VMAX 20k Number of Engines

We have a VMAX 20k with 4 Engines with Open Systems and IBM mainframe volumes.  We have 1 system bay and 2 drive bays.  Soon we will need to add another drive bay.  At what point will we need to add new Engines?  We seem to have good performance.  Our important Storage Groups have response time less than 2 ms. The front end and back end are usually around 18% busy.  The array generally does about 9000 IOPS and 400 MBs (of course we get spikes at night during batch and backups).

I was thinking we could keep from adding engines until we get to 4 or 5 drive bays - probably in 2017.