Wow the wankle is considered a 3.9ltr
KevinK2,
Congratuations on the first complete accurate analysis. It is amasing
how little some owners know about their engines.
http://www.rotaryengineillustrated.com/index.html is a good starting place.
Congratuations on the first complete accurate analysis. It is amasing
how little some owners know about their engines.
http://www.rotaryengineillustrated.com/index.html is a good starting place.
Junior Member
Joined: May 2001
Posts: 33
Likes: 0
From: Sydney, NSW Australia
No a rotary is definitely not 3.9l - pretty unsurprising that a supra owner would be the one to try and tell you this 
The reason it is not a 3.9l motor is due to the fact that a complete engine cycle in a rotary takes 1080 degrees (3 turns of the eccentric shaft) but a complete cycle for a piston engine is only 720 degrees.
In the rotary, for every 720 degrees of eccentric shaft rotation, only 2
combustion cycles occur per rotor - i.e. not all 3 faces perform a
combustion.
Therefore a single rotor in a rotary is equivalent to 2 cylinders of a
piston motor.
Mazda classify a 13b as 1.3l, because they use the formula of a rotary
capacity of a single chamber (i.e. 654cc on a 13b) times the amount of
rotors (2 in a 13b) which gives you 1308cc. This is fundamentally correct
but for the purpose of comparing to a piston motor you go back to the fact i wrote above that the rotary fires twice as often as the piston motor, which then makes it the capacity of 2 rotor chambers times the number of rotors i.e. 2 x 654cc x 2 = 2616cc for a 13b.
So in summary - A 13b rotary is only 1308cc BUT FOR THE PURPOSE OF COMPARING TO A PISTON MOTOR it is efectively 2616cc.
The reason it is not a 3.9l motor is due to the fact that a complete engine cycle in a rotary takes 1080 degrees (3 turns of the eccentric shaft) but a complete cycle for a piston engine is only 720 degrees.
In the rotary, for every 720 degrees of eccentric shaft rotation, only 2
combustion cycles occur per rotor - i.e. not all 3 faces perform a
combustion.
Therefore a single rotor in a rotary is equivalent to 2 cylinders of a
piston motor.
Mazda classify a 13b as 1.3l, because they use the formula of a rotary
capacity of a single chamber (i.e. 654cc on a 13b) times the amount of
rotors (2 in a 13b) which gives you 1308cc. This is fundamentally correct
but for the purpose of comparing to a piston motor you go back to the fact i wrote above that the rotary fires twice as often as the piston motor, which then makes it the capacity of 2 rotor chambers times the number of rotors i.e. 2 x 654cc x 2 = 2616cc for a 13b.
So in summary - A 13b rotary is only 1308cc BUT FOR THE PURPOSE OF COMPARING TO A PISTON MOTOR it is efectively 2616cc.
Hi sir
Joined: Feb 2001
Posts: 443
Likes: 2
From: Modesto/Rancho Cordova CA
Originally posted by sejanus
No a rotary is definitely not 3.9l - pretty unsurprising that a supra owner would be the one to try and tell you this
The reason it is not a 3.9l motor is due to the fact that a complete engine cycle in a rotary takes 1080 degrees (3 turns of the eccentric shaft) but a complete cycle for a piston engine is only 720 degrees.
In the rotary, for every 720 degrees of eccentric shaft rotation, only 2
combustion cycles occur per rotor - i.e. not all 3 faces perform a
combustion.
Therefore a single rotor in a rotary is equivalent to 2 cylinders of a
piston motor.
Mazda classify a 13b as 1.3l, because they use the formula of a rotary
capacity of a single chamber (i.e. 654cc on a 13b) times the amount of
rotors (2 in a 13b) which gives you 1308cc. This is fundamentally correct
but for the purpose of comparing to a piston motor you go back to the fact i wrote above that the rotary fires twice as often as the piston
motor, which then makes it the capacity of 2 rotor chambers times the number of rotors i.e. 2 x 654cc x 2 = 2616cc for a 13b.
