From what recall, both springs will make your throttle more responsive, the 4k noticeably so. I bought a truck with a 3k in it and added the springs to make it a 4k for the throttle response. I don't know how much of a power increase there is, but having the truck not fall on its face and quit accelerating around 2500-2800 rpm was absolutely worth it.
I haven't noticed any additional throttle response in any of the three engines I've put a 4K GSK in. Perhaps the different HP rated pumps respond to them differently in that regard IDK.
What I have noticed is the governor no longer pulling fuel at 25000-ish RPM causing the truck to nose over when trying to merge on the interstate in traffic. It may not be as beneficial to an automatic trans truck , but in an NV4500 manual truck there is a big gear spread between 3rd and 4th gear. Having a 4K GSK just makes the truck easier to drive as you can pull a little higher in 3rd (and the engine will still be fueling / making power) before shifting into 4th.
I currently have 4k springs in my truck. It can be super miserable to drive while towing. If I hit bumps at higher rpm’s the truck turns into a bucking bronc. I’ve loosened them a few times but I’ll most likely be going back to 3ks at some point. Not sure if it’s the same with an auto trans
I sometimes get that governor spring bounce when accelerating with a heavy trailer. It only happens in a narrow RPM band. I either have to floor board it, or let off and try again. It was worse with my 3k springs than the stock springs, but the other day it happened again so I think my stock springs have about timed out. The fix is to back them off a click, but you can only back them off so far.
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