Interesting Stuff On Vital MX

[TMKIWI]

Hey Big Boar.
I know what you mean.
Most of the guys here are pretty good when it comes to keeping it polite.
A bit of ribbing here and there but no insults normally.

Oh i feel a love in happening 

[Chris2T]

A 2 stroke is harder to turn up the rpm's like that because of the back pressure etc.  I don't believe most 2 strokes even have a rev limiter because of that.  It just runs out before it could explode.

I thought the rpm difference was due to the fact that 4 strokes bore x stroke have grown hugely oversquare, while 2 strokes have remained fairly square, not needing high rpm to find their horsepower? Interestingly, evinrude direct-injection 2 strokes are some of the most highly oversquare 2 strokes i've seen

[miedosoracing]

A 2 stroke is harder to turn up the rpm's like that because of the back pressure etc.  I don't believe most 2 strokes even have a rev limiter because of that.  It just runs out before it could explode.

I thought the rpm difference was due to the fact that 4 strokes bore x stroke have grown hugely oversquare, while 2 strokes have remained fairly square, not needing high rpm to find their horsepower? Interestingly, evinrude direct-injection 2 strokes are some of the most highly oversquare 2 strokes i've seen

I'd say you are pretty accurate, but along with what I said. :-)  Meaning, when we put in the 144cc kits and 300cc kits in our 2 strokes, it is bringing the cc's up and not really losing any RPM's from it, and it would be considered overly square. But, even at being square, 4 strokes need a rev limiter like on old XRs, because they will go until they blow. The pistons will probably hold up even higher, but the valve train give's way.  Where as the 2 stroke is somewhat limited via, porting and pipe.  If we were to change the pipe for instance, most likely we could bring the RPM's higher, and along with that, raise the ports.  But then you lose the low end.  I'm theorizing that we could actually have a pipe that had a cavity that could change with RPM, somewhat like a power valve does.  Maybe work together.  I think we have not seen what a 2 stroke can really do yet.

[miedosoracing]

I'll add a correction to myself from what I said. 2 strokes look to be more undersquared. CR250 for instance is 66.4 x 72.  So when we go to the 144cc or 300cc we are probably making them even more square. i will admit I haven't studied that side of it enough.  But me being a porter, I can say I can raise RPM on a 2 stroke with changes to the ports etc. 

[JETZcorp]

Well, I'll just say this about revving two-strokes.  My 250 Husky, obviously, doesn't have a rev limiter, and I feel confident that if I wanted to, I could make that thing grenade by revving it.  When I'm at Vmax on that bike, I'm holding it at something like 1/16 throttle, and you can tell just by listening to the engine that it does not want to go any higher.  What's scary is that when you get to the point that you think, "Shit, better not go any higher!" the bike is still pulling pretty hard.  I think it might have something to do with my pipe not having anything in the silencer, perhaps, because it seems as though the power curve keeps climbing to infinity.  I could just be reading my bike wrong, I mean, I've never taken a Husky (or any bike) so high that it actually blows, and I don't have a tach or anything to look at a red-line, but that's just my experience.  If I was feeling suicidal, I'm pretty sure I could just hold that thing wide open, break the sound barrier, leave a trail of flames, travel back in time, and shatter that motor.  I don't, however, think I could just open it up and get a boom on command, I think it would take some time.

[JohnN]

If you want to see a two-stroke that revs to the moon... check out the little motors they put into remote control airplanes!

They rev over 20,000 rpm... without rings!

JETZcorp, you're Husky, while a cool bike will not rev as far as you think it might. My feeling is that the open silencer probably makes it sound as though it's going to blow... seriously you may want to fix that, your bike will run better.

[JETZcorp]

I think you're over-estimating how much that changes how the thing sounds.  The noise it makes is exactly the same, just with higher volume and a sharper crackle on idle.  As you may know, I rode it around quite a lot on the last ride without anything it it at all, just the silencer can acting like a big Tuba horn.  It was much louder, but it didn't make me feel like I was going faster or revving higher.  You learn to ignore it after 30 seconds and ride normally.  When I rode my dad's 390, which has full packing and a spark-arrestor, it had exactly the same sort sound as the 250.  Incidentally, they both have a very different sound from all the other bikes of the time.  They actually sounded unique back then, OMG.  And of course, I have no delusions that these bikes are made to rev as high as a modern bike, if for no other reason than because I think my rod (so to speak) is still made of 1982 metallurgy.

Anyway, a lot of what determines maximum RPM is the stroke, as I was reading in the Two Stroke Tuner's Handbook.  Rods and pistons and such are limited in the amount of force you can put in them before they go, and of course we know that force goes up with speed.  That means that the faster your piston gets a moving, the lower its max RPM will be for a given strength of construction.  That's why the model airplane motors can rev so high, they may go from BDC to TDC in a very short time, but because the stroke is so short, they can take it because the physical speed of the parts never gets all that high.  This is why four-strokes are so oversquare, so they can rev high and match the two-strokes.  I think the two-strokes tend toward square (or even undersquare) because port timing is very important and you don't want 1/100th of a millimeter error in your port translating into 25 degrees of crankshaft rotation.  That's my theory on that, anyway.

[JohnN]

I think you're over-estimating how much that changes how the thing sounds.  The noise it makes is exactly the same, just with higher volume and a sharper crackle on idle.

For a really smart guy this is not such a smart statement.

How much experience do you have outside of your vintage machines? How many years have you ridden bikes of many differing decades? How many years of race tuning experience do you have?

You seem to forget that I have been around and ridden literally hundreds of different motorcycles. Including Maicos (AW 400 & 490) Husqvarna (CR390 ) air-cooled machines, watercooled machines, drum brake machines, disc brake machines, no brake machines... well you get the idea...

