Remember my thread from 2011? https://www.rx7club.com/1st-generati...-twist-942480/

Here we go again!

No twists and turns this time. Just straight up awesome!

I'm in the planning stages for some engines I'd like to build next year and low and behold it turns out I have enough parts once again to do another half BP!

Parts list:
•RX-2 separate runner intake manifold
•Racing Nikki (26mm primaries and 31mm secondaries, because it's huge!)
•already hogged runner Y plates by Hayes with wannabe 74 spec ports weakly attempted (will get corrected out to true 74 spec and get a pair of eyebrows cut in the secondaries)
•rotor housings with a little edge wear
•Atkins apex seals to deal with the edge wear
•static balanced 83-85 rotating assembly with stock flywheel (to smooth out the rough edges)
•multi-hole stat gear bearings already installed and broken in

The eyebrows will get cut according to my previous findings in 2011. I'll need to go research what I did before.

The exhaust ports will be hogged out to similar timings as before, but this time a little bigger. Maybe out to 12AT spec or something.

Semi-locked if that's what it wants. Otherwise it'll get a stock or nearly stock dizzy. I've got four different stock timing curves to play with ranging from N201, N249, N304 and the rare N305 or N306 (hard to read through the grime).

The exhaust system I'll test it on will just be an RB long primary, which according to peejay, is not restrictive enough to worry about on his BP 13B, so I won't need to worry about it on a 12A.

Air filter will probably be an RB spun aluminum or a stock blue canister. j9fd3s says the stockers aren't all that restrictive. But at this point who knows what it will want. I haven't had a chance to build it and test it yet. I'm just in the planning stages at the moment.

 

Update time!

•different intake manifold
•different carb
•turbo

It turns out the knife edged channel connecting both secondary runners up under the phenolic spacer in the RX-2 manifold would be detrimental to a high overlap engine like a bridge port so I've decided to go with the totally separate runner early RX-3 manifold instead. This is similar to the sep runner RX-2 manifold I used on the half BP back in 2011. Only difference here is the previous owner of the RX-3 mani port matched the primary runners out to the tall runners in a Y intermediate plate. This is a major no-no because it removes the anti-reversion lip which can hurt low end torque and drivability, but it's all I have to work with so I'll go for it.

Likewise I've also decided to go with a hogged out boost prepped Nikki that's about the right size for 74 spec ports on a 13B at 25.2mm which works perfectly on the brown car's 74 ported R5 13B, so it should work similarly as well on a 74 ported 12A, and may cover for the no-no port matching. The only difference compared to the Racing Nikki is that the secondary venturis will remain at their factory stock 28mm as this is a boost prepped carb and I want all the vacuum I can get out of them for when the secondaries open to hopefully prevent a delay/hiccup/bog under boost. The Racing Nikki is all the way out to 31mm and would be fine for NA, but I can't trust it with boost. The way I figure the boost prepped carb is going to work is it'll be a little small for NA use but should still hold its own for break in etc, and then once it gets a turbo, the carb size becomes largely irrelevant anyway (in fact smaller is usually better with boost) so it should work great!

The turbo may be a stock S5 or a small aftermarket. Either way, it should work fine seeing as a bridgie can drive a turbo really well.

I'll save the Racing Nikki for other projects.

 

Just a simple question, how much does it cost to take apart the whole engine to port it?

Meaning, the cost of seals (ones to replace vs ones that can be reused). I have never taken apart a rotary motor before but very familiar with taking apart other engines.

I am very interested in how this half bridge port works! I was thinking of doing a full bridge port but figured I would have to ditch the carb setup to go Fuel injected, which is not out of the question as I know megasquirt pretty well but want to play with the Nikki carb just a little more.

I just like the Idea that with the right know how, stock parts (Nikki Carb, Factory Intake, Removing material) can result in a performance engine.

 

^^^bout $500. That's if you don't need housings or rotors or bearings. Could be over $1500 if you can source the parts.

