OP, Jack, everyone. Nice work.
I still have nothing to add, but will re-note for re-clarity my earlier information, with some simple amplification. It would be nice if people actually read this stuff. I'm trying to help, but its very stressfull seeing people "not get it". But then, its possible yall do, but I'm missing it.
Anyway
I think this covers in part what Jack is saying, graphically. On the basis of what increases lever arm and valve lift, you have to look at the diagrams, and work it out yourself.
MustangSix":edelmeth said:
....
Since at the moment I don't have a rocker to measure, let's do this notionally. Lets' say for example, the center of the rocker shaft is 1.100" away from the pushrod ball and the center of the contact point on the valve stem is 1.650" from the shaft centerline, your ratio is 1.500:1. If we shove the rocker shaft away form the valve so that the contact point is hitting the valve stem .050" further out, the long end is now effectively 1.700" and the new ratio is 1.545:1.
.....
Examples.
NB// On stock Ford small sixes, the rocker arm shafts are not bushed, but can be.
1. B series Volvo engines, ohv fours and sixes with the "nominal" giant 4.165 bore spacing engine
For all engines in general
When D1 is least,
the lever arm gives D2 the most lift.
2. The A series 948/998/1275 engines
http://www.ausmini.com/forums/viewtopic ... ht=rockers
drmini in aust
PostPosted: Fri Oct 16, 2009 8:54 pm
Location: near Baulkham Hills, NSW
I tested S rockers, pressed steel rockers, and the late sintered ones.
I found the S ones gave 1.16:1,
pressed steel 1.21:1, and
sintered 1.18:1.
All were measured on the same 12G940 head, with an RE13 cam (0.290" lobe lift) fitted.
I like the pressed steel (round pad) ones, with 0.44" offset bushes in I can get them up to 1.31:1 ratio. And their pads are harder than the S rockers.
Note:If the rocker shaft stays the same diameter, then you'll have to find rocker arms with a bigger inside diameter. FE rocker arms are bigger ID.
You use easily avaliable bronze bushes but have them offset ground. The lift gain is had by having a difference between the shaft diameter and the rocker bush diameter, and then offsetting the difference. So if you have a typical approx 840 thou diameter rocker (Packard, FE Ford) and a small diameter Small Six (144-170-200-250) of about 748 thou approx, then if everything was in order, you would have an ability to reduce the FE rocker ratio from a stock 1.76 to as little as 1.63 to as much as 1.90 by using a 70 thou offset. In practice, the FE rocker on the small six is no longer 1.76, but 1.65 after its centered on the stock 4.76" tall valve, then bushing can take it up to 1.76 or 1.78:1 again. It just depends where you place the offset bush. There are a host of other matters, but if you can just see what happens as per the diagrams, you'll be able to do it yourself without spending 600 bucks plus.
Remember, the art of pushrod rockers is an American institution that it understood fully by all our OHV race engineers, but it seams the English and Swedes fiddle around with it on there 4 cylinder cars A LOT MORE THAN WE DO OUR SIXES. Its hard for me to fathom that the engines that won Le Mans against Ferraris with "better" engines did so with just stock FE and modfied Windsor valve gear engines which were designed by the same people who threw together the small six in 18 months. It would be better that a few tricks from the FE was used in conjunction with normal small six rebuilding work than a few extra bucks go down the gurgler on roller tip rockers that probably don't even de load the valve tip.
On A and B series BMC engines, (and on Small sixes ) you can re bush
the shaft pedastools (can be rebushed with a single diameter bush, shifted 70 thou in this instance)
or
the rocker arm (shifted 44 thou to create a reduced D1 lever arm so the overal ratio goes up from 1.21:1 to 1.31:1).
offsetting from an FE to a Ford Six bush size (840 thou minus 748 thou ) allows a
potential of nearly 92 thou offset to increase lift by
reducing the pushrod to rocker shaft pivot distance. In fact, 60 to 70 thou is possible.
See
http://www.mgbmga.com/tech/mgb16c.htm