workaholic

Have a 12A I just took apart and the guts are in great condition! It only had about 113k miles on it but it appears to have carbon locked. Anywho the plan is to port and rebuild it to put in my 83 GSL. From my research, the racingbeat streetport template is a bit much. What about the mazdatrix template? Also, should the exhaust port be enlarged a bit? It seems awfully small.

Jeff20B

iTrader: (13)

Go for 74 spec. It is the perfect street port. In other words, it is a perfect port for the street. The RB port is huge and what they call "streetable" and indeed it is, but isn't ideal.

And yes the exhaust ports on a stock 12A are small. You can go up 2mm and recreate the bevel. This is a fine increase in port area and open time. Do not port it downward as it already opens at a good time (at T2 spec in fact). It just closes way too darn early.

workaholic

Is there a template of the 74 12A ports? I don't know precise dimensions.

Jeff20B

iTrader: (13)

No one sells any, but you are in luck if you have access to a stock T2 or FD end plate. They have the same port timing as 74 spec. Just don't bother porting it downward to match the bottom shape as that was updated in '86. You can make a paper template which will last for a while if you are gentle.

chuyler1

iTrader: (1)

My REPU is running stock porting on a 13b which I believe is the port Jeff is suggesting. Even with a free flowing intake (and it's a 13b, not a 12a), it's not all that exciting when you step on it. We're talking barely the performance of a modern economy car off the line, and that's after upgrading to EFI. I'll be opening the block eventually to make the ports bigger. I certainly wouldn't waste time porting other plates to match what I have unless you are planning to slap a turbo on it.

Jeff20B

iTrader: (13)

The REPU is a big heavy vehicle that pretty much requires a turbo in this day and age. They might have been the pickup with "pick-up" back in the '70s but everything was gutless back then, so not much to compare to. And I'm sure EFI isn't doing you any favors over a carb in the power department. Maybe better drivability? But you also have small NO sized primary ports for the primary injectors to have good velocity. In other words, total port area is less than on a classic 74 spec port size on all four ports.

How much bigger do you plan to go?

chuyler1

iTrader: (1)

The stock primaries probably hold me back a little from having all 4 ports the same, but not by much. The truck dyno'd at 115hp with a 2.5" Exhaust and stock intake/carb stripped of emissions. It's definitely quicker now with EFI but I haven't been back to the dyno to prove it.

I've actually switched focus over to my street and track day 1st gen. I just had a motor built and I'm giving it a similar EFI treatment so I can have some fun at the track. The motor won't go in for a few months but here's a peak at what was done. These are pretty big, but still streetable ports.

j_tso

iTrader: (2)

I traced Turbo II ports a few years ago.

Intake port with TII closing - ignore the blotch in the corner. There's an outline for one dowel pin so you can use that and then align it with the port opening and oil seal line since they're the same as a 12A. Ignore the part that extends further down than the stock 12A port.

Here's the 12A exhaust lengthened to TII closing time.

Print at 1:1 scale.

Jeff20B

iTrader: (13)

I think the stock carb, in this case a Hitachi, is what was really hurting your power. Those things are terribly gutless, even with a whopping 22mm set of stock primary venturis. 22 is too small even for a 12A. 24mm is a bare minimum in my opinion. And some Hitachis came with only 20mm like a stock Nikki. Eek!

Wow, those are some big ports! Let us know how well EFI works with them. Any plans on going with modern injectors? You just might get away with running ports that big with the spray pattern from modern injectors.

As for the template j_tso posted, thanks! That gives a nice visual of what a T2 port looks like. Hopefully it can come in handy for people who don't have access to a spare late model iron.

chuyler1

iTrader: (1)

I'll be running a pair of pico ID1050 injectors. I was going to do 725 but they didn't come in the right size for this fuel rail. I suppose I have plenty of overhead for a future turbo setup.

workaholic

My nikki has the primary venturies bored to 24mm, 120 primary jets, mech secondaries. A/F is about 12:1 at idle and ranges 10:1 - 13.5:1 through the band. I think I need to change the secondary main air bleeds, but that's not a huge deal at the moment. Have Rotary Engineering headers and presilencer with Racing Beat street muffler in 2in pipe. I'm going for streetable but can take to autocross meets. Should have probably added these details when I first asked.

Jeff20B

iTrader: (13)

Go for 80 on your secondary main air bleeds. Stock 140 is too big for mechanical secondaries, and can cause a bog when they are opened below about 4k RPM. I can open mine at any RPM and they don't bog. There might not be any power at 1500 RPM, which is just above idle, but I don't bog either hehe.

workaholic

KYPREO

I had a T2 block running a highly modified Nikki carb with enlarged venturi etc and well thought out long twin primary exhaust. It was pretty uninspiring. A bit torquier than a factory tallport 12A in the low-mid rpm but peak power was pretty much the same. Tried with a 4.44 final drive and it still wasn't much chop.

You will be disappointed if you are expecting great things out of this port size and timing on a N/A 13B.

You can go much more aggressive while remaining "streetable".

sa7

iTrader: (13)

peejay

One of my favorite engines was a deeply street ported 12A with a very heavily modified intake manifold. I removed the shutter valve from an '81-85 manifold and used lots and lots of epoxy to block off all of the channels so that it was a full ITB type manifold.

It wasn't particularly high powered, but I could pull 5th gear from 1500rpm smoothly. I guuesstimate it had about 150hp, with a header, stock downpipe/muffler, and 1.5psi fuel pressure on a completely stock Nikki.

t_g_farrell

iTrader: (1)

Was that deeply ported on all the intake ports or mostly the secondaries?

peejay

All.

I ported them such that the inner line was barely touched height-wise but the closing line ended up massively angled relative to the side seal. My theory was that I wanted to present maximum flow to the chamber's center of area when the rotor was at its point of maximum velocity. The port closing line was as close to a right angle as was practical to the center of the chamber when the eccentric shaft was at something like 180ATDC. It's been a while.

I always wonder now what the engine could have done with a better exhaust system, but oh well. Nostalgia ain't what it used to be.

I ported my current engine to similar theory, and it was pretty sweet. I ported it to conventional theory last month when I rebuilt it, and it appears that I didn't gain anything at the top end and I lost power below 4500. So maybe I was on to something in 2001 when I did that 12A?

Jeff20B

iTrader: (13)

peejay, I wonder what your engine would have done with a bigger Nikki.

peejay

Probably worse... Drivability was excellent. I do wonder what I could have done with different exhaust though