Hey. My team is considering building a tachometer and when looking thru legal motors I saw a denso throotle control motor that appeared to meet our required specs but I can not find any pictures or place to buy it
You should have gotten one or two in your kit. They are the small grey motors, with a pair of leads extending from the faceplate. Rated at 5,300 RPM. A helpful resource for finding pictures of any item is the KOP list,
As mentioned you should have got some in the KOP for the last few years if you want to buy one check with your Toyota dealer though I’m not sure which exact model car it is from.
Who uses these motors and what for? 
Our team has never used them in recent history.
They spin and (hopefully) things move.
Really, they’re for what you want them to be for.
The motor are a PIA to use because of the way the leads come out by the shaft. They are also not particularly powerful compared to other legal motors. The motors are used in the denso window motor assembles included in the KOP.
They do, however have a gear preset onto their shafts, which might be an advantage in a gearbox configuration. Our team hasn’t used any, but play around with them.
In 2012, we back-drove one to use (with a custom noise-reduction circuit) as a tachometer on our shooter wheel. Performance in this role was quite acceptable.
What is the advantage of this over a encoder?
To my understanding the reason for using a tachometer over an encoder is that encoders sense position and a tachometer returns velocity in the form of voltage. The idea is that you can have faster and more accurate feedback from motors where you only care about speed not position.
Using a DC brushed motor for a tachometer in 2012 to control shooter RPM was a result of our programming team refusing to deal with the issue digitally. We had a hall effect solution as non-contact 2 pulse per revolution and a 16 pulse per revolution solution. Interestingly when an analog solution was provided the code happened. Originally a small motor was chosen that would give a 0 to 9 volt solution in the range of rpm’s our shooter would be operating taking advantage of the wider range of the c-rio ADC. We asked the GDC about the legality of using a DC motor as a tachometer that was not on the approved motor list. In was a sensor in our view. The GDC nixed our use of the motor and we turned to a KOP motor for legality. We chose the throttle motor. A filter turns the pulsed voltage of the motor (generator) into a smooth voltage proportional to rotational velocity. It worked very well. Also was an excuse to bring in a scope and show the student a little about filtering. We are currently working on eliminating our programming team’s fear of digital encoders (Quadrature). RPM’s where measured by analog solutions for years before the digital solution was economically possible.
The output of a tach is indeed a voltage proportional to speed.
Absolute encoders sense position, but the raw output of incremental encoders (like the US Digital E4P) is temporally-spaced voltage pulses which can be decoded into position or velocity.
The idea is that you can have faster and more accurate feedback from motors where you only care about speed not position.
Dollar-for-dollar, it depends on the application and what decoding capabilities are already in the controller you are using