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half bridge 12A, with a twist

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Old 02-18-11, 02:31 AM
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half bridge 12A, with a twist

That twist being a high flow intake manifold from the '70s.

Are those separate runners I see?



The matching carb; so you know it's calibrated to run on separate runners. What this means is its idle circuit and everything else has been factory tuned to run best on a fully separate runner intake manifold. If you used it with a later channeled manifold, its idle would go lean. Likewise if you were to throw a later Nikki on the separate runner manifold, it would idle pig-rich.

So a plan hatched in my rotor-brain. I've always been curious how a half bridge would run on the street if you had a good way to keep the bridged ports physically separated from the other ports in the intake. trochoid has had success on the street because he had a dual DCD weber setup which keeps them separate.

If keeping them separate is the key to success, what about if you happen to have an old school intake manifold lying around from before Mazda started having pri-sec communication? Would that also work?

Oh and here is the new clutch kit and a set of Atkins apex seals I ordered for it.


More pics coming as progress is made. Please feel free to offer up any tips or pointers you have about half bridge ports. Think a Nikki is too small? And so on.
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half bridge 12A, with a twist-colten01.jpg   half bridge 12A, with a twist-colten02.jpg   half bridge 12A, with a twist-colten03.jpg  
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Old 02-18-11, 05:58 AM
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Originally Posted by Jeff20B View Post
So a plan hatched in my rotor-brain. I've always been curious how a half bridge would run on the street if you had a good way to keep the bridged ports physically separated from the other ports in the intake.
My EFI experience suggests that if you go this way, anything you can do to make it idle and cruise on the bridged ports will make it a whole lot more tolerable/drivable. The drivability problems come from high vacuum during overlap, so killing vacuum on the bridges makes the engine run smoothly.

http://www.youtube.com/watch?v=gJ6nuqPg_6w Cold idle is about 800rpm. S4 N/A throttle body and the secondary throttle stop screw has been replaced with a much longer 10-32 screw, which is where idle speed adjustments are made. Small changes in secondary plate angle make for huge changes in drivability. It could probably have stock like drivability if I could tolerate an 1500-1800rpm idle, as it is it's merely about as good as a street port.

Of course, since you'd be relying on airflow through the primary side to move fuel through the engine, this may not work for you...
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Old 02-18-11, 06:24 AM
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Looking good Jeffrey! I always love to see the projects you put together out of your eclectic pile of forgotten rotary items. What clutch did you go with? Looks like an Exedy?
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Old 02-18-11, 10:42 AM
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Thank you Jeff. Very glad to see someone is finally listening to me. I think you'll be quite amazed how much difference there is with the separate runners and how much more streetable 1/2 bridges are when set up this way. Oh, and hang on to your *** when the secondaries open up.

Peejay, not sure what you mean by high vacuum on bridges. My vacuum readings are stupidly low, 6-10 inches. How high I ported the exhaust ports may have something to do with that though.
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Old 02-18-11, 11:15 AM
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Originally Posted by peejay View Post
My EFI experience suggests that if you go this way, anything you can do to make it idle and cruise on the bridged ports will make it a whole lot more tolerable/drivable. The drivability problems come from high vacuum during overlap, so killing vacuum on the bridges makes the engine run smoothly..
ive found the same on Peepers. intake vacuum goes down = drivability goes up, until the carb stops working.

basically with less intake vacuum it pulls less exhaust gas into the next intake stroke.

i spent a week drilling holes in the carb and then driving it, i decided to quit F-ing around and start tuning the 2000rpm+ range, but i made a HUGE improvement in drivability.
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Old 02-18-11, 12:18 PM
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Originally Posted by trochoid View Post
Peejay, not sure what you mean by high vacuum on bridges. My vacuum readings are stupidly low, 6-10 inches. How high I ported the exhaust ports may have something to do with that though.
It depends on where you measure it.

