This picture shows the difference between the Mechanical Unit Injection (MUI)
and current Electronic Unit Injection (EUI) installation in the cylinder head.
Notice the Helper Spring on the injector pushrod. This arrangement is designed
to keep the follower in constant contact with the camshaft. The helper spring is
required due to the increased injection pressures of 151 MPa (22000 psi) and the
steeper, high lift camshaft lobe profile.
The 3500B has a larger diameter camshaft to accommodate the higher injection
pressures generated in the unit injector pumps.