Cams

I'm looking to get some cams reground and have been doing some investigation of the standard timing and lift.
Measured the FG cams (with the pulse generator) and the max valve lift is 8mm. At full lift I can push the valves down another 2mm easily so there appears to be some room for improvement if I build up the heal end of the tappet with stellite to keep the geometry the same.
However I mainly want to decrease the lobe centres and get more overlap and use similar timing to an RC30.

Does anyone know if the lift on FK cams is the same?

Still can't work out how the FG engines are so fast with the 34mm carbs and smaller inlet valves?

 

Unless you're a cam grinder/designer forget it. Even then it's going to be a crap shoot. There is more to a cam than lift and duration. A lot has to do with lobe profile which is what the designer keeps to themselves. With that said you could change the rocker ratio if you could find different rockers or have the ability to make them. Lobe center angle, cam to crank timing, lift and duration can be played with. Just because the intake valves are smaller doesn't mean they flow less. Things like port length, cross section, shape, angle to valve, placement of valve, combustion shape are a few things needing consideration for flow are just a few things to consider. Over lap is just a by product of cam design. From what I've learned the most important aspect is the closing/opening points of the intake valve.

Why not just get FK cams and or FK heads with cams and save yourself some nightmares?

 

I am currently running and FK engine but just happen to have an FG engine apart so I though I would make some measurements as there is a lot of hearsay on these forums.

I agree totally re cam design and have an experienced cam grinder in mind who has 40 years experience and over 400 profiles, he makes a lot of Honda race cams.
Fortunately VFR cams are chilled iron running on chromed rockers which allows reporfiling as they are toughened throughout.

Also agree with your comment re inlet valves, however the port design and finish is quite poor on FG's although they have a much shorter inlet tract than FK's purely due to the lack of inlet trumpets (as RC30) which is better for high rpm power.

I'm also planning on increasing the comepression ratio to at least 12:1 by using VFR700 pistons which have higher crowns and skimming the cylinder face and shimming the cam carriers as per RC30 tuning. Having visited Tony Scott he confirmed he uses uses this method of machining the cylinder faces flush with piston flats and the gasket thickness of 0.025" as the quish clearance. Currently the barrel is 0.030" higher than the piston, std figures say 10.5:1, will measure this using a burette tomorrow.

 

As long as you have enough clearance for zero deck and you raise the pistons instead of milling head/block because of cam gear tolerances that should work. Except your squish dimension is on the small side. Most I've dealt with uses .060-.070 as optimum. Depending on what you're shooting for as far as power range. You could lengthen the intake port by making a longer manifold piece. Sounds like the ports needs some massaging for better flow.

Your cam guy should be able to regrind the cam for the type of power band you want. Then you could make all other components to work with that. I'm sure your a where without matching parts it's kind of self defeating. Keep us informed how it comes out. Always interested in "hot rod" projects.

 

Milling the block on RC30's is std tuning practice here for that lasy 25years. Scotty said that he has run 13.2:1 comp ratios without problems, I have one of his engines in my race 30 and runs beautifly although its hard to bump start.

Will be using FK heads with some gasflowing and RC30 carbs with HRC superbike slides and needles plus a programable 4 channel ignition.

Hopefully it will be a bit quicker.

Donnington Classic Bike Track Day 2012 AM Part 1 - YouTube

Donnington Classic Bike Track Day 2012 AM Part 2 - YouTube

 

Further explanation to skimming cylinder block, the cam carriers sit on 4 pads which are then shimmed relative to the amount of metal removed, as are the gear cassettes. The std gear meshing is quite loose, but to get this correct its a bit tricky unless the backlash gears are removed or pegged open then you can feel the backlas and ajust accordingly.

 

You'd sacrifice reliability for a slight HP increase, and often different cams need improved valve springs to control things at high speeds. Shim-under-bucket setup might handle it, but with rockers ?????

You'd find a weak point somewhere............coil binding, valve float, something. And low rpm performance on the street might be much worse.

Were Honda engineers just lazy or too conservative and didn't get all they could from the heads and valves ??

 

How about just doing a port job on the inlet ports, people have reported good things when just a port job was done on their CB750s.
I am sure these engines would benefit too.

 

Gottacha on shimming the cams to take care of the gear lash. Didn't know you could do that. But looking at the photo's I see what you mean. So milling head or block isn't that critical as I thought.

 

Jerry Branch

 

Wouldn't there be problems with timing since the cam drives are not sloted for adjustment but must be placed in fixed, stepped positions to fit the gears ??