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Offline Hondacrrider

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How do they do it?
« on: July 28, 2010, 09:30:28 AM »
I was looking into some street bikes(like a GXSR 1000) and supposedly they put out 200 hp, but, a yz 250 only puts out like 45 hp, how are they able to get so much hp out of a four stroke like that, and still have it reliable, but are they very reliable? They are a street bike, so they must be, this is all so confusing, it seems as though a whole dif level of technology is put into the street bikes than the dirt bikes
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Offline 2smoker

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Re: How do they do it?
« Reply #1 on: July 28, 2010, 09:35:36 AM »
I was looking into some street bikes(like a GXSR 1000) and supposedly they put out 200 hp, but, a yz 250 only puts out like 45 hp, how are they able to get so much hp out of a four stroke like that, and still have it reliable, but are they very reliable? They are a street bike, so they must be, this is all so confusing, it seems as though a whole dif level of technology is put into the street bikes than the dirt bikes


They have 3 more pistons.. It is an inline four..
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Offline Coop

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Re: How do they do it?
« Reply #2 on: July 28, 2010, 10:08:11 AM »
Think of it this way. A street bike like a GSXR1000 that has 4 cylinders. If you had 4 YZ250 cylinders to equal a 1000cc two stroke, it would be 180hp, not far off considering it doesn't have cams, valves, etc.
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Offline JohnN

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Re: How do they do it?
« Reply #3 on: July 28, 2010, 10:24:56 AM »
First of all most horsepower figures are exaggerated and sometimes misquoted.

You are comparing an inline 4 cylinder 4 stroke street bike versus a single cylinder two stroke dirt bike. Not a good comparison to start with.

With that cleared up, what is the question you'd like to ask?
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Offline wintrader

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Re: How do they do it?
« Reply #4 on: July 28, 2010, 10:47:22 AM »
http://en.wikipedia.org/wiki/Ducati_Supermono Look at this and reasonable reliable too. The 4 stroke motocross engines are not overly stressed it seems. But still expensive if they blow. I guess normal use is different then racing a four stroke. Ducati does not use valve springs but desmo system that can handle higher revs. 100 ph for one liter car engines is not considered very much but in modern cars kinda normal. Motocross engines are i guess not build for high hp alone but also torque. Not expert shall ask my brother he prepares classic ferrari's for classic races.

Still i think 2 strokes much much better for mx bikes and also for motorracing. I watched motogp and just 15 bikes were there and the startgrid!!! 15!!! only 12 finished. Much too expensive for private teams it killed the sport a bit.

Anyway 35 hp for a 250 motox engine is not really much. As far as i know 12000 revs or so is not much either it depends on pistonspeed. Short or long stroke.

But why race a 4 stroke if a 2 stroke can do the same thing for less money and is lighter and cheaper to repair and more fun to ride. That really beats me. I owned a aprillia rs 250 twim street bike (2 stroke) really fun to ride even more fun then my 4 stroke ducati with a lots of horses.

But as far as i can see the 4 stroke motocross engine are not overstressed.

Shall ask my brother if i see him. He knows all about 4 stroke.

Offline JETZcorp

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Re: How do they do it?
« Reply #5 on: July 28, 2010, 11:40:16 AM »
The nearest I can figure, the street bikes are simply tuned more radically.  I think if you compare the dyno charts of a 1000cc bike you'd probably find it looks a lot more two-strokish than a 250F, with less [relative] low-end and more top-end.  So what I think that means, and remember I'm just speculating, is that they're made to reliably cruise the roads at low-revs for a long time with very little power being laid down.  But then when you want to go nuts for a brief burst to run from the cops, you take it up to the kind of speed where a 250F would normally stay at for an entire moto.  I would guess that if you ran a sport-bike like that as hard as you ran an MX four-stroke, you'd probably see a pretty vastly diminished lifespan.  There may be other things at play, too.  Perhaps they use a much more effective lubrication system because they're not as worried about KISS, that kind of thing.


Is this Maico a 440 or only a 400?  Well in all the confusion, I forgot myself.
But considering this is a 1978 Magnum, the best-handling bike in the world, you have to ask yourself one question.
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Offline Hondacrrider

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Re: How do they do it?
« Reply #6 on: July 28, 2010, 01:25:24 PM »
Interesting, do you guys know if they ever made crotch rockets in 2 stroke versions for the general public, like a 1000 2 stroke?
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Offline wintrader

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Re: How do they do it?
« Reply #7 on: July 28, 2010, 01:36:11 PM »
I remember that i read something about yamaha and yz 250f. Was something like a yz 250f has one quarter of a r1. Stroke and obre also the same by the way. 77x53.6 for yz and r1. Only 2009 and 2010 R1 have other stroke and bore.

Anyway like jetz said one moto stresses an engine alot more then normal roaduse.

The thing is that normal riders want to pet or prepare their motors themselves because that is cool too. 2 strokes can be rebuild or worked on by a lot of people by themselves. 4 strokes is much more difficult. And if a 4 stroke blows much more expensive. This is all part of the fun. We or alot of people do not want lightweight valves and so on. We just want to ride and have fun.

If 4 strokes were so much better and reliable then ok i can understand this all. But in my opinion this is absolutely not the case. The only reason i see is that people THINK they will be faster on a 4 stroke and that the pro riders all ride them. Just fashionable these days. I give it a few more years and it will be over. Allready happening now. With all the fuzz and propaganda 2 strokes should have been dead by now BUT they are not so the only thing that will happen is that more and more go back to 2 strokes.

People are going back to 2 strokes and people like me who never owned a 4 stroke will not even consider a 4 stroke so tell me what can happen?

