Best rotors for street NA 13B build?
#1
Junior Member
Thread Starter
Best rotors for street NA 13B build?
I'm in the process of figuring out my street NA 13B build. 99% sure I want to go with a carb'd setup for mechanical simplicity and some nice throttle response.
I've read that the Renesis rotors have an attractive high compression for NA application, but I've also read that they require custom machining where the apex seals go in order to fit inside a non Renesis 13B block.
I'm guessing exterior wise, that a Renesis 13B won't easily fit/bolt up where an S3/S4/S5/S6/S7/S8 would? Or else everyone would be swapping in whole Renesis blocks for easy NA power right?
Also I'm assuming the Renesis motors didn't get as much love and aftermarket developement as the other 13Bs?
So are the NA S5 rotors the way to go for me? Or are Renesis rotors worth the effort? Or are there other rotors I should be considering that I have yet to.
So far what I know about the pieces to my rotary motor puzzle are:
-13B
-street port-ish (want to try and hear a wee little bit of brapping if I could for fun, still figuring this out)
-want carbs
-want decent idle rpm and peak rpm for street use
-aluminum flywheel (10-ish lbs)
-want to keep some sort of oil injection system for the seals (too lazy for that pre-mix life)
-60mm OD exhaust pipe restriction
Feel fee to point out and criticize where I may be on the wrong path with my ideas. I'm just starting to learn about these motors and may be waaay out to lunch.
Apreciate any input,
brapbrapbrapbrap
I've read that the Renesis rotors have an attractive high compression for NA application, but I've also read that they require custom machining where the apex seals go in order to fit inside a non Renesis 13B block.
I'm guessing exterior wise, that a Renesis 13B won't easily fit/bolt up where an S3/S4/S5/S6/S7/S8 would? Or else everyone would be swapping in whole Renesis blocks for easy NA power right?
Also I'm assuming the Renesis motors didn't get as much love and aftermarket developement as the other 13Bs?
So are the NA S5 rotors the way to go for me? Or are Renesis rotors worth the effort? Or are there other rotors I should be considering that I have yet to.
So far what I know about the pieces to my rotary motor puzzle are:
-13B
-street port-ish (want to try and hear a wee little bit of brapping if I could for fun, still figuring this out)
-want carbs
-want decent idle rpm and peak rpm for street use
-aluminum flywheel (10-ish lbs)
-want to keep some sort of oil injection system for the seals (too lazy for that pre-mix life)
-60mm OD exhaust pipe restriction
Feel fee to point out and criticize where I may be on the wrong path with my ideas. I'm just starting to learn about these motors and may be waaay out to lunch.
Apreciate any input,
brapbrapbrapbrap
#2
Junior Member
Thread Starter
Been doing some more reading and I see that the Renesis block is quite different for compatability with mounting points and intake manifolds and water pumps etc.
So disregard all my questions regarding using a whole Renesis motor where a good old fashioned 13B would normally sit.
Just interested in possibly using the rotors at this point.
So disregard all my questions regarding using a whole Renesis motor where a good old fashioned 13B would normally sit.
Just interested in possibly using the rotors at this point.
#3
Old [Sch|F]ool
There are four different ways of mounting a non RX-8 13B, too. Front cover mount, two different flavors of side mount (one sold in the US, one not), and the FD style.
Scuttlebutt and inference suggests that higher compression is better for mid range torque but low compression rotors are actually best for peak horsepower. It's an internal airflow thing, a problem shared with flathead engines - the things you do to help airflow hurt compression and vice versa.
Mostly, I would not worry about it as long as the apex seal slots are in good shape. This is the most important factor. I'd rather run S4 rotors with .002" seal slot clearance than S5 rotors with .005". Not that the 2mm seal engines seem to have slot wear issues like the old 3mm seal engines had, but always measure.
Scuttlebutt and inference suggests that higher compression is better for mid range torque but low compression rotors are actually best for peak horsepower. It's an internal airflow thing, a problem shared with flathead engines - the things you do to help airflow hurt compression and vice versa.
