Rephasing, What is it?

I think MMM's head is an OW copy.

I didn't know the vintage boys were re-phasing (I'd guess it is the road-racers and not
the flat-trackers). I'll look into that.

You know there's no more power to be had by spinning a motor past its "breathing space"
 
Go to the Aussie club site and there are pages of info about the racers and there success.

I know about breathing space, thats whiy griz and other knowlegable guys talk about head work and not cc's.

I understood the MMM head was the reworking of the 650 head not the recast one.
 
Seems to me that this rephase fad is all about separating naive bikers from their money.
Clever marketing hype by the sellers, suggesting more horsepower and/or more torque, is a sure way to increase profit. The male ego knows no bounds.

I do not agree and I am not a "naive biker". I rephased by XS 7 years and 43,000 miles ago for vibration reduction and it works. There is no loss in low end grunt, there is some loss in the ability to use it because of the more pronounced power impulses caused by the more constant momentum. This makes the drive train unhappy at a higher minimum RPM than a stock engine but IMO this is more than offset by the vibration reduction in the RPM range where the engine lives most of its life.

With only slightly altered gearing (Mikes 5th gear) and Vibranators in the handlebars it is significantly more comfortable for long rides than it was stock. Some riders have reduced vibration by making more drastic gearing changes. I have experimented with that but concluded that Yamaha knew what they were doing when they chose the gear ratio because this compromises performance.

So with a rephased crank, a few more cc's (700) and slightly altered gearing I have a bike that is a blast to ride (like it always was), and can be ridden comfortably for longer stretches than before.
 
Burns, the premier flat track XS engines built in the '70s did not feature rephasing. All manner of crank variations were experimented with, but none provided a sufficient benefit. In talking with Bud Aksland, who built KR's engines, none of them were rephased. Ditto with Harry Lillie, Axtell and other high end builders and tuners. This is true with both the stock-based engines as well as the later race-centric OU engines.

650P, there was all that development done to the head that was/still lost to a generation. The re-phasing would help to make the XS650 a more competitive race bike with the extra revs for top end speed

I'm still waiting to see any proof of difference in performance or torque on the rephased motors and a lot of people saying different things, as here there is 650performance saying the top tuners of the 70's tried different crank configurations with no benefit and stayed with the 360, and 650skull saying rephase will add extra revs for top end speed, which is right? I asked the question of whether the rephase would give my bike more power/speed in a similar thread and was told it wouldn't, it would just alter where the torque is,
With the amount of people doing these engines now, you would think somebody would build two engines with identical pistons, cam, carbs and exhaust one 360 and the other rephased and put them on a dyno to show what the difference is,
 
I'm still waiting to see any proof of difference in performance or torque on the rephased motors and a lot of people saying different things, as here there is 650performance saying the top tuners of the 70's tried different crank configurations with no benefit and stayed with the 360, and 650skull saying rephase will add extra revs for top end speed, which is right? I asked the question of whether the rephase would give my bike more power/speed in a similar thread and was told it wouldn't, it would just alter where the torque is,
With the amount of people doing these engines now, you would think somebody would build two engines with identical pistons, cam, carbs and exhaust one 360 and the other rephased and put them on a dyno to show what the difference is,

A rephased motor will REV TO THE MOON over a stock 360 but your valve train best be up to the task of keeping the upper end in check due to less strain imposed on the bottom end and at those higher than normal RPM's a stock cam would fall flat on it's face offering no power advantages and who in there right mind would consider pushing the XS over 8500 RPM's,the bottom end was never built to sustain those stress loads.

The advantages a rephase offers is less vibrations at certain RPM's,a shift in power distribution and sound.
 
Whenever I read posts from skeptics demanding scientific proof for the benefits of re-phasing, I can't help but think about that scene in the movie Kingpin where the HOT blonde in the tiny dress walks in and the Amish guy chuckles and says, "I didn't want to be the one to tell him, but with those narrow hips, that girl couldn't have more than 6 or 7 children!"

A re-phased XS650 is Vanessa Angel and a stock one is a wide-hip Amish girl.

It doesn't matter if the re-phase makes more [or less] power, torque, rpm, vibration or whatever. It's not about numbers. I prefer a re-phased engine because the sound of it stirs some primal emotions deep inside of me that a stock XS just doesn't.
 