When everything is divided, and you idle almost entirely on the secondaries (my main source of primary air at idle is the injectors' air bleed!), the primary can build quite a lot of vacuum since it does not see the same overlap. It's almost like two separate engines joined at the rotors. My primary will be at 65-50kpa depending on temperature and how high the idle speed is set. (Set it at 1500 and it will idle at 50kpa = about 15" vacuum) I never measure secondary, but I had to move the brake booster vacuum source to a spot on the plenum that sees primary vacuum, because the secondaries never built enough vacuum to build any brake assist.
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Old 02-18-11, 01:17 PM
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peejay, do you think the issues you ran into will be present on this carbed setup? I'm thinking mine will be more similar to trochoid's setup even though it's just one carb.

Now there may be a little bit of vacumm signal communication within the carb, but from what I can tell, it is only for the vacuum secondaries, which will be converted to mech secs anyway, so that little passage can be blocked off if necessary. lol and if not necessary, that's less work for me.

And fortunately this old school manifold has a couple of nipples here and there so I can move the brake booster vacuum source over to one of the primary runners if necessary.

Thanks Glazedham42. It's an NKK clutch kit from Atkins. Looks decent, and the price was right. Hey when are you going to use that nice channeled '70s manifold?

trochoid, your setup always made sense to me. Why anywone would do it differently boggles my mind. But then peejay's findings are noteworthy. Fortunately I don't think they will apply here.

Yeah, I've been looking for a way to do a half bridge for years. Now I can finally give it a shot. Just gotta build one streetported 12A for someone first, which is getting an OER carb, so its intake is known/already figured out, but I will be able to test this old school carb and manifold on it. If everything checks out, it's 1/2BP time!
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Old 02-18-11, 02:08 PM
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This makes me all excited
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Old 02-18-11, 03:23 PM
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In my case, overlap from the ported exhaust may account for most of the lower than normal vacuum readings. When I went from the Mikuni 2 barrel to the dual DCD, vacuum went up a couple inches. It should have gone up more than that since it no longer had the exhaust pulse from the opposing rotor's dynamic pulse. One thing to keep in mind is that the dynamic effect also reduces the vacuum signal as it is, in effect, pressurizing the intake vacuum of one rotor from the exhaust pulse of the other rotor.

The RE intake manifold is like the old Mazda one you found, it has no signal between the primary or secondary ports nor one one between each rotor's primary and secondary runners. This is important. No signal from the secondary runners eliminates the huge overlap of the bridge, thereby allowing the engine to run like a streetport on the primaries. A slot cut between the primary and secondary runners of each single rotor may increase the vacuum signal. That may need further testing/experimenting.
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Old 02-18-11, 03:45 PM
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Originally Posted by peejay View Post
My EFI experience suggests that if you go this way, anything you can do to make it idle and cruise on the bridged ports will make it a whole lot more tolerable/drivable. The drivability problems come from high vacuum during overlap, so killing vacuum on the bridges makes the engine run smoothly.

http://www.youtube.com/watch?v=gJ6nuqPg_6w Cold idle is about 800rpm. S4 N/A throttle body and the secondary throttle stop screw has been replaced with a much longer 10-32 screw, which is where idle speed adjustments are made. Small changes in secondary plate angle make for huge changes in drivability. It could probably have stock like drivability if I could tolerate an 1500-1800rpm idle, as it is it's merely about as good as a street port.

Of course, since you'd be relying on airflow through the primary side to move fuel through the engine, this may not work for you...
OMG she sounds soooo sexy
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Old 02-18-11, 04:04 PM
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All the 12A intake manifolds I've ever messed with have had at least one channel connecting primary runners with secondary runners, up under the carb spacer, so it makes sense that when you port an engine, all four ports should be the same size witht he same timing, so when the ports close, they send a very similarly timed reversion wave back up the carb and meet up at the same time or nearly the same time.

But because this one is totally separate, including the 4-hole carb spacer, I can do a half BP and it should drive like a stocker on primaries, which is one goal, then kick your pants when the secondaries open, being the other goal.

Take a look at your later Nikki carb spacers. You'll see a channel connecting both primary runners. But you won't find that on this carb spacer.

