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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,
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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.

peejay

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.

brapbrapbrapbrap

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.

peejay

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.

Dak

This, and ditch the whole carb idea and go standalone ecu. it's 2023, IMO no point in going carb'd on a streetcar. Only reason go carb these days is if you're racing in a class that doesn't allow fuel injection. FD engine, standalone and the right intake setup and you'll make more power than a 6-port n/a ever will.

diabolical1

iTrader: (3)

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.

fidelity101

iTrader: (11)

I wonder about this every day, why didn't I just buy an REW and send it

rotarygod

iTrader: (1)

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.

peejay

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...

rotarygod

iTrader: (1)

Peejay: It's cool to see you and BlueTII still around after all these years. I stop back in after several years away and some things never change.

TeamRX8

iTrader: (2)

please don’t be starting multiple new threads on the same topic:

https://www.rx7club.com/general-rota.../#post12570377
.

Kenku

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.

Dak

For some reason when this has been asked in his multiple threads, he hasn't answered the question. Unless I missed it which I may have. He's not replied to this thread since the question was asked. Anyway, his profile says S4 N/A

. Also I should have posted this early on but the conversation drifted away from the Renesis pretty quickly. From this thread mounting doesn't look too bad and he's made the templates available.
Renesis 13B-MSP into FC chassis how to - RX7Club.com - Mazda RX7 Forum

fidelity101

iTrader: (11)

here is the cost breakdown of the cosmo 13b build plus ITB setup I did for reference:



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.

j9fd3s

iTrader: (3)

if it is an FC then the FC based engine would bolt in, the FD and Cosmo need mount brackets for ~$400. the new FD engine is actually cheap enough where its only $100 more than an FC engine.

diabolical1

iTrader: (3)

how long ago did you buy those Cosmo side housings or do you have the Motorsport discount?

TeamRX8

iTrader: (2)

Several years ago … maybe

I bought mid & rear FD plates from Motorsports several months ago for about $50 more, but the front plate is ~$120 higher; $1840 for all three
.

fidelity101

iTrader: (11)

2021/2022 time frame and yes but they were out of stock at the time so I grabbed them from mazdatrix, in stock - fair price.
the 20b center was a used part via ebay @ banzai

Dak

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.

diabolical1

iTrader: (3)

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.

Dak

Thanks. I was going to use a S5 front cover as well. I originally wanted a REW. I got a mostly complete S5 JDM engine/ trans combo with failed water seals for cheap locally a few years ago. The front and center irons ended up being bad but the rear iron and everything else except the rotors are good. Thought I'd use REW irons since I had to buy two new ones anyway. Ends up buying the REW parts I'm missing ends up being almost as much as buying a whole engine. Using the Cosmo irons lets me use the good TII parts I already have, but from being filled with water the rotors are a mess. Apex seal groves are good but side seals are currently rusted into their slots so they may be junk.

j9fd3s

iTrader: (3)

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.

Dak

That would be great. I can deal with plugging those holes up as long as it bolts to the engine.

peejay

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.