Powerband questions

[Xpress]

This morning I was intrigued by how the powerband on a 2 stroke engine works. I know the powerband on a 4 stroke is mostly linear, where the power is normally always there, but when you hit the powerband on a 2 stroke, you get slammed with immense acceleration and power, mostly on the upper end of the throttle range, I believe.

But, my question is how does the powerband work on a 2 stroke???? What happens with the engine and exhaust WHEN that powerband occurs, and why?

I've looked around on some articles on google, but they got to the point where they were talking about how 2 strokes don't use oil contained inside of them to lubricate them, but it gets its lubrication from the fuel, and some other things that I don't think are fully true, so I don't want to take what that article wrote as truth. Can anyone shed some light on the matter???

[dcreel]

The powerband is a mythical creature used to describe the power of a 2 stroke. Also known as "on the pipe" The powerband feeling you get is when a 2 stroke motor is doing what it does best. Make power (peak efficiency). Akin to a turbocharged motor with the lag before the boost builds, below a certain rpm depending on how your motor is built the power hits at a certain rpm thus hitting the powerband. Like a single carb on a Banshee there are ways of manipulating the powerband or where a 2 stroke motor makes the most power. Sometimes you see where people say their motor has a powerband in it. I've even seen them for sale a time or two. It would be easier to explain side by side with a dyno sheet, which would show exactly where you hit the proverbial powerband.

Here is a dyno printout of an RZ500. You can see where the line climbs almost vertically, that is where he hit the powerband. It's so much easier to feel on a 2 stroke due to it being shorter and more abrupt, than a 4 stroke which is more spread out and linear. 4 stroke have a powerband too, it's just not as easy to feel on the bike under full throttle.



The top end of a 2 stroke motor is lubricated by the premix in the gas. Without it, the top end will heat up expand and sieze.

[takeuchitb007]

dcreel said it pretty much bang on.. It's just the engine's power range reaching full peak, powerband is a term that was made up years ago to describe the feeling of the 2 stroke engine coming into it's max horsepower range.. Every engine has a "powerband" whether it's a 4cyl car, a lawnmower engine, a tractor etc you can have 2 bikes that equal hp figures, for example let's exaggerate to make a point.. If you had a 125cc 2 stroke making 30hp and a 500cc 4 stroke making 30hp the 2 stroke will feel very peaky in acceleration.. even though they have the same power the 500 will be linear across the rpm range making it feel not as snappy when really it's making the same power as the smaller engine but it's making it more spread out across the rpm whereas the 2 stroke is making it's power in a very narrow range..

Does any of this make sense? I probably could have explained it better but I'm in a rush to leave lol

[dksix]

I would have been interested to see the torque numbers on the dyno chart, decreel. I always consider the "power band" on a 2 stroke to be when everything came together for maximum air flow through the engine. Like the reeds were wide open, the carb was flowing it's maximum rate and the pipe was making just enough back pressure to hold the new mixture in but move the burnt out. Modern day 2 strokes that make the big power aren't as simple as people make 2 strokes out to be, with power valves, reed and reed cage design, pipe design, port angles and such. I would say they are now just as refined as the original idea was simple. Take a late 70's early 80's open bike and compare it to one of the latest 2 strokes and I would guess that a new 250 makes all the power of the old 450 to 550cc bikes, it not then more usable power. Of course, I'm speaking as a trail rider not an MX'er. I was always trying to calm the power down and make some luggable torque, so I may be completely off with my opinion.

[BGP]

A "powerband" is just that; a power "band". You could also say power "range". The french say "plage de puissance" or power "beach" if you will. In other words, it is the rpm range in which you engine makes "useable" power. Having said that, it all depends on the state of tune of the engine or BMEP (brake mean effective pressure) that it can produce and where in the rpm range it'll produce it. For example, old snow mobile engines had long narrow pipes, smallish carbs and very conservative port timing/geometry... i'd even go as far as to say they were "agricultural" or "industrial" in nature. However, those engines made low, soft hitting power over a wide range. Tuned properly, they would putt around all day without fouling or overheating; in fact, power output was so low, they only required air cooling. Today's two-strokes are very different and were it not for variable exhaust timing devices, would suck at anything but the few hundred rpm (or powerband ) for which they were tuned. Take a drag engine for example; barely idles, bogs down low, takes a while to clear it's throat slash heat the pipe and once it reaches the rpm range that corresponds to the design [cone tapers, lengths, diameters,etc] of the pipe/port time area [ size of the ports and exactly when they open and close regarding crank angle ] POW~ hang the EFF ON and shift fast until you fall off the pipe. Of course, power is a function of torque and rpm and on a normally aspirated engine, chances are you'll rev the crap out of it to get the most horsepower. Now, is that power useable? Depends what you're after... hence the old soft snow engine with the broad "useable" powerband or the hard hitting purpose built drag engine with the short fat, narrow stinger pipe that only run WTFO. If you're wondering where I was going with the variable timing devices, here it is: Yamaha calls'em YPVS, Ski-doo (Rotax) calls theirs RAVE, Honda had the HPP, etc, etc. What these do, in essence, is "vary the length of the pipe". Wait, What?! You read it right. Remember the short fat drag pipe that only works up top? What if you made it longer? Yup, you would move your powerband lower in the rpm range. Why? because i pipe works with the exhaust pulses. Imagine you just had combustion and the piston is accelarating towards the bottom of it's stroke, say 81 degrees of crank rotation later, it uncovers the exhaust port... a strong pulse is emitted into the pipe, is expanded by the divergent cone, carried through the body for sometime and reflected back by the convergent cone. The speed at which this occurs is largely based on the "pipe temeperature" ( temp of the gases in it, really) which in turn dictates the speed of the wave traveling through it. During that time, the piston is still traveling and roughly 40 degrees later uncovers the transfer ports... a time that should correspond to the pressure wave being in the divergent section of the pipe as it will assist in scavenging old gases and bringing a fresh charge form the base. Again, all based or pipe desing and rpm that corresponds to said design. Piston continues on past bottom dead center and accelerates back up the cylinder. Now, some or a lot of fresh fuel was "sucked" into the pipe ,i.e out of the cylinder at this point. Every two stoke pipe is designed to "stuff" that charge back into the cylinder before the piston closes the exhaust port again through it's upward stroke; that is the job of the convergent cone at the end of the pipe. That wave I mentionned earlier was reflected by this cone and makes it's way back up the pipe pushing gases with it. Based on the design of the pipe and the rpm, the fresh charge will be pushed back into the cylinder and the piston will shut the port before any spent gases get in to "pollute the charge". Another interesting fact is that a well designed unit will actually "refill" the cylinder to a point above atmospheric pressure... yes, supercharged because a good pipe will also suck inlet through the transfers!. Now imagine all this with the only variables being temperature and rpm. It becomes obvious why pipes are called "tuned pipes". SO what's the deal with the YPVS devices and why did I say the change the length of the pipe? What they actually do is change the time (in crank rotation degrees) at which the exhaust port opens to release the pulse down the pipe to then have it return. For a pipe to work at low(er) rpm, it needs to be long or have agricultural spec port timing where the exhaust opens late. For it to work at high rpm, it needs to be short or have high exhaust port. A YPVS type device will make a short pipe seem long by "blocking" the upper portion of the exaust port at low rpm and uncovering it at higher rpm. Some units will also uncover auxiliary blow down ports on either side of the main exhaust port at high rpm but that's for another day because my intent was to post the first line of this verbage and nothing more! But I LOVE two-stokes.

