To rephase or not to rephase…
I quite often I hear ‘I want to do a rephase on my XS650′, but when I ask why, the reply ‘the internet told me to’ seems to be a common answer. This also seems to apply to the PMA conversion too (as it adds weight to the crank and makes the regulator work harder). So, I thought I would pose this question myself and try to clarify what it is and what it does in simple terms for those thinking about giving it a go, trying to be as unbiased as I can.
WHAT IS A REPHASE?
A rephase is when you change the configuration of a crank or camshaft to change the time an engine fires.
Eg: a stock XS650 has a 360º crank (when the firing rotation is 360º apart from each other, that means that both rods are at the same position on the crank. 180º would mean they are at opposite sides). A rephase on an XS650 is usually at 277º or 270º so that one piston rod is at 12 O’clock and the other at 9 O’clock (depending on which way you hold it). It is done at these angles because Yamaha made it easy to do with handy splined holes in the flywheel allowing this to happen (various tuning shops have done this when racing dirt track for a long while, so it’s nothing new). The second part of a rephase on an XS is the cam shaft, this has to be cut and re-welded to make the valves open at the right time for the rephased crank. Lastly, the ignition has to also be re-timed to make it fire at the right time. There is also one other type of rephase occasionally done on XS650′s… it’s called a ‘twingle’ (which it isn’t a true twingle as it doesn’t share a common combustion chamber). This is done by leaving the stock crank alone and rephasing half of the cam 180º to make both cylinders fire at the same time. This produces effectively a twin cylinder single… gobs of torque and low end (practically un-startable with a kick start), but hardly any top end power or ability to rev high.
WHAT DOES A REPHASE DO?
It will do two things for certain; it will make the engine sound like a 83º or 90º V-Twin (if we are talking about an XS650 rephase) and it will possibly run a hair smoother at higher RPM (but this is arguable). A 180º crank would be the best configuration for a smoother ride, but that’s a lot harder to do on an XS650 (Honda configure their CB350, CB360 and CB450′s to 180º. However, the smaller twins up to 200cc were all 360º cranks and Honda later rephased the CB450′s to this too on the Nighthawk). As for if a rephase gives you any more power? Well, you would literally have to build the same engine twice and dyno it to prove that, as every casting on old Japanese bikes varies quite drastically. Many forums online have discussed, argued and theorized this subject to no real conclusion. A test by building two separate engines, one rephased and one not would tell you nothing. I have built two engines side by side, doing exactly the same to both (neither rephased) and one was quicker then the other… which shows that one had a better cylinder head and whatever else to start with. Not to mention variations from ignition, exhaust, cam and carbs etc. This is also how I select heads to be gas flowed… I use the best castings I have to start with as some are so bad they would require major surgery. The more you counteract what one piston is doing, the smoother an engine will be (like a 4 cylinder, as they are basically two 180º cranks which is the smoothest configuration). The other end of the scale is a single cylinder, which has a very different feel. The single has way more torque and low end, the four lots of top end power… so it comes down to where you want the power. There is always a trade off for any performance modification you do to an engine, except for making sure everything is in good condition and is tuned right… but that’s not modifying it. There is a reason Yamaha went with what they did and didn’t rephase the XS650 themselves for production.
NEED MORE POWER!
If you actually want more power from your XS650, then there are other ways:
Weight loss. There is no down side to weight loss at all, whatsoever. This is called ‘free power’. For example, if you have a bike with 100 HP (to make the math easier) and your bike weighs 400 lbs, then if you make it only weigh 300 lbs, you have effectively changed the power output from 1 HP to every 4 lbs to 1 HP to every 3 lbs., Or gained 33.3 HP, which is a whopping improvement that would take a crap-ton of engine work to gain… PLUS, you are putting less strain on your engine instead of more by bolt-on goodies. The 750cc conversion is always fun. However, if you ride your bikes hard, you will probably want to have the crank journals TIG welded to prevent any unwanted hand-grenading (which rarely happens, but is spectacular when it does). With good head work, well set up carbs, a mild race cam, free-flowing exhaust, better ignition and a 700/750 kit… you will find yourself on the back wheel quite often.
WHAT’S THE BEST BANG FOR YOUR BUCK?
