Two strokes...

[wildpudding]

Hi everyone,

I just have a wee question which has been on my mind lately, and that is, why do two-strokes for a given displacement have a greater power output compared to four-strokes.

Mabye a simple question, but what charastictics about the design of two stroke engines let it produce more power? Is it because they have power strokes twice as often? Or mabye due to the design of the exaust, to use a pressure wave to suck in more fuel and air?

Cheers,

Stefan

[digsaw]

well the exaust has a lot to do with the horse power in two strkes as well as in 4 strokes but the best part is the power stroke per revolution against one per two revolutions per cylinder of o 4 stroke and that a twostroke only has three major moving parts so has less friction to overcome

[Mental Trousers]

2 strokes produce power more often than 4 strokes. Twice as often in fact. However, the 4 stroke extracts more from each power stroke then the 2 stroke does, so twice as many power strokes != twice as much power.

Since power is how often torque is applied the 2 stroke produces more power than a 4 stroke of the same capacity.

[Motu]

A 2 stroke doesn't give twice the horsepower of a 4 stroke,although with a low tuned 4 stroke and a highly tuned 2 stroke it may look that way.A 2 stroke is less effficient,but one bang a revolution makes up for a lot,they don't need a fancy exhaust,but having an expansion chamber makes up for losses in efficiency.They normaly have lighter flywheels too,so the motor accelerate better than a 4 stroke.

Riding style need to be altered a bit too - power comes on hard with smokers,and with the light flywheels they spin up real quick with loss of traction,when they get traction again they snap back instantly....so you gotta be ready for that.There is also no engine braking,it's like going into a corner with the clutch pulled in - so no safety margin with corner entry,it's all done with the brakes,so you better get it right.Up sides are the gearbox requires no skill at all,you just smack the lever around,there is no drama with rear wheel slides,being in the wrong gear is no worry,just smack it down until it goes.They are a shit load of fun to ride!

[pyrocam]

2 smokers are great bikes. but I'll never buy another aircooled one.
I figure, less work to do to get a revolution out of it is always better.

[VasalineWarrior]

Like motu said, strokers rule. Nuff said

[justsomeguy]

Riding style need to be altered a bit too - power comes on hard with smokers,and with the light flywheels they spin up real quick with loss of traction,when they get traction again they snap back instantly....so you gotta be ready for that.There is also no engine braking,it's like going into a corner with the clutch pulled in - so no safety margin with corner entry,it's all done with the brakes,so you better get it right.Up sides are the gearbox requires no skill at all,you just smack the lever around,there is no drama with rear wheel slides,being in the wrong gear is no worry,just smack it down until it goes.They are a shit load of fun to ride!

Hi Motu,

Could you please expand on the above?

After reading NSRPaul's maintenance list and costs - I'm really put off getting a 2 stroke 250 (was thinking of buying one).

Are the maintenance procedures really that expensive??? What about all the horror stories about engines seizing??

Thank You,
JSG

[Ixion]

The NSR250 is about as close to a race bike as you can get on the road. Maintainence costs are high because it is , in effect, a race machine, not because it is a two stroke. Maintainence on a sport-touring two stroke is less than a four stroke - obviously because there are fewer parts!.

Mr Motu's comments on riding style pertain specifically to off road riding - obviously in road riding you would not expect to experience loss of traction - (hopefiully!). In dirt riding , it's the norm, and as Mr Motu says, two smokers behave differently when it happens.

And, yep, they do seize. Some riders have done biggish mileages without it happening, not sure how they manage it, but expect it to happen sooner or later. But it's not such a big deal. Cover the clutch, mostly they free up once you stop (I'm not talking racing here ) , pull the oil pump cable to give max oil and let them rev up and down freely for a bit. Prolly don't even have to pull the barrells.

[What?]

A certain Rolls Royce engineer once stated "Four strokes = 1 stroke for power, 3 to wear the engine out".

Not sure how his career went after that..

