Stupid Question about a Cummins ISB

I picked up an old F800 for personal use. It has the ISB 5.9 210 and a Fuller 6-speed that I believe is rated to 660 lb-ft input and has a 1:1 final drive. I think the ISB has 420 or 440 lb-ft.

I've never messed with one of these before, been watching YouTube, etc., but want to know what way to go for sure before ordering parts. It needs some more oomph before converting into a personal toy hauler. Currently tops out between 65-68, depending upon load and wind, runs out of RPM. It is also a turd below 1750 rpm and has to stay wound up to move anywhere.

I think it has a 2500 rpm governor spring. I also think, mathematically, that it has 4.30:1 in the rear and sits on 10R22.5 tires. The injection pump was replaced with a reman along the way, so I can't guarantee the settings (I assume stock).

I already purchased 11R22.5 drive tires, now searching for some aluminum wheels for the outers and a pair of steels for the inners that are 3/4" wider for the new tires. This should drop the rpm by about 100 at 65, giving me about 70 or so.

Next up, I was thinking about the 3K Governor Spring Kit and changing the fuel plate to a lower number. I don't know what is in there now, though. I was thinking of putting in a #2 or so, as I want more fuel sooner, but don't want the total smoke show of a #0.

Finally, turbo. Is the stock one good enough, or am I looking at an upgrade there, too?

The final result I am hoping for is the little 5.9 to push 550-600 lb-ft (or even a little more) and be able to cruse at 70 with a little more to spare in order to pass if necessary. Am I on the right track, or do I need to look at a different route towards my goals? It is a 26k GVWR, and would probably weigh about 22k with my Land Cruiser and a large slide-in camper permanently attached... with that said, am I just asking too much of a 5.9 ISB to begin with?

 

Before boosting the fuel system I'd go over all the basics to be sure everything is working properly. Check the charge air cooler & hoses for leaks. Check the waste gate capsule for leakage. Some engines had the waste gate line coupled to the air fuel valve on top of the governor. When the waste gate capsule leaks the air fuel valve doesn't see any boost. Check throttle travel & be sure the stop is going into full run.

 

Most of that has already been done/verified. New CAC, boost tubes, clamps, expansion tank, alternator, water pump, belts, A/C system, hydro boost system (complete from pump to master to brakes), etc. The only actual issue with the truck is the oil sending unit was dead when I bought it, I replaced it, and it dies within a couple weeks (I need to get out there and replace it again). I just want more power out of it before I permanently load it heavy.

 

Have you had the overhead done? I have a 12 valve in my daily driver and I was amazed at how much get-up-and-go was restored after I had the overhead done.

Has your 24 valve been p-pumped or is it still running a VP44? If it has a p-pump you can tune the star wheel and the smoke screw to get more power out of it without it being a heavy smoker.

You may want to advance the pump timing a little bit. And if you go for the bigger governor springs talk with an engine builder to make sure your engine can handle the increased RPMs on stock valve springs.

 

We had a ‘95 F800 24’ box truck with a mechanical 215 hp 5.9 6-speed Spicer 4.33 rears and 11r22.5’s. We ran South Texas valley and coastal bend with it. The winds we get down here killed it. Back when Texas speed limit was 55-60 it would get 62mph if pushed hard with palm of hand pushing knee cap into leg harder down on to the peddle. Lol! Hauling 26,000 gross at 220,000 miles and she was wore out. Without those 4:33’s I’m sure she would not move.

Much to small of an engine for a medium duty truck that does mostly highway miles. Great for vocational and local short hauls.

Can’t remember any major problems with it other then we had a few fuel pump issues and they were known for cracking the timing case cover in the Fords back then. So check that for sure. We had ours replaced twice over a 6 year period.
Never replaced clutch,driveline,or brakes in that time and were still good at trade-in.

 

Killer dowel pin. The gear case has to be aligned to the block perfectly, because engine timing depends on it. One of the dowels would squirt out of the block after so many heat cycles, and fall on the gear train. The gear would drag it against the gear case and crack it. You can fix it without pulling the cam, but back when it was a hot issue, most people didn't know that, so it was a pretty big job. That dowel has to be tabbed to prevent it from happening.

