Parts compatibility to go in a 13b PP miata
 

Hi all,

I am sourcing the parts for my new project :

NA miata
13b REW PP with S5 NA rotors (250hp goal)
Miata 5 speed gearbox with FC bellhousing.
12a front cover for front engine mounts

I have been thinking about this for several months and there is one thing i can't figure out.

What i know : Miata 5 speed gearbox will match FC3s bellhousing. FC3s bellhousing will match S4/S5 rear iron (i would need to shorten the gearbox input shaft).

What i don't know : will S4/S5 rear irons match an otherwise stock 13b-REW ?

Because of the position of the engine and because i want to keep the stock miata gearbox, i know i need to use the FC bellhousing.

You might ask : why not go with a S4/S5 engine ? Because for a high reving NA 13b, i would need hardened stationary gears, i would want better chrome coating on housings (which come in S6 and later) etc... The less old the better. Plus, i can buy a rebuildable REW for good price.

a few things ...

1. the bolt pattern for the transmission is pretty much the same for all rotaries. i know the automatic REWs had a different rear housing, but given the fact that people do manual tranny swaps, i tend to think it's not a factor.

2. how are you swapping bell housings? i thought the Miata tranny was related to the smooth case (Type M) trannies. i know people swap the gearsets between the Rx-7 and Miata, but never heard of a "simple" bell housing swap.

3. the REW is a good block to use, but you could get hardened gears with an S5 Turbo block (though i'm not sure about the 3-window bearings). coatings probably aren't going to matter much at this point unless you buy NEW.


as for you actual question ...
it would bolt to it, but you would need to modify the port runners and you would also lose the ability to bolt a manifold up without some modification (how much modification? i don't know.) it's better/easier to leave it alone or swap front and rear as a pair.

wait ... forgive me ....

i totally was not thinking about the fact that you're building a PP. :( side ports will be blocked/filled and manifolds won't be needed.

check out everything else that i said though. you shouldn't need to swap rear housings at all and look into that bell housing swap because there are a few threads (between here and some of the other rotary boards) that show how to transfer the guts and how much to grind off the input shaft afterwards, but i can't recall any that simply swap a bell housing. i suppose in a way that's what you're doing by using the FC case, but i don't know if that's what you actually meant.

the bellhousing bolt pattern is the same from 1974-2012. the automatic 93-012 engines are missing a bolt to add a starter, doesn't seem to cause any issue converted to manual.

the ports are different REW to B-T, but if you're not using those its not an issue.

250HP is a low goal for a peripheral port 13B. That number is achievable with side ports and careful tuning.

Thank you all for the inputs.

Diabolica : That is great news. I will simply PP my REW then and plug it to the tranny. Other than the S5 rotors, it'll be full REW.
Aparently the bellhousing from the FC gearbox (i don't remember if it's from NA or turbo that splits) is plugnplay on the miata 1.8 gearbox. The input shaft mod is findable on mazdatrix site.
I do want to keep the miata box because i will keep the PPF, and moreover, the gearstick will already be at the right place.
I will have to hammer the firewall a bit though to fit the engine that far back but no biggie.

j9fd3s : plain and simple, thanks :)

Aaron Cake : I want to go PP because i find it easier to build properly versus a good bridge, where you have to have a steady hand. Plus, with a PP with round holes, it is easier to get evenly ported housings. A good jig and off you go.
A PP motor will outlast a bridge in my mechanical engineer opinion.

I said 250hp because of the noise that i'll have to cut down, hence the lower power output. But i will build it the right way so if i get more, then great.

my P port was the first one i've done (#2 goes together next week), and i'm totally happy with it.

re the 250hp, it depends on where you're measuring the 250hp... it also misses the part where the PP makes lots of low rpm power.

my 12A PP makes more power by 3500rpm, than a stock port 12A engine ever does. it idles @1000rpm (i've had it as low as 700, but the carb isn't happy). it starts hot and cold, just like stock, and it even gets better gas mileage at the track, than a 13B 6 port.

noise: noise is a problem, the exhaust noise is basically the same as any other rotary, however the intake noise on the PP is much louder. if you have any sound limits where you are, you will want an airbox of some kind.

other than that the PP is a piece of cake, its the simplest setup.

well, considering i have not played around with a Miata tranny (yet :)), this is just speculation at this point on my part, but my guess is you can basically do the Miata-to-FC gearset exchange and then simply add the Miata tail housing to the FC case. depending on how the engine is mounted, that should keep the shifter in the stock location for you.

as far as i know, the Type R (the ribcases: Rx-4/5/REPU, '86-'02 (?) Turbo rotaries, and B-series trucks) are the ones that have the removable bell housings. again, i believe the Miata (Gens I and II) trannies are (or are at least related to) the Type M, which do not. however, it's best if you confirm it with someone more sure about it than myself.