Rearend Ratios & MSD box question

I've recently had my 4.11 rearend mess up so stuck my spare 4.86 ratio in it and I've had issues with rpms.. run same gear ratio I did with the 4.11 and turn less rpms. Ex.. a 6.90 in the w/411 would turn 75 & the same gear ratio for a 486 would turn 7-71.. also happens with different motors (steelhead & 604crate get 64-66 out of crate with same gear ratio I ran with 411). The 411 was all low drag best you can get. 486 stuff is jus standard stuff.

Question i have is... Would the ratios make that big of a difference? Do I just needa try different gears to get the rpms up or could something else be goin on like maybe MSD box or coil going bad? Thanks for any advice/help!

Are you using the same exact quick change gear in both rears, or using a different set of quick change gears that are the same final ratio and getting different results? Do you have a gear chart? You will see that the final ratio is different using the same quick change gears in different ratio rearend.

if using the same gear set with 4:86 ring and pinion , rpm,s would be way higher , not lower , so assuming you are changing gear set , to same final drive, then you have a drag issue of some kind , i would not think there would be that much difference between a low drag and standard , but IDK , with weaker engine , could be......

Using different gear set.. the crate motor is brand new so it's not weak (jus won with it friday) and the steelhead is jus freshened so not weak either.. neither is running nor turning the rpms they should/need to be. And only thing we've changed is the center section. Could the ring gear be to tight? Possible wrong bearings? When we let the guy put the 4.86 setup in he changed the bearings and everything but it's not acted/ran right sense.

didn't mean your particular engine was weak , meant that a crate or limited is weaker than a super engine , if the rear is not heating up or making noise and you won with it like this friday night , leave it alone is all i can say......