rocket 5979
Resident Gearhead
- Joined
- April 22, 2002
- Messages
- 3,994
- Reaction score
- 12
- City, State
- Lake Villa, Illinois
- Year, Model & Trim Level
- '03 XLT 4.6
The Roots type blower (Eaton M90) is very efficient (90%) for low rotor speeds and low boost (1.5 psi). The efficiency decreases slightly (80%) at mild rotor speeds and boost (5 psi). For medium boost (8 psi) and rotor speeds the efficiency drops to about 73%. The efficiency at 15 psi of boost is only 58%. For any given amount of boost a larger rotor will be more efficient than a smaller rotor because the rotor speed will be slower. The output of a Roots blower is pulsating rather than continuous.
I can only assume you are talking about adiabatic efficiency. If that is the case then you are mistaken in your figures. The attached compressor map shows that the maximum adiabatic efficiency the Eaton M90 is able to achieve is 62% at 6.5psi with an overall flow of 510 cubic meters per hour of airflow while spinning the blower to just over 6,000 RPM. You are also mistaken in your statement that "For any given amount of boost a larger rotor will be more efficient than a smaller rotor because the rotor speed will be slower". All compressors, including Eaton roots blowers, have an efficiency range that when they are spun into attain good efficiency. Overspin a blower for the combo it is on and the ratio of hp gained per psi boost will decrease. The same can be said when underspinning it too. You have to match the compressor to the combo it is on if you want to maximize performance while keeping IAT's in check.