i was reading the big bore shootuout between the flagship atv's of each manufacturer. when they got to explain each of the engines in the quads and they got to the polaris they said, that it was a twin with the cylinders sitting next to each other that rise and fall at the same time. BUT heres the catch, they fire alternately, so what the hell is that, isnt that like only 350cc worth of power? the combustion occurs every other time the cylinder reaches full compression, so that doesnt give you 700 true cc's. so i think that a 650 can outrun one of these, but if not can someone explain why polaris used that type of engine? and no im not in the wrong forum either, i posted there too.

 

Multi-cylinder engines do not fire their at the same time, with all that power at once it would be hard to hold the motor together, depending on # of cylinders they will fire so many degrees apart. I believe you are correct that they are firing 180 degrees apart, I dont think that is typical for V-twin engines but it does not reduce the CC's of the engine. You will end up with a cylinder firing every other stroke instead of say every fourth stroke ( single cylinder or all firing at once ) seems to me you would have a more even power cycle every other stroke.

 

same like a twin, i know they dont fire at the same time, but what was the point of the polaris engine rising and falling at the same time, seems to me it would be less balanced with more vibration that way, y didnt they rise and fall oppositly that way there would be more balanced

 

I think that's part of the problem with the Polaris' motors, they're loading the outer bearings alternately on the power stroke, and the oiling problems. I'd still rather have the V-Twin, or a big single, even though it vibrates a little more. The SP700's are making good power though, just aren't reliable as others for the money you have to put out.

 

It is a four stroke engine and they by definition only fire on every other revolution, once every 720 degrees. Firing alternately gives you a power stroke every 360 degrees which makes for smoothness. Back in the 1920's and 1930's Evinrude had some 2 and 4 cylinder engines that had all the pistons fire at the same time and they had very large gear case. When they update to alternate firing engines the same gearcase handled twice the hp because it was an even, smooth application of power. BTW; the V twins are also alternate firing. Both piston reach TDC at the same time, but only one fires each revolution, one is on the power stroke and the other is on the exhaust stroke. Doesn't matter how you stack those 2 cylinders, only one will fire each revolution. Now 2 strokes are a different matter and the pistons are reaching TDC every 180 degrees giving you twice the power strokes per revolution. That's why it is easier to get more power from them per CID. It is also why the EPA is outlawing them, except in very high dollar EFI applications.

 

Actually, They do fire simultaneously. I went to the Polaris web site and reviewed the parts manual for the 700. It clearly shows the crankshaft having both rod journals in the same location. This was a topic here a while back - Its been a while and I wasn't sure so I did confirm it again.

The Polaris quality reputation is a bit skewed from the massive number of units in service. Depending on where you live, Polaris may be the most popular ATV by 20 to 1. Here in Minnesota if 40 guys show up for a trail ride - 30 or more are on Polaris. With that much of a market share, you will hear a lot quality storys. I've had 3 Polaris machines and the only failures have been from something I did to the machine, not a quality issue.

There have been a number of issues with the AC 650 and limp mode. If its been fixed or not - best ask your dealer.

Personally - I think the Polaris 700 beats the AC650 in every category. But thats just my opinion after riding them around a closed trail course a few times. Nothing has the power of the 700. Its awesome.

 

Sure both plugs fire everytime around because it makes for a simple ignition system. However with a 4 stroke with both at TDC on is on exhaust and one on compression anit it will make power. The other will have exhaust gases only and nothing will ignite. I have been a professional marine mech working on 2 and 4 stroke engine and no 2 cylinder 4 stroke has both pistons on compression at the same time. Ther would be no point in having a second cylinder, just make the single bigger. It would be cheaper, less parts and make for a simpler engine. Doesn't anybody understand how a 4 stroke motor works?

 

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I think there is not any question on how a 4 stroke motor works.

I remember this being a topic either here or in another forum and the answer was yes - both plugs fire at the same time. When talking to a dealer, he pointed out several engineering advantages to it - something to do with the water jacket around a large single piston vs. two pistons and not having alternating vibration - but the biggist reason for two cylinders was for fault tolerance. If one plug failed - the engine could keep running and get you back out on the other cylinder.

Best check with a Polaris dealer and he can explan it. I'll try and remember to do the same.

I wonder how big of a single cylinder you really want to go with in a aluminum block engine.

 

I GOT THE ANSWER [img]i/expressions/face-icon-small-smile.gif[/img] ok as some of you know i live about 15 miles from the polaris factory and well know alot of the inside stuff that goes on there and the reason they have this engine where both pistons move up and down at the same time is cause they tried the alternate moving pistons and they couldn't keep the motors together they kept breaking crankshafts and i guess it was a mess i can't remember for sure but i do believe that they tried a v-twin don't quote me on that one not 100 percent sure so they went to this old type engine that doesn't produce anywhere near the amount of horsepower and it is a rougher running motor but it is as reliable as they could get and they wanted to jump in the big bore industry as quick as possible to answer i think it was the 600 grizz for polaris has been trying to catch yamaha in the sales for atv's so you can't blame polaris there but the 650 cat will walk all over the 700 polaris as will the 660 grizz i do hope that this does answer your ? cause i asked the same ? myself right to the engineers there