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Not based on personal experience, but I've been told that the 4K spring can tend to kill some bottom end power while adding more revs top end. This was from some pretty reliable sources so possibly you might want to research that part of it, and also look at where you need to make power? Of course the size/weight of the swap vehicle would have a lot to do with what kind of bottom end power/torque you need. Based on this info I decided on the 3200 rpm #366 spring.
 

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I don't see how a spring change would affect fuel delivery in the low RPM ranges.
 

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What I suggested is research. In my own research a few guys who had installed that spring said it could effect bottom end performance because of a change in the fuel curve or some such thing, and that they had experienced a slight loss in bottom end performance. I've never driven one 'before' and 'after' a spring is installed. I would think that's about the only way to tell is to compare. Just passing on what I was told.

Maybe somebody here can relate personal before and after driving impressions?
 

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I'm running two trucks with them. The biggest advantage is you don't have to use as much throttle angle at cruise. It feels more automotive. TPS works better.
 
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