TS87FC3S

I have an 84 12a that is apart for rebuild, im wanting to port but keep my intake.. rb 465 holley (individual runner) im considering stock primary ports for driveability and torque and bridge port secondary's with race exhaust port. What do you guys think and how hard would this be to tune, car is street car with numerous autox and road course days planned.

diabolical1

iTrader: (3)

so you've settled on a half-bridge, eh? well, two things - first, i would at least do a mild streetport on the primaries and second, i would maybe consider a milder exhaust port. which exhaust port template are you planning to use?

as for tuning, all the cautions from your other thread still apply. that said, you'll probably have to use a different exhaust from the one you said you currently have. evaluate that, then get an assortment of jets, a healthy supply of spark plugs and give it your best shot.

TS87FC3S

I've been told my exhaust should work and I have a 680cfm carb sitting at home I can use also.. I don't want to lose my low end torque that's y I decided on stock primary. And racingbeat race exhaust was what I was thinking

TS87FC3S

shouldn't i retain stock low end with the stock primary port, will the 465 cfm be enough? will timing be difficult seeing as the stock low end and highly modified top end?

TS87FC3S

or should i just go with a large extended port and mild to race exhaust and keep my 465cfm and exhaust since i will be cruising and normally driving this car as well as trck days

diabolical1

iTrader: (3)

no. i understand your logic for doing it (leaving the primaries stock), i just don't particularly agree, that's all. i hear about the rubbery low end of streetports all the time, but it's the only port type that i've been doing for the past few years and i can honestly say, i haven't experienced it. at worst, the streetports i've built and driven feel no different than stock down low.

as for the exhaust port configuration, i was merely saying to look into it closer. again, i understand why you would do it as such, but if you stick with this long enough, you'll come to find that things aren't ALWAYS what they seem with building rotaries.

at the end of the day, it's all your decision though. i think you have enough guidance thus far to try something, the worst thing that can happen is you fail and have to try again.

Jeff20B

iTrader: (13)

diabolical1, are you saying your streetport is small and thus drives about the same as a stockport? Or has it been so long you've forgotten what a nice torquing 12A actually feels like?

I disagree with the advice you're giving this guy. I had awsome results on my recent half bridgeport which had STOCK primary and secondary ports. The bridgeports were huge, like group B size. The exhaust I do agree with however, I kept the opening timing stock and only went up 3 to 5mm (can't recall; check the thread), then recreated the 2mm bevel.

Truth be told, you gotta match your primary porting at least to your carb. If the 465 was calibrated by RB to work on a stockport 12A, then by all means keep the primary ports stock. Then up your secondary jet size if that is possible on an RB carb (don't they use a metering block for that?).

I kept my ports matched to the Nikki and changed the sec air bleeds and fuel jets to reflect the greater flow of the bridged secondaries. I kept the primarys stock. Worked out brilliantly. Better low end and lauching than a streetport I did recently. No rubbery feeling (lol). Also better than another stockport I built a few years ago owned by a friend. All this with an aluminum flywheel to prove the naysayers wrong that say you gotta use a stock flywheel due to decreased low end torque blah blah blah. Well, maybe if you port the engine wrong or mess up elswhere lol.

If I do this next half BP 13B, I'll apply similar logic. This time the engine is already ported to 74 spec, but it actually matches a 74 carb and manifold I've got here. However I might need to switch to a 76 setup so it fits the car (MG Midget). The 76 carb is the same size with Sterlinglike 22mm primary venuries and can be jetted the same as 74, so I think I'll be good. The cool thing is the 74 carbs came stock with 140 secondary jets. I can go to 150 or 160. Actually the 76 carb came with dinky little 130s! I can up them to 150s and probably be perfect. Neat huh?

diabolical1

iTrader: (3)

well, to tell you the truth, yes. it has been a while since i drove a streetported 12A, and yes, the ports were not all that crazy - quite mild. as far as i could tell, i didn't notice any difference from the stock ports - and there were times when i lugged the engine in gear on purpose, just to see how it would "pull". actually, i just built a streetport 13B for my FC. i went bigger with this one, but i chose not to mess with the primary runners other than to debur them. i'm actually quite excited to see what it does. the only downside is this is my first FC and i have nothing to compare it to because i bought it with a blown engine. the last time i drove an FC was sometime in the late 90s.

my brother's streetport 13B pulls like a bloody diesel (at least the rotary-land version of one ), but unfortunately, i can't take credit for porting or building that engine. it was built circa 1990 (4 years before i built my first), but i can take credit for tuning it.

