Elastomer dampers have a relatively narrow range of effectiveness. When I called to order the ATI I gave him the bobweight and RPM I was shifting at and he said it didn't matter. So I asked him if the same damper works for a 1900 gram bobweight and shifting at 6800 or a 1685 gram bobweight shifting at 8500? He said it doesn't matter.
I don't have any data for your engine, but what he said might be true (if explained very badly).
The harmonic orders' RPM (sensitive points in the RPM range) are generally a function of crank length, stroke length, journal overlap, and weight - but not balance details. Which orders are dangerous includes engine design (firing order, L or V, number of cylinders).
Factors that make the crank stiffer (shorter stroke, more overlap, shorter OAL) tend to make all the orders occur at higher RPM.
The engine speeds for these orders can be calculated from the resonant frequency, where “Hz” is the crankshaft’s resonant frequency, and N is the order number, using this formula:
RPM = Hz × 60 ÷ N
A V8 crank may be 300 Hz (he may have a chart that shows this - did he ask your stroke length and journal diameters?), giving RPM points at
1st 18,000 RPM
2nd 9,000
3rd 6,000
4th 4,500
5th 3,600, etc.
Note that the area between 2nd and 3rd spans the entire RPM range 6-9,000, meaning the same damper is correct for any RPM in that range. The ones above and below it may be either irrelevant or too small in amplitude to matter.
My project 235" L6, 3.94" stroke, 2.68" mains, 2.31" rods for 216 Hz will have destructive orders at 2,160, 4,320, 5.184, 6,480, and 12,960 RPM - very different even from another L6.
I love the math and the explanation. No, the dude from ATI didn't get in detail anywhere near that level. All he asked was what the actual bore and stroke were. He didn't care what RPM I shifted at.
At that time, I had a room mate who was big on ATI dampers (because almost everyone and their mother was using them) and I told him what happened. I kept a record of all my phone calls so I had the name of the guy I talked to. My roommate decided to them, talk to the same guy and see if he got a different answer.
He called and said he was running a low 11 second 340 and was going to up the compression and switch to a roller and dip it into the 10's and didn't want to use an OE damper. They gave him the exact same part number damper. Didn't ask what RPM he used or anything else.
He was dumbfounded. I don't know what else to say. It didn't make sense to me at the time, and still doesn't today.
It would be interesting to do the same thing today with the same RPM 340 and see what you get if you call back and see what they say about a 408 turning say 7000.
You can do the math and see if the overlap.
