All bikes have powerbands.

[MacD]

Do all bikes (and cages) have powerbands?

Not all bikes come with powerbands installed, however you can buy after-market ones (as illustrated below).

[marty]

I may be missing the mark here but I think the original debate may have stemmed from a confusion over what is correctly known as a powervalve. A powervalve is a sort of sliding plate that is attatched in front of the exhaust port of two stroke bikes (motocross and some road). The reason is that because high performance two strokes have large expansion chambers to produce high power, they would otherwise suffer greatly in low to mid rpm power because of the lack of exhaust back pressure. Different setups are operated either by cable or electronically. The "valve" is down at low rpm to create more backpressure (and power), and then opens at higher rpm so the bike can produce maximum horsepower.

The effect of all this is that in real riding conditions, it creates a sudden surge in power when the valve opens, which is a trademark of any high performance two stroke.

The confusing bit is that a powervalve is often called a powerband. But that word is also used when describing the power delivery of any motor.

But to be clear, powervalves are only present in the form described on two strokes. There is additional confusion to this saga however as Yamaha, and more recently Suzuki, have used the same principle in their four stroke sports bikes in the form of valve located in the middle of the exhaust system. This setup is used for the same purpose as a powervalve in two strokes

Just my 2c worth

i have to disagree. the powerband is created by gas flow being accelerated by the shape of the exhaust. the powervalve is only a means of allowing more gas flow. you can jam/tie the powervalves in the open position, and you will still get a noticeable powerband, especially in a performance 2 stroke (like an RS250/RGV etc). it is a result of the engine (and corresponding exhaust gas flow) reaching it's optimum tuned pipe flow rate, and the exhaust pipe literally 'sucking' the exhaust gasses out, allowing more air to come in the intake side, and correspondingly more air to go BANG.

4 stroke engines do not have a powerband in the 2 stroke sense. they have a power (torque) curve, and tend to have a much more even power delivery. this of course can be modified by cam lobe cut/valve opening duration/timing etc.

becuase they have twice the piston travel per cycle, they have a period of time when both valves are closed, so they tend to pulse more than a 2 stroke, not allowing the 'sucking' action of a 2 stroke exhaust system to be effective.

[curious george]

We need a big dose of Motu in here...

Failing that, captain sensible himself..... Ixion!
Well done that man

[thealmightytaco]

My understanding of it all is that everything has a power curve, and with 4 strokes it's pretty smooth and gradual and there's good power for much of the rev range cause the valves can close it all off so you get good compression the whole time. But with 2 strokes things need to hit some kind of rhythmic perfection before the same valve effect is working which only comes in during a certain "band" of the power curve, the rest of the curve is schlock cause things aren't humming right and you're letting things escape everywhere. Or something. So the power band is just a name for the useful part of the power curve for 2 strokes, as I have come to see it. But it's all terminology really, open to interpretation.

[TwoSeven]

i have to disagree. the powerband is created by gas flow being accelerated by the shape of the exhaust. the powervalve is only a means of allowing more gas flow. you can jam/tie the powervalves in the open position, and you will still get a noticeable powerband, especially in a performance 2 stroke (like an RS250/RGV etc). it is a result of the engine (and corresponding exhaust gas flow) reaching it's optimum tuned pipe flow rate, and the exhaust pipe literally 'sucking' the exhaust gasses out, allowing more air to come in the intake side, and correspondingly more air to go BANG.

4 stroke engines do not have a powerband in the 2 stroke sense. they have a power (torque) curve, and tend to have a much more even power delivery. this of course can be modified by cam lobe cut/valve opening duration/timing etc.

becuase they have twice the piston travel per cycle, they have a period of time when both valves are closed, so they tend to pulse more than a 2 stroke, not allowing the 'sucking' action of a 2 stroke exhaust system to be effective.

I think these two posts are talking about similar things. While I dont know much about two-smoke tuning. I would suggest that the expansion chamber shape is used for both gas expansion/slowing (start of the expansion chamber) and for resonance [not sure if thats the correct word] tuning of the power wave (the closing part of the expansion chamber). If thats the case, then the rebounding power wave will require something to cause it to rebound back off the exhaust port and either build pressure for extra combustion or scavange the port - which ever affect is desired.

This is the same as in four-smokes and is a part of header/megaphone design. To get the effect occuring at two different RPMs (usually mid-range and high end), you insert a valve at the end of the attenuator (primary pipe) which as the effect of changing its length - thus changing the period (I think thats the correct word) of the rebounding wave.

Hopefully I got it right.

[Motu]

Modern bikes don't have noticable powerbands these days - 2 strokes now have powervalves and are sorta 2 motors in one,with the powervalve in the lowspeed position (shut) the exhaust port has timing suitable for low speed power,although the other ports are unchanged.With the powervalve open the bike makes high speed power,they also have reed valves and crankcase induction.Two strokes make strong hard horsepower everywhere these days - it's seamless....and still scary.Strongest hitting powerband I've ever come across was on a mid 80s Husky 250 MXer,it was the proverbial lightswitch and I was never able to keep the front down when it hit,even on the road where there were no other obsticals to run into.

