Advantages of 12A over 13B?
 

So I was discussing my plan on the RX-7, time and funding providing (Ugh... I hate those things. They dictate EVERYTHING... :( )

One of my plans was to pull the 12A and street port it, (Provided it isn't trashed... Ran when I parked it... Two years ago.

So someone said, "Well since you're pulling the 12A, that's half the work of dropping a 13B in it..."

So I got to thinking... What advantages does the 12A have over the 13B?

Both can be carbed or EFI, so that doesn't matter... Both can be N/A or force fed, so that doesn't matter...

12A doesn't have VDI or dynamic chamber or anything.. Not sure how important that kinda stuff is...

What about weight? Do they weigh considerably different?

If the 12A housings are in fact trashed when I tear it down, should I look into another 12A or a 13B?

If housings are trashed, just get 13B housings, oil pan, e-shaft, and exhaust manifold. Also give irons a race port and housings a street port. Tah-dah! An SP 4 port 13B! Not sure about the intake manifold. Havent quite figured that part out yet.

12a parts are more common and cheaper. 3mm apex seals, 13b only had in 85 and older.

13B parts are plenty common... i'd think 12A parts would be less common now for sure, since they finally discontinued making parts for them... and there were 13B's in the 70's.

it is easier to find a 12a than an old 4port or se motor. since 80+ percent of 1G's came with 12a's you would think they would be more common. unless you want a crappy 6port 2G motor with 2mm apex seals. i am talking about motors. parts you may be right, more 13b stuff available. i should have said motors not parts in previous post, my bad.

its much harder to find a 12A in really good condition, most have a ton of miles on them.

13-B will give you more top end than a 12-A.

Hey pele...i say that if your 12a is in decent condition...do a 1/2 BP!!! I am in the process of rebuilding my new, self-ported, 1/2 BP 12a. I just painted the irons and housings last night...and im planning on putting it all back together tonight. I did the whole project on less than $100 ;) But this is probably not a great idea...i just have 0 income. PM me if you want more info on this :D

With a 1/2 BP 12a, holley 600cfm, RB high rise intake, and custom 2.5" exhaust...im expecting around 170rwhp...

although the 12a is still a strong engine, there's no replacement for displacement. The 13b will always have more potential running the same type of setup. But you can still get some huge numbers out of a 12a, so do the best thing for your budget. If the rotor housings are the only thing thats trashed, id just replace the rotor housings. Not sure what good used ones are going for, but i would think you could find a set of two good ones for under $400.

12a's theoretically will last longer. They have more bearing surface area in relation to the rotor width than the 13b.

This is secondhand knowledge for me and I cannot elaborate much more, but it makes sense. Please correct me if this is not right.

ummm.... you forgot about the rotors.

if you want to know more about 12a potential get a hold of SS124A

lol, i just thought it was funny you used that line for comparing a 12a to a 13b.


BP 12A !!!!!

both have ups and downs. Some of the ups of the 12a are the lighter rotating assembly, quicker throttle response, 3mm apex seals, less rotor tipping than 13b, less weight/size, cheap parts, larger port area (intake) per displacement vs 13b... thus potential for larger volumetric efficiency....


just a few thoughts

Just thought of another one. Power/weight ratio of just the engine. How much lighter is a 12a compared to a 13b and how much more power does the 13b have to make to stay on par with the 12a power/weight ratio?

i don't think a 12A is considerably lighter than a 13B... slap me if i'm wrong, but 20mm really isn't a large difference... especially when with a 13B u can get some pretty light weight rotors... only big difference would be eccentric shaft... rotor housings wouldn't be a huge difference, ur not adding a ton of aluminum...

advantages? cheaper i think, fuel consumption?

The weight differance is for sure isnt noticeable when picking them up. I dont think that should be a worry maybe max extra 20lbs i think at least what it felt like. You shouldnt be worried about the engine itself when thinking of power to weight though.. Think of the rest of the car and all the usless oveweighted metal used there.

ultimatly a 13b motor can make more power, but as far as motor durability goes..... 12a and 13b motors use the same size main bearings as well as rotor bearings , so 12a is stronger... also the stationary gears stick out farther on the B motors , they will break before the 12a motors too.. 12a motors are smaller so there is less motor flexing too..

The 12A also has shorter apex seals which may be the reason that they seem to be able to withstand a bit more abuse under boost. If I was building for boost or high-RPM the motor would be a 12A.
Grant

The price.

