4-Cylinder vs. 2-Cylinder - KawiForums - Kawasaki Motorcycle Forums
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post #1 of 17 (permalink) Old 05-25-2010, 01:03 AM Thread Starter
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4-Cylinder vs. 2-Cylinder

Okay, while I'm really enjoying my Ninja 500R, I'm curious as to what the differences are between a 2 Cylinder like my Sylvia or a 650R, and a 4 Cylinder like a ZX-6R. Clearly from the specs, I can see that the twin in the 650 delivers about 10 more HP, and the 4-banger ZX-6R delivers 40 more HP (too much for me right now), but what else is there?

Is an I4 smoother? Is the power curve flatter? Do they sound vastly different? I'm guessing they rev a lot higher too.

Is there a real reason to go for a 4 Cylinder engine, or can a commuter and weekender get by with a 2 Cylinder bike for their whole lives?

Thanks.
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post #2 of 17 (permalink) Old 05-25-2010, 01:11 AM
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I brought this up one other time and got shit from one guy who just wanted to argue. Basically the cylinders are smaller in the 4 cylinder vs a similarly sized 2 cylinder. This means the "boom" in the 4 cycle cylinder will be less powerful then in a 2-cylinder engine. The upside is that with the shorter stroke and smaller cylinders it MAKES power a lot higher in the RPM band. So while the twin will crap out and stop making power perhaps around 10k RPM the 4 cylinder might go to 14k RPM before running out of power. Since horsepower = (torque x RPM)/5252 the higher you can get an engine (RPM-wise), the more horspower it will create.

So since the twins make a lot more torque (big boom) they are typically quicker off the line and can cruise a lot better then an I4 engine. The 650 for instance makes MORE torque then the 600's, it just craps out much lower in the powerband so the horsepower number suffers. Horsepower engine like the I4's are a lot smoother because you have twice as many "cylinder firings" going on so they rev like butter. For acceleration after you get going the 4-cylinder engine will also take the cake as it will stay in the powerband longer and when you shift it will fall right around maximum torque (as the torque curve for a high revving engine is usually a shit-ton smoother).

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post #3 of 17 (permalink) Old 05-25-2010, 01:20 AM
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The 650R is manly for commuting while the the I4 is built for speed. you will have more low end torque from the Ptwin and better gas millage too. The I4 is all about speed so gas millage... and comfort are secondary... A commuter can most definitely get by with a Ptwin. as for sound, here are some examples.

650R YouTube - Ninja 650 Exhaust Sounds
I4http://www.youtube.com/watch?v=vACXCnGp2Bo

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post #4 of 17 (permalink) Old 05-25-2010, 01:23 AM
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Damn, beat to the punch.... or post

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post #5 of 17 (permalink) Old 05-25-2010, 01:39 AM
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2>4

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post #6 of 17 (permalink) Old 05-25-2010, 08:18 AM
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There are several things to consider. In general a 4 will have a narrower spread of torque, but more power, rev higher and be smoother, but it depends on what the engine was designed to do. All 4s are not sports bikes, and all twins are not commuters.

Personally I find 4s lack character - and I've owned a few over the years - however much extra power they deliver. For smoothness a triple offers better balance, therefore potentially smoother, with an engine not much wider than a twin (but still a lot wider than a V twin) and an exhaust note that's different. 4s are smoother than twins, not really because of the extra power strokes, but because the 4 cylinder crank offers much better balance than the 360 twin. A 180 degree twin offers similar balance to the 4 but has a severe rocking couple and uneven firing interval. Balance shafts can only partially compensate for that. The triple, with perfect primary and secondary balance and an even firing interval still has the rocking couple, but because of the otherwise perfect balance it's easily offset by a small balance shaft.

V twin balance depends on the configuration of the V - the angle between the cylinders. It can be very good, but that's offset by the uneven firing interval. V twins probably have more character than any other engine, but character isn't something that's easy to measure.

