The standard 668 engine produces a claimed 70 CV (70 hp). This can be improved by changing the exhaust system as a first and not too radical step. More radical changes have, however, been suggested by Tombo, who has experience racing with a 668-based motor in the Netherlands.
"Modifying the head is possible, but not as easy as it seems. The block's face is not flat: the steel cylinder bushes protrude about 2 millimeters and take the copper seals there. This means if you want to shave the head, you will also have to mill out the combustion chambers to allow for the cylinders to fit into them. This is not particularly difficult, but you have to work pretty accurately. Apart from getting a higher compression, making the chamber shallower will also increase the squish zones quite a lot. This is a very welcome effect! Also: your cam timing will get earlier (about 10 crank-degrees with a 1 mm shave). This helps the engine run at higher revs. Be sure to check that the valves don't foul the pistons: you should have some 1 mm margin. [See also the High Compression alternative suggested by John Ryti]
"If you feel like giving it the works, get rid of the inlet manifolds. They look like a funnel and restrict air flow at anything over 5000 revs. I replaced them with two copies that have a completely straight bore in them: 40 beautiful mm's to inhale through...
"They are easily made from aluminium on a lathe. (Hey, you probably don't need this kind of explanation:) Of course this mod means that you have (can!) open up the inlets in the head. Best way is using a hand mill on a flexible drive. To keep it safe make a steel template and bolt it in place of the manifold. Now take a day or so and lovingly get rid of all that material in there. You should mill deep enough to meet the original inlet about where they split up into two smaller ones. Finally smoothen things up in there with a sandpaper-ball (sorry, don't know the right word for that in english). You don't have to polish it, a smooth matted surface is perfect. In fact: a high gloss polish will probably lead to fuel condensing on the manifold walls, so...
"With these mods we got about 91hp @ 10.000/min on the dyno, after optimizing the FI unit both on fuel and ignition.
"If you go this road, you would be wise to raise the oil pressure about 1 bar. There is a factory kit for that, it became the standard in the late 668's and onwards. Also, it is possible to fit a 750 clutch. It has an extra friction plate and can take more power. We burnt some stock ones..
"Some final warnings: take care that your bottom end can take this kind of power and revs. Check and ideally replace the crankcase studs with the latest you can get. We made our own from very high tensile steel (it has to be fatigue resistant) with rolled, NOT cut!, thread. Also, get a lot of weight off the counterweights. This will increase vibes, but the original, if you still have that, can get critical around 7500 revs. This means the shaft tears and the block is lost. This was a problem in the early years (95-97) I'm not sure whether current stock parts are modified to prevent this problem. It is not the power, it's the torsional vibrations that kill it.
"After the 97 season, I got the bike from the team and reverted it to street specs. Well..... I put the original fairing and equipment on, but ..eh.... forgot to detune the engine. Just put the airbox back on, with a K&N and a great big hole in the cover, between the ducts. I also adjusted the inlet cam (yes, that's another mod:) to a setting that is a little later in order to get more torque in the midrange. It's a great, if somewhat noisy, bike. Now for more sun and less work..."
Tombo has promised to make more information on his radical mods available on the net - contact him for more details in the mean time.
© Steve Carr, 2001-2007
Site created February, 2000. Last modified Friday, May 15, 2009
30 May 2005