wankel=awesome

Well, this isnt a "What if" or "I might" thread.

I actually AM going to have a custom intake manifold built for my new engine.

Its an s4 6 port with s4 t2 rotor housing and a mild street port. Light steel flywheel, grabbier clutch, no accessory drives, all that jazz.

The induction is going to be one of the 425-465 cfm Nikki's me and Jeff20b have been building.

I have a 24.5 mm primary and 29mm secondary nikki (with a single fuel banjo) that I have dialed in perfectly for a 12A. I like the way it runs. A lot.

Im going to transplant it to the 13B via a custom intake. I spent the afternoon on the phone with Wilson Manifolds regarding my original plan, which was to shorten an RB465 cast manifold, and weld on a nikki flange and blend everything.

They liked the idea, but argued the plenum design. They are suggesting I go with a single plane (open) plenum to get the most out of the smallish nikki carb ill be using.

The logic is that the weber 48 IDA with 44 mm chokes in it is around 392 CFM per throat (their measurement) coming to around 800 total CFM, but each rotor can only access 392 at any given time. By going single plenum on the Nikki, they estimate that per each intake stroke my 13B will have 400+ cfm to work with, which is very similar to the Weber, which we know runs pretty well.

My question is: Is this just theory based on unchecked facts? Or will it really be beneficial to run a single (open) plenum when it comes to getting the most out of the carb? My original design apparently limits the 13B to HALF of what my modded nikki can deliver. So yeah, im really hoping someone who has tried something like this chimes in with their experience.

The custom intake is going to cost around 1200 USD, so I want it not to be a turd.

t_g_farrell

iTrader: (1)

Why not take a 12A nikki intake manifold (79-80) and just have a 1/2 inch or so plate made to
mount it to the block. I saw this done somewhere on here before. You will get the benefits of
the nikki matched intake and it will fit on the 13B. Of course not sure what to do about the 5,6
ports.

Did you talk to Jeff about it already? He may have some ideas or seen this as well.

wankel=awesome

The 12A intake manifolds in my opinion are what's holding back the nikki in terms of high rpm performance. The carb is literally large enough to support more horsepower but the manifolds are designed for a woeful amount of airflow.

That's why I'm trying to build one more similar to a real racing product.

Jeff20B

iTrader: (13)

The forum just ate what I wrote so I'll see if I can type it all out again.

I've tried an open Nikki phenolic spacer and didn't like the drivability compared with stock 4 hole with its little channel on the primaries. The open spacer didn't seem to add any power but did hurt drivability.

Wilson's heart is in the right place but they are thinking with pistons on the brain. They don't know much about reversion inside a rotary intake manifold.

Before investing any more time and energy into this idea, you need to get the 1/2" aluminum plate from Mazdatrix and drill/tap for the 12A pattern to install the intake manifold of your choice for some real work Nikki testing on a 6 port 13B. Sure it won't flow quite as well as a purpose-built manifold like the RB, but will work well enough for testing purposes.

https://www.mazdatrix.com/r-intakefab.htm

I have two manifolds kind of like this:

One is an original 6 port 12A mani welded to a GSL-SE flange. It has been all hogged out and port matched. This mani has very little power below 4k, but comes alive between 4k and 7k+. It needs a really big Nikki to supply it properly.

The second one is far crappier. 79 mani welded to an FC flange. This one is not port matched at all and hurts flow above 4k. However its performance below 4k is outstanding. I feel that a little port matching would really wake this thing up and not hurt low end all that much, if at all. It may be crappy right now, but I feel it could turn out far better than the other one simply due to the fact it isn't all hogged out and is actually very stock in the under-carb plenum area.

Having said all that, I will not invest in any 6 port setup for myself because I just don't believe in them. Seems like a flawed concept to me. They only work properly if the stock intake manifolding is in place. That is, primaries, then secondaries and finally aux ports, that all open in time according to RPM and load. If you don't have all that in place and functional, I just don't see it working all that well at low RPM with all 6 ports wide open all the time with an open plenum. Just does not compute. My hogged out mani is like this. Makes it kind of a dog to drive daily. Like a huge streetport that rarely sees high RPM.

