If the paint you use recommends primer you should use the suggested primer.

I used Duplicolor engine paint (and primer) from NAPA on my engine, turned out very nice.
Most of the work was in the prep, stripping all the old paint and scrubbing every last inch with a brush and lacquer thinner.
Here's a picture of the stripped block
http://rides.webshots.com/photo/2387721890080251109siGDXz
Here are a few in progress and completed.
http://rides.webshots.com/photo/2987531930080251109rscvBI
http://rides.webshots.com/photo/2139348100080251109hatrCb
http://rides.webshots.com/photo/2205440580080251109TNUrGL

In the past I tried just steam cleaning and scrubbing the old paint really good, and painting over it. That worked well for a while, but eventually the old paint started to come off in places, with the new too. From now on I'll completely strip an engine before painting.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-