P7100 is super easy to wake up. After that, just add air as needed. A good stop gap was to create a hybrid turbo from an HX35 and an HX40. You get the benefits of quick spool and stronger shaft from the 35, and the increased air from the 40's impeller. Keep you running until you got hot enough to need an actual turbo upgrade. Super fun engine to mess with.

 

Just to add to what you mentioned in the operating range. It would not pull any kind of weight below 1800 and 2700 rpm is what it needs to maintain hwy speed. Ours ran that everyday all day and I believe that is what wore it out. Anything higher then that will probably hurt it worse. Like I said not enough engine for what we were asking it to do. It was very stressful running mashed into the governor 99% of the time while screaming at it. Lmao!

 

Thanks for that explaination.
The first one that was replaced was when still under warranty. Grande Ford did it and at the time San Antionio/Bexar co. CPS had a fleet of them and we had to wait a month for the parts to get in for the fix because they had a couple dozen others in line.
On the second one Grande had Cummins Southern Plains do it for them.

 

I put a '97 in my '93 Chevy. It had 227k on the engine when I bought it. I tabbed the KDP first thing. It was already sticking a quarter inch out of the gear cover. For reference, it's supposed to be about an eighth inch RECESSED. So it had already moved 3/8". That issue was certainly a "when not if" situation.

Those pumps generally defuel around 2,200 RPM. A 4k governor spring kit wakes them up like no other. Literally has the throttle response of a hopped up small block. And you can take a 12-valve to 4,800 RPM safely as long as you have valve springs and push rods to back it up. The crank could take it. Harmonic destruction was around 5,400 RPM without balancing.

I love them because of the platform to tinker. You've got a bottom end that can handle 1,000 horsepower stock, and a fuel injection pump from a 14-liter engine. Couple tweaks, and you could make gobs of power. After I got mine up and running, I did what we called the "free mods", which was just grinding the AFC foot for full travel, sliding the plate, stuff like that. Made 359/831 on the dyno my first try. You could make it outgrow the stock turbo without even throwing injectors at it. When it was said and done, I was running an HE351cw from a 5.9 Commonrail, over an HT4c from an N14 Cummins. 2,400 CFM and 80 pounds boost, and I'm not joking, in the circles I ran, I had the "mildest" truck.

 

Its a 12 valve with a reman P7100 pump, not a 24 valve VP44. I wouldn't have bought the VP44 setup. Just like the Rams, the early 1998s came with the 12 valve from the factory.

This one was with a concrete company in Southern Cal for the first 15 years, and then a single-man scaffolding outfit afterward (until he hurt his back). Picked up cheap due to emissions law changes. In its recent history was a reman P7100, but I can't say when it was replaced, only identified by fresher paint and the reman tag.

The KDP is on my list of upgrades. While in the timing case, I'll look into the possibility of bumping up the timing on the pump. I'm not sure how far I can take the fuel before I need to upgrade the turbo... all of my prior performance mods were on gassers.

Mine has the 2500 rpm springs from the factory and a Fuller 6-speed with 4.33:1 diffs on 10R22.5 tires. Defuel begins at 2300, and any load (or towing) means 2400 rpm is all its got. Empty, it will cruise at 2500-2600.

I have no interest in valve springs or 4k rpm. I think a 3k rpm spring set will be more than enough to allow me to be able to upshift when accelerating from a stop going up a hill. As of now, 2500 rpm drops to 1800 rpm on the flat, and if going uphill, 1500 is not unheard of (just bogs).

I love the idea of tinkering, but I can't afford to sacrifice reliability while I am doing it. This engine has zero blowby, all the accessories have been replaced, a fresh P7100, new water pump, etc. Its ready to travel as-is, but when I load it up for adventures, I am sure it will be a miserable drive. Who knows, maybe an Allison MD3060 swap would benefit me in the future (0.75 5th and 0.65 6th), but its kind of cost-prohibitive for a toy.