2 stroke power vs. 4 stroke? - Sportbike Forums : Sportbikes Motorcycle Racing Forum
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post #1 of 23 (permalink) Old 06-28-2004, 04:38 PM Thread Starter
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2 stroke power vs. 4 stroke?

I've been reading about the NSR500 lately. I read somewhere that Rossi's MotoGP NSR500 had around 190 horsepower : The bike is 100cc's smaller than my bike and makes about 100 horsepower more?

I'm guessing it's because the NSR has a 2 stroke and my F3 has a 4 stroke??? [img]/wwwthreads/images/graemlins/confused.gif[/img]

What makes 2 strokes more powerful than 4's?

Also, from what I understand, 2 strokes do not do well with meeting the US's emissions regulations.

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post #2 of 23 (permalink) Old 06-28-2004, 05:03 PM
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Re: 2 stroke power vs. 4 stroke?

In very simplistic terms, there are twice as many power producing "bangs" in a two stroke over a four stroke, hence the reason they put out approximately the same power as a four stroke twice its size.

Four stroke: Power, exhaust, intake, compression, so once every four "strokes" do you produce power.
Two stroke: Power/intake, Exhaust/compression, so once every two "strokes" do you produce power.

Two strokes are dirtier because (among other reasons) you don't completely separate the unburnt "intake" charge from the burnt fuel/air mixture leaving through the exhaust, so some of that unburnt fuel goes straight through the engine.

Okay, who dropped a steamer in the gene pool?
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post #3 of 23 (permalink) Old 06-28-2004, 06:56 PM
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Re: 2 stroke power vs. 4 stroke?

Also ryan, these are highly tuned motors, probably only good for one race. Actually I have heard they (incl most internals) will last for longer, but you can be assured they srtip and replace after every race. Do the same to a 1000cc 4 stroke and you'll get similar (if not not more) power.

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post #4 of 23 (permalink) Old 06-29-2004, 04:06 AM
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Re: 2 stroke power vs. 4 stroke?

And the other inportant factor on the 2 smoke bikes is the weight less , a street legal NSR250 has nearly the same capibilities as your F3.

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post #5 of 23 (permalink) Old 06-29-2004, 06:01 AM
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Re: 2 stroke power vs. 4 stroke?

Pleas forgive me for the length of this post guys. I am a 2 stroke "fanatic" and this subject hardly ever gets mentioned in the "Superbike" forums.

To give an example of the awesome power of 2 strokes, in 1987, my Yamaha RZ350YPVS after the extensive mods I did, pushed out just over 120Bhp on the dyno. It only weighed 135 Kg dry/155Kg wet. At the time I weiged 70Kg myself, so the power to weight ratio was awesome.

Although I don't have the bike anymore, I swear that it would beat my RC51 on acceleration from 0-60mph today.

Whilst it is true that the biggest difference between 2-4 stroke is that the 2 stroke fires twice as many times per "cycle" another thing not yet mentioned is that the exhaust pipes (commonly known as expansion chambers) on 2 strokes are another "secret weapon".

It is also the prime culprit of the distinctive "power band" found on 2 strokes.

It was not until the rapid development of the expansion pipes in the 70's and early 80's that the true potential of the 2 stroke motor came to the fore.

The 2 stroke expansion chamber can best be described as a "bolt on 'passive' turbocharger"

Without getting into the technicalities, by making use of pressure waves and not moving parts like a turbo or superchanrger, essentially the 2 stroke expansion chamber "sucks" the burned fuel/air out of the combustion chamber AND a fair amount of UNBURNED fuel/air too. The "suction" generated by the expansion chamber is strong enough to actually suck fuel/air all the way through the carbs, through the inlet and transfer ports, through the combustion chamber and into the exhaust port WITHOUT the aid of the suction created by the piston moving up and down. This unburned or "fresh" fuel/air sits in the header pipe and just before the exhaust port is closed or "blocked" by the piston, it gets "rammed" back into the combustion chamber .. adding to the power.

The 4 into 1 exhaust systems on the 4 stroke motors achieve similar results, but work on a slightly different principle and are nowhere near as effective as the 2 stroke expansion chamber.

Another "plus" for the 2 stroke motor is that the moving parts/weight of the motor is far less than the 4 strokes. In a 2 stroke there are no valves and camshafts etc. 2 strokes work with "ports" which essentially are holes on the side of the cylinder walls. They are opened and closed by the piston "blocking" or "unblocking" them as it moves up and down.

The end result is that a 2 stroke motor is extremely efficient, much more than the 4 stroke.

As far as the power differences go, it was not until the manufacturers came up with new materials/construction methods that allowed 4 stroke superbike motors to reach the rpm's that they do today that the "gap" in the power output between 2-4 stroke closed.

In the 80's it was virtually impossible to get a 4 stroke motor of the same engine capacity to output anywhere near the powere developed by it's 2 stroke counterpart.

In all the years of riding my 350, I was NEVER beaten by another bike on acceleration and only ever got beaten on top end by my mates modified GPZ 750 turbo after 110 mph. I am NOT saying that mine was the fastest bike around on top end, but the bikes i raced against never beat me. I had a true top end of 155 Mph, which for 1987 was quick by anyone's standards. The bike was however built for acceleration ..the top end speed was an unplanned but welcomed "by product".

