550 Hop Up

So winter has arrived and that really sucks for a myriad of reasons, however, it has given me some time to focus on a project I put on the back burner almost 2 years ago - a CB550 / 650 hybrid motor. If you want some particulars about this set up check out paulages sohc 4 post here. The long and the short of it is that in the late 70's Honda needed a better middle weight motorcycle but didn't want to spend the big bucks developing a whole new engine. They simply took the existing 550 engine and made a bunch of improvements. The bigger displacement is a product of a bigger bore and longer stroke but that is just the beginning of the upgrades. The head porting is eons better and the domed pistons fit nicely into hemispherical combustion chambers. The changes mean 64 ponies as opposed to the 550's 50. So the big idea is to swap the top half of a 650 on to the bottom half of a 550 resulting in an engine approaching cb750 power in a bike 3/4 of the weight. I got my sacrificial 1980 cb650 from my friend Paul. Seized, no title, gaping hole in the bottom of the tank and desperately needing to be put out of it's misery. The cool thing about getting the whole bike is you quickly discover other upgrades at are easy bolt ons for the 550. 17" rear wheel, better fork and front brakes, etc . . . Anyway, on with the build:

The sacrificial unit

550s use studs and 650s are bolts so all of these guy have to come out. Without the right tools this is a real son of bitch.

The magic moment.

Here you can see some of the head differences. Size is obvious but the shape of the combustion chamber is a little more subtle. The intake design is entirely different, good bye 550 manifolds! Now I've got short runners that can run the stock 650 pumper carbs or any cb750 big boys. 

Domed pistons for bad boys only.


I'll try to update with some of the machine work my friend Aaron is doing to make everything work, mostly mods to the upper case, points timing advancer and the alternator rotor.  Hopefully I'll be surprising some 750's in the spring! Time will tell!