Mostly, I would not worry about it as long as the apex seal slots are in good shape. This is the most important factor. I'd rather run S4 rotors with .002" seal slot clearance than S5 rotors with .005". Not that the 2mm seal engines seem to have slot wear issues like the old 3mm seal engines had, but always measure.
#4
Junior Member
Thread Starter
I'll look more into the Renesis mounting points, but the more I look into it I don't think the whole motor will work for my application.
Not really chasing peak HP numbers, just whatever breathes best and makes the best powerband for street use out of the combination of parts I choose to go with, so I don't think I have too much of an issue by going with either the 9.7:1 or 10:1 compression NA rotors.
Understood regarding importance of rotor condition. I don't want to run NA S4 rotors as the compression is lower than NA S5. I'd rather find S5 rotors in good/new (if possible) condition. Also S4 rotors are crazy heavy compare to S5 rotors.
I'm not building a fancy ported race or bling motor or anything like that, so it seems as if I'm between S5 NA rotors or Renesis rotors.
Still not sure if the Renesis rotors are worth the extra trouble due to the machine work required to make them fit, or even what that machine work exactly is yet. All that would be for an extra 0.3:1 compression on a what's supposed to just be a fun street motor. To be honest it's the weight of the Renesis rotors that peaks my interest more than the little bump in extra compression. I will be running the lightest off the shelf flywheel on this motor and appreciate any reduction in the motor's rotating assembly.
I know rotor weight reduction via machining is a thing, does anyone know how many grams can be taken off an S5 NA rotor?
Good to know regarding the sleave wear issues on the older 3mm slotted rotors, but again I think I'll be avoiding them anyway due to their weight vs more "modern" and lighter 13B rotors.
I think things will clear up a little bit more to help me narrow it down between the two rotors once I figure out what irons I want to run.
Not really chasing peak HP numbers, just whatever breathes best and makes the best powerband for street use out of the combination of parts I choose to go with, so I don't think I have too much of an issue by going with either the 9.7:1 or 10:1 compression NA rotors.
Understood regarding importance of rotor condition. I don't want to run NA S4 rotors as the compression is lower than NA S5. I'd rather find S5 rotors in good/new (if possible) condition. Also S4 rotors are crazy heavy compare to S5 rotors.
I'm not building a fancy ported race or bling motor or anything like that, so it seems as if I'm between S5 NA rotors or Renesis rotors.
Still not sure if the Renesis rotors are worth the extra trouble due to the machine work required to make them fit, or even what that machine work exactly is yet. All that would be for an extra 0.3:1 compression on a what's supposed to just be a fun street motor. To be honest it's the weight of the Renesis rotors that peaks my interest more than the little bump in extra compression. I will be running the lightest off the shelf flywheel on this motor and appreciate any reduction in the motor's rotating assembly.
I know rotor weight reduction via machining is a thing, does anyone know how many grams can be taken off an S5 NA rotor?
Good to know regarding the sleave wear issues on the older 3mm slotted rotors, but again I think I'll be avoiding them anyway due to their weight vs more "modern" and lighter 13B rotors.
I think things will clear up a little bit more to help me narrow it down between the two rotors once I figure out what irons I want to run.
#5
Old [Sch|F]ool
This is 2023. You use what you can find. If you have the money to buy new parts, it makes the most economic sense to buy a new complete FD engine, it is cheaper than buying parts individually.
When I built a 12A for my '81, I spent about four or five years acquiring parts from here and there trying to find ones good enough for an engine. It literally would have been cheaper to buy an FD engine.
When I built a 12A for my '81, I spent about four or five years acquiring parts from here and there trying to find ones good enough for an engine. It literally would have been cheaper to buy an FD engine.
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#6
Information Regurgitator
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#7
what are you putting the engine in?
just curious as to why?
i see the MSP for what it is, and i look at it as being very viable (with a few small tweaks here and there). it may not be a hugely popular opinion, but i think a decent argument can be made.
the Renesis apex seal is physically "shorter" than the older engines. so the slot needs machining to use the older apex seals because the Renesis seals don't play so nice with the peripheral exhaust ports.
generally speaking, i agree with the two posts above me (peejay and Dak). the writing's been on the wall for years. the REW (and MSP) is/are, or should be, your default choices - depending on what you trying to do.
however, i get that we want what we want, and at times that means doing things against a better judgment. there's nothing necessarily wrong with that as long as you realize that it's against better judgment.
i see the MSP for what it is, and i look at it as being very viable (with a few small tweaks here and there). it may not be a hugely popular opinion, but i think a decent argument can be made.