If you follow jeff you'll have to hogg out your nikki to meet air and fuel demands.

 

Well of course. Every 12A needs a hogged out Nikki.

 

Soft seal kit is about $120 but you won't know until you crack it open which hard parts will need
to be replaced depending on specs. Hard parts include apex seals, oil seals, all springs etc. Its
assumed the rotating assembly, housings and side plates are all good and in spec. A full soft
seal and all hardware can run you about $1K or so depending on where you source it all.

 

Jeff, I just finished installing my 30mm secondary Nikki on the 12A.

For reasons unknown to me, it required a slight adjustment of the air screw on my 74 body, but it RIPS.

I left everything including the jet size the same as my 24.5 primary carb.

Wideband confirms the engine is getting more air at WOT, leaning to around 13.2:1 stable@redline, and then instantly goes rich after 7200 RPM, which indicates that my tiny intake ports are holding it back now.

This will be going on my s5 6 port 13b, (maybe with t4 turbo power )

Will keep you posted, Hope this motivates you to try even bigger Nikki's on that bridgey

 

Now I have some time to respond.

One thing you will need to consider, once you go turbo, is how enlarging the secondary venturis could affect vacuum signal. I have not done anything to my secondaries yet because any enlarging I could do would reduce vacuum signal in exchange for higher CFM. I believe that with a turbo, CFM is not really a problem as the turbo will force feed whatever CFM it's making, through the carb and into the engine. What I'm concerned about here is that the reduced vacuum signal just might cause the dreaded secondary delay I've been fighting since day one on these hogged out+boost prepped carbs.

I've got some key issues to point out:

•I've not ever tested a stock venturi carb with boost. I've driven one with the carb hat on it and the turbo corking up the exhaust, but didn't actually go into boost as it was a stock rebuild only (and I wasn't about to take the turbo off just for a basic drive test).
•From the videos I've seen of stock venturi carbs used with boost, there is only like a split second delay or rather a "hiccup" only when you floor it, not an actual delay of 1 second or more as I've experienced on carbs with stock air bleeds and hogged primaries.
•It seems hogging out the primaries causes a delay until you correct the air bleeds. But you can only go so far until you hit the limits of the carb, as I seem to have discovered through trial and error.
•I have not ever tested hogged secondaries with boost other than the ones in the racing Nikki, which being an SA carb was so terrible for boost in other ways, I aborted the test and didn't explore it any further.
•Of course your mileage may vary.

As for keeping me motivated, yes I may try a set of 30mm venturis on this half BP, unless it's getting a turbo, at which point I'd seriously weigh my options but I'd definitely start with stock secondaries.

 

Major update and good news!

•Decided to go with my original carb choice, the racing Nikki. Did 24.5mm primaries and 32.7mm secondaries for good vacuum at low RPM on an engine which has none, but will still provide enough size to not restrict it on the severely hogged out but sep runner intake manifold and hogged intake runners in the irons. Tested it with boost* on my 13B and it rips! This will definitely pass enough air for a half BP.
•finalized the irons, housings, rotating assembly etc I will use.
•Deciding whether to go with OEM apex seals or maybe Atkins.
•might try an S5 NA flywheel on this thing because a) it is designed for S5/S6 13B rotors which weigh virtually the same as S3 12A. Only weighs 21 pounds which, along with the rotors, comes in at around 40 pounds which ought to drive well and not be too heavy or too light. And we know it can handle the higher redline of S5 NA engines.

*had to swap an FB top on and do an FB tune on this SA carb, which actually provided a nominal tune both at part throttle (13AFR) and in boost (11.3AFR). Because all of my carbs handle boost and NA equally well, I expect its tune to remain this way on an NA half BP, with potential for a future turbo.

As for how well the 32.7mm secondaries performed with boost, well, I have to say it is not necessary to have such a big carb with boost. For a half BP in NA mode, yes go big, but not on a boosted engine. Of course these monsters worked fine with no annoying sec delay, but the stock 28mm secondaries in other carbs perform fine as well when boosted. A little food for thought.

