Jeff20B

iTrader: (13)

If you've seen this thread, you know what I'm talking about. half bridge 12A, with a twist

The half bridge bug bit me again. I've got an R5 13B to play with this time.

Like last time I'll start by posting pics of intake parts. They gotta have separate runners! I can't stress this enough. 76 Cosmo reverse runner on left, 74 REPU normal runner on right

Don't get the wrong idea about the size difference of the middle ports. The manifold on the left uses the tall ports as its secondaries which means they only come on at high RPM. It's a good thing they're tall. Never ever make your primaries this tall. Ever. Unless you like being slow and enjoy nasty reversion.

So which manifold will I use out of these two? Either one really. But I gotta choose one so I'll know which ports to bridge. In case you're just joining, half bridge means you bridge the secondary ports only. The only twist here is I may end up bridging the intermediate plate. Wouldn't that be a trip?

What's next? Oh, the carb. It'll be a Hitachi carb like this.

Why not a Nikki? These Hitachi carbs are slightly shorter and will fit better in the application. I also don't have any more Nikkis with an air screw. The Hitachi has one. I've discovered the air screw is useful when tuning the idle on a half BP.

What's next? How about a pic of the old engine. Now you can see the manifold and the light steel flywheel.

I decided to go with a light steel flywheel this time for improved driveability. Sure an aluminum can be used, but I was going to use a light steel in this application anyway, regardless of porting style.

Hmm, should I talk about the carb jetting next? Or how about the bridge port size? What about the primary and secondary ports? There are so many neat things to discuss, but I've said enough for one post.

Hey I wonder how many pages this thread will generate before the engine is running? Seven like last time?

Jhereg

iTrader: (27)

subscribed

j9fd3s

iTrader: (3)

i say 9 pages, and 190 jets!

PercentSevenC

If you want my opinion, use the reverse-runner manifold. Having the shorter runners on the bridgeport side and the longer ones on the streetport side should result in a really fun powerband. Also, my gut feeling is that it will idle better having as short runners as possible for the high-overlap ports.

Jeff20B

iTrader: (13)

That's good thinkin'!

Glazedham42

iTrader: (4)

Jeff, whatever became of the 4-rotor you were building out of spare 12A parts?

Jeff20B

iTrader: (13)

Only five posts and we're already off topic.

If I was gonna talk about the 4 rotor, I would have made a thread. I'm here to talk about a proposed half bridge R5 13B that anyone with access to an 85 or older 13B 4 port and a separate runner manifold can replicate.

But since you asked, the owner wants to go with peripheral ports. I think that's a stupid idea. P-ports suck ***** for the street. I'd suggest a dual Nikki setup with maybe a large single turbo. Or maybe a half bridge because I know how awesome those are now.

peejay and j9fd3s would probably contradict what I just said. So I'll let you know what the guy has in mind. He'd like to use two weber IDAs, two sets of RB dissassembled header kits, collected to an RB streetport center section, then to a powerpulse muffler. That is not enough flow for even a streetported 4 rotor. He will spend thousands on the intake and exhaust and then not have the time tune it and redo the exhaust. I'm sorry but that's a stupid setup. You can't muffle a 2 rotor p-port enough for the street and still perform like it does on the track. He's not a racer. Doesn't have the time to even pretend. j9fd3s has shown us a p-port needs a lot of time and attention and big bucks spent on jets. The carbs themselves are spendy. He's be better off going EFI, but hates computers in cars. Wouldn't even begin to know how to tune an ECU. That's why he's into Camden superchargers with carbs. He does have an FD, but it's very stock under the hood. All that rats nest and twin turbo crap is still in there and still sorta working for him. I'd have gone large single long ago. So the 4 rotor is on hold indefinitely until he can be educated as to what is best. He doesn't have the time to get into it. And who knows, maybe a simple p-port would be best for him, but it may take ten years. So I've basically lost interest.

