Login Register Sign up for the GRM e-newsletter

Login to post Forums » Grassroots Motorsports » RX7s and Rotary Swaps « 1 2 »
  • ReverendDexter

    Nov. 17, 2009 4:36 p.m. ReverendDexter HalfDork

    JeepinMatt wrote:

    ReverendDexter wrote:

    JeepinMatt wrote:

    With an equal amount of boost and all other things being the same, I would expect the 3-rotor 20b to last longer.

    Why's that?

    Seems more robust, not so high-strung (not that they can't be made to be) I don't know much about rotaries, but the 20b being longer-lived than the 13b seems to be the consensus as well.

    Well, the presumption was "equal amounts of boost and all other things being the same".

    I can totally understand a 20b outlasting a 13b that makes the same amount of power, but I can't understand why a longer eccentric shaft, extra housing and rotor, and all the supporting intake/exhaust/spark/etc parts makes for a MORE reliable motor. All else being the same, a simpler system is more reliable, right?

  • ronbros

    Nov. 17, 2009 5:33 p.m. ronbros Reader

    GRANNYS car is an incredible piece of work, i doubt if any of you guys could understand the work involved in makin two rotarys actually work together!!

    also Dirt cars have some of the best backyard engineers EVER, F1 aint gotta. chance

  • JeepinMatt

    Nov. 17, 2009 6:42 p.m. JeepinMatt Reader

    ReverendDexter wrote:

    Well, the presumption was "equal amounts of boost and all other things being the same".

    I can totally understand a 20b outlasting a 13b that makes the same amount of power, but I can't understand why a longer eccentric shaft, extra housing and rotor, and all the supporting intake/exhaust/spark/etc parts makes for a MORE reliable motor. All else being the same, a simpler system is more reliable, right?

    Too many quote boxes, so I cleaned it up a bit.

    It was a good point about simplicity being more reliable. I've always thought of reliability and longevity as two separate things. One being how often it leaves you stranded in the crappy part of town and the other being how long until you actually wear out the engine. The 13b is a more rev-happy, high-strung engine, which is a plus in fun but a minus in longevity (unless you drive it like Granny). That 20b makes torque lower and doesn't rev quite as high. I'm mistaken that the 20b had thicker rotors and rotor housings than the 13b; they're both 80mm, the 20b just has one more of them. So the perceived difference isn't as much as I had thought.

  • amg_rx7

    Nov. 17, 2009 7:06 p.m. amg_rx7 Reader

    Too many quote boxes, so I cleaned it up a bit.

    It was a good point about simplicity being more reliable. I've always thought of reliability and longevity as two separate things. One being how often it leaves you stranded in the crappy part of town and the other being how long until you actually wear out the engine. The 13b is a more rev-happy, high-strung engine, which is a plus in fun but a minus in longevity (unless you drive it like Granny). That 20b makes torque lower and doesn't rev quite as high. I'm mistaken that the 20b had thicker rotors and rotor housings than the 13b; they're both 80mm, the 20b just has one more of them. So the perceived difference isn't as much as I had thought.

    Actually, both the 20B and 13B are equally as high revving as the other. Neither are "high strung" on their own. The 20B obviously makes more torque thanks to its bigger displacement.

    The center iron of the OEM 20B is thicker - that might have been what you were thinking. I think there is a company in Australia that makes an eccentric shaft that is shorter and doesn't use the thicker OEM center iron.

    As you said, the rotors themselves and most of the rotor housings are similar to the 13B and can in some instances be swapped.

  • amg_rx7

    Nov. 17, 2009 7:11 p.m. amg_rx7 Reader

    Well, the presumption was "equal amounts of boost and all other things being the same".

    I can totally understand a 20b outlasting a 13b that makes the same amount of power, but I can't understand why a longer eccentric shaft, extra housing and rotor, and all the supporting intake/exhaust/spark/etc parts makes for a MORE reliable motor. All else being the same, a simpler system is more reliable, right?

    I think if you compare a turbo 13B running 15 PSI or a Bridge or Peripheral Port 13B making 300 +/- RWHP to a 20B taking 8 PSI or a mild Street Port to make similar HP number, then yes your argument makes some sense

    the 20B won't have the extra heat of a turbo running more boost on the 13B to make the same power level or the 'race' style Bridge or Peripheral porting that usually decreases longevity and makes street driving basically illegal. :)

  • JeepinMatt

    Nov. 17, 2009 7:45 p.m. JeepinMatt Reader

    amg_rx7 wrote:

    Actually, both the 20B and 13B are equally as high revving as the other. Neither are "high strung" on their own. The 20B obviously makes more torque thanks to its bigger displacement.

    I thought the 13b revved like a sonuvabitch? I thought the 20b's powerband and redline were lower as well.

  • amg_rx7

    Nov. 17, 2009 9:23 p.m. amg_rx7 Reader

    Nah, stock they both rev like a sonuvabitch. :) Rotaries love to rev.

    The stock 20B's powerband in the Japan only Cosmo coupe (luxury car, automatic only, smallish twin turbos...) probably redlined around 6500 or 7000 but its not b/c of any built in restrictions. Peak power on most 13Bs motors (besides the Renesis) is around 6500-7000 on stock ports with stock intake manifolds IIRC. Peak torque is around 5500 IIRC. This is partially by design for a consumer oriented car with street manners.

    My 3rd gen makes peak power a bit further up the rev band from 6500 thanks to a mild street port and a free flowing exhaust and... other mods... :)

  • aussiesmg

    Nov. 17, 2009 10:13 p.m. aussiesmg SuperDork

    My 13B bridgeport made power to 9000 rpm and was only limited by the steel apex seals, carbon seals will easily rev to 11000.

    That said my 5.0 with aluminum heads etc is actually within 20lbs than it came from the factory with the 12a, AC, PS etc.

    I also get 350 hp and 350 ft lbs that will rev to 7000 and run for 200,000 miles, the stock 12a can easily do this but can't make anywhere near the power.

    I may not win the 1st race against a turbo rotor but 20 races later I will still be running. Steve

« 1 2 »  

You'll need to log in to post.