Jeff, i respect your (and anyone else who disagrees') opinion. i freely admit that i could be wrong, but i simply have not experienced the soft low-end that i hear so many caution people about. i try to keep in mind that we all have different experiences, levels of understanding and points of view. actually i take great pleasure in reading your build philosophies. there's a certain - quirkiness (for lack of a better word) - to your visions/methods that i truly admire.

i'm always learning and i try not to have ego or any other hindrance to that learning. however, i still maintain that a well-done, mild streetport and stock low end are not mutually exclusive. i feel if he opened earlier, and makes very, minimal changes to the closing edge while retaining stock runners, then it could work. for the exhaust, rather than full race, i would still keep it mild and slightly reshape the closing. of course, working bevels into both intake and exhaust ports should go without saying. what do you think? if i recall, there was a discussion of this in one of your threads a while back - i think it was for a pickup or something. either way, i'm interested in your thoughts.

also, my advice was more of a suggestion for him to research and reconsider, as opposed to telling him that he should do what i said so if that's how it came across, i apologize. it's his decision and i'm not trying to take that away.

this is a fairly valid point and i could easily see myself overlooking it if i were building on his setup.

Jeff20B

iTrader: (13)

I can agree with what you're saying.

You say I have a certain quirkiness? That's good. I just assembled an S4 NA to test run in my REPU with a modded manifold and a Nikki. This is going to be interesting. My first 6 port. Pretty quirky huh? I've done so many 4 ports I've lost count, but this 6 port stuff is new to me. I'll see if I can install it today.

After it's had its four heat cycles it can come out so the half bridge 13B can go in for a test run. That's the plan. Still not 100% committed to the half bridge though, based on a number of factors. It's one of those situations where I know I can do it, all the puzzle pieces just about fit, but should I? Just because I can, it doesn't mean I must, you know? Heck I could fit the 20B in the MG Midget. But just because it will fit and work pretty well and all that, is it really the right way to go? Wouldn't an FB be a much better home for something this extreme? The 20B has been streetported rather largely so could make a lot of high end HP if the exhaust is done right. I'm keeping it NA but may go turbo if things work out. Already have a big turbo sitting here that I'm deciding what to do with. Anyway could I ever extract any kind of fun out of this thing if it went into the MG? Or would I be worried about spinning the tires at every stop light (already had that problem in the GLC with a 5" camden on a 13B and some old tires lol). For that matter, would a half bridge 13B be the best choice for the MG Midget? It's a very tiny car. Not very practical, but neither is a half bridge. It needs a slightly richer idle so gas mileage will suffer over time. They're also harder to start making daily driving duties that much less fun. I might find myself driving something else, and I don't want that to happen.

I was just going to do some stock 76-85 12A size ports in Y plates so they match the stock 76 reverse runner manifold and carb at factory settings from a 1976 Cosmo. But where's the fun in that? Plus the old school MAZDA rotor housings were already boogered pretty bad where one was ported badly with a missing sleeve and the other one's sleeve had been destroyed by the PO trying to remove it or port it or something, so I had to pull the sleeve, port this housing to match the other, and now I'm left with some large exhaust ports (almost to T2 spec) that need large intake ports to match. It's ok to remove sleeves from 74-75 housings by the way. These have good chrome, too. So I've already ported the intake ports to 74 spec and thought that would be enough. But then I had so much success with the 12A half bridge that I'd kinda like to do it all over again. But is it the right choice for such a tiny car? Will the idle brap push the whole car to the side each time? As awesome as that sounds in my imagination, would I still like it in a year once the newness wears off?

I guess the one good quirky thing about this half bridge 13B with a reverse runner manifold is I only have to bridge the intermediate plate. So if it sucks, I can do a quiky rebuild to just swap the one plate. Unfortunately the housings would still be notched so they'd act like a bunch of missing chrome right there. Would it mess up the idle? Not sure.

You know the other option is to streetport the secondaries. Much better idle quality. Much less fuel consumption at idle. I can still go with large secondary fuel jets. Will still tend to lift the front end under hard acceleration. Will still be a good match for the large exhaust ports.