Whenever I read posts from skeptics demanding scientific proof for the benefits of re-phasing, I can't help but think about that scene in the movie Kingpin where the HOT blonde in the tiny dress walks in and the Amish guy chuckles and says, "I didn't want to be the one to tell him, but with those narrow hips, that girl couldn't have more than 6 or 7 children!"

A re-phased XS650 is Vanessa Angel and a stock one is a wide-hip Amish girl.

It doesn't matter if the re-phase makes more [or less] power, torque, rpm, vibration or whatever. It's not about numbers. I prefer a re-phased engine because the sound of it stirs some primal emotions deep inside of me that a stock XS just doesn't.

:thumbsup: For me it's the sound and attach a set of CR500 rods and the primal emotions get stirred even deeper.
 
I'd like to chime in on this with the 'old guy' perspective.
Warning, this may not make sense since it comes from the 'other' side of the fence.

Some of us who grew-up in the '50s-'70s would watch with fascination during auto tire changes, seeing the tire guy do the 'balance' job with the wheel setting on a bubble-level balancing rig, carefully placing wheel weights about the rim to center that indicator bubble. Or later, on those new-fangled spin-balance machines. And then, if you had the opportunity, watch (or perform, if you were in the business) the process of crankshaft balancing.

All this 'balancing act' obsession would give one a love of weight-distribution symmetry, equal sized spokes on wheels, equal sized blades on propellers, equal sized shoes on your feet. This kinda lockstep thinkin' can blind you to alternatives.

Then, along comes this lopsided/wobbergobbled doo-dad called a "rephased engine", with its twin cylinder crankshaft twisted out of idealistic symmetry into a 90° wobbly-gonk.

:wtf: ???

It sure knocked ME for a loop. Sure seems that it would vibrate itself into pieces.

Then, I remembered my ancient '20s-'30s engineering handbooks, with their exhaustive text/formulas/tests on the design of flywheels for use in industrial machinery.

Long story short, the purpose of the flywheel is for the storage of kinetic energy, to be dispensed as necessary to perform some function. The design criteria involves calculating energy containment, energy release, and recovery. The flywheel naturally slows during energy release, and the designer must account for this change in angular velocity, keeping enough in reserve, and ensuring that the bearings and support structure can handle these changes.

Enter the single cylinder engine. The flywheel is the energy storage, and its angular velocity actually changes (faster and slower) as it exchanges kinetic energy with the reciprocating parts (piston, wristpin, rod). When the piston is at TDC or BDC, its kinetic energy is zero, and the flywheel (crankshaft) is at its greatest rotational speed. Conversly, when the piston is near mid-stroke, its speed and kinetic energy is maxed, and the flywheel (crank) is at its slowest.

This varying angular velocity of the crankshaft is significant, and must be accounted for in the main bearings, since they act as the fulcrum point between the mass-center of the crank and the mass-center of the reciprocating parts. This is the true source of rectilinear motion vibration.

Add another crank throw (second cylinder) in the same phase (orientation) and the effect doubles.

The rephase configuration forces these two flywheels (crank halves) to attempt to cancel each other's varying angular velocities, by distributing the varying kinetic energies between them, significantly reducing the spin speed differences, and reducing loads on the crank main bearings, hence reducing vibration. It's like the natural smoothness in 90° throw straight-8's, not so in flat cranks. Of course, this new energy exchange must be transferred through the crankshaft center pin as severe twist/torque moments.

In the idealistic/frictionless analysis, this rephasing has no effect. But, add some reality, and this rephasing will demonstrate some degree of increased output, both by reduced main bearing loads and by reducing irreversible energy losses from vibrating your bike (and your gelationous you).

Hook-up one of those old '20s weight-reducing belt vibrators to your bike (or a commercial paint shaker), adjust its output to duplicate the vibrations you currently receive while bike riding, and that would represent the amount of energy that is robbed from your engine...
 
Burns, the head that MMM offers is based upon the stock head casting, not the OU casting. Its ports are an exact (within .002") CNC copy of a head that Harry Lillie ported for a bike ridden by Steve Eklund - a former AMA #1 - which won at Peoria and placed in the top 3 at several other tracks. The best power we've ever obtained with the head (as measured on Vance & Hines 250 DynoJet) is almost 74 RWHP, which is right at the limit before things like cases and transmissions start breaking in a racing environment.
 