I understand that you love your bike, we all love our bikes, and if we don't we sell them or buy another one that we do like.

Not trying to give you a hard time but most of your knowledge and experience comes from talking to folks that have a lot of experience or from reading... there is something to be said for actual time on multiple machines.

In no way am I claiming that I know all, in fact the more I learn, the more I find out there is to learn (thank you 2T institute for that amazing quote) To me what's amaing is that over the past three months I've learned more about two-stroke tuning that I have in the previous 30 years...

All I'm asking is to keep your mind open to some new ideas, there's still lots to learn!

An open silence is irresponsible, even if there is no one within a 100 miles of you. It also makes LESS power... guaranteed!

[Recovered]

Open silencers cost ponies. No way around that.

And stroke is a factor in RPM limits but unless the pipe will go there you can be as oversquare as physically possible and it means nothing. Making power is like conducting an orchestra. Everything must work in concert to get the max out of the music. 2 stroke,4 stroke,rotary or steam it's all the same.

[JETZcorp]

I was just talking about the psychological effect of the sound.  John has said quite a number of times that he thinks the sound of a loud bike, like one with an open silencer or straight stinger, makes the rider feel like he's going fast, or like the bike is revving higher than it really is.  Well I'm saying, from my personal experience having had the bike TRIPLE in volume over the course of half a second (when the silencer fell apart) that it didn't change my perception of anything.  I didn't feel like it was suddenly powerful, it didn't seem like it was revving real high.  It was just louder, and then I got used to it, and kept riding.  Where in the hell does tuning experience and riding a large variety of bikes come into that?  I didn't say anything about horsepower, or resonance, or back-pressure, or any of that other stuff that's associated with altering a silencer.  I don't contest any of that.  I was just rebutting your speculation about how noise of the bike may have changed my perception of what my bike is doing.

Why does it seem like every time I write something today, people are reading something else?

[JohnN]

I was just talking about the psychological effect of the sound.  John has said quite a number of times that he thinks the sound of a loud bike, like one with an open silencer or straight stinger, makes the rider feel like he's going fast, or like the bike is revving higher than it really is.  Well I'm saying, from my personal experience having had the bike TRIPLE in volume over the course of half a second (when the silencer fell apart) that it didn't change my perception of anything.  I didn't feel like it was suddenly powerful, it didn't seem like it was revving real high.  It was just louder, and then I got used to it, and kept riding.  Where in the hell does tuning experience and riding a large variety of bikes come into that?  I didn't say anything about horsepower, or resonance, or back-pressure, or any of that other stuff that's associated with altering a silencer.  I don't contest any of that.  I was just rebutting your speculation about how noise of the bike may have changed my perception of what my bike is doing.

Why does it seem like every time I write something today, people are reading something else?

Hey really, I'm not trying to pick on you... please re-read some of what you wrote. It sounded like you were contesting that...

As someone that cares deeply about the longevity of our sport, I don't like to see people do things that could jeopardize our collective rights to be able to ride. When you talk about the lack of silencers on your machines, you seem to do so with a great deal of pride. At least that's how I'm reading it.

If you didn't bring it up so much, it wouldn't be an issue...

I don't want younger guys that read this board to think that kind of stuff is what they should be doing too.

Just to kind of clear things up, I'm very glad that you are a part of this community and share your ideas and opinions. It certainly makes me think about my beliefs and understandings in a new way. Your passion for two-strokes is contagious!!

[Paul P]

There is a formula that you can use to determine the limit factor on the RPM a motor will turn.  Engineers use it when designing engines. Short stroke engines will outrev long stroke engines with similar porting layouts. Remember the Can-Am's? They had a 57mm stroke when most 250's had 64mm stroke and Maico's 250 had 70mm stroke. Now, most 250 2S have 70+mm strokes. The old Can-Am Rotax motor can be designed to out rev any of the new design motors, but it will only be an advantage in certain types of riding, like road racing.
  Long stroke engines are generally more tractable and user friendly for MX. The later model 4S 50mm stroke ( + or -) allows them to get the RPM's needed to compete with a 2 stroke, and with only firing every other stroke, the motor gets that long stroke feel.
  I patterned my nitro/methanol pro hillclimber after the model airplane motors. 20,000RPM is only part of the story. The HP these little motors make per cc is incredible.
    Keep your mufflers packed and like new or face what we have faced up here in my area: tracks shut down from too much noise after this 4 stroke revolution.

[Recovered]

The ceiling for a durable engine is 5K FPM average. Depending on stroke/rod length that can mean acceleration rates over 8K FPM. The formula for average piston speed is (stroke (in MM) X 2 X RPM) / (24.2X12). Peak speed is somewhat more complex. I use software now so I can not recall the formula. Well that and I'm old too. Using that formula and a 70 MM stroke you're just a tick over 10K for 5000 FPM.

The short stroke 4 strokes achieve high rpm because the bore is large enough to have the valve area to breathe well at those speeds. The short stroke is necessitated more to allow a large bore for valve area than RPM. High RPM is the area in which VE is optimum for them.

[miedosoracing]

Current 4 stroke "The ceiling for a durable engine" don't go together. LOL

[Recovered]

The current crop of four stroke MX bikes is far exceeding the tune of the 2 strokes. Tune your CR250 to the level of a CRF450R and it will need frequent maintenance as well. The advantage of two strokes is the superior output at a relatively mild state of tune that the diesels simply can't match. That and the relative simplicity inherent in their design.

A CRF at 10K is at 4490 FPM. CR250 at the same is 5206. Wouldn't live long there.

Look I disagree with the current rules too. But the 4 strokes have elevated the state of the art for smokers as well. Try looking at the bright side once in a while.