So now I'm going to ask you an inappropriate question. Why don't you fill in the channels of the RX-3 manifold and go half bridge? They're really great. You may have screwed the pooch on that one. I'm sorry I couldn't advise you back then, but we all learn as we go.

Jager

iTrader: (10)

Have any pictures of your porting?

Do you modify your carbs at all?

Jeff20B

iTrader: (13)

Sorry for the soap opera. As you can see I'm not happy about the 4 rotor stalling out. If it was my engine, I'd have set it up as described above and fall in love with it all over again every day I drive it. Street port or half bridge, twin Nikki. Heaven. The proposed exhaust might even work.

Thinking about it a little more, a stock 12A makes about 100HP. With simple mods, 110 to 120 are easy. That;s about 50 or 60 per rotor. Now multiply that by 4 and you get 200 to 220ish. Not a lot, but it would be ultra reliable and the low end would be amazing. Now think about a streetported 12A. They get peaky with less low end for daily driving. What if you had four rotors? It would still be super reliable and probably get decent gas mileage when driven conservatively. See that's the way I think. I'm not a racer and niether is the owner of the 4 rotor, yet he thinks a p-port is the only way because that's all we knew back when he purchased the parts from Jeff Bruce. Now that I know an alternative exists, I'm all for it but the owner is kinda older, from the old school, doing carbed rotary stuff before many of you were even born. So he's slow to change and it puts me at a disadvantage because I'm the engine builder. If the engine turns out to suck an inapropriately large amount of ***, it's on me. So it's on the back burner. I try not to think about it.

Jager, thanks for the interest. The ports so far have been ported to 74 spec. I could stop now or bridge the secondaries. If there is still genuine interest in this thread, I will post pics of my progress later. You guys will probably like what you see. Just don't be jealous if you're into 12A stuff. I already have shown half bridge 12As work really well. You just need a manifold.

j9fd3s

iTrader: (3)

a dual nikki setup would be kind of cool actually....

but yeah i'm not sure a power pulse muffler flows enough for a TWO rotor P port, double that and it might not even run. plus 4 2" pipes under the car?

the IDA's aren't cheap either. i'm into mine for like $800, PJ and Max, both spent less on their EFI setups, i bet.

j9fd3s

iTrader: (3)

what are the lengths and diameters of the two intakes? cosmo vs um the other one

Jeff20B

iTrader: (13)

Right, the p-port plan, as it sits right now, is crap. So I'll leave it until the owner can be properly educated.

I agree about the expensive carbs. There's no way I could recommend them. They'd be like 2k with some jets. Lame. I've got two good used Nikkis right here.

He'd have two dissassembled header kits, collected to two 2" collecters, then to the RB center section. Then to the powerpulse. This might work if the intake ports are stock or mildly ported. It would sound really good though.

Jeff20B

iTrader: (13)

I'll have to get a tape measure out and see.

j9fd3s

iTrader: (3)

sweet! maybe we learn something.

Jeff20B

iTrader: (13)

Ok I just measured manifold runner lengths.

74 pri 5"
74 sec 7.5"

76 pri 7.25"
76 sec 5.5"

These numbers are approximate. I sort of centered the tape measure.

84stock

iTrader: (5)

You sure you didn't stick something else in the ports? We all know how you love your rotaries, god knows what the thoughts of a bridgeport inspires you to do...

Jeff20B

iTrader: (13)

Those ports are all too small. It's p-port or tailpipe man!

diabolical1

iTrader: (3)

yes, i agree. bridges feel great but they often chafe.



on a more serious note, this is good thinking.
also, giving some thought to something you mentioned in another thread, this way gives you a slightly easier "out" if you find the half-bridge is not what you want for this project. if you keep the bridgeports small enough, you wouldn't have to notch the rotor housings either.

j9fd3s

iTrader: (3)

i like this. the carb and intake are small too, so a small bridge might work rather well?

trochoid

iTrader: (6)

Keeping this one for yourself? Sure hope so. It opens up a whole new way of thinking about porting, overlap and the 'Dynamic Effect'. It would be interesting to know how seperate runners affect the VE. I'd go for the reverse runner. If you decide to go back to a stock port, you only need to replace the center iron and the short runners may really take advantage of the overlap at high rpms.