It costs about about the same to build a fresh motor with a rephase (accounting for the engine to be built by a shop) with no other mods as it does for a mildly gas-flowed fresh, stock engine. including better carbs, ignition, cam and exhaust. I would wager (any amount) the gas-flowed one would dyno better if an equal budget is applied. The power curve would be very different too, having a steeper curve for the stock crank (dropping off at 8,500 RPM) and a more linear curve for the rephase (dropping off 500 RPM later). Most of the XS650′s power comes from the cylinder head. Some castings are significantly better than others too. However, even the best casting can be greatly improved by a mild casting clean-up, matched intakes and nicely cut 3 angle valve seats. Apart from weight loss, this, combined with a nice set of carbs, good ignition, a Shell No.1 cam and a good exhaust is probably the best bang for your buck on engine mods. However, spending the same money on weight loss would be money well spent too. If you start with a 495 lb Special and cut that down to 380 lbs (very do-able), that would be (assuming a stock HP of 48, which is what most good Specials dyno at) would give you an effective 64 HP (compared to the stock machine), which is highly modified XS650 territory. Strawberry Shortcake is about 300 lbs (as it has no swingarm etc.), which would translate to 78 HP in comparison to the stock machine. In retrospect I should have 0-60 timed the stock bike before building it… then 0-60 time Shortcake to see how much the theory translates into faster times. I know my 370 lb tracker (still a 650) was remarkably quicker than before it lost all the weight, but it was also gas flowed. A 750 cc kitted and modified engine will get close to 70 HP if everything is done and done right… but 60-65 is more likely. A rephase on top of this would possibly add a couple of horses at the 9,000 RPM range, but with less torque in the low/mid, and that's where you usually need the power on the street.
Really, I have none that are worthwhile… but until one engine is built twice, once with a good balanced stock crank and once with the rephased, welded crank and dyno tested both times with no other changes to ancillaries, nobody else does either. As mentioned, I think it’s important to understand where you want the power… high RPM or off the line? I feel most people would prefer it off the line unless they are racing it (rephasing in dirt track helps with tractability as it’s less ‘thumpy’ in the low and mid… but that’s on dirt. This also equates to less mid-range torque). If you want an XS650 that’s better on freeways, I can’t help thinking you have the wrong bike. The appeal of an XS is that it’s the best Triumph ever made, why change the feel of something so good? If you want smooth freeway riding, get a Honda four. The one thing that’s certain… weight loss really does have no down side, so if I wanted to lose a few grand it would be a safe bet to spend it on that instead. Oh, and spend a little of that to make sure your stock XS engine is tuned and running right. Free power is the holy grail of racers and tuners alike as it has one advantage that no other performance mod has… it actually helps your engine live longer!
A very interesting addendum: please read this!
This was a letter I received from a nice British guy, who really knows his stuff.
Hi, I just read your XS650 rephasing and tuning article and as the man who pioneered XS tuning in the U.K 30 years ago I have tried every option you mention and more. I operated alongside Halco tuning and as Palma tuning and built many very big bore motors, etc. Then the rephase idea cropped up, so I made a few. I actually did tear down the same motors and tried every combination possible. Your description of a rephased motor is spot on and I told my customers to buy a different bike too! The internet has made the rephase a popular myth/folly. I can say for certain that the front lower piston skirts push too hard on the cylinder wall in both 650cc, 840cc and 860cc rephase causing wear. The power delivery is also in the wrong place! Head work and cams are king. Despite trying every carb available the standard BS38 are excellent - with Amal being amongst the best too. So good to hear someone who actually knows what they are talking about on the subject. Too much internet bunkum out there, So Thank You! Some folks just add parts so they can say that they've tuned the bike. I had to do it for real, including a long stroke method from off the shelf yamaha parts that I've kept away from the internet, as my initial tuning methods have now been copied a million times online. Surprisingly no ones spotted the .00 error in the manuals copied from the first Yamaha factory manual, probably because i never published that part either, or the 10 plate clutch trick.
Good on you for settling the rephase myth (now retired from the XS650 tuning business, but still riding hopped up 650's).
Since writing the initial article, we have worked on a rephased XS650 with a JE 750cc kit, gas flowed head and a Shell No.1 cam... and we can categorically tell you that the rephase didn't really help with vibration, and It was also a complete pig to kick start as you have two compression strokes right next to each other (it may also be noted that I personally only weigh 140 lbs and have the legs of a chicken, but I do also ride an XT500, so I feel I'm used to a firm kicker).
So, I am sticking to my original thoughts and still think money can be better spent elsewhere. Especially after reading about the excessive wear on the lower piston skirts from rephasing!