Hi all, I understand adjusting the star wheel on the fuel pump is a good way to free up horsepower. I heard there is a star wheel under a plug. If this is correct, how much should I turn it and what should I expect? I’m not looking to drag race my truck, but a bit of free horsepower to climb hills sounds great. Is there anything else recommended? Thank you very much.
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8.3 Cummins fuel adjustment
- Thread starter ericp351
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Under the Allen head is strictly “Low-Boost fueling” meaning it adds fuel before boost comes on, but doesn’t alter the end power of the engine. You have a Bosch MW pump most likely, and the main fuel is located on the rear of the pump with a jam nut. You’ll want to give it a little low boost fuel and main fuel to it.
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Lukes_deuce
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Look up nevrenufhp on youtube. He has videos specifically for the Bosch MW pump on the 8.3 in the M939A2. I would start with putting an EGT gauge in the exhaust manifold and a boost gauge. Then start playing with the pump. Some of these trucks run hot from the factory and need to have a baseline established before you turn up the pump.
Once you have your gauges, then you can adjust the preboost screw, under the 3/8 or 10mm allen bolt. I usually start with 2 turns out. Then you can go and move the jam nut on the rear of the pump. You will have to modify a socket to loosen the jam nut. I have 3-4 threads showing after the jam nut. If you do not have any threads showing, I guarantee you it will get very hot on stock truck.
To answer your question on the star wheel, that is another adjustment you can make in the pump. It has less effect than the other two mentioned above. Its more of a fine tune of your rack travel for the jam nut. It can help with fuel up top and come on earlier, but unless you have other modifications, I would leave it alone. You will enjoy the pre boost screw and jam nut alot. Any other questions, ask away.
Once you have your gauges, then you can adjust the preboost screw, under the 3/8 or 10mm allen bolt. I usually start with 2 turns out. Then you can go and move the jam nut on the rear of the pump. You will have to modify a socket to loosen the jam nut. I have 3-4 threads showing after the jam nut. If you do not have any threads showing, I guarantee you it will get very hot on stock truck.
To answer your question on the star wheel, that is another adjustment you can make in the pump. It has less effect than the other two mentioned above. Its more of a fine tune of your rack travel for the jam nut. It can help with fuel up top and come on earlier, but unless you have other modifications, I would leave it alone. You will enjoy the pre boost screw and jam nut alot. Any other questions, ask away.
74M35A2
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Yep. EGT gauge and then the video will be more than enough to get you in trouble. Move the injection timing to 16 degrees if you really want an efficiency boost as well.
Lukes_deuce
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You wont notice much of anything if you bump the timing up on a stock pump. Adjusting the timing helps when you add more fuel. Due to duration of the fuel injection cycle is very retarded due to safe settings of a stock pump. Also on stock settings, your lucky to get 20 psi of boost which wont get you in trouble.
I personally like doing one change at a time, so you can better monitor the results of that change. Good luck.
74M35A2
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Agree with above.
Timing increase is not supposed to add hp to an otherwise stock engine, but it is reported to add efficiency (MPG), and then help additional mods take hold even better than they would with stock timing. This is what the RV group indicates, which appears to be the largest user group of personally owned 8.3L's, and whom are always looking for gains in both fuel economy and power output.
Timing increase is not supposed to add hp to an otherwise stock engine, but it is reported to add efficiency (MPG), and then help additional mods take hold even better than they would with stock timing. This is what the RV group indicates, which appears to be the largest user group of personally owned 8.3L's, and whom are always looking for gains in both fuel economy and power output.
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Wes was by my place earlier this year and adjusted my pump, did not adjust timing. It made a noticeable difference in power off the line. Rolls coal now and the turbo maintains boost longer. Even had a buddy comment he saw me "smoke" some other vehicles leaving a red light. It's much peppier now.
Aneroid adjustment
Aneroid adjustment
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Pyrometer first.
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I’ve been trying to decide for a long time what order to do these performance mods in, in order to get the best data for each mod, especially ability to maintain speed, and EGT on long hard pulls.
Already have pyro, and boost gage installed.
Thinking in this order... governor springs, pre-boost fuel, timing, main fuel, star wheel, air filter, then way out on the wish list air to air cooler.
