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.