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Old 08-14-2014, 09:53 AM   #1
SAM7 OP
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SINGLE vs. 360 Degree crank PARALLEL TWIN???

First, I'm a COMPUTER ADMIN... I think that explains it all! I've VERY LIMITED knowledge outside basic maintenance and oil/filter/coolant changes... Here's an example: Take 2 engines. One is a single, other is a 360 Degree crank parallel twin (BOTH PISTONS MOVE TOGETHER). Both have similiar displacements, same amount of valves (single has 4 valves per cylinder, twin has 2 valves per cylinder), same compression ratio, both fed by ONE carb etc etc... EVERYTHING IS SAME. Even spark plugs. Single has two per cylinder; twin has one per cylinder. Both have same; either SOHC or DOHC or even pushrods, doesn't matter... My QUESTION is: 1: Which one will make more BHP? And why? 2: Which one will have better low-down torque? 3: Which one will vibrate more? 4: Which one will drink less fuel? And why? I really see no reason in a 360 Degree crank parallel twin. As single is cheaper to make, lighter and is literally the same thing IMHO. Can any expert shed some light on this topic? I've googled everywhere but couldn't find a thing on this specific topic... Thanks a lot for your time.
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Old 08-14-2014, 10:28 AM   #2
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My take (and it's just that) is that a parallel twin has more surface area for cooling, which allows for a higher compression ratio before detonation occurs, as well as a longer service life before rebuild.
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Old 08-14-2014, 10:41 AM   #3
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I think the twin would be a more efficient breather. You're moving more air volume in the twin because you've got twice the number of valves. Smaller combustion chambers are probably more efficient as well.

Also, possible the twin would be able to spin faster as there would be less reciprocating mass on the conrods.

YMMV
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Old 08-14-2014, 10:42 AM   #4
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Just because the pistons in a parallel twin move in unison doesn't mean the firing order is the same. One will probably be on the exhaust stroke while the other is on the compression stroke. This really smooths out and increases horsepower delivery.
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Old 08-14-2014, 10:52 AM   #5
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If you limit the twin to 2 valves per cylinder, well, why not limit it to lower compression too? Yes, in an unrealistic comparison with one type handicapped, the other can be better. Is that what you want to hear?
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Old 08-14-2014, 12:07 PM   #6
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Originally Posted by Navin View Post
If you limit the twin to 2 valves per cylinder, well, why not limit it to lower compression too? Yes, in an unrealistic comparison with one type handicapped, the other can be better. Is that what you want to hear?
This is it. No engineer would ever approach a design with these constraints. They would take advantage of the characteristics each type of engine has to offer and design it to make it perform in the manner desired.

While your question is interesting at some level, there is no single answer. You would have to fill in all the specifics to get there. Bore, stroke, engine capacity, valve area, valve timinmg, carb size, compression ratio, ignition timing, piston mass, exhaust diameter & backpressure, and all the rest.
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Old 08-14-2014, 12:09 PM   #7
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The twin will have more valve area per cc and so should breathe better. There will also be twice as many power strokes per degrees of crank revolution, given a firing order where the cylinders fire at 180 degrees apart from each other. Vibration will be similar between a 360deg twin and a single. This is an old design mainly used by the British marques of yore to minimize parts and simplify the crank construction. The British tended to be quite, ah, frugal in their design philosophy, which certainly made sense as motorcycles were seen as working class transportation rather then play toys or racetrack refugees. If you design the twin so that one crank throw is 90 degrees out of parallel with the other, a 270 degree design, they can be made very smooth indeed, and this has been done with the XS650, which has a built up crank pressed together with roller bearings, like a two-stroke motor. Turns out, the center pin of the crank has 13 splines, and you can press apart the crank, rotate the right crank throw forward 3 splines and get a 277 degree crank for (relatively) little effort. This smooths out the motor considerably, and changes the exhaust note to something resembling a 90 degree v-twin. All you need to do then is have the cam suitably rephased, and change the ignition system to points with an 83deg separation for the proper firing timing. If you are less of the do it yourselfer, all the parts and crank work are readily purchased for a proper 90/270 degree setup. Now the vibration between the single and the twin are nowhere near comparable.