I also find that bikes - and you have to look at the whole package, not just the engine - with 4 cylinder motors are less agile than twins. Sports 4s are more extreme, less comfortable and less suited to non-sports uses, but that only applies to sports 4s, and not to bikes intended for commuting or touring.

Middleweight twins are probably the best all rounders. You can't regularly do 150 mph in normal riding and expect to live, and you can't cut through congested traffic on a 600 lb lump of cruiser or on a sports bike where you drag your knuckles on the road reaching down for the handlebars.

Last point. Engines dont make power. They only make torque. Power is the expression of the work done by the application of that torque. 4s do make more torque than twins at the wheel (though often less at the crank) - it's just further up the engine rev band and develops over a narrower spread of revs.

There's an often misquoted equation that says power = torque x revs. Thats an over simplification, but it is basically true. What people forget though, is when you're using a dyno it's the torque at the wheel (the gearbox is a torque multiplier) and the revs that the wheel, not the engine, is making. Torque and revs at the crank are back calculated for the chart and the results are distorted by the characteristics of the dyno and by transmission losses.

Rob

Last edited by williamr; 05-25-2010 at 08:22 AM.
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post #7 of 17 (permalink) Old 05-25-2010, 08:53 AM
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You're good with the twins. I commuted on a 650R for about a year. Great bike, really good mileage, and very comfortable. After adding a slip-on, it was a real thumper. In fact, after using a 636 and ZX14 for the same commute, I'm ready to find another twin.

Just because you don't know what I'm talking about, that doesn't mean I don't know what I'm talking about.
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post #8 of 17 (permalink) Old 05-25-2010, 11:08 PM Thread Starter
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Quote:
Originally Posted by Mayhem43 View Post
650R YouTube - Ninja 650 Exhaust Sounds
I4http://www.youtube.com/watch?v=vACXCnGp2Bo
Thanks for the links! I can totally hear the difference between the smooth I4 (fire-fire-fire-fire) and the 2 Cylinder (fire-fire-skip-skip).

Yeah, if I stick with sport bikes, I'll definitely keep this in mind for whatever is next. I'm sure I'll be on Sylvia for a while to come, though.

Thanks again everyone!
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post #9 of 17 (permalink) Old 05-25-2010, 11:36 PM
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Twins are more like (Fire.... .... skip ... ... (run out to the store real quick to get some milk) ... Fire! ... skip ... ... ... ... ... )

Weight is a big factor as well. Torque is a good thing up to a point. One thing I point out is Formula 1 cars. They make 800+ hp right? They also rev to like 18,000 RPM. Doing a bit of calculation they usually have around 250 ft/lbs of torque (and probably less then 200 ft/lbs below 10,000 RPM or so). For a super light F1 car that's all fine and dandy and they can hold gears for a loooong time. Meanwhile drop an F1 engine into a F250 and see what happens It's going to be a dog! It'll accelerate like crap until 12,000+ RPM and by the time it starts pulling anything decent you have to shift. On the flip side if you take a modern 500+ ft/lbs F250 engine and drop it into an F1 car, it'll spin the tires, like crazy and not really go anywhere in a hurry. If you regear it accordingly the RPM drop between the gears will be extreme.

So yeah Sorry to beat a subject into the ground.

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post #10 of 17 (permalink) Old 05-26-2010, 12:48 PM Thread Starter
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Quote:
Originally Posted by VeX View Post
Twins are more like (Fire.... .... skip ... ... (run out to the store real quick to get some milk) ... Fire! ... skip ... ... ... ... ... )
Well, with a 180 degree offset, isn't the sequence:

Cyl 1 at top FIRES, Cyl2 at bottom pulled in air & fuel
Cyl 1 at bottom pushes down driveshaft, Cyl 2 at top FIRES
Cyl 1 at top expels exhaust (SKIP), Cyl 2 at bottom pushes down driveshaft
Cyl 1 at bottom pulled in air and fuel, Cyl 2 at top expels exhaust (SKIP)

?
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