So in summary - A 13b rotary is only 1308cc BUT FOR THE PURPOSE OF COMPARING TO A PISTON MOTOR it is efectively 2616cc.
No a rotary is definitely not 3.9l - pretty unsurprising that a supra owner would be the one to try and tell you this
The reason it is not a 3.9l motor is due to the fact that a complete engine cycle in a rotary takes 1080 degrees (3 turns of the eccentric shaft) but a complete cycle for a piston engine is only 720 degrees.
In the rotary, for every 720 degrees of eccentric shaft rotation, only 2
combustion cycles occur per rotor - i.e. not all 3 faces perform a
combustion.
Therefore a single rotor in a rotary is equivalent to 2 cylinders of a
piston motor.
Mazda classify a 13b as 1.3l, because they use the formula of a rotary
capacity of a single chamber (i.e. 654cc on a 13b) times the amount of
rotors (2 in a 13b) which gives you 1308cc. This is fundamentally correct
but for the purpose of comparing to a piston motor you go back to the fact i wrote above that the rotary fires twice as often as the piston
motor, which then makes it the capacity of 2 rotor chambers times the number of rotors i.e. 2 x 654cc x 2 = 2616cc for a 13b.
So in summary - A 13b rotary is only 1308cc BUT FOR THE PURPOSE OF COMPARING TO A PISTON MOTOR it is efectively 2616cc.
WWFSMD
Joined: Jun 2001
Posts: 5,035
Likes: 4
From: SoCal
Originally posted by Alex
Many people realize the rotor in the rotary engine fires three times per rotor revolution. Thus they multiply the displacement by three to compare to piston engines. This isn't very appropriate, since the crankshaft in a rotary spins only once per three revolutions of the rotor. Thus the number of combustion cycles per rev is 1. A normal piston engine is .5. So in terms of combustion cycles per rev, the rotary is most similar to a 2.6L 2 cylinder. Check out http://www.howstuffworks.com/rotary-engine4.htm for a more detailed explanation.
Many people realize the rotor in the rotary engine fires three times per rotor revolution. Thus they multiply the displacement by three to compare to piston engines. This isn't very appropriate, since the crankshaft in a rotary spins only once per three revolutions of the rotor. Thus the number of combustion cycles per rev is 1. A normal piston engine is .5. So in terms of combustion cycles per rev, the rotary is most similar to a 2.6L 2 cylinder. Check out http://www.howstuffworks.com/rotary-engine4.htm for a more detailed explanation.
And for the purposes of comparison, it makes the most sense to judge displacement relative to the main shaft (eccentric or crank). Which as you (and others) have pointed out, makes the 13B a 2.6L engine. This is an appropriate comparison for the amount of air the engine ingests and is also somewhat reasonable for power output.
The club racing organizations around here classify an FD as a 5.2L, since it gets doubled for being rotary and doubled again for being turbo.
And just to keep the posts going... the power "stroke" or phase for each chamber on the rotary covers 3/4 of an eccentric shaft revolution. (Hint: power phase is 1/4 of a rotor's revolution.) On a piston engine, it is of course 1/2 of a crankshaft revolution. SUGGESTION: Don't use this as an example of how a rotary is better than a piston engine, or there'll be more "stupid RX-7 owners!" posts over on the Supra forums. It is just a difference; it does not make the rotary clearly better or worse than a piston engine.
-Max
Full Member
Joined: Dec 2001
Posts: 73
Likes: 0
From: Bordentown, NJ
those numbers in that car test arent all true i have a similar test wich puts the rx7 fast than the supra in all the same tests, if i can find ill post it, i know for a fact that 0-60 isnt right the 7 is faster than that stock and ive seen a stock 7 run 13.6
Senior Member
Joined: Feb 2001
Posts: 260
Likes: 0
From: Topeka, Kansas, USA
Well then hell, I'm going off to my dream world and saying that it's only a ONE HALF LITER engine! You can look at it ANY way you want, but those that built/mass produced it say it is a 1.3L. I trust them to know-wrong or right. . .
PEACE,
-Derek
PEACE,
-Derek
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