Jeff20B

iTrader: (13)

I'm glad there's a little interest still in this project. I felt like abandoning it and just going with 74 spec ports. It makes a much better daily driver. But then I figured I could keep porting it out to streetport levels and use an RB holley 600, which should work fine with a carb adaptor on this reverse runner manifold, because it's rumored to flow so well, kinda like an RB holley manifold. Not saying I'd do it, but the motivation was simple:

Stock 76 Cosmo reverse runner manifold and 22mm hitachi jetted lean on stock 76-85 12A sized intake/exhaust port timing made 110hp at 6k and 120 torque at 4k in the 76 Cosmo.

Compare these numbers with a true 74 spec setup:

Stock 74 REPU standard runner manifold and 22mm hitachi jetted rich on 74 spec intake ports but still small exhaust port timing same as 76-85 12A and all US-sepc 12As and 13Bs until the GSL-SE (meaning early opening and early closing, kinda restrictive). Power was 110hp at 6k and 117 torque at 3500.

Huh?

I'll say that again. 76 Cosmo small ports = 110HP at 6k, 120 torque at 4k with small fuel jets.
74 REPU = 110HP at 6k, 117 torque at 3500 with big fuel jets. Now I thought bigger ports + bigger jets should = bigger power, but it doesn't here? Someone's not playing by the rules!

Why would a smaller ported engine make more torque at a slightly higher RPM? Makes no sense! Why do they make the same HP at the same RPM? I thought porting increases torque and raises the RPM at which it peaks? Makes no sense unless the intake manifolds are really the main difference. They probably are. It's the only major thing that's different considering similar ignitions (points), similar exhausts (thermal reactor), similar carbs, well same size carbs anyway but leaner jets on the engine making more torque!

Now compare raw port sizes. I don't have pics handy so I'll list the closing timing. Note: all opening timings are the same on all OEM ports (includsing secondary ports for T2/FD). Stock 76-85 4 ports all close at 40°. All 74-75 and all T2/FD close at 50°. Racing Beat streetports close at 60°. Not sure about RB's open timing so I'll ignore it for now.

So getting back the simple part of the motivation I was refering to earlier, I figured I could get away with an RB Holley 600 either with streetports or 74 ports. I was thinking 74 ports based on the differences listed above between 76 and 74 engines, and how smaller fuel jets and smaller ports on the 76 engines netted the same HP and slightly more torque at a slightly higher RPM. I figured the reverse runner manifold flows so well I could get away with the RB holley 600 and not even need to port the engine to streetport levels. That's how good the reverse runner apparently is. But I could always go bigger than 74... As long as it's not too big. It is still a stock manifold after all.

Let me say it this way. If I was to take a 74 carb and intake manifold and plunk it on this now 74 ported engine, I'd only be making the same power as the stock REPU listed above. Well, more or less as we're not considering free flowing exhaust or direct fire ignition (make that more hehe). If I had left it stock ported and gone with the reverse runner manifold I'd be making the same power as the REPU and require less fuel to do it. But since it's now 74 ported, I've created a condition where flow demand is so high it requires a bigger carb if I use the reverse runner manifold. And because my application requires the use of a reverse runner manifold, I may have screwed the pooch... unless I go with a bigger carb. That's why RB holley is talking to me. That's why I then began to consider streetports, but should I listen?

As for going half bridge or not, again the engine was already ported to 74 spec before even considering a half bridge so it may not work out. But I do agree with how neat it would be to test the reverse runner manifold on it, and if it sucks I could easily swap to another intermediate plate, but the whole 74 port plus reverse runner manifold = needing a bigger carb keeps coming up, and who knows how well the Hoelly would like half bridge ports. I certainly don't know. RB calibrates 650 double pumpers for full bridges... I'm not interested in any of that.

