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Discussion Starter #1
I liked what you have written up on Twins, one thing I'd like to add to your thoughts is that, for the most part that a long stroke doesn't determine how much torque an engine will make.
It does affect a number of parameters that make a large torque output likely, but the long stroke itself doesn't do it.
It mostly turns out to be breathing capacity, the Shorter the stroke/Larger the bore, means that there's more room for valves and ports. That is the main determining factor, not stroke.
Longer strokes will increase sideloading of the piston, which makes for a lot of frictional losses at high rpm's, a longer rod will reduce sideloading, but raises deck height, increasing size of the engine.
 

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I've seen this in the VW VR6 motor. Most people that know how to build them and want to keep them normally aspirated(sp?) (N/A) will increase the bore size and keep the stock stroke. They say it doesn't compromise low end torque as much.

Just what I've heard.....trust me I'm no expert!!!!
 

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Good points, all... But I don't think I said anything about stroke length in my post. :confused I limited my comments on cylinders/RPM to "4 smaller pistons are easier to spin that 2 big ones."

Scott :draggin

p.s. - If anyone is confused about what we are talking about, it's my long post I trot out whenever someone brings up the eternal "Twins versus Fours" debate. I save stuff like that on my computer 'cause someone ALWAYS brings it up, and I get tired of typing the same thing again and again.

Here's the post for reference:


I apologize if this sounds preachy... I just don't know any other way to explain it on short notice.

Contrary to popular belief, twins generally do *not* make *more* torque than fours - A 1,000cc motor in a similar state of tune will make around the same amount of torque regardless of the number of cylinders. Torque has more to do with the relationship between airflow and compression ratio than the number of cylinders doing the compressing. That belief stems from the fact that most racing organizations permit 1,000cc twins to compete with 750cc inline 4s.

So, why do 750s compete with 1K twins? Remember that horsepower is torque multiplied by RPM divided by 5252. So, the faster you can spin a motor the more horsepower it will make. Since it is MUCH easier to spin 4 small pistons than it is to spin 2 big ones, a 1K twin makes less horsepower than a 750cc four. But it makes more torque because it's a bigger motor.

Twins are more complex to manufacture and heavier - Why? Because there are two independent cylinder heads, instead of one. Two separate valve trains, cam chains etc etc etc. Twins also have a disadvantage related to weight/balance because the designers often cannot put the motor as close to the front wheel as they would like.

So twins are heavy, more complex to make/maintain and don't even make as much horsepower as an I4 with 3/4ths the displacement.

Under the current rules, there are some significant advantages to using a twin, though: The best is how WIDE the powerband is. Look at the dyno graph of an I-4 versus a twin - Specifically the amount of the powerband between peak torque and peak horsepower. That's where racers live because that's where you accelerate the best/fastest. On an I4 the difference is usually 20% or so at the top. Meaning a racer has to stay in the upper 4th of the RPM range for best acceleration. On a twin the difference is usually 50% or better. This means that, in practice, the rider of the twin has more gear choices and an advantage in cornering speed and (especially) an advantage coming off the corners:

For example, my bike makes 68 lb/ft at 6K rpm. That equates to around 78 horsepower at the wheel. I can open the throttle pretty hard and not have to worry (so much) about the rear breaking away on me. Especially since the power pulses are spaced farther apart and give the tire a tiny bit more time to recover from one before the next hits. I also have LOTS of room before I hit my 115HP peak so I can carry the gear for a while.

A GSX-R 750's torque peak is about 58 lb/ft, but at 10,000 RPM, which puts 110 horsepower at the wheel. 110HP is MUCH more likely to highside it's rider than 78, the power comes on all at once so there a much finer line between acceleration and crashing, and the Gixxer pilot hits his 123HP peak just 2,500RPM later so he has little room to carry a gear.

So, who do you think got out of the corner faster? The contest is now is the straight long enough for the four to catch up on it's superior horsepower.

There's also another really, really, really big reason that twins have dominated over fours in racing: Ducati and Honda have ENORMOUS budgets, and to a much lesser extent Aprilia have also put in a lot of cash and expertise. The old rule “Speed Costs Money. How fast do you want to go?” VERY much applies.

Hope that helps....
 

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Discussion Starter #5
You have such a good post, I was hoping to get you to expand on it a little really, that's all.

It just seems that everyone thinks of Twins as being torque engines, and there's some magical reason for it(like stroke), when it's really just high rpm breathing differences.

That's all, carry on:D
 
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