 

hmmmm


So separated runner intake mani? Im still toying with the open plenum stuff over here.

Had very good success with a stock nikki on my same 12A.

The stocker doesnt seem to have lost any torque, or power on my open spacer mani. I have to do some real side by side comparisons on 1/4 mile times and whatnot, but even with vac secondaries its a very strong engine this way.

 

My thoughts on the half BP subject is you need to keep the primary runners in the mani separated from the secondaries. Only the really old school manifolds had sep runners like this.

Thus you need REALLY big runners and REALLY big venturis to make an intake configuration like this flow enough to work and have power.

However because the half BP will have lousy vacuum signal at low RPM, I didn't want to go too large on the primary venturis because then it would be lousy to drive. So I chose the same size you went with at 24.5mm just to sort of level the playing field. We shall see how it goes. Of course I'll post pics of everything as I work.

As for the secondaries, I went over the top on these at 32.7mm. I'm never doing another set this large again. Sheesh. Think they'll be big enough? Especially on a sep runner mani?

One more question. What about an intake manifold that had both secondaries channeled? That's what the PO did to it. He hogged out the runner and created kind of a knife edge up under the phenolic spacer. Peejay said to not use it on a half BP, but I think I want to use it. What are your thoughts?

 

I would try it. If it doesnt run right, its just a manifold swap.

I would think it wouldnt be a bad thing for it in terms of airflow demands, but there could be some bridge port overlap black magic I dont have experience with at work there.

Im still waiting to see results on a 13B with nikki on the dyno. I really want mine to be finished already >_>

 

Thankyou. I've got three hogged out old manis: RX-2, RX-3 and '79. Only the '79 has had its stock channels widened and opened up as you've seen before, and would do well on a big streetport or maybe a full bridge, which I intend to use it on (probably just a streetport for now as the engine is already together and test fired with zero miles on it so far). The RX-2 and RX-3 are still sep runner, but the RX-2 has the knife edge channel added on the secondaries only. All three manis have gaping primary runners so low RPM velocity could be a real problem for daily driving duties, however I have test driven the hogged 79 mani with one of my hogged Nikkis and it did well enough on a basic 74 ported Y plate 12A, so perhaps it's fine. It wouldn't be hard to cut basic 79 style channels in the left-over mani when my half BP testing is done.

The big streetported 12A with the big channeled 79 mani will get a Nikki with 24.6mm primaries and some semi-hogged secondaries at only about 28.5mm, so not much change over the stock 28.0mm size, but the shape has been reworked*. I built this carb alongside the other one, hence the similarities. I chose this venturi size combo here due to the open plenum spacer and the way the 79 mani was channeled. It's my hope this will make the carb seem a lot bigger, like you are thinking. Thus a huge carb is not necessary, so I can avoid all its drivability problems etc. I think this setup would flow well on a 6 port 13B like you were thinking, so it should be enough for a big streetported 12A on a hogged 79 mani.

By the way, both carbs are '79 with an FB boost prepped tune but won't handle boost with a 79 top. They would require an FB top for that, according to the testing I've done thus far. More testing is needed to double check whether or not the 79 tops can work with boost but my testing so far says no, on that.

*they are a pair of crappy SVT secondaries I got out of Jamie's carb last year. I have already tested a set of these crappy secondaries in Jingkun's carb, and it did well along with a pair of my primaries swapped in, so here's to hoping these things do well in this carb for a big streetport. They only work as secondaries due to the fact secondaries only open when there is already a lot of air flowing into the carb, so it gets over just how badly these are cut. If they turn out to be a problem, I can always get back in there and recut these into something I would be proud of, but for now they were just sitting and I needed to get this carb buttoned up. Well, I needed to get away from carbs and back into engine work, like yesterday. Last week. Last month lol. I'm way over time here.