I've never had a crank comp tell me not to use a certain balancer

The top guy at SCAT told me once to only use bonded type dampers. He said no FD and no ATI. He only wanted me to use the BHJ style bonded damper with his crank. He said anything else could break the crank. I kind of doubt it is true since ATI dampers are almost every pro built engine I've seen over the last 20 years but that was his story and he stuck with it. Now the story might be different with different engines since the original SBC had a fairly weak snout on it. But it is super rare to see a Mopar big block crank snap off at the front cover. I have seen SB Chevy cranks snap there on multiple occasions. Even on street cars I've seen them snap off.

There are several harmonic orders in which dangerous vibration will occur at different RPM levels. Operating an engine rapidly through the RPM range is almost safe even wih no damper at all (blower pulley only) since the exposure is literally 10ths of a second for each.
However: if your orders are very close to your stall speed or trap speed: problem.
The number of orders that will occur (both above and below your RPM range) varies with the engine design (V or in line), number of cylinders, overall crank length, size and number of main and rod bearings, stroke, number of main bearings (not all the same value contribution).
Things that raise the RPM of each harmonic order:
Short crank (V4 obviously best, L8 worst)
Big rod and main journals
Short stroke
5 mains for V8, 7 for L6

Sample calculations for my book project (Chevy 235" stovebolt L6) with 3.9375" stroke, 2.3125" rods, 4 x 2.73" (average) mains, a relatively weak and flexible crank, has a crank frequency of 216 Hz (cycles per second), giving significant harmonic orders at:
9th order occurs at 1,440 RPM (even higher orders are below idle speed)
6th @ 2,160 (weak, but avoid it anyway)
3rd @ 4,320 (very strong and dangerous)
2-1/2 @ 5,184 (strong, but above typical use)
2nd @ 6,480 (never reached)

Never gear the car to sustain these engine speeds! Avoid them, or pass through them.

I, absolutely, do not question your information here or knowledge of this (this is beyond my knowledge). I am just curious if this is based only on mathematical models, or was this or can this be confirmed by measuring the harmonics with sensor based equipment? I’m just asking, because i have no idea; and i would love to know how they could actually measure this. What kind of sensors, and where are they placed? Very interesting info. Thx for that.

Nope. Tom is still pedaling that nonsense. He did a webinar recently and he said the exact same thing. I can’t recall seeing a damper break a crank, with the exception of junk like the Moroso aluminum solid damper and I think when the rule came out requiring aftermarket dampers there was one or two other solid dampers. Those killed lots of cranks. And blocks.

You see a damper not doing it’s job very well in the bearings mostly. I suppose if you let your ATI damper go forever and a day without getting it freshened up like they recommend you could have some issues, but that’s not the fault of the damper. That’s a negligent end user.

I, absolutely, do not question your information here or knowledge of this (this is beyond my knowledge). I am just curious if this is based only on mathematical models, or was this or can this be confirmed by measuring the harmonics with sensor based equipment? I’m just asking, because i have no idea; and i would love to know how they could actually measure this. What kind of sensors, and where are they placed? Very interesting info. Thx for that. [/quote]

Yes thats pretty easy to measure.. just use a variable strobe light and match the speed.. that will tell
you what the RPM is.. I did a lot of this stuff back at work

Great info, now do a 426 with a 4.15 crank and stock journals, rods

I gave a person smarter than me data from the factory service manual as to measurements, weight, CW diameter etc.
He has an expensive program that approximates the frequency, and that figure is about where you would expect to find it vs. a shorter stroke 7 bearing engine (like its replacement 250).
Quoting myself (from my book):
"I suspect that a crankshaft may be tested to detect its natural resonant frequency by simply treating it as a tuning fork: strike it smartly with a brass or lead (soft) hammer. A microphone and oscilloscope will show the frequency of the “ring” sound. Different locations should be almost identical.
Once the frequency is known, the formula* can be used to predict the RPM at the critical orders.
Alternatively, with the crankshaft suspended, a tone generator can be used to excite it by slowly dialing through the likely frequencies (between perhaps 180 and 250 Hz), and observing a fine wire rigidly attached in various places for vibration."
*"L6 engines generally resonate at frequencies between 200 and 300 Hz (source: Heldt). These orders occur throughout, and well above the maximum RPM of the stovebolt crankshaft.
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
For standard 235 and 261 stovebolts, RPM = 12,960 ÷ N."

Very, very interesting. Thanks for the explanation. Maybe the frequency could be altered by changing some properties of the crank, if it turned out to be a problem and occur during rpms required to be extended use periods. I’m thinking cutting down the cw radius and filling with some heavy metal, to change the frequency range. Only worth it on very expensive cranks though, i presume.

Interesting, I got a crank with my engines pieces that had something done to it like that, the counter weights had a strap welded on and each sounded dead inside when tapped on.

Its pretty easy to change the frequency just by changing weight.. I cut down most of my
cranks to bring the rpm up(changing the frequency)

Very cool. Thx mr P. What about 2 , otherwise identical cranks, but 1 has regular shaped radiused counterweights, and the other has pendulum cut cw? They are identical weights, lets say, but they woukd have totally diff frequencies, right?

Not really.. weight is the factor so if they are the same weight then the frequency should be the same.. I
never did any of the fancy shapes but if one allows the oil to slid off easier it would be less weight...
less weight would allow the crank to accelerate quicker.. one thing I did was to knife both ends of
edges of the crank so it could cut through the air easier... think of a sail boat hull in the water

I have a 25+ year old Fluidamper on my 446" wedge motor. Freshened it up a couple times, no problems w/ it.
Have an ATI on my hemi...no problems there.
I had a BHJ damper years ago on one of my 440s...slipped the ring about an inch after the first trip to the track with it.

I have all 3 but I’ll use the FD one, storing it flat till I use it. I just like the looks of it better, almost stock looking