I'm trying to find out if my center iron is from an early tall-port engine (Rx4 or REPU), or if the builder just ported them out to that taller style. I also have an old RB 13B 4-port intake that matches the tall primaries perfectly, but I think it is also ported. Here's what I know and some pictures...

Builder/seller told me it was a combination of 84/85 FB engines - a GSL-SE 13B reassembled with street ported 12A irons to make an early-style 13B 4-port.

I bought an old Racing Beat 2-part DCOE/DHLA intake at the same time from a different seller in the same part of Florida. The ports match up perfectly and I didn't think anything of it, until I went to install a modern RB IDA intake. Now I think these might have been from the same build, even though I bought them separately.

The RB IDA intake runners are Tiny by comparison, and not enough wall thickness to port. Here are pictures and details..


The port heights are 38-48-38 mm.
Standard 13B 4-port gasket is 38-35-38.


This RB DCOE intake looks port-matched to the engine with 38-48-38 mm openings. The RB IDA intake has smaller 35-33-35 openings and would work fine with the standard gasket.

 

So I'm trying to figure out if these irons are from a 12A and were just ported bigger when the street porting was done, or if they're from an earlier tall-port engine.

The front and rear Irons are "Y" castings, which I thought was 12A. But the water outlet on rear iron is just a press-fit stub instead of a hex fitting Y like my stock FB.


I don't see any letters cast near the oil tube on the center iron, so not sure how to ID it.

 

Im not sure if the DCOE manifold was just hugely ported or was made for a tall-port engine. The lower manifold is long out of production, but in pics the ports look smaller -



But they also are cast much lower on the outside/longside than RB's current IDA manifold...


So I don't know if it was cast for tall ports or cast with plenty of meat so you could port it for tall ports, if needed. RaceCast in Australia makes them like that, so you can port them to suit your needs. Next two pics are of RaceCast 13B 4-Port manifold, showing how thick the casting is around the runners.


They are cast for "normal" ports but could easily be opened up for tall ports and still have thicker walls than the Racing Beat intake.

**PORTING** I know when the subject of port matching comes up everyone says not to do it because of reversion and velocity loss. I have read those discussions with flow testing results by Paul Yaw, RotaryGod, Jeff20B and many others who I respect. But both RB and RaceCast tell me that doesn't really apply to Weber since they are "always on" and flowing through all 4 ports. The small primaries on a 4 bbl (which is what most port-match threads are about) affect low speed throttle response and vac secondary operation, but go out the window when using an IDA or DCOE. This is what I'm told anyway - I am not able to flow test myself.

Another arguement i read is that many/most of the tall port irons actually squeeze down again as they head towards the rotors, so the large area where the mani and iron meet is just a big velocity robbing pocket. Well, on my center iron the 48mm tall ports stay tall as they run into large street port. They don't squeeze down much, so I don't think I have that issue. It was hard to get a good picture but I can see light sanding/grinding/polishing marks in the iron runner from the port job.




So anyway, back to my original question.....

Do you think these are ported-out 12A Y plates or are they from an earlier tall-port 13B?

Sorry for the long story, just trying to get the right manifold for the job (I'm committed to using an IDA at this point).

 

Well after all that typing and picturing I feel like an idiot. I looked up 12A irons and they look just like mine. So I can believe that I have 12A, street ported irons like the builder told me.

But why do the aftermarket 13B manifolds have such puny port sizes? 33mm vs the 12A's 48mm primary ports? Really?

 

Yes definitely 12A nitrided end housings which will show the "Y" stamping which look to nicely massaged and cleaned up . I used to import the RaceCast ( Simply Maz ) manifolds and still have one in use. So you have the choice of match porting the intake runners identically to the ports on the engine if you desire max flow - all out power. Or you can take a cue from Mazda and retain the smaller primaries on the manifold and let them flow into the taller ports creating a venturi effect at low speed ( improving torque) . But considering you plan on a 48IDA , you are essentially feeding all four ports at the same time ; venturi sizing and jetting will play a big role in your power band . I have a similarly built engine and run Rotary Engineering's dual 36DCD Weber set up currently which I find to be ideal .