Man I'd be nuts not to do a half BP with a set of parts like this!
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Old 02-18-11, 04:09 PM
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Oh by the way I don't plan to open the exhaust as much as trochoid. I'll instead open them as much as I did on Percent's nice 12A we built a few years ago. That was the engine that behaved like it had a camden on it even though it didn't. Its secrect weapon was an old school channeled manifold from the '70s just like the one glazedham has. This sep-runner manifold has the same runner size and length, just without channels.
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Old 02-18-11, 04:13 PM
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Originally Posted by Verneuil
OMG she sounds soooo sexy
Do you guys predict/know if this engine will have a brapping idle? trochoid? If it does, cool, but if not, that's ok too. Just curious.

Well, those parts won't clean themselves...
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Old 02-18-11, 04:51 PM
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When do you anticipate this to be running??
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Old 02-18-11, 05:45 PM
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It won't brap like usual. That's one thing I lost going with the DCDs, the brap and the body shook to the brap like it had a huge cam in it. Some people thought it had a monster V8 the way the engine loped and the car twitched at idle. As much as I liked the brap, I'll willingly trade it for the primary port streetablity.

As far as the exhaust ports, open them up to match the TII race port template, 5 mm narrower, i.e., keep the same side gap to the edge of the housing.
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Old 02-18-11, 07:27 PM
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Originally Posted by Jeff20B View Post
But because this one is totally separate, including the 4-hole carb spacer, I can do a half BP and it should drive like a stocker on primaries, which is one goal, then kick your pants when the secondaries open, being the other goal.
Well, see, that's the thing. It sounds backwards but you want to to your main idle/low speed drivability on the BRIDGE ports. You don't want to let them see much vacuum, or it will drive... well, like a bridge port And that is what will happen if you drive on the primaries only while keeping the secondaries shut. Bridge ports actually drive very well at low vacuum, it's when you try to run them at low/no throttle that things get hairy.

What gets the mix confusing is the fact that the airflow isn't dry with a carb, so there may be effects there that I haven't considered/dealt with.
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Old 02-18-11, 07:38 PM
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Very cool. I will be watching this.
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Old 02-18-11, 08:20 PM
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go for full bridge and not a half-*** bridge
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Old 02-18-11, 08:42 PM
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Originally Posted by WackyRicer View Post
go for full bridge and not a half-*** bridge
+1

the trick to drivability is tuning, give the engine what it wants and it'll run like a stocker.
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Old 02-18-11, 08:49 PM
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Originally Posted by WackyRicer View Post
go for full bridge and not a half-*** bridge
But then you can't use two piece apex seals.

I have enough makin's to make a full bridge 13B 4-port except for the intake manifold, so I may actually go this route. I don't think 210hp is quite enough anymore. (210 HBP horsepower is waaay different than 210 streetport HP... lots more area in the curve)

But I keep going back to something soul assassin posted about nine years ago after he went fullbridge on his FC, and the subject of "what port" came up.... https://www.rx7club.com/showpost.php...9&postcount=12

Then again he also drove his car as a daily driver too, if you're going to spend a few hours a day in a car you want it to be a pleasant experience, you know?
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Old 02-19-11, 01:27 PM
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84stock, I have one engine scheduled to build right before this one, so pretty soon. I'll keep updating this thread.

trochoid, yes, primary port streetability is at the top of my list, but even if there is a slight brap at idle, I'll be happy.

peejay, a carbed half BP might be at an advantage over an EFI setup because it's not a dry air charge.

Yep, using two piece apex seals. Gotta flip the rear rotor's seals so the triangle tips are next to the primary port. Should I use superglue? Lay it out on aluminum foil so it is easy to peal off if some glue oozes out? Never done this before so any help is greatly appreciated.
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Old 02-19-11, 04:18 PM
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Super glue works great. I glue up on a sheet of glass. Glass keeps the joint dead straight with no stick. Keep a fresh razor/utility blade handy to trim off excess glue.
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Old 02-19-11, 07:48 PM
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Glass, check!

Thanks, man!
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Old 02-19-11, 09:10 PM
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can wait till this is complete
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Old 02-20-11, 01:20 PM
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I know you guys are talking about a turbo setup but does the lower vac with a full bridge NA still make for better drivability? Daily driving a full bridge can be tricky.... but worth it.
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