All things considered, more cylinders make more horsepower for a given displacement (more valve area, more power strokes per crank revolutions etc.). Rejecting the antiquated 360 parallell twin, a modern twin should rev higher, breathe better, be better balanced and make more horsepower than a single of comparable sophistication. Large singles make good low end torque, since that is, broadly speaking, all they are really capable of. Heavy components are less willing to rev stratospherically to make gobs of horsepower high up in the rpm range, so they usually get tuned to make thier best power at lower revs, within the design and material limits of their components. Yes, I'm sure there are speedway bikes running nitro in Rotax singles that rev their nuts off, but I'm talking motors that are built to go more than a few races between rebuilds. Twins, or triples or fours or sixes or anything can be tuned to make as much or more torque as a single at low revs, but that is wasting the design potential, given their (usually) much greater headroom for revs. Generally speaking, tuning that makes good power down low tends to strangle upper rpm performance. Good cylinder filling ( and good power) at low rpm requires fairly meek cam lift and timing to keep up the intake velocity. This is pure poison for high rpm breathing and would kill power. Likewise, wild, high lift cams tend to concentrate all the power up high, great for peak power, but making a motor quite gutless for most real-world driving.
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Old 08-14-2014, 12:30 PM   #8
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There have been a lot of good answers, part of it really is, multi cylinder engines do breath better. But, one thing overlooked is surface area of the pistons. A twin has more surface area. Power in any thing mechanical using a piston is simply force X area.

I will use 2 "antiquated" engines I am familiar with....BSA Gold Star, which is one of the kings of producing power out of a single and a humble BSA A50 which can be coaxed into making good power and only has one carb....

Gold Star 85 mm X 88 MM, piston surface area = 2269mm˛

A50 65 mm X 74 mm, piston surface area = 2652mm˛

Reality is, there are better examples for a twin, I just couldn't remember any other bore/stroke dimensions......

The humble A50 has more surface area to produce power since the force from the fuel burn will be virtually the same. A lot of other factors come into the equation - compression ratio, cams, valve size, exhaust, mechanical losses, etc, but in this case, the twin inherently has more surface area to work with to produce power.

Twins do have a vibration that is different than singles and why they are sometimes thought to vibrate more. The 2 pistons produce a rocking couple in the crank. Singles don't deal with rocking couples.
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Old 08-14-2014, 12:41 PM   #9
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Originally Posted by Navin View Post
If you limit the twin to 2 valves per cylinder, well, why not limit it to lower compression too? Yes, in an unrealistic comparison with one type handicapped, the other can be better. Is that what you want to hear?
One type handicapped? Well, single has one LESS cylinder so its fair and square IMO... And most importantly, I just wanted to understand what will be the major difference between the two if EVERYTHING being equal, thats all. I've nothing against parallel twins or any other engine layout in that matter... Hope you understand. Thanks for the time.
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Old 08-14-2014, 01:16 PM   #10
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Originally Posted by MacMcMacmac View Post
The twin will have more valve area per cc and so should breathe better. There will also be twice as many power strokes per degrees of crank revolution, given a firing order where the cylinders fire at 180 degrees apart from each other. Vibration will be similar between a 360deg twin and a single. This is an old design mainly used by the British marques of yore to minimize parts and simplify the crank construction. The British tended to be quite, ah, frugal in their design philosophy, which certainly made sense as motorcycles were seen as working class transportation rather then play toys or racetrack refugees. If you design the twin so that one crank throw is 90 degrees out of parallel with the other, a 270 degree design, they can be made very smooth indeed, and this has been done with the XS650, which has a built up crank pressed together with roller bearings, like a two-stroke motor. Turns out, the center pin of the crank has 13 splines, and you can press apart the crank, rotate the right crank throw forward 3 splines and get a 277 degree crank for (relatively) little effort. This smooths out the motor considerably, and changes the exhaust note to something resembling a 90 degree v-twin. All you need to do then is have the cam suitably rephased, and change the ignition system to points with an 83deg separation for the proper firing timing. If you are less of the do it yourselfer, all the parts and crank work are readily purchased for a proper 90/270 degree setup. Now the vibration between the single and the twin are nowhere near comparable. All things considered, more cylinders make more horsepower for a given displacement (more valve area, more power strokes per crank revolutions etc.). Rejecting the antiquated 360 parallell twin, a modern twin should rev higher, breathe better, be better balanced and make more horsepower than a single of comparable sophistication. Large singles make good low end torque, since that is, broadly speaking, all they are really capable of. Heavy components are less willing to rev stratospherically to make gobs of horsepower high up in the rpm range, so they usually get tuned to make thier best power at lower revs, within the design and material limits of their components. Yes, I'm sure there are speedway bikes running nitro in Rotax singles that rev their nuts off, but I'm talking motors that are built to go more than a few races between rebuilds. Twins, or triples or fours or sixes or anything can be tuned to make as much or more torque as a single at low revs, but that is wasting the design potential, given their (usually) much greater headroom for revs. Generally speaking, tuning that makes good power down low tends to strangle upper rpm performance. Good cylinder filling ( and good power) at low rpm requires fairly meek cam lift and timing to keep up the intake velocity. This is pure poison for high rpm breathing and would kill power. Likewise, wild, high lift cams tend to concentrate all the power up high, great for peak power, but making a motor quite gutless for most real-world driving.
Very interesting readout... So Twin is always better then a single... But, It just make me wonder one thing. Is it possible to make high performance engine out of a 270 or 180 Degree Crank parallel twin that could rev up to 12,000rpms like a Ducati L-Twin??? The biggest problem will be the 'rocking couple' I believe but is it possible at all???? Its a pretty foolish question no doubt but I've been wondering about a parallel twin 1.2L ' Superbike' for quite a while now! If Kawasaki Ninja 300 (which I believe has a 180 degree crank) can rev past 13,000rpm then I think that a BIGGER engine can rev that high too with some REALLY REALLY good Counterbalancers?
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Old 08-14-2014, 01:28 PM   #11
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At the bottom of the new bike gene pool, you can find a freshly designed 300cc P2 180 degree from Kawasaki making 39 crank HP, and a 300cc Honda single making about 31. As 250cc bikes, each made less, the Euro/Thai Ninja 250 made around 36 iirc, the Honda CBR250 near 26.

