Hello, i recently installed pacbrake 4k gsk in a 96 cummins that i swapped into a 97 gmc dually and i can maybe get 2800rpm. That is even with the springs tightened down a bit more than spec. Engine idle is about 150rpm higher but still starts as good as before. I have a steady 35psi fuel pressure at wot from an electric lift. Any ideas as to why rpm is so low? i measured timing the other day and it was approx 11deg by the measurement charts i had found, from what im told stock is 12, would this have an effect on higher rpm to this extent? any advice would be great, thanks!

 

I'd say your doing pretty good if you can hit 2800 at 11°, I've seen trucks that will barely run at 10°. I'm not saying that's your problem, but I suggest you bump it to at least 15-17 and see what happens. Do you have stronger valve springs? Because you'll float your valves if you spin it anywhere near 4K with stock ones.

 

I do not have stiffer springs yet but am aware and would be staying away 3300ish plus for the time being.. just seemed that 2800 was a bit low. Illadjust my timing and seen what its like after that. Thanks for the feedback.

 

Good luck