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Custom Manifold for 6 Carburetor Setup


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Hey everyone,


This is my first post to the forum, and I have a project and a few questions to share.


I'm currently working on building a 6 carburetor setup for my l28 and have decided to use 6 30mm Mikuni motorcycle carburetors. I've selected these based on the airflow requirements of the engine while accounting for volumetric efficiency, engine speed, and of course engine size, and by comparison to some other carburetor setups (triple webers, various SU setups, etc). Needless to say, the Mikunis won't be compatible with the existing N33 intake manifold I'm currently running, so I plan to fabricate one from steel sheet and tube.


My question relates to the ID sizing of the new intake manifold. I've read a rule of thumb that says the throttle body at the base of the carburetor should have an area that makes up 80% of the intake manifold cross sectional area. Based on this, I'm approximating an intake manifold ID of 33.54mm (see calculations below)

Area of throttle body at carburetor base= (15mm)^2*3.14=706.5sqmm

Area of intake manifold =.706.5/.8=883.125sqmm

Radius of Intake manifold=(883.125sqmm/3.14)^(.5)=16.77mm

Diameter of intake manifold=33.54mm


Can anyone confirm whether this 80% rule of thumb is safe to go by? I know there are other considerations to make when it comes to resonance tuning and airflow, but I've had little luck finding anything conclusive. 


I'm also considering sizing the new intake manifold to match the ID of my existing N33 manifold and N42 head. Given the simple construction of the new manifold (going with steel tube and steel sheet), fewer changes in diameter might be better. Abrupt changes in diameter of the tube will cause turbulent airflow, which, while potentially useful for achieving an even fuel mixture, could also reduce throttle response (thus my reasoning for matching the new manifold to the head). Does anyone know the size of the holes where the head meets the intake manifold?  I've done some searching around the forums but am unable to find this bit of information.


Thanks for your help! Any and all advice is welcome.



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I'm not really clear on your idea here but if you are going to use six individual carbs, then you wouldn't want a fabricated plenum manifold. That defeats your purpose. You would be better off using a readily available triple side-draft (Webers) manifold and adapting your six Mikuni carbs to it. There are some rubber mount adapters that match the individual motorcycle carbs to Weber spec inlet bolt pattern (with minimal modification) that could make it possible. Ebay source for the adapters. You would need to research this, I saw it somewhere recently.


Linkage would be a nightmare though.


If you are going to design and fabricate an intake manifold it may make more sense to use an OEM L28E efi manifold mounting base, cut the runners off just upstream of the injectors and go from there. You could design your manifold to interface with these stubs using neoprene boots and clamps. Maybe something plastic/3-D printed, IDK.


I've never seen much attention paid re datsun L6 induction to equal length runners pair by firing intervals. i.e., 3-4; 2-5; 1-6, to extract low end torque performance through resonance tuning. Look at BMW inline six efi runners as an example. You could run three individual throttle plates for each pair of runners, or one throttle plate and a plenum.


If your goal is to optimize throttle response and WOT performance both, then just go with available triple side drafts in 40mm or 45mm sized throttle plates(Weber, Mikuni, Solex, Delleroto; i.e., the work-a-likes that all share the same mounting base/bolt spreads). 40mm's are far more common and available used. Or ITB's if you can afford them.


You could do something creative in tuning paired airflow runners upstream of the carbs and maybe incorporate a two-stage induction setup that you design. Look at other high-tech inline six two stage OEM manifolds: BMW, Mercedes, etc. for ideas.

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I have six FCR41's Keihin flat slide carbs on my car, It's currently off the road while I build a 3.1 stroker. The 'rattle' from these at idle is very unique and the 'whistle' at part throttle is awesome!


I used a 12mm aluminium plate laser cut to fit around the stainless headers, It then has 40mm I.D. aluminium tube TIG welded to match the spacing of the carbs. Doing it this way allowed me to keep the intake runners straighter than a regular side draft manifold and allow the carbs to sit a little higher up.


As for linkages the FCR's run from the factory an 8mm shaft directly coupled to the slides with roller bearings in each carb, I would think the Mikuni's are very similar setup. All I did was make one long enough to go through all six and drill it to suit. It's a cable operated throttle to the gas peddle using factory FCR connections on the carbs.


I don't know how to attach links here but I have a few videos on you tube of the setup. Look for 'riceburner46' The vids are with the old worked L28.


I also have a build thread on 'Auszcar' titled 'the riceburner'. It has more info and pics.




Edited by Gareth. J.
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Thanks for the input guys. A few more questions


DAW, when you say "plenum manifold," are you referring to running 1 throttle plate with 6 runners? Also, I did take a look at the existing Weber manifolds but given they they would need an adapter or modification to fit my application, they don't seem right for my application, especially given their cost. My current plan is to fabricate a manifold consisting of 6 individual runners and 6 throttle plates.


Gareth, Thanks for the advice. Your setup looks great! The throttle linkage on the mikunis is directly on the throttle shaft. Which means to run the same setup you have I would have to pull the throttle shafts and drill/tap holes for each throttle plate to mount on. I see accuracy being a big obstacle there, so instead I'll probably start by trying to connect the ends of the shafts together using rod couplers. I really do love the cable setup though. 


As Xnke said, I chose steel for the fabrication benefits. Don't currently have a tig welder, just a simple Lincoln flux core welder. I figure the manifold gasket should act as an insulator between the alloy head and the steel intake. If not, some high temperature paint should protect it well enough.


I'm currently running stock internals and 2 flattop carbs. I think the extra flow will give me a bit better throttle response for now (each Mikuni carb flows about 100 CFM), and flow enough where I could run slightly hotter, still street oriented cam if I decide to. 40mm+ seems like overkill to me. 600 CFM, even in a built l28, seems plenty. 

