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2 Cylinder Vs. 4 Cylinder..... Why?

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gkotlin

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Discussion starter · #1 ·
Ok. Why?

If a racing class allows up to 600cc I-4's, why do they need to allow a larger 2 cylinder to be competitive in the same class?

If you have 600cc's why can't the 2 cylinder make enough power to be competitive with a 4 cylinder? The 2 cylinder has less mass, should have less weight.

Why does Ducati for example need these special rules to allow them to race and be competitive? If they can't compete with the same displacement, why don't they start making I-4's. It seems that the 2 cylinder is apparently less efficient. So other then heritage, why do they still run twins?
 
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#3 ·
its kinda like 2stroke vs 4stroke I think.. Twin has a considerably different powerband, and to get to even numbers with an inline, extra capacity is required..

I think there's more to that, but obviously the people putting the series on, want all the brands, which will draw more spectators..
 
#4 ·
well, due to large cylinder volume, longer stroke, heavier pistons and other factors, twins just can't rev as fast as multi cylinder engines. HP is a derivative of torque and revs - HP = torque * RPM / 5252. since revs are lower, torque has to be higher to compensate for the low rev, and how do you get extra torque - primarily by displacement...
 
#5 ·
samal said:
well, due to large cylinder volume, longer stroke, heavier pistons and other factors, twins just can't rev as fast as multi cylinder engines. HP is a derivative of torque and revs - HP = torque * RPM / 5252. since revs are lower, torque has to be higher to compensate for the low rev, and how do you get extra torque - primarily by displacement...
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I as so gonna say that but I couldnt figure out how to explain it. Nice job.
Ride a 916 on the track, you find out real quick that although it's basically a litre bike, it is only marginally faster down the straights the the 600's. and if they are really new they will either be even or slightly faster.
 
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#6 ·
gkotlin said:
Ok. Why?

If a racing class allows up to 600cc I-4's, why do they need to allow a larger 2 cylinder to be competitive in the same class?

If you have 600cc's why can't the 2 cylinder make enough power to be competitive with a 4 cylinder? The 2 cylinder has less mass, should have less weight.

Why does Ducati for example need these special rules to allow them to race and be competitive? If they can't compete with the same displacement, why don't they start making I-4's. It seems that the 2 cylinder is apparently less efficient. So other then heritage, why do they still run twins?
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Simple answer, valve area. A 4 cylinder can have greater valve area, meaning more fuel and air can be pumped in and out. Also, smaller lighter pistons allow higher revs, and horsepower is a a function of RPM. (Torque x RPM)/5250, IIRC. Asa rule, motors that can rev higher will make more HP all things being equal.
 
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Discussion starter · #7 ·
I guess the simple answer to why is because they can.

They stick with the twins for their heritage. The race organizations continue to work with them. So why change a good thing huh.
 
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#8 ·
Rob said:
Simple answer, valve area. A 4 cylinder can have greater valve area, meaning more fuel and air can be pumped in and out. Also, smaller lighter pistons allow higher revs, and horsepower is a a function of RPM. (Torque x RPM)/5250, IIRC. Asa rule, motors that can rev higher will make more HP all things being equal.
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+1
engines are essentially air pumps

maximize valve area per displacement volume and you'll maximize HP

the higher the number of cylinders, the larger possible valve area
 
#9 ·
gkotlin said:
I guess the simple answer to why is because they can.

They stick with the twins for their heritage. The race organizations continue to work with them. So why change a good thing huh.
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I don't think it's just the heritage. Competing with Japanese manufacturers is tough, Ducati needs something to set them apart. They are creating a niche market.

But twins are nice per se, though. Because you don't need to rev them high, you get all the power right away. A lot of people simply prefer them.
 
#10 ·
As was aready stated, valve area (intake/exhaust volume) and piston weight.

Another consideration is force distribution, an inline pounds the cases in two directions and can easily be balanced without adding much weight to the rotating mass of the engine. A V or L config. distributes force in quite a few more directions which caused stress on the cases (they must be beefier) and it will take a lot of additional mass on the rotating assembly to smooth out and balance that motor. This hinders max revs even more than piston weight.

This is the reason why Hodgson bitched about not being able to use the SBK crank on his Duc in AMA. it was much lighter and helped the motor rev higher without failure to acheive more juice at the black rubber donut.
 
#12 ·
DefyInertia said:
I think it's mostly about power delivery like Downforce said.



Can someone explain the difference betwen this 4 banging "twin" and a V-4? Is it the firing order/timing?
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Yes. The oringinal D16 GP bike fired 2 cylinders at the same time (big bang) in order to get the power advantage of the 4 cyl configuration, but the power delivery of a twin. They then changed to a 'long bang' configuration which fired 1,2..3,4, if that makes sense. I may be mistaken, but I think they may e using a screamer configuration now.
 
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#13 ·
DefyInertia said:
Can someone explain the difference betwen this 4 banging "twin" and a V-4? Is it the firing order/timing?
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Yes, in a traditional V-4 the cyclinders fire in opposite "corners". While with a "big bang" V-4, the cyclinders fire in forward or aft pairs. I-4 bike experimanted with the "Big Bang" firing order, firing two cyclingers at the same exact time. Giving a different power pulse, all in the search for traction.
 
#14 ·
All correct above.

I'm not sure if some said above but the piston size and weight of a twin vs 4 is very different. Twins have normally heavier cranks and piston for the rotational mass where as 4's have light weight parts.
 
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#15 · (Edited)
DefyInertia said:
Can someone explain the difference betwen this 4 banging "twin" and a V-4? Is it the firing order/timing?
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There is nothing to explain. It is a V4. HRC already showed that a twin could only be somewhat competitve in GP. Ducati knew they needed to abandon the twin and run a V4. But for marketing reasons, they called it a twin pulse.

What bike won the most races in GP history? The NSR500 and that was a V4. The V4 is the best motor config for road racing.
 
Discussion starter · #17 ·
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#18 · (Edited)
i remember reading something a long time ago, about power pulses going to the rear tire.. basically with a twin, in a 1:1 world, which equals 4th gear, there'd only be two power pulses put to the tire in each revolution of the tire, where with a 4cyl, there'd be 4 power pulses put to the tire.

1:1 would be 1 turn of the crank, for one turn of the countershaft. Usually 4th gear is 1:1. however the countershaft would have to rotate about 3 or 4 times to make the rear sprocket turn once... so i dont understand anyways..

i think a bigger advantage the twins have, is being much narrower, and are able to acheive a much bigger lean angle, with a lower center of gravity.. my $.02
 
#19 ·
gkotlin said:
If you have 600cc's why can't the 2 cylinder make enough power to be competitive with a 4 cylinder? The 2 cylinder has less mass, should have less weight.

Why does Ducati for example need these special rules to allow them to race and be competitive? If they can't compete with the same displacement, why don't they start making I-4's. It seems that the 2 cylinder is apparently less efficient. So other then heritage, why do they still run twins?
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Ducati was very famous for singles at one time. Dental equipment too.

A two cylinder doesn't have less mass necessarily. And you'll have to keep those pistons from throwing that mass around enough to bust rods, etc. You give and take.

But I think allowing a twin cylinder a 40% increase in displacement over the previous smaller displacement advantages...well, I think someone must be getting a blow job or something.

You know, at one time, displacement was kind of absolute. 250, 500's, etc. Two stroke, four stroke. Build your best design and race it. If it sucks, change it.


Was nice meeting you too, Greg! :)
 
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