[Motu]

Actualy I was thinking more of my experiance on a dirt track and gravel roads where the speeds are higher than off road Ixion - My TS400 is still the only bike that's dumped me on a gravel road,and when I set it up for dirt track with a TM top end I purposely kept the heavy TS crank and flywheel to tame the nasty snap.I was very wary of that bike,and only rode it to 80%,but that was enough to win easily and they often handicapped me a substantial amount.

The same thing is stopping me giving the DT230 it's head on sealed roads,I'm very hesitant of getting on the gas too hard in a blind corner early (good move anyway eh?) because if the exit has a bump or it's a drop away downhill I know what will happen,and it won't be nice.The thing is an unleashed 2 stroke is not something you can control with the throttle like a 4 stroke,you can't just roll off a tad to control wheelspin,it's on or off,thems the choices.

I've done a strange thing in my bike choices the last few years - gone to a 4 stroke trials bike,and a 2 stroke street bike....against the trend - so what's new eh? But it's given me a huge insight on 2 strokes,the advantages and disadvantages,especially with this modern state of the art job with electronic powervalve and traction control.On a 4 stroke when you throttle off it puts weight on the front end,and this is where a lot of people get into trouble,and I'd say the major proportion of sportsbike ''accidents'' are from ''losing'' the front.Off road and in gravel this gives a front end you have to ''fight'' as the weight goes on the front wheel,and you have to ''get on the gas'' to get control again.A 2 stroke doesn't do this and you can come into a potentialy tricky situation with the gas off and just roll over the problem,the front wheel staying light and in control,and if the front does get into trouble there is less weight on it,so it's easier to get back under control.

On the road...like yesterday when I was in one of those not clicking phases....I've found you can make a huge mess of a corner and get away with it - if you overcook the entry,or suddenly find yourself in a tightening radius corner,you can throttle off,brake,pick the bike up,put it down,whatever - it doesn't seem to matter as much as it would on a 4 stroke.It's because the front end doesn't get heavy,you don't ''push'' the front,there is no need to ''get on the gas'' - you sail through a complete cock up and think ''oh boy,that was messy you dork!'' On gravel the thick stuff is now a breeze,you can throttle off mid corner on an off camber with thick gravel without fear of plunging down the bank,corner entry is all brakes and you can be stupidly out of control coming into a corner and tidy it up.

I'm lovin' my smoker!

[F5 Dave]

And, yep, they do seize. Some riders have done biggish mileages without it happening, not sure how they manage it, but expect it to happen sooner or later. But it's not such a big deal. Cover the clutch, mostly they free up once you stop (I'm not talking racing here ) , pull the oil pump cable to give max oil and let them rev up and down freely for a bit. Prolly don't even have to pull the barrells.

2 strokes only seize if you do something silly, like run it out of oil / they are left beyond their maintenance schedule or you’ve bought some dubious manufacturer kit.

There are zillions of kdx’s out there going great guns as long as you remember to add oil to the gas they will continue to. Race tuned engines of which the NSR/RGV etc & MX bikes are compromising performance for maintenance. You have to keep replacing parts & servicing them.

The equivalent is the latest 4 stroke MX bikes. They have gone in leaps & bounds over the old XRs, but owners are finding they are now where near as reliable & in fact many are changing back to two strokes as they are cheaper to maintain.

Road going 2 strokes are now so old that they are gong to be worn out & need a rebuild to the extent that it is probably not economically viable.

Some powervalve systems didn’t help that as they wear they drop into the piston. On an MX bike this isn’t likely to wear that much, but with thousands of ks a road bike will. RG150s shared the same gremlins the RGV250s did. Yamaha’s system at least spared that particular failure.

With dubious manufacture the RG & RGV150s are Thai specials & show it. I have one of the later on my bench with a cracked piston. I not it is 1st oversize so it has happened before. I suspect oil tank has been run dry, but have heard of another oil pump failure. Not normal Japanese behaviour.