After getting a lot of pm`s about my 12AT, i thought i would share some of the info i have, and of course, take some pictures! The 12AT was available both in the RX7, and the Cosmo in Japan. The early engines produced 160 HP, and the later ones 165. My Japanese registration says 165, so all my pictures/info is from this engine. The difference is basically just the turbo..

BTW, i suck at photos..

ENGINE

First of, lets start with the engine basics. Its a 12A, so its still 1146cc, but it has its quirks. Im guessing it shares the center iron with the -SE 13B, but we never got those, so its just a guess.. It has bungs for the injectors in the center iron. The ports in the center plate are really small! Front and rear iron has the same ports as 83-85 12A`s. Front iron has the same oil outlet (for the turbo) as the 13B. The big difference is the housings. By the first glance, they look like other 83-85 12A housings, but there are a few differences:

- Since its a EFI engine, it has oil injection holes (like the later 13B) on top of the housing. Only the 12AT and the 6 port 12A have these.
- It has bigger exhaust ports than a normal 12A.
- Its said to have better chrome than other 12A`s. Seeing that my engine had about 60K miles on it when i opened it, and not a sign of flaking or wear, i can believe this..

Centre iron, compared to a old school R5 iron:





I also took some pictures of the housing, compared to some other 12A housings. The one on the left is the 12AT housing.







What you cant see, because i glass bead blasted the outside, is that the Mazda logo on the housing had a black background. There are also a couple of other hints that should be easy to spot on a genuine 12AT:

- The clutch fan is orange, not white like the rest.
- The front cover has a return line from the turbo.
- The oil pump is the large one, same as the 13B
- It has 2 piece steel apex seals.. Sorry, no pictures, could not find them right now..

 

INTAKE/EXHAUST

The intake looks like the carb one, but with a couple of differences. It has basically the same external gadgets as a TII, but its as stated above one piece with a carb looking throttle body on top. The TB itself i a 3 barrel, that divides into 4 runners in the intake. One single barrel for the centre ports, and one for each secondary, much like the later TII versions. It does not however, have any vacuum operated barrels, like the later. Enough said, picture time!

 

Now for the turbo! Its basically a smaller version of the later TII units. It has a relativly small exhaust, to aid spool. Its been a long time since i ran it stock, but i seem to recall that it reached full boost very early. It kinda died with the revs, but it didnt boost very much either! I dont remember exactly how much it boosted, but i dont think that it was more than 5-6 psi.. I also snapped some pictures of the manifold, its similar to the S4 TII, undivided.















And of course, the famous red turbo hat! Its just a pipe going from the turbo, to the TB. It has an air temp sensor, and a some sort of pressure valve..

 

EFI

Yes, as stated above, it is fuel injected! It is a old school system, with two ECU`s, one for fuel, and one for emission. Spark is handled by a dizzy, that i could not find right now. Ill come back to it later.. Its a mass air flow system, like the later TII ones. It has a MAP sensor, but its for the boost gauge, and not connected to the ECU, like some believe. Like the TII it uses a flapper door style sensor to determine how much air it gets, and how much fuel it needs.It was also one of the first engines to have a basic knock retard system. I have never bothered to look into it, but if i have understood correctly, it uses a resonatic sensor to detect ping/knock and retards ignition. Pics of the ECU`s:










I know there has been a lot of discussion about the injectors. Well here they are, part number 195500-0830. As you can see, they are light green and Denso style. When i flow tested them, they flowed about 750cc`s at 3 bars. These are the original ones. Again, sorry about the lousy picture quality!





Thats about it for now, ill get some more pics/info when i have the time!

 

What is the compression ratio of the 12A turbo?

 

Great! I'll archive this in a few days.

 

Nice work man. Great info.

 

The rotors are 8.5:1

 

Everything I've been reading say they have 800cc injectors, but they only use 2.

 

is the 12a turbo carbed?

 

no

 

The throttle body does look like a carburetor, but it is fi.

 

the TB looks just like one from a GSL-SE only mounted vertical.

the turbo appears to be very similar to a T2 unit, except that it doesn't have water cooling and the turbine housing is different.

 

Thank you for the super valuable info on this very rare engine!!


I vote archive archive ARCHIVE!

 

Very nice Morten! Thank you for this (no doubt) valuable information.


.

 

i would love to bolt that turbo on my 12a

 

great info....

and im glad to see mortenf is still around these forums

i always loved his car....

mortenf, years ago you were loosing "loose head bolts" as you called them on 4x108 wheels....are they still holding up? or did you change your setup

 

Changed the whole car! Running ReSpeed conversion kit, and 5 lug Watanabes..

 

Wow, great writeup. So, can you run the 12aT with just the injection computer?

 

the valve on the side of the orange hat is called a pop off valve. it releases air once it gets to a certain pressure.


also the exhaust ports are huge from the factory on the 12A turbo motors. they come streetported from the factory!!
the intake ports are tiny though, we used to call it the jellybean port, lol

 

Very interesting info, thank you. The compressor side of the turbo has the same cast number as the S4, HT18S.

 

^^ the 300zx turbo cars had that I believe.

Somebody should attach these pics to the thread though so that there is no risk of losing them. we lost a lot of pics in the 2nd gen section when Icemark died.

 

Also, looking at this I now realize how many advances were made in the 13B turbo engines, especially compared to the series 5 turbo cars.

The series 5 had

--Intercooler
--An upper intake manifold with a plenum chamber to take advantage of pressure waves as the intake ports open and close
--Fully divided exhaust manifold and turbine housing
--electronic boost control
--larger intake and exhaust ports
--electric OMP, except it is failure-prone
--faster more simplified ECU
--injector staging

 

The s4 turbo says HT18S-2S

 

is it bad a$$ bein fuel injected? i would love to do mine turbo