That's it.

MY main advantage of the 12a is that you can say "I have almost 1.2L of displacement, and I kicked your ass!" or "My GS beat your TII" (if you reallywork it).

If you build a 12a right, it can be a reliable "beat the shit out of me and I'll come back for more" powerhouse. It's not lacking *that* much compared to the 13b.

If you have a 12a right now, you have to think that by switching to a 13b you'll have to change intake and exhaust manifolds, engine mount, and a bunch of other stuff.

Stick with the 12a. I'm always going for the victory of the smaller guy.

Jon

I havent seen a 12-A streetported n/a that can beat my 13-B streetport. :D

lol thats an interesting way of looking at it i guess... the engine mount wasn't much of a task tho, i hammered mine flat turns out it was perfect fitment... i think i'd much rather have .1l and 30hp to boot hehe, i can still say my 13B beat ur 5.0 or something like that

True enough. I guess I just fell in love with the smaller, carbureted car. Hell, I'd run a 10a if I had the chance... Of course it would be a 10a BP with custom everything and a 10k redline.... but it would get cool looks from people, especially rotary people who *know* what a 10a is when they see it under my hood ;)

I've just heard a lot of talk about the 'bulletproff' reliability of the 12a, and the fact that they have 4 big intake ports with lots of possibility for expansion.

I drive an Rx7 partly because it's a rare car that few people have. The 12a is even rarer when you look at the fact that almost every rotary before and since has had a 13b (except the r100 with it's 10a). I really wish that there had been more research and expansion on the 12a base, but like everything else with my 7, if you want something better, custom build it yourself.

Either way, my vote is for the 12a because I'm a crazy Canadian nut of a moderator, and I say so ;)

Jon

I hear ya. Im just spoiled and picky. ;) It has to be a 4port 13-B.

first rx7 i ever drove was my bud's 79... it was a blast, but i didn't like his clutch setup... it was all stock, and having come from a 85 GMC 3/4 ton conversion van it seemed super fast to me haha... my first 7 was a 87 TII... fell in love with it... then wrecked it, next was a 88 TII pushing 17 psi in 4th gear, definately could fall in love with that! then got a REPU, it had a 76-78 13B, 4 port of course, and that thing was sweet.... till it lost an apex seal 3 days after i got it.... then got a 84 GSL... my 84 had no motor, so i put in a 12A that the shop owner(a rotary shop in the area) told me was a working motor... he couldn't be further from the truth... got that thing installed, and when i did get it running it overheated(radiators fault) and leaked/burned more oil than i could put in it... so that motor was a dud, it was also very sluggish, very tired... so i got my motor i have now... a 85 SE engine... took it apart, ported it, had atkins assemble it, and now its sitting in my car waiting for a friggin oil pressure sender, after fixing the carb, replacing the starter, wiring a new switch in for the fuel pump since the relay didn't work, etc etc... been a lot of work going into this motor just trying to get it running(and i also have about 9 quarts of oil sitting in my driveway, 4.5 from the oil pressure sender being MIA, and 4.5 from the oil level sender being mia(don't have one, tried to plug the holes, unsuccessful).... so yeah, i'm about to be running my first 6 port... and hopefully its pretty sweet lol...i don't think i'd want to go back to a 12A, especially with how easy it was to get the 13B in(initially, the rest is just a side track lol)... no need really, when i can have that 30 extra hp

I had a SP 4port 13B in my 81 about 12 years ago. It was nice to make new Mustang GT's go :eek: .
I think it would have been hard pressed to get a SP12A to do that..

If my 12A ever goes kaput, I'm pretty sure I'll build a 4port 13B. It might be a while- I've only got 72,000 miles on the original motor..

a 12A is great,when you want to build a high HP ,high revving engine.

The biggest problem with the 12a is : Oil passages.They are not able to flow enough when building a high revving engine.The 13B has a great improvement in the passages,and the oil pump is allso way better.

With high i dont mean 9k rpm.When going over 10k ,it is really hard to keep the bearings lubricated,as the passages arent up to it.

Their rotating assembly is much lighter than the 13B`s,but that only helps to some extent.The thing with the 13B is the amount of torque that it has more ,its easier to get the engine revving up quicker if you have more torque. ( i dont mean low down grunt,but torque in general).

If i had to build an engine,i would build a 12A 6 port. I have seen what insane hp numbers they are capable of na.

Its not just the PP 12A that makes hp....

karis