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Grunt??

buffmaster

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#1
I'm not 100% posittive this should go here, but I'm sure Rich'll let me know if not. Anyway, here goes. I own a '98 KX250. If any of you have thrown a leg across one, or knows someone who has, you know it has a ton of torque, at least for a 2-stroke MX'er. Now, my question is, how does this compare, relatively, to the torque of a 4-stroke? This is the one single aspect that I love about the bike(I love everything about it(except that the engine is at Eric's), but this aspect is my fav.). I plan on keeping this thing around as long as possible, but the day is going to come where she's going to need replacement(the way I keep screwing her up, it may be sooner than later) and I'm wondering how big a difference the torque is between the 2-&4-strokes? BTW, I do realize that to compare my KX250 to a 520mxc is unrealistic.
 
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#2
I've never ridden a '98, but I've ridden a '99 KX250.....right after I hopped off my old XR600. The low-end on the KX was pretty much non-existent compared to my XR's.
 
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#3
I also rode a 99 KX, and riding it after a 99 XR400, I found it to be a bussy and peaky bike. Much faster bike, but I was much slower on it.
Most likely just because I hate 2 smokes.
Although, I did love the RMX 250.
 
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#4
I ride a WR426 currently. Before that was an XR600R. Anyway, I rode a friends 98 KX250 not too long ago and although it seemed fairly torquey for a 2 stroke MX bike, and fun to ride (light and fast!), it was pretty weak down low compared to the thumpers.
Its just really two different worlds. If you've never ridden one and get the chance sometime, ride one of the 5-valve Yamahas, or any newer large displacement 4-stroke for that matter. People could try all day long to explain the difference in the "feel" of the torque,, and you could read every test article and comparison that has been done, but I think the only way to truly understand the difference is to ride them and feel it for yourself.
 
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#5
The KX has smoothly delivered low end horsepower.

It does not have torque.

This type of power allows you to moderate the horsepower better than something like the CR250, which builds power more explosively.

Here's the difference. Lets take an XR400 and your KX250 (i'm trying to find a good raw-torque engine against something you know). Okay, mud hole in front of ya - lots of slimey gooey stuff, really sticky. About 2' deep.

Okay, ride through it on the KX. What's your plan of attack? Feet up, second gear with your fingers on the clutch, hit it hard and fast and hope for the best.

Okay, I come up to this mud hole on the XR400 - I'm behind you because I'm a slow and completely dorky rider. I see the mud hole, but I'm scared to hit it fast. I accidently shift to third gear (because I'm a dork, and I'm scared). The XR bogs down, and rolls slowly into the muck. It descends to the bottom - but the rear tire keeps moving. The bike hunkers down and drags itself through. I emerge on the other side - unscathed.

I'm still a dork, I looked like a dweeb going through the mudhole, but I made it - because the four stroke had the torque to do it.

If I'd tried the same dummy maneuver on the KX, you'd be helping remove me and your bike from the muck, cussing me thouroughly!

That's the difference. It doesn't make a fast rider faster (in most cases), but it does allow a tired rider to ride sloppy and survive - and a goofy rider to make it through rough stuff more often.

Now, in talking about torque and the MODERN four stroke, we have a little anomoly - because the YZF, the CRF and the KTM have torque, but it's explosive stuff. Not for the faint of heart or the novice. In the right hands though, it's a VERY potent weapon.
 
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#6
I guess I am leaving myself open to attack, but a two stroke generally makes more power (HP) and torque than the equivalent cc four stroke.

Most people make the mistake of confusing torque with the flywheel effect.

The reason that a two stroke "feels" like it has no torque is because most two strokes have less rotating mass - lighter flywheel - than four strokes, as they fire twice as often and don't need the extra rotating mass to keep the engine running. Due to this extra rotating mass a four stroke will pull from a lower rpm and feel "torquier" than a two stroke.

If you ride a YZ426F and then jump on a WR426F, which one feels like it has the most torque – of course its the WR, it has a heavier flywheel, which stores power – also makes it slower reving.
 
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#7
Rich, a little help here???

I could respond to Aust520exc, but I'm hoping Professor Rohrich will take us all to school :p


okay, okay, just one question Aust -- where, exactly, is the modern four stroke gaining the additional rotating mass? The crank? Nope, same or lighter than a two stroke of similar displacement. The flywheel? Not on the modern thumpers I've seen. Clutch - those are normally more beefy, that's correct - they make them bigger to handle the increased TORQUE of the four stroke - you know, that non-existent torque we're talking about. Oh, the valve train, yes, the additional rotating mass of moving the valves. That's like adding a whole four or five ounces to a flywheel at least!

Okay, by your theory then, a counter balanced two stroke will feel more....er....torquey....than a non counter balanced two stroke of the same displacement. Since the counter balanced engine has all that extra rotating mass....er...."storing" energy. Is this a valid argument?

I'm wondering - have you been dyno testing these engines a bunch, and extrapolating data from these tests? I hope you've been using a hydro style dyno, since the drum dyno does not actually test torque. I really want to see this data, since it's counter-intuitive to every test I've seen in the last 5 years.
 