However, if done right, kind of like the crappy attempted 79 mani I described that retains its stock channels and runner size, it could yield some decent results. Maybe. Only dedicated experiments will tell (just needs to be port matched at the 6 port flange). And I'm not done experimenting with either mani at this point, so some future data could potentially be shared once I get it up and running later this spring/early summer. We'll see.

wankel=awesome

Well, I have done a lot of research on stock and street port 6 ports making really good power and decent power bands running downdraft carbs with no aux pods.

The open plenum thought is just a natural step in the direction of getting the most out of these carburetors.

If I can't figure out how to make one comparable to weber, (Yaw accomplished this) I'll just switch to an IDA.

If not a single plane, what about a 4 runner setup that comes into a clover shape under the carb, so instead of a plenum area it's just a more modest channeled area so rotors 1 and 2 have access to all 4 barrels.

I know Yaw built an open plenum mani that peaked torque at 8900 rpm, and no doubt had an insane curve of horsepower alongside it.

Now I'm not looking for 9000 rpm torque peak, but I am looking for under 1.5" vacuum at the manifold at 8000 rpm under WOT.

The math is there, and it has great potential. All I need to figure out is the actual ideal runner length, plenum/channel design and height, and how to package it so the relatively tall nikki setup fits under the hood.

Jeff20B

iTrader: (13)

Ok. You've got my interest.

Have you seen the top of my 79 manifold? I posted pics of it somewhere on the forum. It has the most plenum area of any rotary mani I've ever seen, but retained the divider between front and rear rotors.

This is where the open plenum spacer came from, as well as the Racing Nikki with its monstrous (and poorly cut/shaped) 26mm primary and 31mm secondary venturis done by a guy calling himself Dave *******. It was on a pineapple racing engine that has some RB-sized streetports.

I'll have to do a search for pics in a bit so you can see what I'm talking about.

wankel=awesome

Alright, so I found a ton of ancient information about this subject hidden deep inside the internet. Names like rx7carl, oneiros, mar3, 84stock, j9fd3s, peejay, sterling, jeff20b, bumpstart, Jager etc have frequented this topic over the years.

So my conclusion is that based on everything I've read, this trick has been done professionally on 12A's for racing using modded nikki's and hogged factory manifolds. Many rumored in the 200+ HP range.

So here it is: The nikki is set up from the factory to operate on an individual style manifold where the front and rear rotors are separated. By adding anywhere from half an inch to a full inch of plenum to the manifold it weakens the fuel signal to the point of starvation at high rpms. The carb would need recalibrated andjetted, as that plenum is actually a massive change to the carb.

So far besides Yaw, nobody has gone as far as recalibration the nikki for the open plenum and talked about it. Thus, he's the only one who really ever saw the benefit of the plenum at all. Every other experiment or attempt at this ended in "it doesn't run right" after the plenum was added. And that was it. I can't even find documentation where basic jet changes were made, let alone a ir bleed calibrations or relative changes to AFR's on tip in, or cruise.

So I know I can do it, and that so far open plenum is taboo because nobody has properly executed it (and shared). But, I'm wondering if it's worth the trouble, or if I should just go 48 IDA.

Jeff20B

iTrader: (13)

wankel=awesome... you are awesome!! You just lit a fire under my ***. I want to tackle this subject with you, if you are going to keep using the Nikki. I've got the parts available, and a little time this year (a couple months) to throw at this topic. Plus I have tuning knowledge under my belt that goes beyond what Mr Yaw knew, and way beyond anything Sterling and Carl ever did.

I mean talk about a recalibrated Nikki... sheesh. That was goal A #1 for me; solved that back in early 2014 right after I hogged out my first Nikki. I did things I've never seen on any Yaw or Sterling carb. Not even the Racing Nikki had this configuration. It seems they all missed the boat on that one. Or maybe I'm the one who's wrong? Doubt it.

It took the next several months (about a year) to get these carbs to handle boost and still drive fine whether boosted or NA. Along with several other finesse improvements here and there.

Now that I've tackled and beaten the boost thing, I want to try NA now. lol

Oh and here is a picture of the open plenum spacer and manifold. 79 mani, smoothed hogged out runners, holes filled with quicksteel etc.

Jeff20B

iTrader: (13)

Rereading your post, what you say about the open plenum weakening the signal at high RPM, is akin to how adding boost weakens the vacuum signal at any RPM. And at the higher CFM flow rates of boost, I'd say 8000rpm with boost could be similar to theoretical NA flow rates of around 11k or 12k, maybe. But then it's not really linear (j9fd3s knows more about this subject than I do). Plus we know carbs were designed to work with vacuum differentials, not necessarily with boost, lol. But by shrinking the secondary air bleeds, it provided enough differential for the carb to work again, and eliminated the sec delay. It even eliminated an annoying bog that ALL Sterling mech sec carbs had. Even with that stupid flipantmatic mod.