Each has a very different feel, the Honda vibrates at a different frequency and redlines much lower.

2 extra valves and the second intake on the twin are the only difference besides cylinder count and neither is in a very hot state of tune. The twin spins to 13k RPM, the single would be in MX 250f tune and design/weight, etc. to pull that off and then give similar HP.

What does it all mean? Could we shove 6 valves and a second intake tract/injector in the CBR300 head and get 39 HP with low compression and gentle cams? Maybe with an advanced V-tech design and variable cam lift?

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Old 08-14-2014, 01:30 PM   #12
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[QUOTE=SAM7;24847547]Very interesting readout... So Twin is always better then a single...

No, or the Aprilia 450/550 twins would be dominating every form of racing where they compete with singles. Weight, gyro mass, packaging and power delivery are all factors.
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Old 08-14-2014, 02:07 PM   #13
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Very interesting readout... So Twin is always better then a single...

No, or the Aprilia 450/550 twins would be dominating every form of racing where they compete with singles. Weight, gyro mass, packaging and power delivery are all factors.
... And reliabilty.
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Old 08-14-2014, 02:21 PM   #14
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All things being equal, a hi po twin is just as much a hand grenade as a 250/450f.
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Old 08-14-2014, 07:01 PM   #15
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Originally Posted by SAM7 View Post
Very interesting readout... So Twin is always better then a single... But, It just make me wonder one thing. Is it possible to make high performance engine out of a 270 or 180 Degree Crank parallel twin that could rev up to 12,000rpms like a Ducati L-Twin??? The biggest problem will be the 'rocking couple' I believe but is it possible at all???? Its a pretty foolish question no doubt but I've been wondering about a parallel twin 1.2L ' Superbike' for quite a while now! If Kawasaki Ninja 300 (which I believe has a 180 degree crank) can rev past 13,000rpm then I think that a BIGGER engine can rev that high too with some REALLY REALLY good Counterbalancers?
My old as dirt EX500 had a 180 degree crank, 8 valves, 2 carbs, was fairly smooth so long as it wasn't idling, and had a 11,000 rpm redline. No idea on the horsepower.

Trying to make a large twin (like your hypothetical 1200 cc superbike engine) rev as high as a four cylinder is going to be difficult - As has been mentioned before, the higher the reciprocating mass, the harder it is to move it faster - twice per revolution, you have to stop that big lump of a piston, and accelerate it in the other direction. Also, assuming you are using a 180 degree crank as the 300 ninja, 500 ninja, and I assume the 250 ninja does, you have a firing order problem. The power stroke is every other time the piston comes up, so you have one piston firing 180 degrees after the other, with a bunch of "space" - similar to a Harley, where it fires one after the other making that pop-pop, pop-pop, pop-pop sound. The parallel twin with the 360 degree crank has an even firing order, BUT of course has a mechanical balance just as bad as a single.

You mention counterweighting your large twin. This will also cause problems when trying to make a sportbike - counterweights are WEIGHTS. The engine has to spin them. Rev up a 600cc sportbike - then rev up my 1400 intruder. The sportbike screams up to it's redline almost instantly, compared to the Intruder's V-twin 1400.

A good article about engine balancing that was written by someone who HASN'T spent 12 hours at work, an hour in the shop wrestling a tire off a rim, and is now falling asleep. Informative and very extensive, and will answer many of the questions you have.

http://www.autozine.org/technical_sc...ne/smooth1.htm
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