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#8
Horsepower is a measure of torque and time. I don't know the equation, but because of this, torque and horsepower cross at 5252 revs (I think).

I don't know how much relevance this provides, but it goes to show that I did learn something in year 12 Physics.
 
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#9
Plucked this from cyberspace

First of all, from a driver's perspective, torque, to use the vernacular, RULES :-). Any given car, in any given gear, will accelerate at a rate that *exactly* matches its torque curve (allowing for increased air and rolling resistance as speeds climb). Another way of saying this is that a car will accelerate hardest at its torque peak in any given gear, and will not accelerate as hard below that peak, or above it. Torque is the only thing that a driver feels, and horsepower is just sort of an esoteric measurement in that context. 300 foot pounds of torque will accelerate you just as hard at 2000 rpm as it would if you were making that torque at 4000 rpm in the same gear, yet, per the formula, the horsepower would be *double* at 4000 rpm. Therefore, horsepower isn't particularly meaningful from a driver's perspective, and the two numbers only get friendly at 5252 rpm, where horsepower and torque always come out the same.
p.s. replace driver with rider, & car with bike

What was that about two strokes having no torque?? Without it, the smokey little screamer's wouldn't work.

Dirk, very good - 5252rpm - its amazing what you remember from school
 
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#10
torque, as measured by a hydraulic dyno, is the amount of force the engine can "push" against the load (which is generated by the dyno) at specific rpm's.

Horsepower, as measured by a drum style dyno, is the amount of time it takes to get to a given rpm.

If you put a two stroke on a torque measuring dyno, it will not "hold the load" at lower rpms - which means it is not generating torque. It will start to "hold the load" at higher rpms - which shows the bike is able to generate power in this range.

Call it whatever you want - but an engine which generates lots of "push" at low rpms will create what the general public would construe at "torque", and an engine which generates no "push" at low rpms, but is able to create power at higher rpm's in a quick manner is considered to have lots of "horsepower".

It's a given that all engines generate some torque and some horsepower - it's just the percentages that change. Admittedly, two strokes do generate torque, since it's all power per burn cycle, but they are more efficient at generating power at higher engine speeds. Four strokes are typically more efficient at generating power at lower engine speeds.
 
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#11
I guess I am leaving myself open to attack, but a two stroke generally makes more power (HP) and torque than the equivalent cc four stroke.
DualSportr did you misunderstand me?

As for the rotating mass, all I said is a four stroke has more of it compared to a two stroke - logically half the power strokes is going to require some additional form of inertia to keep the engine running, a four stroke therefore benefits from this at low rpms, giving it more "pull" not more TORQUE - no matter what the general public call it.
 
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#12
Originally posted by DualSportr

Four strokes are typically more efficient at generating power at lower engine speeds.
I think this is what this is all about. People tend to call torque at low revs and tractability "torque" and torque at higher revs "power".
 
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#13
I've always thought about it like this -

The reason we think 4 strokes feel more torquey than two strokes is because they are generally a larger displacement when the two are compared. A 426cc four vs. a 250cc two, a 250cc four vs. a 125 two, etc. A 4 stroke generally has to be about twice the displacement of a 2 stroke to get roughly the same peak power output.
Both engines will deliver their high output across a certain RPM ranges. The 2 stroke typically produces its high amount of power over a narrower range of RPM's (higher RPM's), whereas the 4 stroke can produce the higher power beginning at much lower RPM's.

Like I said, we could talk about this all day but I think the only way to really understand is to ride the bikes and feel it ;)
 

buffmaster

Mi. Trail Riders
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#14
Thanks guys, that's what I was looking for. I basically wanted to know if the diff. between the two were significant. I love that my bike can rip on top, but that it can also go into a corner a gear high and chug on out (and pass the guy next to me, throwing roost all over the place). But, seeing as how I love torque, I'll hold off on trying out one of the 'f's, at least until I'm ready to get one.:)
 

wayneg

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#15
Hello

The other thing is that you're comparing a two stroke (KX250) that is designed for motocross/supercross and and four stroke (520MXC) that is designed more as a cross country bike. Obviously the design purpose and state of tune of the motor is a pretty important consideration when comparing different bikes. I would think that you'd be better off comparing the KTM 520 with a KX500, or the KX250 with a YZ250F to get back to comparing apples with apples.

I have a Beta trials bike that is a two stroke with lots of low end tractability, and a huge flywheel (hence great low end "torque"). This bike is 250cc, and puts out around 15 horsepower, so it is in a very mild state of tune. The KX however has a very small flywheel, and is tuned for mid to top end power. The difference in power is around 30 horsepower, and the difference in the bike's power curves and torque curves is also pretty big. The tuning is something that will aways be a trade-off, with a good compromise being easiest to have with larger capacity bikes.

Probably something like a Gasgas enduro bike would be a good idea if you really need a two stroke with low end tractability

Cheers
Wayne