The only real world high RPM experience I have is that at 8000rpm in boost, my carbs still work and won't go lean on me. However this is still with a stock phenolic spacer and only regular channels in the mani like any stock 79. I have no recent experience with the open plenum 79 mani seen above as the last time I used it was in late 2013 back before I got into Nikkis by early 2014. It drove ok with a stock 4 hole spacer but the engine was a fresh rebuild so I wasn't able to take it to high RPMs and didn't bother testing with the open spacer. This is one of the reasons for me to get back into this because I've got a nice 12A here that needed just a quickie freshening up. All the seals are fine and will go back into their homes. Thus very little break-in time will be required. Thus high RPM should be available rather quickly as long as everything checks out. This should fit within my 2 month window.

Awesome parts available:
RB spun aluminum air cleaner assembly with K&N filter
hogged out boost prepped Nikkis of various venturi sizes calibrated the way I always do them for boost and NA.
74 ported 12A
light steel flywheel
RB long primary exhaust
same fueling as with boost: mallory 4309 and MSD 2225 pump, just with 4309 sensing nipple open to atmosphere
stock dizzy (no need for semi-locked here)
DLIDFIS, of course
AEM wideband to keep an eye on fueling at high RPM, as you mentioned

And the icing on the cake will be that open plenum 79 mani and spacer. (I've since trimmed off the shield part because the manifold is cool due to plugs installed in the rotor housings and no active ACV port).

Here is another pic of said mani and air filter on the dead pineapple engine.

wankel=awesome

That intake manifold and spacer acting as a plenum is exactly what im trying to achieve with a higher performance manifold as a base.

That spacer and manifold is allowing whatever engine it goes on to have the full CFM of the carb available per intake stroke. Meaning, which effectively in the case of the beloved Nikki, doubles the amount of air the engine can use at any RPM.

Do you have that manifold still? And spacer? Id be curious to see how many inches of vacuum was at the manifold@8000 RPM WOT.

The goal is under 1.5", so that I can be sure the carburetor is no longer the bottleneck.

I'm talking to Defined Autoworks now for the fabrication of the manifold, and im hoping they can shed some insight as we move forward as well.

Modded exchangeable air bleeds are in my near future.

Also, I selected the 13B 6 port because its
1) what I have and
2) very good at high RPM performance AND
3) has more than a few 180 rwhp+ dyno sheets floating around using stock ports and manifolds with the right tune. a few in the 190-200 range with mild street ports and exhaust baffles removed.

So my goal is a smooth running, smooth vacuum signal, smooth idle and smooth power band all the way to my rev limiter. If the peak torque gets moved up pretty high, so be it I guess. It cant feel anymore torqueless than my stock 12A.

Im keeping the long primary exhaust and going light flywheel. I need it to remain somewhat street friendly but my real goal is to take it to track days and blast around a track.

I also have a goal for making it more fun than a BRZ or S2000. But hey, well see.

Jeff20B

iTrader: (13)

I do still have them both. And they will be going on the 12A I'll be stacking in a few weeks. I can use my boost/vacuum gauge on to see whether it pulls any vacuum at high RPM etc.

Ah, I just like using stock 70 primary air bleeds and modded 80 secondary air bleeds. I've modded them both but find the primaries run best at around 70. I will use a .68mm drill bit in a Hitachi pilot jet for the primaries if I have a set of SA 90s or crappy already drilled out Yaw, Dave *******, or SVT ones. .68mm is the closest I have to 70 without going over. The drilled ones are already ruined so I've got nothing to lose at that point. I also have a set of 60 and 80 stock pri air bleeds for testing and tuning as well. Basically all the stock factory sizes.

Cool info about the engine. I'm going 12A because I've got another turbo project that needs a running well known/tested 13B right now, so the brown car's 13B can come out for its clutch job and go in the other car while I sort the turbo install, and I've got this 12A that will allow me to test the hogged out manifold and do some much needed 12A testing and tuning. Plus I've been neglecting 12As lately. They need some lovin' too! I think Ray would agree.

