre-mix.

 

[XRpredator]

okay, I been thinkin', and that's dangerous . . .

The argument is that a two stroke is cheaper to rebuild than a four. Now, my feeble mind tells me that (in general) a 4 stroke needs fewer top end jobs than a two stroke, so in my head the voices tell me that you are gonna spend about the same money in the same time frame.

Plus I save money not having to buy mix oil.

but then again, I'm me.

 

[Rich Rohrich]

okay, I been thinkin', and that's dangerous . . .

The argument is that a two stroke is cheaper to rebuild than a four. Now, my feeble mind tells me that (in general) a 4 stroke needs fewer top end jobs than a two stroke, so in my head the voices tell me that you are gonna spend about the same money in the same time frame.

Plus I save money not having to buy mix oil.

but then again, I'm me.

If you are talking about the current crop of four-strokes (CRFs, RMZ, KXF) with single compression ring pistons that are being raced then the top end (piston and ring) change frequency is about the same as a two-stroke engine that is being raced. Cranks and rods on the four-strokes tend to last longer , except in the case of riders who actually HIT the rev limiter frequently. Hitting the rev-limiter on a 13,500 rpm four-stroke shuts off the ignition and causes the connecting rod and the cranks bearings to see HUGE tensile loads at TDC because their is no cylinder pressure above the piston at this point to act as a cushion at TDC. Guys who ride 30 rpm UNDER the rev limiter all day long are less likely to break a rod than the guys who hit the rev limiter frequently, but run mostly lower rpm. People have the unfortunate habit of treating the rev limiter like a throttle stop, and they are breaking parts more often than they should as a result.

 

[WillyM]

Everyone forgets those emissions laws don't effect race machines.

 

[oldgoat]

Yes, two-stroke dirtbikes have fallen out of favor with the OEM's because of emissions. But truth be told the clean answer to the 2S emission problem is the direct-injection two-stroke (DI2S) engine.

The popularity of four-stroke dirtbikes has more to do with marketing than anything else. At the end of the 1990's, sales of 2S bikes had reached a plateau and it would have been difficult for the OEM's to move riders (buyers) from the current two-strokes to DI2S bikes. So they introduced powerful four stroke bikes to the buying public. As soon as the market is flooded with 4S bikes and sales begin to tapper off, the next "new thing" will be the 2S engine.

Two-stroke dirtbikes will come back in a big way. It's just a matter of time.

I'm in full agreement. Why would you want more moving parts and expensive when the 2 stroke are cheaper and easier to work on ? To each his own , but I'm staying with 2 strokes till they quit making them. My 2 cents :yeehaw:

 

[Micahdawg]

In the case of an XR motor vs. a two stroke....in the end you will probably spend way more money on a two stroke than an XR. XR motors are low revvers and if properly maintained should last forever (low HP, low revving = happy conditions). BUT, you ain't gonna win races with an XR motor.

When you consider the newer four strokes? I don't know how this new breed of high revving machines will last or what it will take to rebuild them. I know what it costs to rebuild a typical single cam XR motor though, and when you double the HP output, double the rpm range, double the cams and put 5 valves in the head I just cringe at the cost factor to replace all that stuff when something lets go.

Not to mention the fact that I would refuse to pay someone to work on my bike. And I don't want to fuss with all that crap. I've got a pushrod hotrod in the garage and messing with valvetrain geometry and valve jobs, etc.. suck. My two stroke bike is actualy fun to work on by contrast. :)

Micah

 

[Chris_S]

I love riding both 2 strokes and 4 strokes, though I prefer the 2 stroke fun factor and lightweight flickability. The main reason for me to switch was for reduced and simpler maintenance...I wanted an engine that I could do almost all the work on, and I was intimidated going beyond valve checks on 4 strokes.

 

[mtk]

I It's funny to hear F1 and WSB engine guys talk about having to run 50 degrees of ignition lead just to get everything to burn as engine speeds reach above 15000 rpm.

