The problem with two strokes is they don't rev very much. People think they do because they sound like they are, but actually they don't. The ones which were the highest revving were the late '80s / early '90s 250 sportsbike class.
Kawasaki KR1-S 59.13 HP (43.2 kW) @ 10500 RPM and 40.20 Nm (4.1 kgf-m or 29.7 ft. lbs) @ 10500 RPM .
Displacement is relevant, we are discussing the advantages of the two stroke here. You need a much greater displacement with a four stroke to achieve the same power output and the motorbike becomes much heavier.
People think that four strokes have more torque, they don't, but the mediocre torque curve tends to be flatter, giving the illusion of more torque.
Of course there is a big disadvantage to two strokes: the outrageous fuel consumption.
As to reliability, well that is a myth. I've pulled apart a RGV250 with 60k on the clock and it still looked new inside, and that thing is making 240hp per litre.
And as to motorcross bikes, well the modern four stroke ones require just as much rebuilding as the old two strokes, but there is a lot more parts and labour involved.
ronaldo8,
Jun 25, 9:26pm
Blither.
"People think that four strokes have more torque, they don't, but the mediocre torque curve tends to be flatter, giving the illusion of more torque."
Your opinion on the matter is as good as the one you had re photoshop. Look at some dyno charts of equal power two and four strokes, the fours produce most of their power lower in the range, that's flatter unlike a peaky 2 stroke, peaky because it has ONE tuned pipe, not one for each rev band, power valves were an attempt to fudge that, they adjusted the tuning of that single system a bit. a bit. Do you understand the functioning of an expansion chamber or don't you?
This lays it out
" Torque = hp x 5252 . over speed {RPM} so take 40 hp 2 stroke rpm 7200 and 40hp 4 stroke RPM 5300
40x5252=210080 then divide by RPM 7200 = 29.177 ft lbs of torque two stroke
40x5252=210080 then divide by RPM 5300 = 39.637 ft lbs of torque four stroke.
"
ronaldo8,
Jun 25, 9:38pm
That's equal power, not displacement or headlight radius or tyre pressure or any other two irrelevant data points you want to compare,maybe you'd like to use fuel used per watt produced as the metric for comparison? No? thought not.
O and add to that the resulting larger displacement 4 stroke has a lot larger reciprocating mass for equivalent power output and your thoughts on the matter can be simply summed up. All talk, not torque.
absolute_detail,
Jun 25, 9:40pm
Comparing them by peak hp is stupid, you can make a 40hp from a small displacement 4 stroke and its going to have zero low down torque because it needs to rev to the moon. A 250cc 2 stroke is going to make more power down low than a 250cc 4 stroke
tygertung,
Jun 25, 9:41pm
Yes, I know how an expansion chamber works; I have read the Gordon Jennings book and the A. Graham Bell book several times. You can have a very similar design on a four stroke too, but it does become very peaky. When I was racing Honda 150 singles, I put a tuned megaphone type pipe (rather similar to a two stroke expansion chamber) on my bike which my friend had made, and it made a big difference at a certain RPM, but below that the engine didn't like it and especially just before the pipe tended to "come on song" would misfire somewhat, I think due to the air going back out the wrong way through the carburetor and "triple fueling", leading to an excessively rich mixture.
Now your figures, well, they seem to be made up. Please provide a real world example.
Now, I will admit that there is a very low RPM which many two strokes produce lower power, but that is generally at a very low RPM, and four strokes have the same problem.
Power valves cam operate in many different ways, sometimes they change the port timing, sometimes they change the expansion chamber volume. They have a similar thing in four strokes now with variable valve timing.
tygertung,
Jun 25, 9:42pm
Four strokes have to be tuned to rev a lot higher to make any power as they are only firing once every four cycles.
tygertung,
Jun 25, 10:04pm
A motorbike with equivalent power output is going to tend to be a lot heavier.
ronaldo8,
Jun 25, 10:38pm
"They seem to be made up" . fing clueless
tygertung,
Jun 26, 12:14am
Please, which motorcycles did you get those power and torque figures from?
tsjcf,
Jun 26, 1:12am
So what rev range do you think your rgv250 produces most of it power?
tygertung,
Jun 26, 1:45am
Suzuki RGV250 produced over 60 bhp between 8,000 and 11,000 rpm 1993 model: 61 hp @ 11000 RPM 29.5 ft-lb @ 8000 RPM
As you can see the two stroke revs a lot less than its four stroke sister (these are both sports bikes in the same 250cc sports bike class).
The 250cc two stroke sports bikes of this era are unusually high revving for two stroke engines. Of course dedicated two stroke road racing bikes will rev higher I think, but they are fantastically expensive, and cannot be used on the road.
TZ250 The engine produces a maximum peak output power of 86.90 HP (63.4 kW) @ 12250 RPM and a maximum torque of 50.00 Nm (5.1 kgf-m or 36.9 ft. lbs) @ 11750 RPM .
So pretty similar power and torque output, but at much lower RPM.
But the four stroke engine is much bigger capacity and the bike is much heavier at 167 kg, whereas the two stroke is only 138 kg
apollo11,
Jun 26, 2:24am
If capacity is the same, and peak horse power is the same, the two stroke will be able to be tuned for greater low and mid range torque, or greater mid and upper end torque. Double the bangs wins. The RGV250 is tuned for top end power, and they are a ton of fun to ride. I've owned a gsxr250, 19000 redline and similar torque to the RGV, but without the hustle at the top.
bumfacingdown,
Jun 26, 3:04am
Now at the low end of the RPM range "You can't beat that low down torque "
tygertung,
Jun 26, 3:12am
The max torque is lower on the two stroke than the four stroke. 2000 RPM lower. Therefore it has better low down torque.
slarty45,
Jun 26, 3:23am
I had a water cooled Zundapp KS50 that had sod all torque. Sounded like a mosquito when spun above 10
bumfacingdown,
Jun 26, 4:06am
Interesting extrapolation, how about at 2000rpm, that would be low down torque, yes?
tygertung,
Jun 26, 4:40am
Yeah, but those are pretty old, I looked it up and it was from maybe 1970, there has been a lot of development since here.
I've got a Yamaha Jog in the driveway here which has heaps of torque, it is outrageous fast actually.
tygertung,
Jun 26, 4:48am
I can't tell you what the torque output is at that level as dyno charts are not very easy to find, and they don't usually measure the output at that RPM.
But if you can have a look, that would be great.
two9s,
Jun 26, 4:56am
Don't tell my Suzuki TF185 Mudbug that. My mate and then I have owned since brand new. Dec 1978. Been used and used and used. Up till last year it was my daily Beef farm ride. It has not had a single engine component changed in 42 years service! Cases never split, head and barrel untouched since it was assembled in Japan in 1978. Try that with a 4 stroke.
two9s,
Jun 26, 5:00am
'Cause they do'. Go read some boating mags, or hard Enduro mags. Its there in words of one syllable. A bang every crank rotation produces more torque than a bang every 2, engine size, bore and stroke being equivalent
bumfacingdown,
Jun 26, 5:03am
Altho the claim was about motorcycles so I am not sure why outboards sneak into the equation
gunhand,
Jun 26, 5:30am
250s 400s, 500s, 185s? good grief some of you wouldn't know power if you tripped over it, 2 stroke or not. Oh, add electric push bike thing in there as well, equally as woeful.
apollo11,
Jun 26, 5:44am
And what fire snorting behemoth do you currently straddle between your knobbly knees? A TS185?
apollo11,
Jun 26, 5:46am
And all of this is moot anyway. It looks like electric motorcycles will be the future, whether we like it or not.
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