Inline 4 vs V-twin - Sportbike Forum: Sportbike Motorcycle Forums
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post #1 of 57 (permalink) Old 03-11-2002, 05:07 PM Thread Starter
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Inline 4 vs V-twin

In superbike racing, they race 750cc 4 cylinder againts 1000cc V-twins. Also 600cc 4 cylinders againts 750cc V-twins.

Why do they consider this a fair race?
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post #2 of 57 (permalink) Old 03-11-2002, 05:17 PM
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I ride inline 4 right now, but I have every intention of buying an RC-51. It only makes sence that the I-4 has a power advantage over the V-2. 4 cylinders vs. 2 will always be more powerfull at the top end. Reason being that they can support much higher RPM's do to the smaller piston size. However the V-2 has the tourque advantage due to the larger displacement. I think the V-2 will offer a better balance in the turns, but I'm only going by what I've been told. I know for sure though that the V-2 has it all over the I-4 in the tourque section. .

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post #3 of 57 (permalink) Old 03-11-2002, 05:27 PM
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Because the Flammini Group that started WSB are Italian, not unlike certain V-twin stalwarts

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post #4 of 57 (permalink) Old 03-11-2002, 06:15 PM
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V-Twins, to have good combustion, have to have longer strokes than 4 cylinders for the same displacement and so rev slower. If a given motor cannot rev as high as another of the same torque it makes less hp proportional to the the decrease in rpm. 1000cc V-twins however make approximatley 33% more torque than 750cc 4's by the increase in displacment. Nowadays V-twins have been getting shorter and shorter strokes. The current Duke has a 58mm stroke which I do believe is the same as a GSXR1000, and have been getting 13000rpm plus. Ten years ago this was not concievable and hence the rules for the class. Twins now are hp and torque superior. Look at Nicky's Honda doing 190's on the straights of Daytona.
But in 2004 WSB is going to 1000cc for any cylinder number, and multis will probably dominate

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post #5 of 57 (permalink) Old 03-11-2002, 06:19 PM
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Based on theory the 1000cc twin, all else equal, should be able to put out almost as much horsepower as the 750cc four cylinder bike. The FIM rules differ in comparison to current AMA rules because FIM allows twins to be a slight bit lighter to account for the lower horsepower. I think the current FIM rules are a bit behind the times though with regards to the Superbikes. Twins have enough other advantages such as narrower giving better aerodynamics. And more torque as mentioned.


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post #6 of 57 (permalink) Old 03-11-2002, 08:21 PM Thread Starter
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Here is why the 1000cc v-twin is in the same class as a 750cc inline 4.

v-twin: 1000cc x 10,500 RPM = 10,500,000
inline 4: 750cc x 14000 RPM = 10,500,000

Each engine uses the same amount of air and fuel.


Last edited by Andy; 03-27-2002 at 05:41 AM.
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post #7 of 57 (permalink) Old 03-12-2002, 01:15 AM
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This problem was put to a British University 5 years ago, in order to find a formula that would allow the 4s to compete with the twins fairly.
Titomike it was not the Flamini that decided the rules originally
The rules existed from the beginning, it was a small window left by the rules to invite other manufacturrs. There was no Ducati to compete then really! The RC30 ruled WSB back then.
But Ducati saw this and as they were able to put their act together, they built the 888 superbike and got their first championship! Raymond Roche was the Ducati rider that used the potential first. Until 5 years ago the twins also had a 15 kg weight advantage over the 4s! Things had to change as the Ducati (no other twin existed) was getting better and better.
After that the balance started changing and the twins got to level weight with the 4s, they had to use lead inserts many times during rules changing.
Remember that Ducati only used the full capacity allowed (1000 cc) in the last two years, they could still win with less cc before that!
Even Honda had to revert to a V twin to win again. The RC45 only got one championship and that was with Kocinski, after a lot of money was spent.
The main problem now is that although the 4s have about the same power with the twins, and sometimes even better top speeds, they lack a lot in acceleration out of the corners as they have a much better midrange power than the 4s.
This translates in easy overtaking at the end of the straight that folow any curve

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post #8 of 57 (permalink) Old 03-26-2002, 12:49 PM
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the reason for the displacement difference is the amount of air each type of motor can take in at certain rpm which in turn is the amount of fuel a motor can burn and horsepower it can produce. twins cant rev as high but pull in as much fuel/air mix as the high reving 4s.

there is no substitute for horsepower
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post #9 of 57 (permalink) Old 03-26-2002, 01:20 PM
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Twins can rev a lot higher then they used to. Both Ducati and Honda are getting 13,000+. Fours havn't gotten any higher than 15000 and I don't think the Kaw even gets that high. .75liters *15000 is only 12000 units of air a minute while the twin takes in 13000 units of air a minute. The difference of these numbers is pretty accurate to the advantage of a twin, of course power delivery is another serious advantage already mentioned.

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post #10 of 57 (permalink) Old 03-26-2002, 01:28 PM
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You are all wrong.

The 4s have the advantage because they have more surface area of intake and exhaust. The surface area of 16 valves is greater than the surface area of 8 slightly larger valves in the twins. Thus 4s can THEORETICALLY make more horsepower.

Of course this the theory, but when the twins started to turn more revs the disadvantage became smaller. Then there is head design and valves angles to improve flow. However, the theoretcal flow of air through more surface area is the advantage, not rpm.

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