I recall reading once that the F1 guys didn't like the huge amounts of advance they needed to work at the RPMs they were turning, so they turned to their fuel suppliers for a fuel that would allow them to run more "normal" amounts of advance in their engines. Being F1, they threw HUGE sums of money at the problem and sure enough, came up with a fuel that would allow it. This was probably a decade ago or thereabouts and this was the era when guys wore HAZMAT suits to fuel an F1 car.

It's amazing what you can accomplish with a container ship full of cash.

And I agree with you 100% that the current 125/250 and 250/450 rules aren't getting it done. I'd like to see them rework it so the pingers aren't at the disadvantage they are today. Adjust the two-stroke displacement up a bit to give them parity with the fours and then let the best rider win.

 

[Rich Rohrich]

I recall reading once that the F1 guys didn't like the huge amounts of advance they needed to work at the RPMs they were turning, so they turned to their fuel suppliers for a fuel that would allow them to run more "normal" amounts of advance in their engines. Being F1, they threw HUGE sums of money at the problem and sure enough, came up with a fuel that would allow it. This was probably a decade ago or thereabouts and this was the era when guys wore HAZMAT suits to fuel an F1 car.

I think that's what ultimately forced the rules makers to go with an RON limit on the fuel and a stipulation that only components that currently exist in road legal fuel can be used in the fuel. The fuel that Shell and some of the others produce for F1 is still pretty trick, but nothing like it was in the early 90s. The funny thing is, with current rpm levels they are forced to run compression ratios in the 12.5:1 range.

 

[XRpredator]

In the case of an XR motor vs. a two stroke....in the end you will probably spend way more money on a two stroke than an XR. XR motors are low revvers and if properly maintained should last forever (low HP, low revving = happy conditions). BUT, you ain't gonna win races with an XR motor.

I ain't gonna, but I've seen it done. And I have a feeling Vic will last plenty long. As Rich said, there's the former 125 buzz bomb pilots that jump onto the 250F's and bang against the limiter. Then it's all the bike's fault, according to these goobers.

Maybe if these guys would learn to ride 'em, they would discover the superiority of the thumper

and they will join me in saying "2 strokes suck"

 

[Rich Rohrich]

BUT, you ain't gonna win races with an XR motor.

I'll make sure and call Scott Summers and tell him to give back all those damn trophys cluttering up his garage, and I'll have him send back the CRF450 head we did for him that is filled with epoxy to make the engine more XR like in it's power production. :rotfl:


.... ever notice how every four-stroke goof evokes the name of Scott Summers to prove some weak point and every two-stroke goob points to James Stewart as they cling to that last bit of hope? We are all hopeless. ;)

 

[mtk]

I think that's what ultimately forced the rules makers to go with an RON limit on the fuel and a stipulation that only components that currently exist in road legal fuel can be used in the fuel. The fuel that Shell and some of the others produce for F1 is still pretty trick, but nothing like it was in the early 90s. The funny thing is, with current rpm levels they are forced to run compression ratios in the 12.5:1 range.

That and the fact that a lot of crewmembers were asking for clauses in their contracts that they didn't have to have anything to do with the fuel. Something about two-headed babies and all that. :yikes:

No matter what, there's something wonderful about a 3.0L V10 wailing at 19,000rpm, particularly if it's Ferrari red with Schumy at the wheel.

I just wish we could get a USGP somewhere other than Laguna Seca. If it were at Barber Motorsports Park, I'd be making a road trip to Alabama this summer to watch Valentino Rossi hand out the whoop ass to the world. :cool:

 

[Sayntmatt]

I cant add anything educational to this topic because some of this tech talk goes way over my head. What does make me cringe tho is when you see people at the local race sat waiting on the start gate bouncing there 4 stroke machines off the limiter because they think like a 2 stroke it needs to have its throat cleared! I think they do this because they have been brought up on 2 strokes and old habbits die hard etc. Now I dont claim to be an engine guru by any stretch but am I right in thinking 4 strokes dont need to be cleared out like 2 strokes?