[dksix]

BGP, nice. The "power valve" (as I've always called it) was what you are referring to as a "YPVS type device". The part that I misunderstood was the way it worked with the pipe. I had always thought it varied compression stroke. That was a great explanation, thanks. I understand more tonight than I did this morning, it was a good day.

[Xpress]

I think I get how the powerband works a lot more, but one thing still is a big foggy is why the insane acceleration at the topend? I am understanding it has something to do with the expanding exhaust gasses blowing unburnt fuel back into the cylinder before the piston closes the port?

[Jason Hall]

When the engine Is ON the Pipe, It's using the pipe to make It's self bigger! The pipe's front cone shape (along with the exiting exhaust expansion) create's low pressure In the cases, that draw's fresh air and fuel straight through the engine and Into the pipe. The boost port helps aim the air directly at the exhaust port. Once the pulse of the exhaust wave hit's the back cone It force's the good usable air back Into the exhaust port. Seems wrong, but the exhaust port has burnt gas going out, and fresh air/fuel coming In LOL. The hard pulling power Is when everything Is In sync or the Carb, Crankcase, Reeds, Pipe, Ignition are all working together properly. Port timing's and angle's can help fill the cylinder correctly for different power range's. My goal lately Is to get smooth power through a wide range! It's a lot harder than building a screamer

[BGP]

... I had always thought it varied compression stroke...

Well, the rings do not come into contact with these mechanisms at all, and therefore wouldn't really up the compression noticeably... However, there is what is called a trapped compression ratio. That is to say the ratio of cylinder volume right as the exhaust port closes to that of TDC (or combustion chamber) volume. That is why you see compression ratios in the order of 6~8:1 on two strokes as opposed to 10~12:1 as per regular 4 strokes... Power valves only vary the amount of time the port remains open, or it's height or it's area, which ever term you prefer.

[BGP]

..., but one thing still is a big foggy is why the insane acceleration at the topend?

Not so much that it's insane at the top end as much as how it really sucks anywhere other than on the proverbial pipe! 4 strokes seem lame because they have linear torque curves; they produce OK power over thousands of rpm so there's no "hit" to speak of ( on stock units ). 2 Strokes will produce all their power a over narrower rpm range so they have not choice but to produce it in a hurry! Hence the wicked pull you feel when it all comes together into full song.

[Xpress]

I think I get it. When it hits the powerband, the engine can basically breathe fully and cleanly, no?

[HuffieVA]

In laymans terms... "The Perfect Storm", basically to ride a smoker effectively (meaning fast) you need to learn to keep it in the "Peak Power Range" through shifting and slipping the clutch if necessary you can tune them to some extent to your riding style (low end/mid range/top end) they take a bit of effort but they can be made to fit any riding style, Tecates are historically top end monsters but with some tuning along with some adjustment to your riding style they can be a good trail/play bike as well.

This is why alot of the younger riders think the newer four stroke mx bikes are so fast, basically they arent faster they are just easier to ride...

[Xpress]

My T3 is pretty well setup for the trail riding I do, but it also will do this 8 track I have nearby pretty easily, but like you said, knowing how to keep it in the "peak power range" is something I gotta learn. With a thumper, I leave it in 3rd and cruise around, whereas with the T3 it's all changing gears.

[dksix]

BGP and Jason, thanks again for the explanations. A couple more questions. What effect does the 2 strokes lack of valves, cam(s) and timing chains have? With those scavenging power that does get to the crank, it looks like that would help get more power out of less displacement. And with each down stroke of the piston being a power stroke, does that aid in building RPM's faster?

[Dirtcrasher]

Less turning mass, 1 crank revolution completing the intake/exhaust cycle versus 2 crank revolutions for every cam revolution to do the same job. No valves to push open etc etc.

Thats why some knuckleheads can't figure out the 2 TDC's on 4 stroke engines.

Less weight and until some years ago, all thumpers were air cooled and heavy......