Difference between 360 and 270 degrees firing intervals

[RoyNC]

I was just curious why the Bonnies/thruxtons are 360 degrees and the Scrambler is 270 degrees. What exactly does this mean in regards to firing intervals? Does it change horse power?

All I have found so far is that it effects the Scrambler exhaust note to sound more aggressive.

 

[Thruxtonian]

The cruisers (speedmaster and america) also use the 270 crank. It is supposed to produce more torque at lower revs as opposed to making higher RPM horsepower. Everthing of course is a compromise, but the reasoning was that the lower RPM torque would be more usable power off-road.

 

[BlueJ]

Haven't ridden a Scram, so don't know the practical difference. But I agree, it's supposed to favor a quicker rise in the torque curve (low-end torque) but at price of a quicker drop-off (thus less high-end power).

As to the geometry: Imagine the piston is the second hand on a clock. One full cycle of suck-squeeze-bang-blow is two revolutions around the clock. If we start with the second hand on the 12, then the next half-revolution (180 degrees) is suck, followed by a half-revolution of squeeze, with the spark coming roughly at the 12 on the next time around. The next half-revolution another 180 degrees) is bang, followed by the final half-revolution of blow and then we're back where we started. Clear enough?

Now, on the 360 degree twins, the two second hands are always pointing at the same place on the clock face, but they are one full revolution (360 degrees) out of phase, so that when one is starting to suck, the other is starting bang, and so on. So one is doing suck-squeeze-bang-blow and the other is doing bang-blow-suck-squeeze.

In the 270 degree motors, when one second hand (piston) is pointing at the 12, the other is pointing at the 9. So when the one at the starts to suck, the other is halfway through squeeze. Or, to talk about the actual firing, when one of them starts the bang, the other one is halfway through blow.

If you break each of the 4 strokes into 2 parts, so that there are a total of 8 parts per full cycle, then, representing the left cylinder with an L and the right one with an R, and no cylinder with a -, then the firing pattern for the 360's is like this:

L---R---L---R---L---R---L---R--- etc.

whereas for the 270's it's like this:

L--R----L--R----L--R----L--R---- etc.

Clear as mud! I have no idea why this change has an impact on torque curves.

 

[Deano]

+1 on what the others have said. From purely a sound point of view, I have a scrambler, my mate has a bonny. The difference is quite noticeable side by side. The 360 sounds very even, like a marching band to a drum beat. The 270 is more lumpy running, a bit like the way a Harley goes 'potatoe-potatoe-potatoe' This combined with open type pipes produces a sound that I find incredibly addictive. I start up my bike on a rainy day in the shed, just to listen to it. Entirely a personal preference from a sound point of view.:T

 

[dschief]

When the America came out, I remember reading that they used the 270 firning order to get a more "cruiser-esque" sound. I guess so that it would be more audibly acceptable to someone looking for that kind of bike. Who knows????

 

[thearcticsea]

If you break each of the 4 strokes into 2 parts, so that there are a total of 8 parts per full cycle, then, representing the left cylinder with an L and the right one with an R, and no cylinder with a -, then the firing pattern for the 360's is like this:

L---R---L---R---L---R---L---R--- etc.

whereas for the 270's it's like this:

L--R----L--R----L--R----L--R---- etc.

Clear as mud! I have no idea why this change has an impact on torque curves.

I think with 360 the pistons rise and fall at the exact same time. So it's more like LR----LR----LR----etc.

 

[Nick Morey]

I think with 360 the pistons rise and fall at the exact same time. So it's more like LR----LR----LR----etc.

Nope, BlueJ's right... :whip:

 

[ivar]

270 vs 360 in itself has a very little impact on torque/HP , the 270 motors are fitted with milder cams which make for the differences.

The difference is in power delivery - the 270 has 2 power stroke pretty close, then 450 degrees of rotation where the rear wheel is able to retain traction. Where on a bike with 360 , if one looses traction, its harder to retain without backing off the throttle.

This theory is used both in flat track racing and e.g Honda's "Big Bang" 2-stroke GP500 racer. And in Yamahas newest R1 with its "crossplane crank"

 

[RoyNC]

Guys.. this is great information. It is starting to make more sense to me now. All great answers. Thanks.

 

[ivar]

more reading:
http://en.wikipedia.org/wiki/Big-bang_firing_order

 

[ktmmiker]

power delivery different

I believe that the power specs from the factory are also a bit different.
The 270 deg. at the crank peaks at around 48-50hp with similar torque peak.
The 360 deg. is in the neighborhood of 64 hp and torque in the mid 40's.
The 360 deg. on the Thrux specs about 68 hp at the crank.

 

[BlueJ]

I think with 360 the pistons rise and fall at the exact same time.

They do indeed rise and fall at the exact same time. However, at the top of the rise, only one fires each time (they alternate, one full up-and-down or 360 degrees apart). 4 stroke motor equals 2 up-and-downs per cycle (and I think that both these up-and-downs happen in one turn of the crankshaft - I didn't make that clear in my post, sorry).

Interesting about the shape of the cams being the cause of the torque changes - so not only are the lobes at different angles (to get 270 vs. 360) but they are different shapes, too??

 

[Nick Morey]

...and I think that both these up-and-downs happen in one turn of the crankshaft...

No. It's two revolutions of crankshaft for 4 strokes, one revolution for 2 strokes. In a 360 degree 4 stroke twin however, alternating cylinders fire on each revolution.

Notice a Bonneville 360 degree crankshaft:

Here is a Scrambler 270 degree crankshaft (notice where connecting rods connect is 90 degrees apart):

Check out the beginning of this link. The one thing this article fails to mention is that the reason for a 270 degree engine in a Scrambler is so the rear wheel will hook up better, a lesson learned from previous dirt racing experience.

So if one stroke is 180 degrees, 4 strokes is 720 degrees then:

360 degree - fires @ 0/720 & 360 (360 or 1 revolution apart)
270 degree - fires @ 0/720 & 270 (270 or 3/4 revolution, then 450 or 1 1/4 revolution apart)

:cheers:

 

[nohawk]

What happened to that animated video of the Triumph twin that someone posted a while back? It might have been sweatmachine? I dont get on very well with the search feature.

 

[thearcticsea]

They do indeed rise and fall at the exact same time. However, at the top of the rise, only one fires each time (they alternate, one full up-and-down or 360 degrees apart). 4 stroke motor equals 2 up-and-downs per cycle (and I think that both these up-and-downs happen in one turn of the crankshaft - I didn't make that clear in my post, sorry).

Interesting about the shape of the cams being the cause of the torque changes - so not only are the lobes at different angles (to get 270 vs. 360) but they are different shapes, too??

Oh man, awesome! Thanks for the info. I knew they rose and fell at the same time but I didn't know about the alternating firing. That makes total sense though. I guess I never thought about it hard enough. : )

 

[BlueJ]

No. It's two revolutions of crankshaft for 4 strokes

Awesome, thanks!!

 

[Nick Morey]

Awesome, thanks!!

:up:

 

[dschief]

Actually, both cylinders do fire at the same time, that's why you only have one coil. It's called wasted spark. The cams are phased 180 degrees at half crank speed which equals 360 degree firing order.

 

[RoyNC]

Nick's link was an excellent read and not it all makes sense to me regarding the difference between a 360 and 270 (and even the 360 V twin).

As Nick pointed out, the link does not go into any detail about the different "control" (for lack of a better term) that the 270 provides to the rear wheel. So... can anyone explain to me how the 270 is able provide more control or stability to the rear wheel?