But if the cylinders fire opposite of each other on the polaris, then why do they call it an even-firing parallel twin? To me, that means they fire at the same time, but I could be wrong. It make sence to have them fire oposite of each other. Thanks for the good information.

 

We have dyno'd a 700 with Mods and a V with Mods.Both of these are in the 35 HP range. I'll try to dig up th dyno sheets.

 

I was under the understanding that a parallel twin fires once on every revolution of the crank. A v-twin fires twice on every 2nd revolution. remember the v-twin puts both connecting rods on the same crankshaft journal. The difference is the v-twin pushes the crankshaft from 2 different directons, The result is more torque with less rpm's out of the v-twin. Hope this helps. (I prefer the v-twin myself)

 

4 stroke engines fire once every 2 revolutions. It requires two upstokes and two downstrokes to complete a cycle. That means a cycle is 720 degrees (twice around).

A single fires once per 720 (ie 0 degrees, 720 degrees, 1440 degrees and so on). These are obviously even firing, since the interval between fires is even.

A V8 fires 8 times per 720, or once every 90 degrees. As far as I know every V8 ever made fired evenly.

My 4 banger fires 4 times per 720, or once per 180 degrees. Inline 4 bangers fire evenly.

An even firing twin fires twice per 720, or once per 360 (ie 0,360,720,1080, 1440).

An odd firing v-twin fires like the Prairie fires at 0, 270, 720, 990, 1440. Note the interval is not the same between all fires. Also note the semi similarity with the even firing 2.


Now, when I say fire, I mean combust (ignite). I do not mean spark, since the 650 sparks at 0,90, 360, 450 etc. Many modern ignition systems spark when the piston is at TDC for exhaust. This is a wasted spark and has nothing to do with the price of rice.

The even firing twin sounds smoother than an odd firing twin, but they have similar shaking forces as a single cylinder. The odd 90* V-twin we have is primarly balanced (greatly reduced shaking), but still not as smooth as a horizontally opposed twin like a BMW. I for one do not wish have heads sticking out into my floorboard area so the V-twin is the best engine overall. The inline 2 is smaller and more compact.

Polaris' inline 2 is a pushrod model with many inherent weaknesses (crank), so you can't bump up it's horsepower. The 2 valve per cylinder pushrod design is lighter and simpler but can't produce as much power per CC with everything else even. If the Kawi 700 was efi and not detuned, it would have more horses than the SP700EFI.

 

Thanks Nyroc, that makes a lot of sence. Sounds like you know a lot about atv's, so I have just two more questions for you to help he fully understand.

1. On the 90* v-twin, do the pistons move out at the same time or is one on the down stroke while the other on the up stroke?
2. On the parallel twin, do the pistons move up at the same time or is one on the down stroke while the other is on the up stroke?

Thanks for the good information.

 

On our Prairies, the rear cylinder is aproximately halfway up the stroke when the front cyl is at TDC. Basically the crankshaft has one crankpin with two connecting rods on it. This is exactly like cutting off a fourth of a V8. All our V8s have two rods per crankpin. Some V6s do, some don't. Fords V10 has one rod per crankpin, but they did that so they could make ther V 78degrees. Our 90 degree V could fire at 0, 90, 720, 810, but that would be very uneven firing and would be stupid. I am not sure but I think Harleys do that kind of thing, but at a 45 degree scheme. Not that hogs are stupid, but from a vibration perspective some of their engines aren't the best design.


The polaris pistons move up and down at the same time. If you think that makes it vibrate like a one cylinder, you are right. However, engine vibration is not only caused by inbalanced rotating and reciprocating mass, but also by torque pulses. HAving two smaller torque pulses at even spacing is better than one big single pulse.

Any V twin can be made to fire evenly, but that makes them vibrate more. I have a copy of a research document of V twin engine vibration in the tech tips on my website. It covers many different two cylinder designs at different angles and firings. I didn't write it, but I understand the math behind it. I took a class on it in school.

 

Hey Nyroc.....thanks for the great information. That makes a lot more sence now. Appreciate guys like you who help educate us nebiees in the atv world. I have been riding for about 4 years, but I still have a lot to learn, esp. on the mechanical side of how they work. I studied chemistry in college, not mechanics, but a lot of it is just a matter of educating myself. Again, thanks for the info.

 

Mr. Nyroc has great knowledge of physics and mechanical engineering. He is just applying that knowledge to the ATV world.

Brian