The two-speed heater motor and large impeller kit is the most comprehensive single upgrade available for the heating system, replacing the original single-speed motor with a two-speed unit paired with an oversized impeller that takes full advantage of the available space within the heater box casing. The original single-speed motor operates at a fixed rate that was designed as a compromise between adequate airflow and acceptable noise, too quiet to be truly effective in cold conditions yet too loud for comfortable background use during touring. The result of the upgrade is significantly increased airflow through the matrix at both speed settings, faster cabin warm-up, more effective windscreen demisting, and noticeably better heat distribution.
Two-Speed Control
The two-speed control allows a lower setting for quiet background warmth during touring, running the motor at reduced speed for a gentle stream of warm air, and a higher setting for rapid cabin warm-up on a cold morning or vigorous windscreen demisting in fog or heavy rain. The increased airflow is particularly effective when combined with the high-efficiency heater matrix, as the standard motor often cannot push enough air through the matrix to extract its full heat potential, the matrix producing ample heat but the air moving through it too slowly to carry that heat into the cabin.
Fitment & Switching
All MGBs manufactured prior to 1976 were fitted with single-speed heater motors, and the kit is suitable for the full production run, fitting within the existing heater casing using the original mounting points. Cars from 1976 onwards were fitted with a factory two-speed motor, and on these later cars the supplied rubber blanking plug covers the now-redundant resistor aperture. The oversized impeller is supplied with a locking R-clip, and the kit includes an adapter plate, harness P-clip, and all necessary fixings. To take full advantage of two-speed switching on pre-1976 cars, a separate switch is required, a three-position toggle switch being available for chrome bumper cars to 1971 and a modified rocker switch for 1972 to 1976 cars.
Installation requires partial or full removal of the heater box, and for maximum thermal efficiency, applying Dynamat Xtreme to the inside and bulkhead face of the heater box and Dynaliner to both sides of the air control flap during reassembly is recommended.