For interests sake, some of the "organised" races I had included the following bikes:
(where indicated the year refers to the model of bike not when we raced) All races were prior to 1989 when I essentially stopped riding after a serious wreck

1. 1988 GSXR750/1100, in race trim
2. Kawa GPZ 750 Turbo,
3. Modified Honda CBX 1000 (6 cylinder)
4. 1982 Honda CB 750/900
5. Kawa 900 Ninja
6. Yamaha 1.1
7. Yamaha XJ 650 Turbo
8. Honda CX 500 Turbo
9 Kawasaki Z1300
10 Yamaha FJ 1100
11 Yamaha FJ 900
12 Yamaha FZ 750

The list of "victims" in unplanned "races" on the roads is as long as my arm :

The reason why the MotoGP emerged out of what was once the 500CC World Formula 1 motorcycle championship and why 2 strokes are not raced in MotoGp anymore was primarily because of safety.

It started with Wayne Raineys tragic accident.

The power levels being achieved on the 2 strokes were essentially still higher than the 4 strokes, but it reached the point that this power, because of the nature of 2 stroke power characteristics, became virtually unuseable, unlike the very effective power transmission properties of the 4 stroke bikes. In a 2 stroke, the power "comes on" in a sudden rush and then, depending on the tuning of the motor, dies out just as dramatically. The power band, even with brilliant inventions such as the Yamaha Power valve system, was still restricted to a few thousand RPM, typically at the top range of the rev limit.

Eventually, the 2 stroke bikes just became too dangerous to ride. Imagine coming out of a corner with 245 rear wheel HP that worked on an on/off switch!. That sort of describes what the 2 stroke 500 cc riders had to contend with.

The decision was therefore made to phase them out completely. A sad day, but the best decision after all.

They are still incredible fun though. I understand that many States in the US have had a ban on 2 stroke motors for years, so many of you guys won't ever of had an opportunity to ride one of these amazing machines.

It's not the speed that kills ... it's the sudden stop.
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post #6 of 23 (permalink) Old 06-29-2004, 06:28 AM
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Re: 2 stroke power vs. 4 stroke?

Also, from what I understand, 2 strokes do not do well with meeting the US's emissions regulations.

[/ QUOTE ]

Apart from what has been mentioned about the unburned fuel, another reason of emission problems is the way a 2 stroke motor is "lubricated".

On our 4 stroke motors, the drivetrain (crankshaft/conrod/bigend/smallend assembly)and the rest of the moving components are lubricated by the oil in the sump. Underneath the pistons (crankcase) in the 4 stroke motor is oil, and lots of it. There is an oil pump and oil filter to keep it all clean.

A 2 stroke is different. Underneath the piston on a 2 stroke, it is DRY, very dry : No litres of oil there. There is also no oil filter.

The oil that lubricates the crankshaft, pistons, piton rings, main bearings, big end bearings and small end bearings is found only in the fuel/air mixture as it moves through the entire system in a 2 stroke motor. This oil is mixed with the fuel by a device called an autolube. The autolube is essentially a type of oil pump which forces oil through small nozzles into the fuel/air mixture coming from the carbs. Some bikes have the nozzles in the venturi of the carb and others inject the oil directly into the crankcase. On racing 2 strokes, the autolube is completely removed and we mix the oil with the gas in the gas tank.

Typically we use a ratio of 20:1 Fuel/Oil (it varies a lot based on type of oil/fuel etc. so this figure is just an example. I have on occasion used 16:1 and gone as low as 48:1 with synthetic oils)

So, the fuel/air mixture in a 2 stroke ALSO has a high oil content. Same type of thing as would be found if the piston rings or valve stem seals on your car or superbike were shot.

This oil is then burned and expelled into the atmosphere, hence the problems with a number of countries emission control laws.

If you ever wondered why a 2 stroke normally "smokes", that's why.

It's not the speed that kills ... it's the sudden stop.
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post #7 of 23 (permalink) Old 06-29-2004, 07:30 AM
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Re: 2 stroke power vs. 4 stroke?

Audix_Dude- Great info, I never completely understood how the expansion chamber on 2-strokes worked. As far as 2 strokes being banned in the US, the older models are grandfathered in so we can still legally ride them (exept California). Most states still allow 2 stroke dirtbikes as well, thank God because i love my KX250 2smoker, there is just nothing like a 2 stroke as it comes into the power. : :

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post #8 of 23 (permalink) Old 06-29-2004, 07:49 AM
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Re: 2 stroke power vs. 4 stroke?

That about sums it up : :
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post #9 of 23 (permalink) Old 06-29-2004, 08:05 AM
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Re: 2 stroke power vs. 4 stroke?

Yep almost bought a RG500 but decided it wouldnt be a good beginning streetbike : Actually the guy just wanted too much $$ for it. I love 2-strokes it's too bad they are becoming so rare.

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post #10 of 23 (permalink) Old 06-29-2004, 08:07 AM
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Re: 2 stroke power vs. 4 stroke?

GREAT POST Audix_Dude!!! : :

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