Stupid Question about a Cummins ISB

You don't run 4k springs so you can run higher RPM, really. They completely change how the throttle behaves. Makes it way more "driveable". They're so cheap and easy to install, you could give both a try and see which one you like.

You can do the timing whenever. You don't have to open up the case to do it.

 

Fuel plate just changes the max fuel delivered based on throttle position and RPM. RPM changes the height of the AFC foot. As RPM increases, the AFC foot slides against the profile on the plate. That's how they can change fuel based on where the average shift point might be, for example. Throttle will move it forwards and backwards based on boost pressure.

You get a certain amount of "pre-boost" fuel based on how tight or loose the starwheel is. You need this fuel to light the turbo. After that, boost can move the AFC back and forth. Say your starwheel is set to give enough pre-boost fuel to get you to 7 psi. If you unhook your boost reference line from the back of the AFC, then you'll only ever be able to get pre-boost fueling. The pump will only deliver "real" fuel when it knows you have enough boost present to be able to actually use it. Basically the mechanical version of a computer.

Once you get your plate slid where you want it, or install a different profile, or grind the face, or even completely remove it, go on the road with an 8mm allen wrench and a flat head screwdriver, and a wrench for the jamb nut on the back of the AFC, and tune it. Loosening the starwheel will increase pre-boost fuel, and cause the turbo to light faster, but you will be smokier leaving a stoplight. Tune as necessary. This tuning really doesn't affect EGT's. That's all in restrictor plate setup.

And whatever you do, it's easy to return right back to the way it was if you don't like it, or it runs too hot. (EGT's, not coolant) You definitely want a pyro.

 

Also, check out www.cumminsforum.com

There's a lot of info there. A whole section dedicated to the 94-98 engines. A tech section, etc.