I was recently sat on the gate waiting for practice next to a guy on an 05 CRF250 who was doing this then all of a sudden his engine started spewing oil AND coolant out of the engine!! I dont know if he had boiled it up or what? Its no wonder they are going pop!

 

[Micahdawg]

yeah, that is another seriously annoying problem with 4 strokes. Overheating at the gates. A friends 400F did it religiously and I've heard this is an ongoing battle with KXF's and RMZ's. Can't they strap a catch can to these things...geeze.

As for the superiority of the thumpers....I don't want to rub it in, but I did handsomly walk away from a 2004 YZ250F on my 1989 KX250...on the street, from a dead stop, all out drag race. He had a 50 tooth rear sprocket and I have a 47. Even with his tighter gearing I had him from the line.

It wasn't an all out crushing kill, but as fast as that 250F felt when I rode it, i wasn't sure if I had enough. 250 vs. 250 straight up. 1989 vs. 2004 top of the line technology. Which just emphasizes the point that if two strokes weren't handicapped to give the thumpers a chance....they would still be winning continually.

Micah

 

[Lissa]

I used to race a XR. At that time alot of off-road riders did, there was nothing better 4str wise available yet. I loved the power delivery but the bikes weren't very happy when modified. Since the XR's were designed as a trail bike and not a racer, any performance modifications put a lot of stress on all the other stock engine internals. We (boyfriend and I) have both new and old 4str's in the garage. One is a '02 YZF426 and the other a '92 DR435 (it was a 350 at one time). The YZF has revalved suspension. The DR has RM suspension machined to fit. The YZF makes cool sounds when revved to the moon and the DR is a tractor on two wheels. Between the two, my boyfriend and I would both rather ride the old DR! What I loved about riding a 4str was low-end grunt; the power delivery that allowed you to short shift. When these new 4str were developed the manufactures made them lighter (thank goodness), start easier, and better suspended but they also tried to make the power delivery more 2str like. The YZF lacks bottom-end. It loves to rev, but for those of us who like more traditional 4str grunt, the new crop of 4str scoots just don't have it. Yea I could buy a XR650R or a KTM525 but neither of those are going to scoot between the tree's the way I expect my bikes too. I'm staying with my 2str's for racing. They have been cheaper to maintain and with less moving parts to break, more reliable for my application.

 

[Rich Rohrich]

As for the superiority of the thumpers....I don't want to rub it in, but I did handsomly walk away from a 2004 YZ250F on my 1989 KX250...on the street, from a dead stop, all out drag race. He had a 50 tooth rear sprocket and I have a 47. Even with his tighter gearing I had him from the line.

Well that sums it up. :coocoo:

 

[XRpredator]

Well that sums it up. :coocoo:

yep, 2 strokes suck. :nener:

 

[Micahdawg]

OK, so the YZ rider weighs 160lbs and I weigh 210lbs. My KX has fresh boyeson reeds, 174 main jet, 52 pilot jet, Uni filter and FMF pipe/silencer. The YZ was stock. We are both at 500 ft over sea level.

So now that you have all of the data...and we did 5 runs back to back through the approx. length of 1/8th mile....can you tell me any MX four stroke that can beat any same cc MX two stroke?

Because at least back in 1989 two stroke technology was better powerwise than Yamaha fourstroke technology in 2004.

Micah

 

[Lissa]

2str riders, Do not let these people get you wound-up. When I last seriously raced a 4str, everyone would belly-ache to me. "Why don't you ride a 2str?" "Why do you wanna race that?" "4str are a disadvantage" etc., etc., etc. How times have changed. No sooner I jump ship everyone else changes their tune. You know what, I couldn't care less. I know what works for me and thats what I race. This arguement, I'm sure will continue till the end of time so ride what you like.