12a Porting Options - Good References, But Questions Galore!
 

Hey guys, I'm now researching into porting my extra 12a engine and I'm wondering what the best port for my budget and skillset would be.

Listed below are the different pages I found in my research, each line is clickable to a link:

Differences between porting styles

4-port intake porting

Exhaust porting

4-Port Porting Templates - Mazdatrix

Intake Street Port template - $51

Exhaust Street Port template - $32

Mazdatrix Street Porting service

SCCA porting rules

So I'm looking for advice on my porting future.

I'm thinking Street Port right now because it's middle of the road, somewhat cheap and easy.

Would like to get some opinions on what's worked, best for a 12a, those kinds of things.

Thanks in advance guys,
Austin

Street porting is the simplest to do and will not require a lot lot of sporting mods like bridging or bigger ports will.

Start watching some vids on it. pinnaple racing has some on there site, and you can find some on youtube by aaron cake

it may be a little more complex and time consuming (but not really more expensive) but you could try for a small full bridge. like lots of air flow, but not neccessarily nasty amount of timing/overlap


... just a thought :)

I recommend street port for your first shot. Just take your time. My favorite is the Judge Ito aggressive template. Then just decide if it will be n/a or turbo to decide the porting options for exhaust.

I would go street port if budget and streetability is a concern. You can always go back and port more in the future. I used the racing beat templates and watched the Mazdatrix DVDs for mine. Pretty easy to do.

74 spec

if you're set on porting it, you have a couple options.

first would just be to open the ports up to 74 spec timings, intake closes @50 instead of 40, and exhaust closes @48 instead of 38. its a small change, so power increase is small, but it won't impact driveability, and it'll work with the stock intake/exhaust if you need it.

second would be a "street port", the difference from the 74 style is that it opens the intake earlier, bigger jump from stock, it'll make more power everywhere, you could run the stock intake/exhaust, but its holding you back.

third; small bridge. you cut the bridges so that the port timing stays sane, you get the brap brap idle, but it'll work with a hot rodded stock carb. kind of the IMSA RS setup, power depends but it should put down 150-160rwhp.

big bridge. this needs an after market intake, and full exhaust. its going to be lumpy on the street, and loud, potential for 180+ hp. you need to bump the oil pressure and think about using carbon apex seals and things like that too, so its getting to be $$$ to build

This thread rocks! Great suggestions and information.

Do you guys think a medium sized half bridge would be something to consider?

So the consensus is a street port is probably the easiest to accomplish for a first-timer, requires little modification, is streetable, affordable, and makes sense for my situation.

My next question with street porting is what mods should be done to ensure proper flow through intake and exhaust to make it worth the work?

I'm planning on deleting the entire emissions system this winter...again...long story...then I also plan on blocking off the OMP and going to premix, Nikki mechanical secondaries mod, FC FMOC swap, and exhaust system is past due for an RB header to 2 1/2" with two Magnaflows.

Is there anything else I should consider doing?

Almost a thread jack, but I'll build upon it - what mods would be required for a medium half bridge port? I know there would be intake and exhaust necessities to allow enough air in, and a well-flowing exhaust to make it worth the work. I guess the question I have with it is how much more is required to be able to run this port instead of a 74 spec or street port?

Thanks for all the help guys!

74 spec is easier for a first-timer.

Just FYI a 74 spec 12A has less low end torque and driveability than a streetported 13B (old school 4 port R5 specifically). PercentSevenC and I found this out in his FB when we swapped from his nice 74 ported 12A to the 13B from the GLC. A hill which he always needed to be in 2nd gear to go up with the 12A, he could now be in 3rd with the 13B. Yep, even though it's streetported, it had more low end. I suspect due to the larger engine. The exhaust was the same using a stock manifold and a free flow 2" exhaust (tiny!). He later swapped from a stock Hitachi to a Weber DCOE and lost all his low end torque. Then he converted it to blow through with an S5 turbo and gained way more low end than he ever had before, along with all the high end he could handle at the time. Only lacking was the midrange as the 2 bbl carb only had idle and full throttle once boost prepped. We later swapped in an FD T2 hybrid with a MegaSquirt.

Once you start modding for more power, you won't stop. :)

Maybe I can answer your bridge question. I built a successful half bridge last year. I wouldn't recommend it for a noob.

You may remember the thread? Anyway it was tough. Maybe because I'd never done one before. You have to be a lot more careful. And a lot more can go wrong if your intake isn't up to par. I fortunately had an old school stock intake manifold that had fully separate runners. I think it was from an RX-2. The matching carb didn't like the bridge so I had to use one from an RX-3 (glazedham42 got the matching channeled manifold from the RX-3 set by the way - you can find pictures of it as well in his thread). Stange that a channeled carb worked better on a sep runner manifold. Maybe because it boosts the richness - makes it almost pig rich which is required for a half bridge to idle.

The port timing was stock. I didn't bother with 74 spec or streetporting for this. I left them at stock open and close times. The bridge ended up around the same height and as wide as the coolant seal would allow. I had to bevel the rotor housings. The exhaust ports were enlarged to about 12A turbo spec.

The way it drove was let's say decent enough, for me. I could drive it, but not sure I'd want to daily. I tested it with an RB aluminum flywheel just to prove folks wrong that it required a stock flywheel. That would be BDC who recommended a stock flywheel on all his half bridgeport FC engines. So I proved him wrong, but then again the 1st gen is lighter and I suspect the R5 12A I used was a better platform than any FC engine.

