How high can it go?

I am interested if anyone has spun these 650,700,750s past 10krpm and has had any luck or made more power? I was kind of wondering today what is the limit of these engines so I figured I would ask if anyone has tried or is successfully running past 10k? I know with my current setup I have really increase amount of the air flow going in and out of the engine? I have hit the redline a few times and the engine just wanted to keep going.


So how high can it go?

 

Before it blows up?

I believe the limiting factor will be the valves ability to close fast enough to not get smacked. I know my X will easily hit 11K but my valves are much smaller.
As I'm sure you know....it would be VERY bad to find out you went a little TOO high 8O

The 85/86 700s will probably be different than the 650/750s mostly due to the bore/stroke ratio is significantly different than the earlier XJs. IIRC, the earlier XJ were smaller bore longer stroke meaning your connecting rod is longer and potentially more flex along with higher velocity....but I'm not an engine expert....just throwing it out there.

 

65.0 x 52.4 mm is the bore/stroke. I was wondering about the valve springs starting to float also. I was thinking of the 11k to 11.5k range of rpm.

In a perfect world I would like to have the rods cryrotreated and fluxed. Also some new stiffer valve springs install and I could see it possible.

What I would really like to do is have some 67mm 10.5:1 CR pistons made. But that would be around 500 to 600 bucks to have that done. That would also include having the engine bored 2mm over. Also thinking of going with a turbo. Always tons of ideas.

I was wondering Mike (if that is your name) what you did to make your engine you told me about you wanted to FI.

 

http://www.youtube.com/watch?v=fXScchw3 ... F8B583D049 Its no right to do such a thing but it proves a point.
http://www.youtube.com/watch?v=8BEcRzxr ... re=related

 

The motor was designed to make peak power AT redline, 10K. I'm sure you could run it to 12K without it blowing up; push 14K and it will likely explode for sure.

 

Unfortunately, the tach doesn't have an " I'm fixing to blow up warning"

 

Thanks for the replies guys! Hopefully someone has but most people can't without modification to the TCI to allow to go higher then 10krpm.

So really I guess this is for people who took to dive to do fuel injection and major breathing mods.

I know cams will come into play when trying to make more power at higher rpms and would best to run on a dyno to see when the cams drop off.

When I had my XJ700 engine apart I noticed the rods and pistons are very similar in design to my 81 XJ650 parts bike so I'll examine the XJ650 internals to see if there may be any weak points which to cause failure.

This next winter I will be taking the top end apart to clean/polish the ports, CC chambers, and piston tops. Along with that new valve stem seals, piston rings, and testing the valve springs. I am also thinking of sourcing some larger bore pistons and having the cylinders bored out. Some XJ900 pistons seems to be a good canidate for this. I will have to do some calculations to figure out what the new compression ratio and CC increase.

 

I talked to a guy, at the BMW rally of all places, that used to race one of these motors. He mentioned that after installing stiffer springs the next point of failure he encountered at RPM was a tendency to spit shims which could be adequately solved with silver solder.

I'm presently gathering the ambition to tear into a pair of XS400R that I've got. intend to make one quite stock and one as hotted up as I can imagine. Partly as a test bed for some mods I would eventually do on my 750. The thing that comes to mind right off the bat is carving some weight out of the valve spring buckets. You can get stiffer springs and that's all well and good, reducing the mass of the valves, springs, buckets, (shims?) all together has the same effect of allowing the springs to keep them in contact with the cams.
As ever, jetting jetting jetting. They lean out at the top end of the range so to run higher RPM ya gotta feed it.
Knifing cranks should do good things both for HP losses and response.

 

A lot of that stuff has crossed my mind. There are shimless buckets........


I don't have to worry about jetting though. =)