Make a slow bike fast - KZ750 Twin

The crank balancer on these engines is not terribly well engineered. Its heavy and complicated, and probably one of the reasons why the bikes were never really raced. 

I was alerted to a problem with the crank balancer sprockets. These sprockets are broached and the broached slot holds springs and pins that act as a dampner for shocks coming through the balancer chain. The problem is that these broached slots leave the sprocket quite thin and brittle in this area and it is possible to shatter the gear on a hard shift. The solution is to machine a ring with an angle and press it on the sprocket boss. Attention must be paid to provide adequate clearance for the balancer chain. I used 4140 as it is incredible strong and is typically used in applications like these. I shot for a slight interference fit (0.0005"-0.001") and then also tacked the ring on with silicon bronze. I lapped the face on a slab of precision granite to make sure the welds were not proud of the sprocket boss. 

The other issue is with the balance weights. The slots broached here leave a very sharp edge. The material is iron so it is brittle and this edge is prone to chipping. Every 750 twin engine i have torn down has had chipping here. I used a dremel with a grinding stone to put a very slight chamfer on the edge to prevent further chipping.

 

Is it possible to completely remove the balancer and just blueprint the rotating mass ?  I've seen this done in other engines before...

The balancer takes care of the secondary imbalance.  The porkchop weights on the crank are for the primary imbalance.

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The balancer takes care of the secondary imbalance.  The porkchop weights on the crank are for the primary imbalance.


 

Yes, I understand that.  I'm sure I read a thread somewhere {not here}, about a German KZ750 Twin race bike where they removed the balancer as . As I said, I've seen secondary balancers removed from engines successfully before and was just curious if it is possible on these engines, plenty of twins don't run secondary balance shafts or balancers...

wrote:

wrote:

The balancer takes care of the secondary imbalance.  The porkchop weights on the crank are for the primary imbalance.



 

Yes, I understand that.  I'm sure I read a thread somewhere {not here}, about a German KZ750 Twin race bike where they removed the balancer as . As I said, I've seen secondary balancers removed from engines successfully before and was just curious if it is possible on these engines, plenty of twins don't run secondary balance shafts or balancers...

A balance shaft is not mandatory, but it makes the engine much smoother.  There is no amount of crank balancing that can replace a secondary balance shaft (or two.)  Balance shafts are even used on a lot of bigger L4 engines.

The problem is the oil pump runs off one of the balance shafts, so if you removed it completely you would need a stand alone oil pump which presents its own set of hurdles. This will make the balancer setup good enough to race. and is comparatively easier than removing the whole system. plus it only cost me $20 in material, two afternoons, and a ton of swearing because i screwed up the tolerances on both rings the first time.  .00025"+/- is some of the most accurate stuff i have ever machined. it was difficult. The boss was deformed about .001"-.002" around the slots from broaching so i had to guess on the fit. The worry was too much clamping force from the press fit might deform it and not allow it to spin, and too little wont do anything.

wrote:

wrote:

wrote:

The balancer takes care of the secondary imbalance.  The porkchop weights on the crank are for the primary imbalance.




 

Yes, I understand that.  I'm sure I read a thread somewhere {not here}, about a German KZ750 Twin race bike where they removed the balancer as . As I said, I've seen secondary balancers removed from engines successfully before and was just curious if it is possible on these engines, plenty of twins don't run secondary balance shafts or balancers...

A balance shaft is not mandatory, but it makes the engine much smoother.  There is no amount of crank balancing that can replace a secondary balance shaft (or two.)  Balance shafts are even used on a lot of bigger L4 engines.

Depends entirely on the application, we have a home built version of the Buick V6 in Australian Holden's, these engines have a balance shaft, they were fitted to basically an unbalanced engine package to smooth them out, you could remove the balance shaft , blueprint the rotating parts and still have an engine that didn't vibrate, and gain more HP, I was told It was cheaper and faster to mass produce the balance shaft and parts than it was to continuously balance the engines rotating parts on the production line...   I'm sure the KZ750 I touched on earlier still used the balance shaft for the oil pump, just modified, maybe weight removed ?, If I can find the story I'll post a link here....
 

