NoPistons -Mazda Rx7 & Rx8 Rotary Forum - Can You Help Answer This Question?

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I received this e-mail today and I do not have an answer for him but thought you intelligent, brilliant folks may have one. I do not know this guy and I do not know how he got my e-mail address. If this is posted in the wrong section please feel free to move it.

This is what he asks........

Hi.

I am not a RX7 enthusiast so I know nothing about them; however I am using a old alternator from a late 80s RX7 in another car, and need some help selecting the correct pulley size. I have managed to find out that the engines rev at a similar speed to `normal ' engines but I am not sure that the shaft that the drive pulley is on rotates at the speed that is quoted as the RPM. In other word does that shaft rotate at the same speed as the rotor ? if not which rotation speed is being referred to when rpm is mentioned , and what is the ratio of the two. From pictures I have found the drive shaft pulley appears to be only a little larger than the pulley on the alternator.

Hope that makes sense.

thanks Steve G

Any answers for this guy.

Thanks.

Rotors spin at 1/3 the speed of the shaft. The Tach is reading the shaft speed, so the rotors are spinning slower than what your Tach says. The smaller the pulley you use on the alternator the quicker it will spin. Since rotary engines have a big RPM range as compared to other motors, they spin pretty slow at idle and rather quick at around 7500 RPM. Pulley selection is much easier to work with on something that idles at 1000 rpm and redlines at 5500 to 6000. But it's also dependant on the size of this guys main crank pulley. If his main crank pulley is smaller, that makes accessories spin slower. So he would need a really small alternator pulley to make it spin at a normal speed.

no the "shaft" rotates 3 times faster than the rotor, but all rpm measurments are taken from the "shaft" not the rotor. The alternator is a dumb device it works the same for a rotary as it would for any other car, but of course it's geared a little "higher" than the eccentric shaft, I'm thinking it runs a little over twice the speed of the eccentric shaft.