So, rotors are usually for the RX-7 are cast iron? The housings are
    aluminium with a thin layer of nickel for the wear surface?

    Does anyone know what the Johnson or Sachs small snowmobile engines
    used for their rotors and housing? I have been told by one person
    that the rotors are cast iron too but the housings are just
    aluminium.

    Does anyone know if just plain aluminium housing are used? Any other
    material used for rotors besides cast iron that works?


    David Jr.

    Yes the rotors are cast iron. Very tough and robust. The housing are
    lined with hardened steel and then chrome plated.

    There have been many different coatings applied to the aluminium
    with out as much success as the Mazda housings. 2.5 million engines
    have been sold.

    Paul Lamar

    Great pictures, thanks Paul. I would take as many of those
    production pictures and such that you would like do send.

    David Jr.

    Here are a bunch more. We were the first round eyes to get a tour
    through the factory :-) Took 18 months of cajoling.

    Paul Lamar



    Great pictures!  I've been working on some investment casting (steel
    and aluminum) using sterolithography to generate the wax molds. It
    appears that the rotor would be an ideal part to make using that
    process. For ones or twos, it is much cheaper than having to make a
    mold.  If larger quantities are needed, then they make a wax mold
    from the stereo model. This method sure beats the old way of making
    patterns.

    Gary Knutson

    I made some intake patterns on the local college stero lith machine
    back in 2003. In those days the stero lith machine cost $30,000.


    Paul Lamar

    Could the rotors be 3D printed from say Titanium or other materials
    at a realistic cost? The weight of the rotor would be so reduced that
    RPM could be increased. The printing would allow a design that would
    not be practical or even possible with conventional machining or
    casting. I think that NASA is able to print in different metals
    without the conventional welding process.

    Andrew Campbell.


    As long as it can hold the tolerances to + or - .001 inch.

    Paul Lamar

    It would appear that the tolerances and surface finish of EBDM
    printing are of a very high magnitude, the amount of material that
    can be deposited in an hour is also high so hopefully costs may be
    acceptable. Given the ultimate tensile strength of Titanium alloys
    being in the region of 2 times cast iron and a lower SG and the
    ability to remove most of the unwanted material within the rotor we
    could end up with a %75 reduction in weight.

    Titanium has poor thermal conductivity, this would help to keep the
    rotor cool, the extra heat not absorbed into the rotor is going to
    have to go somewhere though. Would this reduction in rotor weight
    translate into a significant power increase?

    Andrew campbell

    Only if you have a gear box to handle higher RPM.

    Paul Lamar


    Oooh!  Do I see an opportunity for compound planetary gear reduction?
    With two 2.85 planetary gears in series you could run your hyped up
    rotary at 14250 RPM.

    -  Mark LaPierre


If you stack two planetary units together you increase the problem of
gear clearances allowing chatter. The ratio would be 8.12:1. This would
give a 2000RPM propeller speed which would necessitate a larger diameter
or wide chord. It would be better in my mind to look at an alternate
planetary source. Allison Transmissions has built thousands of heavy duty
transmissions for trucks, buses and construction equipment.
Transmission series with ratios and total transmission weights:
2500/2550RDS       3.51:1      330#
4000RDS                3.51:1       ?
3500RDS                4.59:1      535#
4500RDS                4.70:1      890#
4700RDS                7.63:1      1083#
Transmission weight should give an indication of the gear set sizes inside
and the power/torque capability designed in. The gear sets will not be close
to the transmission weights listed but heavier than the Ford planetaries.
If you need high power transmission with a high duty cycle this is what is
required. These pats are readily available which is good.

Dale Davies

Here is some info on Allison transmissions.

Paul Lamar

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