As for 4 stroke, a by product of car makers having to meet emission controls,there have been huge advances in combustion chamber design and camshaft profiles.There are even motors with variable cam timing,so sorta like a 2 stroke powervalve - so a street 4 stroke now makes much more HP than a ropey almost unriderble motor did 25 yrs ago,but idles around town like a Morris Minor.The race tuned singles and twins from the 50s and 60s had rather narrow powerbands and were quite hard to ride with close ratio 4 speed boxes - they had megaphonitis when the dropped ''off the cam'' and would 8 stroke and splutter,you'd have to slip the clutch into the powerband and get the speed back up.

Powerbands are a thing of the past...for old fart to reminiss about.

[gav]

Rather than the term "powerband" think of it more as a a "sweetspot", that rpm range that your engine is working at its best, you know how sometimes you may need to shift down one or two gears to pass something, or how you know you should change up a gear when you feel the power start to flatten out after hitting peak pwer. "Powerband" is more a two stroke term as a tuned two stroke will barely function outside its sweetspot or powerband, so you tend to ride them on the track just knowing where that maximum hit or rush of power is going to come in, ie you'd prefer it just as your exiting the corner rather than entering normally, or just as youre about to pull out to pass that car or ya mate! Equally a two stroke will almost feel like it hits a wall when you over rev them. Look at the hp curves in magazines, look to see if the power has a "kick" at a certain revs and also how the power drops away quickly after hitting maximum hp. Four strokes tend to have a flatter curve and more of an over rev ability, see how the power curve slowly drops, you've got a wider power curve to use, hence a wider "powerband" or "sweetspot".
Hope this helps!

[Jamiepo]

Yes. The faster they go, the further up and down in the sleeves the pistons go. If you go too fast the piston either impacts the head or drops out the bottom into the sump. Be careful.

I have been pondering this staement for a wee while and am wondering how much truth there is is its?? I just can't see how unless the metal stretches it would do this, I know that in a 4 stroke the piston can sometimes hit the overhead valves at high RPM especially with sticky valves but going higher and lower in the sleeves at a high RPM, I just can't fathome how that works. Or was hitcher just being sarcastic??

[avgas]

everything has a 'powerband', not everything has powervalves.
Of course in some machines the powerband is all over the rev range.

[MisterD]

I have been pondering this staement for a wee while and am wondering how much truth there is is its?? I just can't see how unless the metal stretches it would do this, I know that in a 4 stroke the piston can sometimes hit the overhead valves at high RPM especially with sticky valves but going higher and lower in the sleeves at a high RPM, I just can't fathome how that works. Or was hitcher just being sarcastic??

I think it's called exaggerating for comic effect....it certainly is possible for the piston to hit the cylinder head at high revs, especially with 2-strokes where the shape of the head and the squish band is important. If your piston is too heavy then it certainly is possible for the crank to stretch enough to cause an expensive problem.

[*sic]

Vt@k Y0...

[Lou Girardin]

The Rocket has a power band, starts at 800 rpm and goes till the limiter stops play.

[Motu]

I think it's called exaggerating for comic effect....it certainly is possible for the piston to hit the cylinder head at high revs, especially with 2-strokes where the shape of the head and the squish band is important. If your piston is too heavy then it certainly is possible for the crank to stretch enough to cause an expensive problem.

Back in the 60s when 2 stroke design was being pushed to the limits,they were getting some astounding rpm,it would still considered doing pretty well these days.Because a 2 stroke fires everytime the piston is at TDC,there is a resistance to the pistons desire to reach for the sky - this was taken into account in the design,everything as light as possible....but if the engine just misfired the piston could fly apart from the forces.So in a way Hitcher is right....but you knew he couldn't be wrong eh?

[MisterD]

Back in the 60s when 2 stroke design was being pushed to the limits,they were getting some astounding rpm,it would still considered doing pretty well these days.Because a 2 stroke fires everytime the piston is at TDC,there is a resistance to the pistons desire to reach for the sky - this was taken into account in the design,everything as light as possible....but if the engine just misfired the piston could fly apart from the forces.So in a way Hitcher is right....but you knew he couldn't be wrong eh?

I was actually wondering if he knew how right he was! I've seen some real horror-show pictures of what happens when you over-do your 2-stroke tuning.

I have a grand total of 2mm clearance between my piston and cylinder head squish band on my Lambretta, which is fine with the alloy piston I run but would be dodgy with heavier jap motorbike pistons....

I had noticed that no-one seems to be recommending up-rating your piston-return springs, what's with that?

[TwoSeven]

I think it's called exaggerating for comic effect....it certainly is possible for the piston to hit the cylinder head at high revs, especially with 2-strokes where the shape of the head and the squish band is important. If your piston is too heavy then it certainly is possible for the crank to stretch enough to cause an expensive problem.

Actually, con-rods are known to stretch. How much so these days I havn't bothered to check what with forged titanium being used in high end applications its probably almost zero, but it is an amount thats big enough to require taking it into account when tuning engine.

From memory, its to do with having the weight of the piston on top thats accelerating from stop to x-miles an hour in half a nats whisker of a second and then trying to stop again.

I think there is a special engineering formula around thats used to calculate it all. But I dont know what it is.