Turning up a 903

Putting around with the door plate rating of 290HP is not working out. When I bought the truck, it was listed as a "350 cummins", so don't even get started on the "well why did you buy such a weak engine" bit. Running up to Pueblo, CO at around 7,600ft was no pleasure holding onto 1st gear for dear life at about 4mph and watching the engine bog 300rpm every few seconds between 1100 and 1500rpm. Getting stuck at whatever speed I was going up anything that is slightly steeper than flat is no treat either. I've also read here and elsewhere that these engines can see a bit of an economy gain with a bit more power up the hills, which would be welcome, since I do about 5.5mpg on flat ground.

The engine was rebuilt less than 25,000 miles ago, so my only concern with turning it up is the driveline. The transmission has around 250,000 miles on it, and the pumpkins aren't stock (door plate is 4.56, and differential tags read 4.44 after I scraped off 2 layers of paint), but I don't know how much life is left in them. I've got a misfiring cylinder, and the turbo is running in the single digits, so I need to get that sorted out first, but I'd like to sort out my options in case I don't find the power I need getting those two fixed.

I've talked to a few former 903 owners from back in the day (all wrecker operators now, coincidentally) and they all had different methods of getting more power out of the engine.

1) Twin turbo setup @ 850hp
He didn't recall the specifics of the upgrades, but he did note that this power rating caused more trouble than its worth, and I'm really not interested in that kind of power, since I won't be doing over 80k gross.

2) Drilled out bearing @ 390hp
This was a roadside story about how he drilled out a ball bearing where the throttle rod enters the fuel pump. I wasn't entirely clear on the details, but he did note that this modification made the engine competitive with the other big V8s of the day. Kind of iffy on how sound this modification is.

3) Shimmed and buttoned/springs
Heard this one most commonly, running anywhere from 400hp-600hp. Simple fuel pump work for simple gains, backed up by the field service manual I have from the U.S. army on this engine.

I'd like to see 350-400hp out of this engine, and spare the bottom end the strain of a bigger gain. I'm not trying to drag race a semi, and I'm not big on passing everyone in the right lane, but I don't think pushing the engine to its mechanical limits all day every day is a good way to run it. I'd also like to give my 4 way flasher relay a break a few times a day, since I've overheated it more than once.

Any experience/other suggested methods for achieving this?

 

If it isn't being temperamental and sticking between 1800 and 2000 (stays between 1900 and 2100 if I kick it down to direct) I'll usually run it around 2150, which is somewhere around 66mph (speedometer cable binds), but I'll run it all the way up to 2500rpm up and down hills, and that just falls of going up most hills, and it'll start to hold around 1700-1900rpm if I'm in the right climbing gear.

 

I'm working on the misfiring cylinder. Been talking to some boat guys that run 903's, and they said plugged air filter elements can cause misfire in these engines, and the date on the one I've got is 7/08. Got a new filter ordered and waiting for me back in bloomington, so I'll give that a shot when I can get to it.

Beyond that, kenworth and cummins have both told me they only have one type of injector for this engine. Would I have to go aftermarket for a higher flow?

In addition, I have two spare injectors, but haven't found anyone that will rebuild them, or even test them. I've rebuilt injectors before, but they were less than half the size of these ones. Anyone have a resource for rebuilding diesel injectors? Any special tools I need to worry about? Any part suppliers if I find something wrong going through it?

I'll definitely get the fuel lines swapped when I get the misfire looked at again. Nothing came up on a pressure test, but I've had a hell of a time priming the filter, so I think something might be up with them.