Core Shift ???

[Sizzler]

Originally posted by Richard Hudgins
Hi Ron,

Offset bores are a very common alteration to motors.

I cannot give you details as I am still involved with some engine folk and they would not look kindly on free information.

But, think of the offsets and the burn timing of the mixture in low rpm engines. (less than 10K) and you will understand the possibilities of offset bores.

You must think of overall timing events and the effective moment arms.

That is all I can say due to Non-Disclosure agreements.

OK, this is a new idea to me, but not, apparently to others. So searching this is what I'm understanding so far:

1) An easy way to see this effect, and banned in NHRA events, is reversing the piston in the bore. This reverses the built-in piston pin offset the manufacturers design in to quiet piston-slap in the bore and effectively, offsets the piston-rod assembly.

2) A more involved process is increasing the effective angle of the engine from say, a 90 degree V-8 to a 95 degree V-8. The effect of this has been documented, but as near as I can tell, most feel the effect is more from reduced friction on the thrust walls than from any "moment arm" effect. This has also been suggested as a means of unshrouding valves and better arranging things topside within the firing area...sort of advancing a cam if the grind is slightly off...more involved and nowadays, why not just get a decent head instead of disassembling an engine and going through all this.

3) A still more involved process, and an ancient one is this:

Originally posted by H. Franks

Ford/Merc flatheads had the cranks offset more than 1/4 inch to the right side. In other words when you intersected the centerlines of the 90 degree bores, it was NOT on the centerline of the crank. Naturally the cam was offset as well to be over the crank, and the valves on the left bank had to be canted 5 degrees and the deck on the right bank had to be slightly higher to compensate for the offset.
Look at it this way; when a bicycle pedal is at the top (perpendicular to the ground) it's hard as hell to get it moving. But if you move it forward an inch or two it is much easier because you have a better angle on the pedal crank. So the flathead was not wasting a lot of energy trying to get over the hump, but was pushing the rod downward instead of trying to push it sideways.Also the piston goes down noticeably faster on the down stroke than coming up.
Obviously you can't offset a crank if the block wasn't cast that way, but you can get the same effect by offsetting the bores. I guess you can do it two ways. Either keep the the same centerline and bore off center; left bank toward the outside of the block and right bank toward the inside, or cock the bores a few degrees off the center line and both toward the right. Either way you would stay 90 degrees.
In an earlier posting I talked about a guy who is running an under 300 inch flathead dragster at 7.7 sec and 175 mph; maybe faster now. I doubt that he is making 500-550 hp even on nitro, so the only thing I can think of is the natural advantage of the offset crank + the incredibly long stroke and rods of the flatheads. You would probably have to make 2-300 more horsepower on an overhead to get times like that.

Now does any of the above make any kind of sense?

How does the effect graph out? Does it increase from 90 degrees to 135 degrees, or go up steadily all the way to Boxster configuration?

Help?