So we have a good looking 91 vert in pick n pull around here and my brother and i are probably about to raid it. We are thinking of pulling the engine and transmission. i am about to pull my transmission to do a new clutch and replace my rear main seal.

So the questions I have are how would the performance of said transmission be on a 12a and what all would i need to pull to fit it on my car? how much would it affect gas mileage and acceleration and would it be positive or negative?

 

It's not a direct fit but it is a bolt-in in the sense that you don't need to do any fabrication or purchase any extra parts.

The engine side is fine - bolts right in. If you have the beehive oil cooler, there is a breather on the transmission that will need to be removed to clear the oil line.

The tailhousing is where you need to get dirty. On the S5 trans, you need to remove the tailhousing/shift tower assembly, the speedometer drive gear, and the damper mass. Then put all of the equivalent parts from the FB trans onto it.

The damper mass will not fit under the FB tailhousing, the drive gear will not work properly with *some* (but not all) FB driven gears, the FB tailhousing will bolt to your existing transmission crossmember, and the shifter will come up in the correct spot.

If you wanted to make a crossmember and hack your floor, you could just put the trans in as-is.

There's no real gain to be had, other than being slightly easier to find than 12A stuff because it's ten years newer. The gearing is so similar that you wouldn't notice a difference, except for 5th, which you may notice is too tall to be useful, depending on what year your car is. (Some later 12As had tall 5ths, unlike the earlier cars which had usable ones)

I think I've fairly well proven that carbureted stockport 12As get best fuel economy when cruising at 4000-5000rpm...

 

Im with peejay on the stock gearing. The 12A likes to spin, and the gearing helps it be more peppy all the time. I used to think I wanted longer gears until I drove a turbo 12a with a t2 trans. Wasnt fun.

 

Thanks for all of the info peejay. that makes a lot of sense. and is very understandable. I have noticed that my gas mileage seems to always be 22 mpg on the free way between 55 and 70 but around 75 to 80 which is around 4k rpm in my car it seems to hop up to about 24 to 25 mpg. I would try cruising higher than that to test but we have too many police around here looking to pull people over. I thought that was related to having the short rb headers on my car originally until i started reading up and others seemed to have similar results. It is also quieter at around 4k rpm as opposed to 3k to 3.5k? (unless i am flooring the gas pedal)

i am thinking about having a shop around here just do a rebuild of my trans. they quoted $500. The guy said he would give me $100 off solely because he is a supporter of nice antiques and he says a manual transmission rebuild would be a nice break from all the darn automatics he sees now days.

However you say something that raises another question. I thought all 12a's had the same gearing ratio through all gears in manual transmissions on the rx7's? or are you talking about all 12a's in that post?

 

Transmission

 

ah cool link. thanks for that. i haven't come across that the many times i have been on mazdatrix. XD

So why would they put a taller gear on 5th in the s3 if it got worse gas mileage? maybe it got better gas mileage at 55 mph which would have been the common speed limit at that time?

 

I don't really know. The 4 speed models got the same highway fuel economy as the 5 speed models, too.

It could be something as basic as emissions. The OEM emissions testing is done on a precise drive cycle, so it is standard operating procedure at all OEMs, especially back then, to only make the emissions controls work during THAT part of the engine's envelope. Thus why the air pump will divert to atmosphere at a certain engine speed, it's to save the catalyst by letting it "go to sleep". That engine speed is never seen on the Federal or California test loops. So maybe lower cruise RPM means they can divert earlier and thereby allow the cats to live longer, or alternatively they can use cats that will heat up faster (most emissions even back then was during warmup, faster warmup = dramatically reduced emissions since they are graded as total emissions not PPM) but maybe burn out faster too, so it's a way to compensate.

OEMs do things for reasons that may not be immediately obvious.

Note, too, that the 4 speed was discontinued at the same time the thermal reactor emissions system was...

 

Ummmm 4 speed auto or 4 manual from a SA? Cause NO FRIGGIN WAY IN HELL the 4 speed manual gets the same gas mileage as a 5 speed. Hell you red line the damn thing at 80 mph lol

 

actually at 80 in 4th it is around 5k.

but the thing is the speed limit when these cars were made was 55 so yeah the gas mileage wouldn't have been much different.

so in theory at 70 i would get better gas mileage in 4th than 5th?