Do a light steel flywheel, then. You can't go wrong.

wankel=awesome

This is a shot of a failed Yaw plenum.

This one moved the torque peak to 8900 RPM. So this can be the guide of what "too much" looks like.

Jeff20B

iTrader: (13)

You know, it's been years since I saw that on his website. Back then I took whatever Yaw said and placed it up on high as gospel. But I can say these days Yaw was kind of misguided and silver tongued. Kind of a hack, really.

The reality is his carbs suck!!! I had the chance to mess with my first Yaw carb over the winter and it had so much wrong with it, it was only slightly less horrible than the utter garbage SVT carb Jingkun sent to me last year for repair. Basically everything that SVT guy touched in the carb had to be fixed. Far more time consuming than if Jingkun had just sent a stock carb.

As for the Yaw, it was a close second as far as what was wrong with it. The lack of attention to detail was disturbing. As if a rank ameture was taking on more than he should have. The booster supports were cut and ground down too far, damaging the outside of the booster and then only the primaries were loctited in place. One primary was very crooked leading to a vibrating engine. Both secondaries were loose and could be lifted out of the carb easily, one by fingertips!! Talk about a lack of vacuum signal!! And don't get me started on Yaw's thinned throttle shafts. All four butterflies showed some form of damage that was so bad it lead to a vacuum leak large enough that no amount of mixture screw tuning could fix it. I had to crank it all the way in and it still wasn't enough to make up for the vacuum leaks passed the butterflies. I also backed the throttle stop screw all the way out and it still idled kinda high. I recommended that the owner look for a stock baseplate to swap in. I included a set of 118 drilled nickel plated short slows for when he does the swap. In the mean time with the garbage Yaw baseplate, I had use a set of stock SA 150s in it because it kept the carb mostly driveable in its dilapidated state. The owner was made well aware of this.

As can be seen in the failed Yaw manifold you posted, the Yaw venturis were cut the same way. They measured out at 24mm but flowed like 22.5mm to maybe 23.0mm if you're lucky. Now don't get me wrong as 23mm is fine for most 12As but when you are paying for 24mm but only getting the performance of 23mm or less, that's what gets under my skin. Sterling stepped away from the Yaw shape and started experimenting with his own cuts and, while he kept them at only 22mm, they flowed like a Yaw 24mm, Sterling claimed. Still isn't saying much, but at least he was trying something new/different. And Sterling still failed to grasp the concept of the strong reversion pulse inside the intake manifold that you can't mimic on a flow bench, and why 22mm is not a very good idea in practice. It works in theory, on a flow bench sure, but the real world can be a harsh mistress. Peejay understood this. I think j9fd3s does too. Peejay said something about the amount of time it takes for the reversion pulse to travel all the way up and bounce off of the butterfly or venturis and boosters, that by the time it makes its way up there and starts to reverse itself, another wave is on its way. Thus tiny stock 20mm and Sterling's too-small 22mm venturis just don't make as much low end torque as 23mm+. Moreover, Sterling was 100% wrong when he recommended against going very big on venturis claiming that the bigger you go, the greater the reduction in low end torque. Remember all that? I still do. He couldn't be more perfectly 180 degrees out of whack here. lol

The way I cut venturis these days is closer to Sterling style, but up to date with what we know now, and as such, when I do a 24mm, it actually flows like a friggin 24mm! Same with 23mm, 25mm etc.

It seems 26mm is the physical limit of these carbs. You get transition circuit problems when you go this large. You gotta have a healthy non-ported 13B that pulls a good strong vacuum and a full long primary exhaust before you should even think of doing a carb this big. And then if the engine isn't ported, what is the point for doing such big venturis? Just keep them smaller and enjoy better driveability. No need to chase the unicorn horn of high CFM numbers at the expense of the carb itself. If you want serious power, add a turbo. It's not that hard with a Nikki anymore.

Oh and it seems that Yaw exhaust ports were worked over with the same misguided mindset and they would actually lose flow the bigger he went when the anti-reversion lip is eliminated. Don't ever get rid of the anti-reversion lip.

Because you will be doing an FC engine that has diffusers, only mod the sleeves to get rid of them and leave the port size and shape (timing) stock. It's already at T2 spec so plenty big enough in my opinion. Much bigger than any 12A. And bigger than a GSL-SE or older 13B housing as well.

