I’ve been anywhere from 3800-4200 and it seems to pull through just fine. We normally run 11-13 drivers. If you were on a bigger driver I might stay on the low side of that swing. I just ran my stocker this past weekend on a 11/63 set at 4500 ish and it never missed a beat.
Horsepower is a calculation,(torque x rpm / 5252.1) so for the maximum horsepower at the axle, you want maximum torque at the axle. The clutch should hold the engine at its peak torque RPM on the starts and/or when coming out of a corner, depending on what kind racing you're doing. This is a hard and fast rule!
This will depend on your cam, ignition timing, pipe, and so much more...but for years now, peak torque on most every builder's WKA legal flathead has been right @ 4000 rpm.
Bully clutch with red springs @ .225" height with one bolt & nut in the outside of each weight lever will get you awfully close.
Memory ain't that great for the time period but I seem to remember 4200 or 4300(ain't sure) from flat head sprint lefty righty racing, using Turner flat heads. I think it was what I was told to do and doesn't mean at that time I had the skill to set the clutch to a particular engagement. Seems to me on dirt when we would go from a 1/5th mile to an 1/8th and took gear off the clutch driver, i'd turn each MDC spring screw in 1/4 turn which would slightly raise engagement on a smaller track. What it was set at then I don't know.
I have a question for you all with fancy recording tachs. Is there a reading or pattern you can look for on tach read out to know what your clutch engagement is? And from Al's post I think the word 'hold' verses 'clutch engagement is very significant. How do you know if your setting it near initial clutch engagement or where it is able to 'hold'?
Racepromotor, I'm curious to know if you asked because of wanting to know something new or if you asked to verify your thinking? It doesn't matter to me either way because I enjoy reading, writing and offering input on here, it just seems interesting to ask now with my morning coffee. ...
Al, you must of read my posting on another thread. This is the FIRST time I heard you go from engage at peak torque to HOLD at peak torque. Even your scale method for setting a clutch sets it to engage at peak torque, NOT hold at peak torque. On the plus side Al, until learning your scale method I had no way to be sure of where the clutch engaged no matter if I was setting at initial clutch engagement or at it's hold point. Thanks for pointing me to the scale method you got from someone else.
If you watch the tach there is a point where it will ramp up near or past your set clutch rpm then rpm will fall slightly before going 1:1 and locking up. But as I said depending on weather, track conditions, and tire I’d feel safe in saying you got a 200-400 rpm window to get it close enough to “ be right”
If you have a tach with replay feature - it's pretty easy to see the clutch engagement.
With the analog graph display it's even easier still.
With the driver in the seat, the engine (and clutch) warm at an idle; from a standing start, go wide open throttle and accelerate while holding the steering wheel straight (ie drive in a straight line..)
Watch the engine rpm climb on the tach (3400, 3600, 3800, 4000, 4200, 4400, 4000, 4200, 4400, etc.) The rpm will climb/drop several times in very small increments (obviously not 200 rpm per my typed example.) The very first time that the rpm drops is your clutch engage. Now, the rpm may climb and drop several more times after that initial drop -- this is typical of clutch chatter. Some chatter is acceptable, too much means that the clutch is probably ready for a rebuild (close the air gap, etc.)