Wikipedia has an article about engine balance... en.wikipedia.org/wiki/Engine_balance

Bottom line: you can't eliminate secondary imbalance in some engine configurations, including a parallel twin, without a counter balancer shaft setup.  

Guys running race bikes sometimes would rather deal with some extra vibration vs. running a power robbing balance shaft.  That's a decision the race bike designers make.  I personally have a lot of miles under my belt running smaller parallel twins and I can tell you that those that ran balance shafts are worlds smoother than those that didn't.

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 I'm sure the KZ750 I touched on earlier still used the balance shaft for the oil pump, just modified, maybe weight removed ?, If I can find the story I'll post a link here....




 

Please do, finding info on engine modifications for this platform is rare.

In theory you could remove the balancer weights or cut them down, but you would have the keep both shafts, sprockets and chains to keep the pump. At that point id still be inclined to keep the balancer weights as i doubt removing them would increase power significantly, and smooth is good. I rode my buddies XS650 and could not believe how badly it shook at high RPM -  it does not have a balancer. The Canadian guy I talked to is still running them and he is making crazy power, so it will stay in for now.

wrote:

Wikipedia has an article about engine balance... en.wikipedia.org/wiki/Engine_balance

Bottom line: you can't eliminate secondary imbalance in some engine configurations, including a parallel twin, without a counter balancer shaft setup.  

Guys running race bikes sometimes would rather deal with some extra vibration vs. running a power robbing balance shaft.  That's a decision the race bike designers make.  I personally have a lot of miles under my belt running smaller parallel twins and I can tell you that those that ran balance shafts are worlds smoother than those that didn't.

I agree that most balance shaft engines are smoother but, my old mates Norton was an awesome thing to ride when it was going well, yes it vibrated , but we liked to call it "character", still put a smile on your face...

wrote:

wrote:

 I'm sure the KZ750 I touched on earlier still used the balance shaft for the oil pump, just modified, maybe weight removed ?, If I can find the story I'll post a link here....







 

Please do, finding info on engine modifications for this platform is rare.

In theory you could remove the balancer weights or cut them down, but you would have the keep both shafts, sprockets and chains to keep the pump. At that point id still be inclined to keep the balancer weights as i doubt removing them would increase power significantly, and smooth is good. I rode my buddies XS650 and could not believe how badly it shook at high RPM -  it does not have a balancer. The Canadian guy I talked to is still running them and he is making crazy power, so it will stay in for now.

I was just thinking out loud about the balance shafts, more with experience with car engines as mentioned. I've searched for that article a couple of times, I had it bookmarked on my old computer but hadn't updated the bookmarks when it crashed. I remember stumbling over it when researching performance mods for the Kawasaki Twin, I think they had it out to near 1000cc...   

Its possible to get close to 1000cc. The guy in Canada is at 948cc and he achieved that through Nikasil plating the block and getting rid of the sleeves. He said it helped with cooling slightly but he mostly did it for the weight reduction. 

In retrospct I probably should have gone with nikasil  over iron but at this point its moot. I took the block to LA sleeve and asked them how big i could go. They said 86mm is about as big as you can go and stay with a iron liner, so thats what I did

There has to be a point in any build where you say enough is enough, otherwise I would end up trying to make a custom big bore block or something else. The possibilities are endless so its hard to decide when to stop. Part of the allure of building this platform for me is there is no tried and true way and everybody approaches it a little different - i have learned a ton along the way doing it.  You'd be hard pressed to find any new ways to make big power in the z1/KZ1000 platform as so many people have done R&D there. Being as I've never done a experimental engine build before I am approaching it a little more cautiously as this shits expensive and laborious - basically the entire engine is custom at this point.

Right now I pretty much have my engine planned and parts acquired, so I wont be making any major deviations from that moving forward. I am very close to assembling the bottom end of this engine and I will outline my engine plan then.

I appreciate the interest from anyone paying attention!