Heck I've got an FC 6 port sitting here with cut out diffusers and a hogged out Nikki to 6 port manifold like I described above, but it's going in another project so the 12A will have to do.

wankel=awesome

The plenum/spacer that you have pictured I think would be ideal for the airflow demands of the 13b. It's not possible for the nikki to be "too small" on that type of manifold. I'm actually worried it might be a bit large in that configuration.

If the 48 IDA is really just under 400 cfm per throat, a 400 cfm nikki on a shared plenum is effectively the same size.

Rotor 1 will draw up to a maximum of 400 cfm (probably too much) and rotor 2 will too, since they draw at different times.

The weber does the same around 400 cfm, open to both primary and secondary ports all the time. If you put the weber on a shared plenum you'd have each rotor feeding on 400x2 cfm throats, and likely would not make enough signal to tune at all.

Meaning, the nikki modestly hogged on a shared plenum is actually in theory very high performance in terms of available cfm, at least compared to the 48 IDA

wankel=awesome

TheSamba.com :: Performance - Engines/Transmissions - View topic - What is the CFM of popular carbs?

Very good read here on this theory. Maybe the nikki doesn't have to be large in the first place jeff, it's looking like it's just the right size.

I'd be very interested in seeing that manifold on somethimg with a much greater airflow demand, like a full bridge. 400+ cfm per rotor should be plenty, and the bridges power band should be perfect.

j9fd3s

iTrader: (3)

the hard part with using the nikki is that its a carb, and this limits the runner lengths you're allowed to run. EFI would give you the freedom to follow Mazda's example with the Rx8 and run optimal lengths for each set of runners.

runner diameter is pretty easy though, just think of it like extending the stock port up to the carb.

as far as a plenum goes, you are correct that sharing all 4 barrels amongst the whole engine will help up top, however you will need to re jet (and or re air) the carb, as the airflow characteristics will be totally different up top.

just a though, but maybe keep the primaries separate, and make a small plenum for the secondaries? this should keep the low end response we like, and add some flow up top. hopefully since its the secondaries, by the time they are open, there is enough airflow to not need a huge fueling correction?

wankel=awesome

^I considered this, and it will have to be experimented with honestly. I think if I separated the primary from the secondary it would complicate how well the carb transitions into the secondaries, since its pump shot is small and ultimately designed to be shared with both primary and secondary ports.

Jeff20B

iTrader: (13)

I'm thinking more about this project. I might just skip the 12A and go straight to the 6 port 13B and the hogged out manifold I mentioned that is like the hogged 12A one pictured (mine, not Yaw's failed one).

I think if we both did 6 port 13Bs, it would be closer to comparing apples to apples. Plus the 13B is already together and would be easier to swap in/get results quicker than having me do all the work on the 12A to get it back together, only to leave it in for a couple months then have to pull it again. I'd rather spend that building time on tuning time with the 6 port 13B, you know?

I'm really interested in what the differences are between an open plenum spacer and a 4 hole stock one. And I'd like to see if the tuning requirements need to change etc.

Thoughts?

lastphaseofthis

iTrader: (8)

semi p port tied to the aux ports.(6 port) going to a 2nd nikki that only opens after 4-5k.

wankel=awesome

As much as Id love to have 2 4 barrels, I think that much fab work is out of the question here haha.

lastphaseofthis

iTrader: (8)

it would be better with one nikki and then a downdraft two barrel for the p port.

for you, do they not yet make an adapter for nikki to 6 port lower... just make one of those... there are 4 holes comin out of the nikki and 4 holes going into the lim

wankel=awesome

Jeff20B really needs to try this for me haha. I'm thinking more and about adapting a reworked 12a manifold to the 6 port. And I'm only considering this for the runner diameters. My 79 intake manifold would need a ton of porting to be high performance.

Jeff20B

iTrader: (13)

I'd like to try all three engines. 6 port with a nice plenum Nikki manifold like the 12A one pictured. And of course a 12A with the pictured manifold. I currently have a 4 port 13B installed that needs a clutch job. So I've decided to pull the engine to change the clutch, which, for me is easier since I own an engine crane but not a tranny jack, so when it comes out, I'll swap in the 6 port 13B. Then see about the 12A next month. That's the plan, anyway.

wankel=awesome

Anyone know where I could find a older 13B intake manifold? Might do that instead of cutting up a 12A mani or RB mani

lastphaseofthis

iTrader: (8)

you mean an s4 lower i have two but i needs them...