rocker geometry

kiwicar · Post by **kiwicar** » Fri Feb 08, 2008 3:41 pm

Hi I need some help here in setting up my valve train.
I have a set of RHS heads with effectivly .15" over length valves.
My cam and rockers will give me .667" of lift on the inlet.
I have pro comp shaft mounted roller rocker system, I have started to put everything together and have measured up to see how close to this I am to this,
http://www.compcams.com/Technical/FAQ/V ... ometry.asp
the answer is that base circle is about half way between 1 and 2. mid lift is a little past 2 and full lift is virtually off the valve.
as far as I see it there are three ways to correct this
1/Add a set of lash caps on top of the valve stems and use a slightly shorter push rod so in theory using a different part of the arc of travel of the rocker.
2/ Shorten the pillers that suport the shafts, use slightly shorter push rods to achieve the same as above.
3/ remake the support pillers with the mounting holes that attach them to the heads off set to move the rocker arm pivots about 1mm toward the centre of the V of the engine so moving the roller tip of the rocker to more over the centre of the valve.
I supose this also leads to the question "how do I know I have the rockers at the best angle to the valves centre line to start with?" as I can't find a "correct" angle anywhere.

Many thanks in advance.
Mike

sidecar · Post by **sidecar** » Fri Feb 08, 2008 4:20 pm

Blimey you lost me on most of what you've said but I want to keep an eye on the replies that you get (hence this reply!)

I'm setting up an 4.6 with a typhoon cam that gives 0.5 lift. The valve train and cam were in my 3.5 lump and I can see from the wear pattern on the rocker pads that the pad is right on the edge of the valve stem at max lift but is no where near the edge at the start of the lift.

I am going to remove 1.5mm off the pedestal bosses (not the pedestals) but it sounds like your setup is a lot more complicated than mine!

Good luck.

Pete

kiwicar · Post by **kiwicar** » Fri Feb 08, 2008 5:03 pm

Hi Sidecar
the set up on mine is now vey similar to the Rover, some of the angles are different but basically the same. With what you have just said I think you are beginning to answer my questions. Still not there yet though.
Mike

CastleMGBV8 · Post by **CastleMGBV8** » Fri Feb 08, 2008 7:42 pm

Mike,

It's one of those questions that sounds simple but has a complicated answer.

Because roller rocker have a very small contact area the geometry is much more critical than say a standard Rover rocker with it's larger convex contact pad.

The answer is all to do with the starting point of the rocker. One solution is to have the roller central to the valve stem with the pivot point of the roller slightly higher than the central pivot of the rocker arm itself, then as the rocker starts it downward path it will move initially towards the outer edge of the valve tip and as it passes the point of maximum extension the roller will move back towards the the inner edge of the valve tip at full lift. Cranes solution appear to be to have the initial roller pivot position higher in relation to the centre of the rocker shaft, the roller then starts at the inner edge of the valve stem and then moves towards the outer edge at full lift, both methods should work well it all depends on how the rockers relate to the position of the valve stems.

So I think it will be a matter of trial and error in setting up the rocker pillars to achieve the correct starting point and angle.

Hope thats some help and hasn't given you a bigger headache.

Kevin.

Kevin.