Packaging power - 2T Vs 4T

[Graeme M]

Just reading the comparo in the latest Dirt Action of the 250Fs and the 150 2-strokes. I am curious about something. The 150s are almost as fast but lack in that they don't hook up as well, are 'busy' in the rear etc. They appear to develop about 2-4 HP less at the back wheel. The 250Fs are faster. However, a 250 2-stroke develops about 10 HP MORE than a 250F, yet the same applies. Too much power that is too skittery, so the average rider is a bit slower on the 2-stroke. Yet going back a few years people happily raced the 2-strokes and sought more power to be competitive. If you'd sold a 250 2-stroke that gave away 10 HP in favour of a broad enduro like power spread, you'd have been laughed out of the place. So, what gives?
 
Why all of a sudden are broad flat powerbands with low horsepower the hot setup? I'm talking 250s here cos I think the 450s are approaching the old 500s in terms of outright power.
 
What if someone built a 250cc 2T with say 35-38 HP (slightly more than a 4T) but a broad tractor like powerband combined with a 2-strokes quick response? Would that be competitive with a 250 4T? Or is it just that a 10,000rpm powerband will always be easier to ride than a 5,000rpm one?

[Stan S]

Hi Graeme, I know quite a few younger good riders that are choosing to go back to a 250 2 stroke after spending a few years compeditively racing 250 four strokes and stints on 450's. They like the extra torque of a good 250 2 stroke over the 250 four stroke and find them easier to ride than a 450. Who knows maybe we will have to dust off Cr500's and Kx500's again.

Regards Stan.

[monaro308]

After growing up on 2 strokes then a sabitical of almost 20 years and then jumping straight on a WR450F,
i reckon i have become a lazy rider and almost depending on the engine to get me out of trouble instead of thinking about getting out of trouble.....i was never a good rider but i am noticing that i am becoming bored with riding.
Jump on Thumpertalk.com and you will notice ALOT more people are asking about 2 stroke bike questions.
The ones that have jumped ship are having a ball again and are having to stay switched on and more alert.
Plus when things go bang on a 4banger.....they are not impressed with the rebuild costs.
I know its not a 250 Graeme,but it won't be long before i give the WR the flick.....the top heaviness and the fact that i ever hardly use that power in the bush makes me want to go back to the 250 2 smoke.

[vandy010]

a few years back when i was a bit poorer, i rode a white TTR250 Yammy evrywhere.
MX, trail rides and to and from work.
i never had the spark plug out of it and as you could imagine the valves were never shimmed either. just changed the oil and cleaned the filter and that was it.
i loved it!
on the MX track, it cornered beautifully and predictably and i found the corners were where i could make up ground.
after i sold it, i bought a 2000 model CR250 and phuck me!
what a weapon.
it took me ages to figure out how to ride it properly and i reckon i was slower on that than i was on my little TTR for a long time.
i grew to love the CR but the 250 2 bangers from where i stand command respect.
if your not fit or just after a lazy ride they can bite you big time.
now if i had my TTR donk in a YZ125 chassis... mmm, now that'd be a nice bike.
now in the vintage scene and the pre~75 125's are a ton of fun!
close to the ground if any trouble presents, so thats cool.
but i also like the bottom end grunt of my 360 2 banger as well.
not sure if this sheds any light on the topic, just raving on over 2 v's 4 i guess.
i liked my TTR.

[ba-02-xr]

To ride a 2 stroke you need better throttle control. Then it all comes together. I raced a 250 4 stroke last year. This year I will race a 150 2 stroke. I will stay on 2 strokes for a while I think.

[Nathan S]

I think a big part of it, is that we all run 2-strokes with too-light flywheel weights. Oh, the 'zing' feels great, but the reality is that its costing us traction and is increasing lap times.

And the fact that a 4T is basically a 'big bang' style of motor (to bastardise the Moto GP term), which is better at finding traction on dirt.

[frostype400]

Still it all depends what you like I had no trouble keeping with a new wr450f on my pe400 and the pe is so much more fun to ride when I finish my project one with the disc front end it will be just as good as a new bike.

[evo550]

Graeme,
ADB have also done a similar test.
I think the reason a 2 t is more "Skittery" is that they are much lighter therefore tend to ride across the bumps, where a heavier 4t will ride through them.
As for the 2 t's being slower, I think that is just a case of test riders spending all their life on a 4t and not being able to ride a 2t properly.
I think if one of these test hot shots had the opportunity to spend 12 months on a 2t before these tests were carried out, the lap times might read a little different.
ADB also said MA is on the verge of allowing 250t's to compete in the lites class nation wide. (sorry not really vintage I know)

[Graeme M]

yeah, I tend to that point of view. I can't see that the 4-stroke style of power is so much better that it makes up for a lot LESS power. Maybe it does, but if not, then you'd think that 15 years ago, building say a CR250 2T with an extra 15 lbs, 10 less HP and a broader powerband would have been a winner. But of course it wasn't. So, IF we could bring a gun 18 year old from 1995 into 2010 and put him on a YZ250 2T and let him race a gun 18 year old on a 250F, I wonder who'd win? Or even up against a 450F?  Hmmm...

[Lozza]

It is the same as a 125GP is 1/7th the  capacity of a MotoGP bike but is definantly isn't 7 times slower around the track.
Part of it is the traction a 4T gets from 1 power  pulse every 720deg of crank rotation, where it's 2 power pulses in the same amount of crankshaft rotation from a 2T. So for the same rpm from the 2 engines (on a micro level) the 2T will have more tyre slippage than the 4T, although they both would appear to be spinning the rear.
I don't think less HP would be the go what would be a good idea and easy to try would be a flywheel weight on the 2T or better still a crankshaft with a higher inertia rating. As would some electronic control of the power valve.
EDIT
Ooops I glossed over Nathan's post

[JohnnyO]

It's the 4 strokes power delivery and extra traction that makes them so good. A 250f has a lot more grunt out of corners than a 150 2t.
The 250 2 strokes power is snappy and more aggressive than a 250 4 stroke making it harder to ride even though it has more power it doesn't put it to the ground like a 4 stroke.
It's not the less power making them go faster.. it's the 4 stroke.

[frostype400]

imagine a 2t with traction control like the moto gp bikes have???

[evo550]

A 250f has a lot more grunt out of corners than a 150 2t.

Because it has an extra 100cc.



It's not the less power making them go faster.. it's the 4 stroke.

No, It's because they are almost double the capacity of a 2t in the same class. This is the only way manufacturers where able to con the public that these were good bikes.

[JohnnyO]

Evo i was meaning less power comparing a 250f to a 250 2t.
The 4 stroke will run quicker lap times most of the time because of the easier to use power delivery and increased traction.

[holeshot buddy]

what i have noticed riding my 500 2 smoker against
modern 4 bangers is corner speed
i dont know how many times i have stalled or nearly stalled
or nearly hit 4 strokes mid corner
i find on the 2 stroke you tend more too ride the whole corner
where the 4 strokes stop and go and point and shoot
 took a helmet cam of a race and they seem to go like shit down the straights
brake early ride into corner square it and power out at a great rate of knots
where on  a 2 stroke you brake later get on power early and hold it through turn
thats why i nearly hit them no doubt they have great drive out of corners
i think this is the main differance
in the usa jason lawrence (tool) rode a new yz250 2 stroke
and then a 4 stroke and was 3 seconds faster on the 2 stroke
when i ride the new 4 strokes i find them boring cant wait too get back on my bike
must admit i liked the 2010 RM250F handled great and you could ride it like a 2 banger