I think the deciding factor is turning out to be the already 74 ported intakes. Unfortunately I have no others that would work as well because I picked these plates out of several based on weight (yes I weighed them all and found the lightest ones lol), then based on other features like tall oil fill tube vs short (I need short in this car due to a low hood opening). The rear plate requires an 81 rear plate with a threaded heater core fitting, so I can block it off with a bolt as the car does not have a heater core. I only had one Y and a couple of R5s like this, but the R5s are heavier castings and more suited for boost. If I had known what I now know about half bridges on stock intakes and carbs back when I was working on these plates, I would not have ported them at all. I would have left them stock with just the casting flash and sharp edges removed/cleaned up like on the 12A. Then I would have bridged the secondaries and it probably would have worked out similarly as nice as the 12A. But since the plates are already ported to 74 spec, I feel I'd be better off leaving them alone, and maybe, just maybe streetporting the secondaries.

On second thought maybe I shouldn't streetport the secondaries. See if you can follow this logic. Did you know a T2/FD opens and closes its secondary ports at 74 spec? That's right. If you even were currious about these fabled 74 spec ports I keep raving about, and would like to make template but don't have access to a rare 3B plate, just grab an FD/T2 end plate and have at it. Only the very bottom of the port is slightly lower, which Mazda changed for all 86+ engines, but doesn't really affect anything so we'll ignore it. The T2/FD primaries are smaller, of course, for better port velocity at the primary injectors. Plus since they're boosted, they don't need big ports to make big power anyway. The point is the total intake port area compared to total exhaust port area is a certain size. My 13B with full 74 spec ports already has more intake port area than a T2/FD! Yet its exhaust ports are slightly smaller. So with smaller exhaust ports than a T2, shoud I really be making my intake ports any bigger? Well I have a completely stock 74 engine here to take notes from. Its exhaust ports are very small. They open at T2 spec but close way early (same timing as US-spec 12A housings). So with that logic, I'd be just fine increasing intake port area since my exhaust ports are so much bigger now. But then again US-spec exhaust ports are very restrictive, so with mine as opened up as they are, they might be right in line flow-wise with the 74 intakes and maybe I should leave them alone now? I feel a little like Italo Calvino's Mr. Palomar, but with rotaries.

Getting back on topic, the guy's Holley 465 has some little channels on the bottom which allow a shared vacuum signal. He desperately wants to do a half bridge, but I've made it abundantly clear that there is to be no communication from primary to secondary if you want the half bridge to work, otherwise you risk having all the problems of a full bridge, but none of the advantages.

I don't know enough about holleys to know whether blocking the tiny channels will work or ruin the carb. If he had a way to test run this carb with them blocked before doing the bridge work, he should.

diabolical1

iTrader: (3)

honestly, i kinda figured you would be able to.

i've come a long way from the days when my idea of building a ported engine was simply to make them as big as i could without dropping seals or sucking oil. i've learned to try and use timing to tailor-make the engine's character. i've learned that many times bigger is just ... well ... bigger, and nothing more. that said, i still have plans for an old school (replica, not original) 13B with some - let's say - healthy streetports, and that engine will, coincidentally, give me the chance to put together the 6-port 12A i've always wanted to build - hopefully by the end of this year.

it appears to me that you and i seem to have more engines (and engine projects) than we do cars to put them in? am i mistaken? at any rate, i know it's certainly the case with me.

for your MG ... boy, this is a tough one. assuming it's not going to be used daily, fuel consumption and manners, to a point, may not be paramount considerations (though they would obviously be important). however, when i think of an old British chassis that weighs less than 1800 pounds and (as far as i know) came with - what? - maybe 60 to 80 HP??? stock ... and you're planning to chuck a half-bridged 13B in it, to be honest the prospect is sort of almost scary. i get this vision of that MG having little to no traction BEFORE the secondaries open and then wanting to totally pitch itself sideways when they do. i lived in Seattle for a little over a year ... remember that stuff that comes around October-November-ish and doesn't really go away until late spring/early summer? LOL life could be interesting, indeed.

a stock 12A with the (thermal reactor in place) could probably make that thing fly. seriously though, i think if you want to go with the half-bridge, then go for it. however, if it were me, i would definitely do it with that reversed manifold you have because of what you said above, in the quote. you can always substitute a stock (or streetported) intermediate back in and call it good.

to the OP ... i apologize. i get carried away at times, so i'll continue with Jeff in his thread. go ahead and start looking into those things mentioned, decide carefully and get that 12A going as soon as you can.

TS87FC3S