From Hughes Website:
1. The E-Street head uses a less expensive (a little heavier) valve than the RPM head.
2. The E-Street head has powder metal valve seats whereas the RPM heads use ductile iron seats, which in our opinion is a better seat material - it is harder and lasts longer but is harder to machine.
3. The E-Street heads do not get hand blended valve bowls from Edelbrock. The RPM heads do.
