More Rocker Geometry Considerations

[MadBill]

There have been a number of threads on the subject of correctly determining pushrod length/rocker shaft height. At the risk of generating more controversy, I'll summarize that the most widely-held is that the fulcrum-to-roller centerline should be at 90° to the valve stem at half lift, resulting in minimum overall rocker sweep across the valve tip.

Other theories do abound however, e.g. adjusting to obtain maximum lift (which, depending on pushrod socket location is usually with the centerline at right angles to the stem at max lift), or to minimize side loading in the vicinity of full lift, when the spring loads and thus sweep-induced side loads are assumed to max out the latter. (Of course, this results in the same geometry as the 'max lift' approach...)

Half a dozen other existing philosophies come to mind, but I have a couple of seemingly-plausible ones of my own that I haven't seen before and wondered if anyone else has. Note, I'm not embracing them, just presenting them!

Theory I:
The maximum pressure drop across the intake valve (apart from the huge pulse that results from exhaust scavenging) occurs shortly after maximum piston velocity; say ~ 80° ATDC. At this point the valve lift is usually ~ 85% of max. and further lift is of little value from a flow standpoint, just necessary to ease the velocity down to zero and begin the closing action. Therefore, adjustment of pushrod length/rocker shaft height to maximize the instantaneous rocker ratio/valve lift at this point would be ideal for cylinder filling.

Theory II:
The greatest side loading and thus potential wear per thous. of sweep/scrub will occur at the highest rocker/valve interface force, so this is where sweep should be minimized. At high RPM this is not, as is often assumed, at peak spring load/maximum lift but rather at the point of highest total force due to a combination of spring force plus valve acceleration. This is typically ~ 60° after the opening lash ramp, so to minimize guide wear this is the angle at which the centerline/valve stem angle should be at 90°

OK, I've buckled my flak jacket, so comment away!

[bigjoe1]

I help you lite the fire here too ! on a drag race engine-- non of this seems to be very important at all. I spend very little time on this subject


JOE SHERMAN RACING

[cjperformance]

Drag race- aim for max lift.
Street/endurance- aim for min sweep.

[lada ok]

If you use OHC with bucket followers, you can ignore this problem

[cjperformance]

If you use OHC with bucket followers, you can ignore this problem

tis true !

[ap72]

depending on the rocker shape, it is entirely possible to have the maximum lift AND the minimum amount of scrub.

Your "Theory I" idea sounds interesting though, and an area that could be tweaked according to heads, valve job, chamber, etc- things that aren't directly valvetrain related. I suspect wear would be atrocious though.

[wyrmrider]

there is no free lunch
the "drag race" max lift solution implies rocker tangent at max lift
which gives a big pressure angle at start of lift/ acceleration even if the rocker does not come off the valve
good way to bend a pushrod or worse
some camgrinders recommended this method
they were and are wrong

[ap72]

the "drag race" max lift solution implies rocker tangent at max lift

Does it?

[tenxal]

If you use OHC with bucket followers, you can ignore this problem

That's really helpful.......

[wyrmrider]

Do the math
max ratio is at tangent
so max lift at the pushrod x max ratio = max lift at the valve

[les327]

One theory goes max lift at max piston acceleration.

[ap72]

Do the math
max ratio is at tangent
so max lift at the pushrod x max ratio = max lift at the valve

Not quite that simple. You're assuming only a singular point matters. Even if you assume the pushrod cup is optimized for your particular arrangement, which is a huge assumption- The greatest valve left will occur when your rocker travel is actually centered in about the point where it is perpendicular to the valve stem. Basically forming a perfect 90° angle at mid lift also giving the shortest sweep pattern. But this assumes that you have an optimized pushrod cup location for the rocker

[lada ok]

If you use OHC with bucket followers, you can ignore this problem

That's really helpful.......

Thanks ..... if you had access to a dyno, ... wouldn't you shagg around for max power

[utmost]

depending on the rocker shape, it is entirely possible to have the maximum lift AND the minimum amount of scrub.

.

most rocker arms don't have the right shape (geometry) and we have to compromise-split the difference

[tenxal]

Thanks ..... if you had access to a dyno,

I do.

... wouldn't you shagg around for max power

I do.

Your original reply did nothing to address the question posed. You only gave your opinion on a particular valve train configuration. This thread has a lot of potential so let's stay on track. Thanks!