Street-port 12A or stock-port 13B?
 

I am trying to build a fun, somewhat fast streetable machine out of my 83 GSL, and on a budget. I have a 12A in my basement with a blown side-seal I was wanted to rebuild and street port. However, someone I know is selling a freshly rebuilt 13B (from a FC) for a reasonable price. What would the pros and cons be of each?

Do a 12A with a hogged out Nikki. Forget about the 13B because you will have to deal with EFI or have to get some kind of aftermarket intake manifold and carb. Then have to put up with tuning either system which is expensive and time consuming. Then if it's a Holley or Edelbrock, you can't even autocross it. Fuel slosh will ruin your day. If weber, say goodbye to your secondaries, if you happen to like secondaries like I do.

When you port the 12A, do 74 spec ports. Don't go any larger because you will lose your low end. Do not get the irons lapped. If you do, you're just shortening the engine's usable life and it leads to smoking at startup and other increased wear of side seals and oil seals etc. Lapping went obsolete in 1979 with the advent of nitrided irons. Don't ruin what Mazda ingeniously invented. Only crooked shops recommend lapping/resurfacing these days. Why? Because it costs you more. Don't waste your money. Leave them alone.

I'm going to embark on a nice 12A project next year. I will port the factory nitrided Y irons to 74 spec and open up the exhaust ports by only cutting upward a few mm to mimic 12A turbo spec ports. All US-spec 81-85 housings already open at T2/12A turbo spec from the factory so no need to port downward. It will get one of my hogged out boost prepped Nikkis to run it NA which is fine seeing as I recently discovered that a boost tune is probably at least 80% compatible or more with an NA tune, if not 100% the way I prep them. :) Of course I'll test it with a wideband on an RB long primary. Maybe a light steel flywheel but some lighter options like aluminum might be available. Or a stock flywheel if necessary. Direct fire of the DLIDFIS variety and some kind of air cleaner assembly will round out the package. It will make a very affordable and very quick streetable machine that should have came from the factory like that.

+1 What Jeff says.

Thanks for the tips! I was just going to send my housings to the machine shop with the RB Street port templates. I don't know what 74 spec means. Is that like the 12A from the RX4?

^^ yeah 74 was the best street port Mazda did... in 1974 before fuel was an issue.

Man, 13b with the hogged nikki!!! and Yeah, down with the edelbrocks & holleys.

RB street port templates are too big for most applications.

74 spec is the same as stock T2 and FD secondaries. Grab one of those and make a paper template.

i like this. i have a 85 7 with a 12a that is street ported with a weber IDA 48 and a full RB exhaust and i can tell you its like a bat out of hell.