It was hard to start. I tried several sets of spark plugs and the BR8EQ-14 were consistantly the easiest to start. Still difficult though. It only had stock FB ignition. I suspect DLIDFIS would have made things easier.

Cliff note: don't bridge or half bridge

the real trick to this is to have a good combination. the intake/ports/exhaust are all one system.

so the 74 spec works well, because it is stock. so it'll work well with a nikki.

the more you port it, the more the stock carb will hold you back. the racers in the 70's ran "stock" carbs with a bridgeport because they had rules, if they didn't have rules the competition book stage 1 is a weber IDA clone

Yeah exactly. I can add something to what I said earlier. If you decide to go 74 spec, I'd recommend modding your Nikki like a Sterling. The enlarged primary venturis will increase low end torque. Heck even my overly enlarged venturi race prepped Nikki has better low end than even a 22mm hitachi. Someone enlarged the primaries to 26mm. They're huge! Sterling only goes to 22mm, up from the 20mm of a stock Nikki. It also worked really well on a 74 ported 13B and a full RB long primary exhaust.

I'd say anywhere from 22 to 26mm would be fine to experiment with. But my carb has some small glitches and flat spots at part throttle cruise caused by the large venturis - the only drawback to such a big increase over stock. Not sure they can be tuned out. The idle circuit is fine though, and full throttle is amazing.

Next test for this Nikki is on a stock ported R5 13B with a questionable exhaust. Should be similar in flow to an FC exhaust manifold, which is ok at stock power levels of an S4 NA FC which should be 145hp at 6k and whatever the torque was at 4k on those things. However the small ports of my engine probably won't make anywhere near 145. It'll tide me over until I can go blow through turbo with the stock S5 turbo PercentSevenC is using, at which point I'll be glad my engine has stock ports because it will have good low end off boost performance which a streetport just wouldn't have. The plan is to swap Nikkis to an 84-85 unit which Sterling said has a better idle circuit and the best boost prep candidate. Something tells me I might not need to enlarge the venturis on this carb, as smaller carbs tend to do better when boosted, but then again if it's easy to do, I'll go ahead because it is for a 13B, so I'd say at least 22mm but what do I know? :)

Sorry to thread jack. Hope this carb info helps you.

Oops my apologies I didn't mean to thread jack, I just figured since drivability and cost are the biggest factors, a hbp might be something to add to the list of suggestions if bridge porting is an option.
Jeff's post a few above this perfectly answered my question, as I got wrapped up in power options, forgetting that for a first time port... Keeping things as simple as possible would be the most beneficial. Next time I'll just keep my mouth shut haha
Also on behalf of lurkers like myself, all this information including the carb stuff is really helpful. Again Sorry to be off topic ill stop now

Nope! I needed the info and I'm glad I got you talking about it! You and j9 have helped out immensely on this thread and I appreciate it, you've given me many things to think about for my build. :nod:

No apologies! I actually liked the question, I was just messing with you. I'm not the kind to freak out over a related question on a thread I started, that's what a forum is for! Good question :icon_tup:

So I'm definitely going to start writing up an engine rebuild list and put it together here soon. Will scour the forum here to find as much info as possible on rebuilding.

Thanks guys, I'll post up some more questions when they come into my scattered brain later!

for a rebuild the least expensive, with best results way is to take the engine apart, measure EVERYTHING, and replace as needed. it actually makes life really simple

I've heard this from a local rotary friend as well, he's willing to help me work on my build when we both have some free time and I'll make sure to utilize his knowledge without fully relying on it.

Another thing I'll ask here as well, not to change the topic, just looking for some more advice.

How many engines should I have in order to make sure the build is successful with parts that will pass spec and continue running well?

I currently have two full engines:
1. My 12a, running perfectly with 86,001 miles on it in my 84 GS.
2. Another 12a, stated as running with 234,XXX miles on it in the 82 GS parts car I just picked up. Spins freely, isn't carbon locked as far as I know so far.

I'm looking into getting at least one more from someone locally for cheap, or free if he wants to trade for some junk of mine :nod: He says it runs but I emailed him to get details.

As it relates to porting - I could use an iron for practice if it's destroyed internally upon cracking it open, doesn't pass specs, or won't be able to be used for some other reason.

Just my 2 cents, but if that 12a is the original with 234k...I would suspect that very little will be useable for a rebuild, but I bet you could get it to run. I once took apart a 12a with almost as much mileage (which ran btw)...the apex seals were as thin as toothpicks and neither rotor housing had hardly any chrome on it. It was slightly difficult to start on a cold winter day, but ran fine once started. Gotta love the 12a...it surely won me over with that experience.

My rx7 is bridge ported i love it. But street port is probably best for your first time. Good luck.!!!

you're right, pull the high mile one apart, and see. you have about a 90% chance of getting a good E shaft, stat gears. the irons are a maybe, but can be resurfaced if needed. rotors are a question mark, but they aren't hard to come by, and nowadays goopy makes oversize seals, so you should be able to reuse any rotor.

the real problem with 12A's is the rotor housings, the are not available new, and even in the 90's i was pulling 5 engines apart to get 2 so/so housings... milling down a 13B housing is an option (5mm each side)...