i was reading the big bore 4x4 shootout and the polaris 700 was in there. and they were explaining the different engines in all the quads. Well when they got to the polaris, they stated that the cylinders are situated next to each other and rise and fall together, but they fire alternately, not at the same time, so technically arent you only getting 350 cc worth of power during each combustion? how the heck does that work?

 

im not sure about that, but i can tell you that ive rode it alot and for how much it weighs... it does move out pretty good

 

yes it's really a 700, that's how much displacement it has.

 

NO, its a 683

 

You are essentially right about 350cc (actually 341.5 cc's) worth of fuel/air per combustion. BUT, it is doing this every time one of the 2 pistons is near Top Dead Center. So, while one is bringing new air/fuel into the cylinder the other piston is on the power stroke (and vice versa). So you have 2 cylinders helping each other out. If it were just a single cylinder 350cc engine it would not get help during any of its cycle from another cylinder. So what's happening is you are firiing both cylinders in 720 degrees of crank rotation, instead of once (like a single cylinder).

 

i guess your right, but wouldnt it be more vibration and less balance rising and falling together, i thought if they would rise and fall oppositly there would be a smoother engine, the big advantage of haveing multi cylinders

 

Polaris engines fire this way due to many hours of testing. One of the main reasons is horsepower/torque. If you checked the torque on a Polaris vs the same size motor of another, you will find that the Polaris is a beast.

I just got back riding in the mountains. My friend on his 2005 Kawaski Brute Force was along. He couldn't keep up with my riding, yet he is experienced and has owned several ATV's. His problem was diving in the corners on the Brute Force. He couldn't get it to spin the rear tire around the corner to slide it. In his words it felt like it was going to flip over. I told him I read about this being a problem with the BF.

We switched machines, I felt the same thing. So we did some adjustments on the shock settings. Got it alittle better but not much and we tried all different types of settings.

He also noted that my machine felt more comfortable to ride over the bumps and steered much better. We drove up and arroyo that was pure water and mud, and he also stated that it was easier to pull from the mud with mine. Granted, I have better tires than him. We switched machines again, I hated driving that thing compared to my '05 700 EFI. It has power and can pull me off the line, but can't pull away from me. He gets a bike length on me and then its dead even and I have 27" tires.

Oh well, goes to show that Polaris knows what it is doing when they designed the Liberty engines.

 

frndinalowplace - you are right in some ways about the balance issue. There are 2 forces (primary and secondary) when talking about the forces by the piston in relation to the crank. The notion of primary and seondary balance is far too deep for my puny head to understand at any deep level. But, very generally, you are questioning the amount of mass (2 pistons) being thrown up and down at the same time being more 'brutal' then if they were 180 apart and alternated the time at which they get to TDC. While that part of what you are saying is right, it is easy to remedy that with a counter balancer shaft (and the counter weight of the crank). The other aspect to consider though, is the force that happens when combustion occurs. The crank design needed to bring the piston up to TDC while the other is at BDC has a more harsh firing pulse than that of the Polaris design. In the cylinder configuration you describe, there are 2 rapid combustions (180 apart) then you wait 540 degrees of crank rotation until you do it again. In the Polaris, you space out the combustion every 360 degrees. Thus, the more drastic force (the one you will feel more vibration from) is the one resulting from the combustion (power stroke). So, from that perspective, the Polaris is smoother. And to make the primary force more manageable on the Polaris, if you design a good counter balancer, you can negate the extra force due to moving both pistons up and down at the same time. Hope that helps and hope that made even a little bit of sense.