I have a set of rotors which I would like to identify. Part # is 4801-78-130. Judging by info I have found by now these are either 74-78 13B 9.2:1 or 84-85 9.4:1 rotors. I would appreciate any hint.

Thank you!

 

welcome to the board.

i would have to go with a Series 4 rotor because with a 2 mm apex slot, but a non-machined recess, it can't be anything earlier (unless it's some weird Japan-thing that i don't know about). all earlier rotors, to my knowledge, are 3 mm apexes/apices.

another idea is weigh them.

 

Glad to be on the board!
These have 3 mm slots, each rotor weighs 4900 grams.

 

the calipers .... tricked.

okay, here is a chart Mazdatrix made: https://mazdatrix.com/rotor-weight-a...ression-chart/

although, according to the chart you're coming up about 0.7 pounds (roughly 300 kg) short. i guess your next metric would be to get some measurements of the recess and extrapolate an approximate compression ratio based on the volume you measure. they listed two on that chart, so it might be worth a try. of course, this is all provided you can find a formula to calculate compression ratio of a rotary. i'm running out of ideas though.

if i could locate my old Rotary Engineering catalog, then i might be able to find an old part number for you, but alas, i can't find it.

 

4801-78-130 is the competition rotor for a 13B. they are a matched set, so the stamping is probably the weight, in grams. the rotor gear should be retained with a snap ring instead of roll pins
9.4:1, 3mm seal. if you're building the "correct" 4801 engine, it would have been the carbon seals.

 

Thank you for the info!
I have 2 sets of rotors - stock 8,5:1 and 4801. Which one should I use to build a reliable 400-450 HP engine?