There is one more thing that comes into play and that is the vibration cancellation that results from the reciprocating masses not moving in unison. With a stock crank there is the primary imbalance that occurs at the same frequency as the crankshaft rotation caused by the pistons and rods. This is partially compensated for by the offset weight in the crank. There is also a secondary imbalance caused by the relatively low rod length to stroke ratio. The short connecting rod causes the piston motion to be asymmetrical at the top of the stroke relative to the bottom of the stroke which results in a fair amount of imbalance that has a frequency twice the rate of crankshaft rotation. With a 360 degree crank there is no inherent cancellation of these components, the only thing you can do is reduce and redistribute the primary component with crankshaft counterweights. The secondary component has no cancellation whatsoever.

If you have ridden a conventional inline four you can feel the effects of this secondary imbalance. With this arrangement if the component weights are properly matched (balanced) the primary imbalance can be completely canceled out, sounds good right? The only problem is there is absolutely no cancellation of the high frequency secondary imbalance. With a 360 degree crank or a 180 degree crank the secondary component from the pistons is all in phase and therefore additive. Riders of inline fours often complain of this buzziness and it can be quite annoying.

With a 90 degree crankshaft the secondary imbalance from the 2 pistons is exactly out of phase so aside from a new rocking component this imbalance is eliminated. When you combine this with the significant cancellation of the primary imbalance caused by the pistons no longer arriving at TDC and BDC together, and the rotational smoothness described by TooMany the change is quite dramatic IMO.
 
The mechanical benefits of reducing vibration are valid for a 90° V-twin, but a 270° I-twin is not a 90° V-twin. Each cylinder will still produce the same vibration no matter how the crank is phased. The only difference is that the cylinders will push against each other more and push on you less. So the 270° twin may feel smoother than a 360° twin but the internal stresses haven't changed.

Reducing the balance factor to improve the feel will actually increase the internal stresses.

PVibe277.jpg


SVibe277.jpg


BLoad277.jpg
 
A 90 degree V twin does not cancel secondary imbalance like an inline twin with a 90 degree crank. With the correct counterweight the 90 degree V has perfect primary balance but not so with secondary imbalance.

Here is a link with good illustrations of primary and secondary forces in various twin cylinder arrangements without the effects of counterweights.

http://sense.net//~blaine/twin/twin.html
 
A 90 degree V twin does not cancel secondary imbalance like an inline twin with a 90 degree crank.

That is true when both crank throws are operating on the same plane but that is impossible with a single-crank parallel twin. When you look at the end of the crank it would appear that the secondary forces cancel out on a 270° twin, but if you look at it from the front, one side is pushing up and the other side is pulling down. The secondary forces are acting together to rock the engine, not canceling each other out. The vibration is shifted a different direction but it is still present.
 
Hi Mrriggs,

Can you tell me what the percentage curves, 45 to 63, are representing? Each is labeled for a 277.

Thanks
 
Hi Mrriggs,

Can you tell me what the percentage curves, 45 to 63, are representing? Each is labeled for a 277.

Thanks

The percentages are the crankshaft balance factor.

The balance factor for a stock XS is about 63%. I'm sure it's no coincidence that a 63% balance factor puts the lowest strain on the crank bearings.

Re-balancing the crank to a 51% balance factor when re-phasing to 277° will reduce the vibes to the rider but increases the load on the crank bearings.
 
That is true when both crank throws are operating on the same plane but that is impossible with a single-crank parallel twin. When you look at the end of the crank it would appear that the secondary forces cancel out on a 270° twin, but if you look at it from the front, one side is pushing up and the other side is pulling down. The secondary forces are acting together to rock the engine, not canceling each other out. The vibration is shifted a different direction but it is still present.

So, from what you are saying there is no advantage to offsetting the crank pins on an inline twin? Then why did Honda make so many twins with 180 degree cranks? To change the sound? With the pistons in different planes the cancellation is incomplete but the reduction in vibration is still significant.

In the 70's I rode snowmobiles a lot and my first couple were singles. When I first rode a twin the vibration reduction due to the 180 degree crank was obvious. A 180 degree crank has none of the conservation of motion advantages of a 90 and the most rocking possible but yet still has very significant vibration reduction.

I have a friend who has worked as a mechanic at Yamaha dealers since these things were new test ride my bike and he said it was the smoothest one he had ever ridden.
 
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