I ran 3 different units of those, and every one did that in o/d. And I blew all three of them up. Two of them stripped off the O/D teeth and on a third I stripped all the teeth off the input gear.So take it easy on the o/d part.
In od the engine torque goes into the input gear and down to the cluster. Then over to the third gear drive , and then up into "third" gear (which is now OD). From there the torque exits the tranny thru the mainshaft. This means that the input gear and the mainshaft are turning at different speeds. 37% different! When the crank is going 2890 rpm@60 mph, the M/S is going about 2110.
It was my conclusion that,that speed difference was setting up an oscillation right inside the tranny.It also means that the cluster gear is constantly under load. And so are the front rollers,and the back bearing. The back bearing. I put a new one in to no effect. $138 Canadian, plus taxs. I have pulled od boxes apart,in the which, the front nose of the M/S was all chewed up. When this happens the input torque forces the cluster and the M/S apart. This creates noise as the teeth are no longer in mesh, and a huge amount of force is pushed into the back bearing. You can bet it is going to complain. I'm not saying your tranny has this issue, nor did mine when I got them, nor when I yanked them. I'm just saying it happens. If you find silver streaks in your oil, as you agitate it, it's time to drop that box and see what's going on inside.No streaks, means carry on.
My combo seemed to work Ok with 3.55s or less. It did not like 4.10s,4.30s, 4.86s,5.13s, nor my 3.91s.
I got out of od boxes.
If you think about it, at 2890 rpm, Our hotrod engines are putting out a pretty good amount of torque.Probably siting fairly near the fattest part of the torque curve. I wonder what harmonics are running up and down the powertrain that find their origins in no mechanical issue at all, but in the tune.
I got out of od boxes.
If you are running the OD box, you are running a starter gear of 3.09 x 3.91=12.08. This is a great starter gear for drag-racing. I found it much to deep for street.But I understand that to run that box, second gear needs that stinking 3.91, or even more. Second thus is 1.67x3.91=6.53, which is the same as (in the std box) 1.92x3.40s. That is the price you pay to get overdrive,namely; too much starter gear to get; a so-so second,a third you almost never use, and a great od gear.
I got out of od boxes.
The only way that box ever worked for me was with a GVOD, used as a spitter. The ratios were 3.09-2.41-1.67-1.30-1.00-.78-.57 GV in red.
The splits were---.78---.68---.78--.77---.78--.73
I ran this with almost every gear in my arsenal. For street,I liked 3.55s, for a 10.97 starter gear and a 2.02 double overdrive.I ran it with a 223/230/110 cam; what a hoot! 65=1630rpm .....
I also tried it with 4.86s,and with 4.30s, in an attempt to have a dual purpose tranny, skipping first and attempting to start in second or first over, on the street. But that didn't work out at all.
That was a fun two summers.
But I got out of those boxes
I'm sure you meant something different than what you wrote here,lol. Cuz the driveshaft rpm at 60mph is the same no matter what gear you are in. And the point that there is less vibration in direct at about 2750,vs, 2000 in o/d, kindof answers your question.
It's either in the box, or in the tune; is my guess,cuz I have never seen engine/flywheel vibration get better with an increase in rpm.
