Quick Question on how the GS motor is setup

[mass-hole]

So I have a Polaris 4 Wheeler that is a parrellel twin similar to the GS but the pistons on it rise and fall together and they just alternate firing. is this how the GS is?

Thanks,

Jay

[007brendan]

I'm pretty certain the GS, like most parallel twins, is a 360 offset.

[utgunslinger13]

When the left piston is up, the right piston is down.

[the mole]

When the left piston is up, the right piston is down.

This gives better mechanical balance, but uneven firing. A lot of older British bikes are like your Polaris, gives even firing pulses but the balance of a single. Some Yamaha parallel twins used a 270 crank so they'd sound like a V-twin.

[twocool]

I'm pretty certain the GS, like most parallel twins, is a 360 offset.

You can't say "most" parallel twins...........many are 360/360 while many others are 180/540

Some say that 360 is used if you have only one carburettor, 180 would starve the second cylinder.....

Some say 180 is smoother engine

Some say 360 has better exhaust note

GS 500 is 180/540......look at microfishe drawing on parts drawing on Internet.

Cookie

[mass-hole]

So I have never heard it before like this. I assume 180/540 is referring to the degrees of crank rotation before each respective cylinder fires again, cause it appears that the parts page is showing that they are physically 180 degrees apart on the crank. I do like the how the polaris runs, its got almost like a two stroke sound to it when you rev it.

Thanks for helping me out with this,

Jay

[Agent4573]

The pistons are set 180 degrees apart on the crankshaft, however the cylinders fire 180/540 degrees of crankshaft rotation after one another. A 4 cylinder motor operates on a 720 degree circle, not a 360 circle.

[twocool]

So I have never heard it before like this. I assume 180/540 is referring to the degrees of crank rotation before each respective cylinder fires again, cause it appears that the parts page is showing that they are physically 180 degrees apart on the crank. I do like the how the polaris runs, its got almost like a two stroke sound to it when you rev it.

Thanks for helping me out with this,

Jay

Polaris fires at 0 degrees, 360 degrees and again at 720 etc.....even spacing between firing

720 completes the cycle, so you could call 720..... 0 again.........


GS 500 fires at 0, 180, then again at 720, (or you could call it zero again)  Uneven firing, but balanced better

Most say the 360 firing gives a nicer exhaust note...throatier........and dven......but the sound depends on other things too .....like exhaust system..


Cookie

[twocool]

The pistons are set 180 degrees apart on the crankshaft, however the cylinders fire 180/540 degrees of crankshaft rotation after one another. A 4 cylinder motor operates on a 720 degree circle, not a 360 circle.

Sorry to be anal again.............a 4 cycle engine operates on a 720 degree cycle..........regardless of the number of cylinders..........1, 2 4, 6, 8, 12, 16 all run through the 4 parts of the cycle in 720 degrees......
More proper to call it a "4 stroke" engine...rather than a 4 cycle engine as some do.....

strokes are

1) intake, 2) compression, 3) power, 4) exhaust

Those four strokes make only ONE CYCLE!!!!    720 degrees, or twice around for the crank.

Cookie

[Agent4573]

You're right. "Cylinder" should be "cycle". I typed the wrong word.

[The Buddha]

The gs is set unlike a brit bike.
The only bike I know of set like a brit bike is a yammie xs650.
Guess what the holy grain mod of in the xs motor is.
Pulling the crank apart and re setting it as a 277 degree motor (which is mainly due to yamaha's brilliant idea of using a 13 tooth spline to hold the crank 1/2's together)
There are people who make a 7 degree center pin and use that and you get a 270. Looks like a parallel twin, runs like a 90 V twin.

The GR650 is much like a GS a 180 degree. As a result it will easily out run a XS and shake and act up a lot less. Which obviously is why it flopped.

Cool.
Buddha.

[twocool]

The gs is set unlike a brit bike.
The only bike I know of set like a brit bike is a yammie xs650.



Cool.
Buddha.

I'm pretty sure the Honda Rebel 250 (just got rid of one) is like the Brit bikes too.......both pistons rise together....360 timing.........one carburettor.....

But I just looked at a microfische of the 450 Rebel and it is also like a Brit.........I think it has two carbs however....

Cookie

[twocool]

Let's get a little more anal (precise) here.......the gs 500 does not "fire" at 0....180......720....etc....

It reaches "TDC of the compression stroke" at   0....180....720....etc....

Since the engine "fires"   some degrees   BTDC..........like say 20 degrees BTDC...........

It would fire at    -20..........160.............700......etc........

I am not sure exctly what the spark advance curve looks like.....20 degrees is  just a guess....at a particular RPM...


Cookie

[black and silver twin]

Let's get a little more anal (precise) here.......the gs 500 does not "fire" at 0....180......720....etc....

It reaches "TDC of the compression stroke" at   0....180....720....etc....

Since the engine "fires"   some degrees   BTDC..........like say 20 degrees BTDC...........

It would fire at    -20..........160.............700......etc........

I am not sure exctly what the spark advance curve looks like.....20 degrees is  just a guess....at a particular RPM...


Cookie

the gsf is 5* at <1300rpm that ramps to 40* at 4000rpm+. that is with stock timing of course. also the f models throttle position sensor changes timing but Im not sure to what at any throttle.

[twocool]

Let's get a little more anal (precise) here.......the gs 500 does not "fire" at 0....180......720....etc....

It reaches "TDC of the compression stroke" at   0....180....720....etc....

Since the engine "fires"   some degrees   BTDC..........like say 20 degrees BTDC...........

It would fire at    -20..........160.............700......etc........

I am not sure exctly what the spark advance curve looks like.....20 degrees is  just a guess....at a particular RPM...


Cookie

the gsf is 5* at <1300rpm that ramps to 40* at 4000rpm+. that is with stock timing of course. also the f models throttle position sensor changes timing but Im not sure to what at any throttle.

Cool....thanks for the info........40 degrees is a lot of advance, but figures right for that kind of RPM.........I am used to airplane engines which run at only 2700 RPM redline......and about 20 something advance.....

Cookie