Still not sure if the Renesis rotors are worth the extra trouble due to the machine work required to make them fit, or even what that machine work exactly is yet.
generally speaking, i agree with the two posts above me (peejay and Dak). the writing's been on the wall for years. the REW (and MSP) is/are, or should be, your default choices - depending on what you trying to do.
however, i get that we want what we want, and at times that means doing things against a better judgment. there's nothing necessarily wrong with that as long as you realize that it's against better judgment.
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#8
This is 2023. You use what you can find. If you have the money to buy new parts, it makes the most economic sense to buy a new complete FD engine, it is cheaper than buying parts individually.
When I built a 12A for my '81, I spent about four or five years acquiring parts from here and there trying to find ones good enough for an engine. It literally would have been cheaper to buy an FD engine.
When I built a 12A for my '81, I spent about four or five years acquiring parts from here and there trying to find ones good enough for an engine. It literally would have been cheaper to buy an FD engine.
#9
Rotors still spinning
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For an na build the 9.7 are your best shot. The Renesis rotors aren't worth it. You need to machine the apex seal slots deeper for the RX-7 and earlier style seals. You also have the bypass rings to deal with that the other rotors don't have. You need them even if it's with a peripheral exhaust port and not a side exhaust port. My friend, the late Rick Engman, confirmed this. He was one of the most experienced rotary race engine builders in Mazda racing history. That negates any weight savings. The small compression bump doesn't actually translate to more power though so stick with the direct drop in 9.7:1. Chearance them or lighten them but just stick to those for an na.
If you want to build the ultimate street rotary na and don't care about price and can adapt different motor mounts the Cosmo 13B with 9.7 rotors would be my choice.
If you want to build the ultimate street rotary na and don't care about price and can adapt different motor mounts the Cosmo 13B with 9.7 rotors would be my choice.
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#10
Old [Sch|F]ool
We are not smart people.
I love driving my '81 - feels like when I was 20 and had a really nice SA. At the same time, i think, you know, I have TWO 2.0 Duratecs. One is an early low port engine and the other is a 12:1 compression Ti-VCT engine. Either one of those would be lighter and more powerful, and might have better fuel economy. (I can knock down 30mpg with the carbed 12A, which still impresses me) An NC Miata 5 speed with an FB tailhousing would make the expensive part of a drivetrain swap cheap.
But no, I gotta be a PURIST, gotta go for the NOSTALGIA, want to be PREPARED class legal...
Last edited by peejay; 08-05-23 at 06:35 AM.
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#13
spoon!
Thing that got lost is question of motor mounts, which is why people were asking what this is going into. 1st gen, you can swap the 12A front cover onto more or less any later 13B, buy an oil pan, and voila, it bolts up. Later cars, IDK, I bet someone could make new mounts to put a Renesis into an FC without too much trouble but I don't mess with FC stuff. My point though is that unless you're deadset on building, "just find and freshen a Renesis and run a standalone with with a Mustang throttle body" has a lot of appeal for a street car 1st gen.
S5 NA rotors are "generally accepted" as best for NA power. Personally I'd like to actually do controlled tests on an engine dyno with lightened S4 NA rotors and S5 turbo rotors but nobody pays me to do this stuff.
S5 NA rotors are "generally accepted" as best for NA power. Personally I'd like to actually do controlled tests on an engine dyno with lightened S4 NA rotors and S5 turbo rotors but nobody pays me to do this stuff.
#14
Information Regurgitator
I bet someone could make new mounts to put a Renesis into an FC without too much trouble but I don't mess with FC stuff. My point though is that unless you're deadset on building, "just find and freshen a Renesis and run a standalone with with a Mustang throttle body" has a lot of appeal for a street car 1st gen.
.