Already have pyro, and boost gage installed.
Thinking in this order... governor springs, pre-boost fuel, timing, main fuel, star wheel, air filter, then way out on the wish list air to air cooler.
74M35A2
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I think pre-boost fuel and star wheel is same thing? Just don’t go too many amps.
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Yeah, not real sure who’s calling what, what, or if I have any idea what pump adjustment does what. Maybe I’ll just turn everything 2 turns counterclockwise at the same time and see if it will start and run
.
Maybe someone that knows what they’re doing will let me know if I’m guessing right?
Main fuel/rack travel. I think these guys back the jam nut off until they have as little threads as possible but still keep the nut on, or melt the pistons down, whichever comes first. 
Star wheel/aneroid. Used to clean up main fuel delivery under boost? Rotating toward motor adds fuel/smoke, away from motor decreases?
Pre-boost fuel delivery? I think this is the screw they turn out a few turns that really helps the a2 trucks off idle?
Hopefully someone that knows for sure can verify, or correct me on this. This is an MW pump on our 8.3, not the P pump on your Dodge
.Maybe someone that knows what they’re doing will let me know if I’m guessing right?
Main fuel/rack travel. I think these guys back the jam nut off until they have as little threads as possible but still keep the nut on, or melt the pistons down, whichever comes first. Star wheel/aneroid. Used to clean up main fuel delivery under boost? Rotating toward motor adds fuel/smoke, away from motor decreases? Pre-boost fuel delivery? I think this is the screw they turn out a few turns that really helps the a2 trucks off idle?Hopefully someone that knows for sure can verify, or correct me on this. This is an MW pump on our 8.3, not the P pump on your Dodge
74M35A2
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Yeah, not real sure who’s calling what, what, or if I have any idea what pump adjustment does what. Maybe I’ll just turn everything 2 turns counterclockwise at the same time and see if it will start and run
Maybe someone that knows what they’re doing will let me know if I’m guessing right?
View attachment 795505Main fuel/rack travel. I think these guys back the jam nut off until they have as little threads as possible but still keep the nut on, or melt the pistons down, whichever comes first. View attachment 795506 Star wheel/aneroid. Used to clean up main fuel delivery under boost? Rotating toward motor adds fuel/smoke, away from motor decreases?View attachment 795507 Pre-boost fuel delivery? I think this is the screw they turn out a few turns that really helps the a2 trucks off idle?
Hopefully someone that knows for sure can verify, or correct me on this. This is an MW pump on our 8.3, not the P pump on your Dodge
You’re right, screw is initial, and star wheel is rate. Some have taken the jamb nuts right off the back of the main fuel rack and been fine, depends on how much load you will pull and if you want to drive by the EGT gauge or not.Yeah, not real sure who’s calling what, what, or if I have any idea what pump adjustment does what. Maybe I’ll just turn everything 2 turns counterclockwise at the same time and see if it will start and run
Maybe someone that knows what they’re doing will let me know if I’m guessing right?
View attachment 795505Main fuel/rack travel. I think these guys back the jam nut off until they have as little threads as possible but still keep the nut on, or melt the pistons down, whichever comes first. View attachment 795506 Star wheel/aneroid. Used to clean up main fuel delivery under boost? Rotating toward motor adds fuel/smoke, away from motor decreases?View attachment 795507 Pre-boost fuel delivery? I think this is the screw they turn out a few turns that really helps the a2 trucks off idle?
Hopefully someone that knows for sure can verify, or correct me on this. This is an MW pump on our 8.3, not the P pump on your Dodge
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Just don’t tell my wife, she get’s goofy
when I’m right!
Steelreaper80
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How do you change the pump timing to 16 degrees?
74M35A2
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You can do it yourself on the engine by pulling the injection pump gear off and backing the engine up or by “spill timing” it, or send the injection pump out to any pump shop and have it changed internally so you can then just easily install and pin time it (can’t miss). Details for doing it yourself are in the 8.3L power up thread.
Steelreaper80
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How do you get the tamper proof screw off the back of the pump? I am about ready to get into the pump and turn it up a tad.
74M35A2
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That’s why it’s tamper-proof. 
Steelreaper80
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Exactly.......but I WANT to tamper with it....lol.That’s why it’s tamper-proof.