Oh another thing is the exhaust ports were already opened up a lot (that's another story), so opening up the intake ports was kind of necessary. I'm not sure how this will affect the half bridge's running at low RPM. I also don't like the thought of having to notch these rotor housings because if I do go back to a stock intermediate plate, I've still got notched housings. I'm also not interested in making small bridges so the housings wouldn't have to be notched, because they'd be so small they wouldn't flow worth much. If I'm going bridge I'm going as big as I went on the 12A, or not at all.

Lastly one peice of good news is the 76 carb has been proven to like high RPM, like a Nikki, and came calibrated with small are bleeds specifically for the reverse runner manifold for high RPM use. So there is a chance it could be used after all on the already 74 ported plates. I'd just exchange some high RPM flow potential for more CFM at a slightly lower PRM, and save the holley for another extend ported 13B I've got sitting here that needs a big intake. It could actually use an RB holley manifold where this engine could not. I could live with that. But how would the 76 carb work on a half bridge? Probably run out of flow a lot earlier than on just 74 ports.

One more thing. I compared a bunch of templates yesterday while considering going streetport and it seems the biggest jump in size is from going to 74 spec up from stock. It seems going to streetport from 74 spec is less of a size change, but maybe it's my templates. I do have an extend port template I copied from a pineapple racing engine, which is about the same size change as going from stock to 74 spec, but it's a little big. I'd rather rely on the higher breathing capability of the reverse runner manifold on either 74 spec or a mild streetport, with the Holley 600 or the 76 carb, as this is to be a daily driver. It's also getting a light steel flywheel for those reasons.

There I think that covers it all. Sorry if it was hard to follow.

cliff notes:
compare 76 to 74 power/port timing/manifolds
compare RB Holley 600 to 76 hitachi carb
compare stock ports, 74 ports, streetports, half bridge
daily driver so gotta do what's best for long term

Jeff20B

iTrader: (13)

This is where I got my power and torque info

splat_mat

iTrader: (3)

I'm very interested in your builds and it has got me thinking. I was original going to use a s4 13b that I picked up with my 12a plates to make a 4 port and bridge the center plate and flip my holley so the primary has the longest runners with the racingbeat intake..... I found out I was mistaken and I would need gsl-se housings to get it to go together.

New idea is to run with your separate runner theorie and either remove the seperation between the separation between the 2 ports or just take out the sleeves and let them flow wide open....

I'm kinda looking for your input and a little guidance since you seem to be the 1/2 bridge master.

Jeff20B

iTrader: (13)

I'd ditch the six port crap and go with 12A Y plates if you're using GSL-SE housings. Remember coolant seal location moved in '86 until the RX-8 put them back in their proper location.

I've got an update for all. The half BP 13B will go ahead with a slightly modified plan. I'll use a 20mm Hitachi because my 76 Hitachi died. All I need is a carb with an air screw, which the 20mm version has. Not sure which mani to use yet as it will dictate whether the int gets bridged or the end plates get bridged. The side plates will be Y with stock ports, only casting flash and sharp edges will be smoothed. The rotating assembly will be R5. Rotor housings will be old school. flywheel will be either light steel or OEM GSL-SE. It'll basically be a clone of the very succesful R5 12A, only in 13B form. Not sure what I'll break it in, in. Just gotta get a gasket set and oil o-rings... and some time. I'm booked solid for a month and a half.

dream_garage

iTrader: (8)

Jeff, if you use the 12a Y plates and GSL-SE housings you would then use a 4port 13b intake manifold not a 12a correct ? I'm interested in ditching the 6port for a 4port for my GSL-SE, as I'd like to have a half bridge as well. Thanks for the info you've provided, I love all your projects.