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Renesis 13B-MSP into FC chassis how to - RX7Club.com - Mazda RX7 Forum
#15
We are not smart people.
I love driving my '81 - feels like when I was 20 and had a really nice SA. At the same time, i think, you know, I have TWO 2.0 Duratecs. One is an early low port engine and the other is a 12:1 compression Ti-VCT engine. Either one of those would be lighter and more powerful, and might have better fuel economy. (I can knock down 30mpg with the carbed 12A, which still impresses me) An NC Miata 5 speed with an FB tailhousing would make the expensive part of a drivetrain swap cheap.
But no, I gotta be a PURIST, gotta go for the NOSTALGIA, want to be PREPARED class legal...
I love driving my '81 - feels like when I was 20 and had a really nice SA. At the same time, i think, you know, I have TWO 2.0 Duratecs. One is an early low port engine and the other is a 12:1 compression Ti-VCT engine. Either one of those would be lighter and more powerful, and might have better fuel economy. (I can knock down 30mpg with the carbed 12A, which still impresses me) An NC Miata 5 speed with an FB tailhousing would make the expensive part of a drivetrain swap cheap.
But no, I gotta be a PURIST, gotta go for the NOSTALGIA, want to be PREPARED class legal...
and I didn't pay anything for the aluminum printed parts, and ended up having to ditch the RA seals in favor of OEM ones with new springs so that cost is closer to 10k after misc purchasings and things the rotor modifications were 2000 dollars alone done at Chips and this was more money than I had in the last 2 motors combined which got me through about 11 years worth. I already want to tear down my cosmo motor and start over. different corner seals? too much porting? just fill it and go PP???
Rotary god you should pop in more often then but nothing really changes here, lots of money for low horsepower.
#16
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#20
Information Regurgitator
Sorry if this is a slight thread hijack, but what oil pan did you use? Does the FC pan work or does one have to use an FD pan. I am thinking of doing something similar to your engine by using Cosmo irons but with S5 TII rotor housings and rotating assembly. If I can salvage the S5 rotors I have. If not, and I have to buy rotors, buying a new 13B-REW may begin to make sense.
#21
at the risk of speaking out of turn, (and if i am, Fidelity101, you have my apology). i asked him a few questions about his setup and that was one of them. he used an S5 front cover and FC oil pan.
as an aside, i'm putting similar plans together. there is no doubt the new REW makes more sense. that said, we want what we want, and sometimes sense need not apply.
as an aside, i'm putting similar plans together. there is no doubt the new REW makes more sense. that said, we want what we want, and sometimes sense need not apply.
Last edited by diabolical1; 08-15-23 at 11:49 AM.
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#22
Information Regurgitator
at the risk of speaking out of turn, (and if i am, Fidelity101, you have my apology). i asked him a few questions about his setup and that was one of them. he used an S5 front cover and FC oil pan.
as an aside, i'm putting similar plans together. there is no doubt the new REW makes more sense. that said, we want what we want, and sometimes sense need not apply.
as an aside, i'm putting similar plans together. there is no doubt the new REW makes more sense. that said, we want what we want, and sometimes sense need not apply.
#23
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the Cosmo and S5 FC actually use the same front cover, although the oil pans are different, because the Cosmo does not have the mounts going through the pan like the FC.
both pans fit the chassis. i think the FC pan would fit the Cosmo engine, but you would have 3 holes for the engine mount that would need to be plugged, or something.
the Cosmo pan is NLA from Mazda, but should be really cheap used if you find one.
both pans fit the chassis. i think the FC pan would fit the Cosmo engine, but you would have 3 holes for the engine mount that would need to be plugged, or something.
the Cosmo pan is NLA from Mazda, but should be really cheap used if you find one.
#24
Information Regurgitator
#25
Old [Sch|F]ool
How does the Cosmo engine mount, if it doesn't use the under mount and an FC pan bolts to the rear?
I have only ever seen two Cosmo engines and they both were on the ground. Pretty sure they both had GSL-SE front covers on them, too.
I have only ever seen two Cosmo engines and they both were on the ground. Pretty sure they both had GSL-SE front covers on them, too.
Last edited by peejay; 08-15-23 at 03:12 PM.