Offline mrhp

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Re: How do they do it?
« Reply #8 on: July 28, 2010, 01:41:21 PM »
First:

200hp out a 1000 GSXR  (or any other 1000) at the wheels is only a myth for almost anyone.

Really good mod 1000's are putting out about 180-185 at the wheel.

Next:

A stock moto 250f is 35ish at the wheel.
X4 that's about 140 at the wheel.  Definately well below a street bike.

A good mod 250f these days is in the range of 43 rear wheel hp.
X4 works out to 172...much more respectable.

The limits in power on the dirtbike, while not fully explored, are typically due to a couple major reasons.  One being the intake tract length.  It's much too long for high rpm inertia/resonance tuning.

The other is head shape.  Because the intake tract goes back towards the air box on the typical bike, the intake port has a fairly large turn in it. Air velocity can not be maximized as would be desired because the air must turn...

The new yami 450 is not limited as much in this respect with its down draft style head.

And lastly cam design and valve train longevity.  High end sport bikes ARE revving higher.  Their peak torque numbers, or BMEP is NOT higher than a good moto engine...but they just spin it higher for more power.  This is not proving feasable on a production motocross bike for reliability concerns.

Now:  A stock 250 two stroke is as stated, about 45 wheel hp.  That's a far cry from what it can make..
a stock 125 easily produces 30 wheel hp...usually 33.  Times that by 8 and you would get 240 plus wheel hp.  

Now THAT is impressive.  The high end motogp engines at 800 cc are rumoured to make approx that type of power at an impressive 18000 RPM.  Technology, materials, and ingenuity at its finest to make that happen.

The worlds best 2 stroke 125s are over 50 hp...still competive at CC per CC at a 100th the cost...

But range of power on the two stroke will sadly NEVER compete at half displacement..and at absolute highest level it would seem even at same displacement to a truly trick 4 stroke..that avg joe will never be able to afford.



Offline TMKIWI

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Re: How do they do it?
« Reply #9 on: July 28, 2010, 01:46:15 PM »
Sorry but you guys are missing the point.
Multi cylinder engines are easier to pull higher revs because the forces of the pistons are acting against each other smothing out the oposing forces.
The more pistons you have the greater the piston area. More piston area you have the more horsepower you can get.
If you use F1 as an example, Back in the 70's the Ferrari V12's had more horse power then the Cosworth V8's but the V8's had more mid range tourqe.
Ducati built a Super single in the 90's which was 600cc i think. They cut 1 of their V-Twins in half but they left in the unused conrod to act as a balancer.Other wise it would never have been able to pull such high rev's.

So in short, the more pistons you have the more power you have & the more pistons you have the higher the revs you can pull & the higher the revs you have the more power you have. In theory.

A modern 250F pulling 13,000 rpm is definatly over stressed   >:D
« Last Edit: July 28, 2010, 01:50:19 PM by TMKIWI »
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Offline wintrader

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Re: How do they do it?
« Reply #10 on: July 28, 2010, 02:01:01 PM »
sounds good.

By the way to add. My brother prepares racing engines for classic races. Like for instance ferrari 250 swb or gto. He always says horsepower is for customers and torque wins races. Sometimes customers want special engines for special tracks. Tracks with long straight high hp more important then torque. So he build special engines for those tracks with shorter stroke for other tracks they use engines with more torque and longer stroke.

Being fast around a track does not always mean you need more horsepower and certainly in mx this is true.
Laptimes of 250f are often as fast or faster then 450's. So if experts or ama says 2 strokes 250 are faster on a track then 250f then 250 2 strokes should also be faster the 450f or as fast.

i think it is the rider and not so much the engine 450 or 250. Open class 500 cc 2 strokes were always slower then 250 cc 2 strokes when i raced. Sometimes 125 were fastest. All not that clear.

I think it is just a money thing and manufacturers want to sell 4 strokes becuase then they will sell more parts too.

Offline luthier269

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Re: How do they do it?
« Reply #11 on: July 28, 2010, 04:01:54 PM »
Hondacrrider
   I'm suprised being from Canada you hav'nt seen any two stroke street bikes. I know in the 1980's and maybe in the 1990's you could get a Honda two stroke street bike in Canada.
Motocross is a real sport all the rest are just games

Offline maicoman009

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Re: How do they do it?
« Reply #12 on: July 28, 2010, 04:24:31 PM »
Yamaha also made a two cylinder 350cc. 2-stroke street bike in the 80's.They were'nt very big but they ran great & had decent power.

Offline JETZcorp

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Re: How do they do it?
« Reply #13 on: July 28, 2010, 04:30:57 PM »
I managed to find an article on, what I think anyway, is the coolest street-bike of all time.


A TZ750, made street-legal after years of battling with the DMV and California Highway Patrol!  And yes, children, it's a two-stroke.
Four cylinders
Three disk brakes
Two brass balls
And a sound that would echo off trees!
http://motorbikearchives.com/Bike-Tests/Road-Race/Yamaha-TZ750-The-Bumblebee-Special-1978.html


Is this Maico a 440 or only a 400?  Well in all the confusion, I forgot myself.
But considering this is a 1978 Magnum, the best-handling bike in the world, you have to ask yourself one question.
Do you feel lucky, punk?

Offline TxTechRedRider

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Re: How do they do it?
« Reply #14 on: July 28, 2010, 04:33:21 PM »
When I was a kid, and my Dad was at work, I would uncover his Gold Wing, start it up(electric start), run beside it until it got going (it was heavy and the speed would balance it) and then hop on for a speedy ride around the block a few times, man that thing had way more power than my little dirt bike.  :o
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