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Old 07-20-2021, 02:36 PM   #1
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5.9 ISB Cummins Performance upgrade

I have a 1999 Diplomat with a 275 ISB it has 31,000 miles on it. It does ok on flat ground, but for mountains or towing my little tracker it’s a little underpowered. I don’t want to make it unreliable. Just wondering what some members have tried. It shows on factory gauge 26lbs of boost TIA…
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Old 07-28-2021, 12:14 PM   #2
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I had one of the 99 Itasca's with the Freightliner Chassis'. I checked into going from 275 up to the 315 hp version. After talking with Cummins, it appears that there was/is a problem with keeping the (factory)manifold cool since the engine compartment is so closed in. In any event, the manifold did crack on mine and had to be replaced. Fortunately, under warranty. I understand however, there is a way to go up in HP by replacing the manifold, turbo, and exhaust. The fix is/was around $3,500+. Hopefully, someone who remembers one of the companies that specializes in this type of Service will chime in. Good luck.
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Old 07-28-2021, 07:48 PM   #3
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99 5.9 should be a VP44 pumped 24 valve Cummins. There are TONS of upgrades you can do. It’ll probably have an HX35 turbo, which can be easily upgraded, larger injectors, performance tune, you name it, it can be done. There’s plenty of folks who crank these motors up. First step would be to read the stickies at the top of this forum:

https://www.cumminsforum.com/forums/...discussion.18/

Then, go read some performance threads there and pick your poison.

As to reliability, you need to know how hard you’re pushing the motor in real time. A set of gauges will be in order in addition to mods. EGT, boost (which you have), temperature (actual numbers, not a H/L), etc. Not sure how much room you have, but fuel and air to match will be a huge jump for you. Larger injectors and turbo or compounds will get you there in a hurry. As long as you aren’t running high egt for extended periods, cranking the timing past a certain point, or overheating, that motor can take a lot. Cylinder pressure is a killer of head gaskets (lots of timing).
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