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I think 30mm is too small. You really should consider larger carbs. If you use CV style carbs it will hard to over do it since they would simply never lift all the way. The stock carbs are 46mm. The fact that it feeds three cylinders does not mean much. What matters is the peak velocity thourgh the carb. The cam timing is such that there is only one cylinder at a time with a large valve opening, meaning the stock carbs only feed one cylinder at a time.

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For the throttle linkage if the Mikuni's are anything like the FCR's there's a 12mm slot in the link that connects the slide to the internal arm on the throttle shaft. It gives you a generous amount of room for DIY drilling and tapping.


Will you have any heat soak issues with a steel manifold?


I'm looking forward to the updates, It sounds like your well planned so it should be an awesome setup!

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Bearmanpete, you make a good point regarding peak flow and cam timing. Unfortunately I purchased the 30mm Mikunis about a week ago, so I'll probably be going with those and seeing how they work for now. I'll be looking around for a set of 40mms and planning to do the carburetor mounting and throttle linkage last (maybe I can find a cheap set of 40's before I get to the carburetor mounting, since the carbs will be evenly spaced anyway I don't see it being a huge issue).


The Mikuni throttle shafts have a 6mm slot with a little flange on the end to hold the arm on. It's pretty small, I don't see doing any tapping being an option. At best I could see drilling a small hole and using a tight fitting pin to hold the new rod in place. 




Here's a picture of the Mikuni 30mm. Sorry the picture's so big, but the extra detail probably helps anyway. The throttle shaft is located in front of the vertical bronze vacuum port. As you can see there's not a ton to work with.


I'll probably shop around for some sheet steel today. I'll go with some 40mm tube and some thin plate. I haven't investigated the effects of heat soak in detail. Could you explain? I know hot air is worse for performance, but will a steel part inherently get hotter than an aluminum one?


I'll keep you updated with my progress in the next day or so.



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I bought the carbs on ebay a few weeks ago so I'm actually not sure where I could find the rubber couplers. However at the very least I'll use a rubber gasket material to insulate the carbs from the intake manifold.


I did a comparison of the 30 and 40mm carburetor sizes using SolidWorks flow simulation. Both were modeled on the same 40mm ID manifold and had a velocity stack. By measuring the flow velocity at the head-side of the intake manifold, and leaving the manifold geometry unchanged, I was able to compare the flow capabilities of the two carburetors. What I found was interesting. The 30mm and 40mm carburetors had the same flow capability when mounted on the 40mm manifold. It wasn't until I decreased the carburetor throat size to 26mm that I saw a drop in airflow. 


I also ran a several different simulations which included a plenum box and changes in carburetor throat length, since I was concerned that leaving out the plenum box might skew the results toward the smaller carburetor, and that changes in throat diameter might not affect airflow unless the throat length changed as well. After running about 10 different simulations, I was satisfied that my model wasn't biased in favor of the 30mm carburetors.


40mm Carburetor Model:


30mm Carburetor Model:



Based on my simple flow sim models, I'm more confident the 30mm carburetor should flow comparably well to the 40mm given my current setup. More updates to come once the metal comes in early next week.

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If you know which model motorcylce the carbs are from you should be able to buy the insulators used on ebay or at a local motorcycle salvage yard. If the carbs are not from too old of a model you might be able to get new parts from the dealer. Anyhow, home made insulators should work fine. The original insulators on the Z are simple phenolic sheets cut to the shapes necessary. Any highish temp phenolic should work. Garolite or an equivalent is relatively cheap and machines easily.

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Considering the head to manifold juncture is Nominally 35mm...how will you work that taper? Just a big anti-reversionary Notch?

The L20A heads have a 30mm Head-Manifold juncture, though. Those 30mms might work well on those.


Saying something is overkill depends on your power goals. A 44 PHH Mikuni with 32mm or 34mm chokes will work well on the street and make decent power, but more is to be had higher in the rev range with 36/38mm chokes. Then you switch to 45ITB's with no restrictions and pick up more (or go with 50 or 55mm Carbs with 45mm chokes...)

30 mm is very small for an L28, you can feel the engine "flatten out" when using 40PHH versus 44PHH on a stock L28.


Remember, the Nissan Designed, tested and proven combination of 40PHH's for 165 Production HP in the never-produced but exhaustively tested "240Z Sports Package" was only 2.4 Liters...

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Tony D. Thanks for the info. I didn't know the head-manifold hole sizes (that was one bit of information I asked for in the original post). The only way I could think to get rid of the notch would be to machine an insert to set into the bottom of the manifold tube where it meets the head. Wouldn't even have to be welded (or metal even!), I could just press it into the bottom of the tube. I'll look into options, obviously it has to be temperature safe if it's going to be that close to the head.


You've all made good points with regard to throttle size. The cost of the larger throttle bodies is a tough pill to swallow, although I suppose that comes with the territory of trying to run a performance setup. Right now my total cost is still under 300$ including carbs, sheet metal, and welding wire. If I can get noticeably better performance compared the dual flattops (73) that I run now, I'll be happy, even if my throttle response isn't competitive with some of the true performance setups. 


By the way, I ran a few more flow simulations using different vacuum pressures and started to see significant differences in flow between the 30s and 40s. At higher vacuum the 30s stacked up pretty well against the 40s, but the lower the vacuum the better the 40s flowed in comparison.


I spent a little time machining throttle arms today. I'll be welding up the head side of the manifold soon. I'll post some pictures next time!

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I knew of ONE guy who ran 36PHH's on his Fairlady Z. It's the only set I've ever seen. 
They will work, and probably give decent economy like 40's do on an L24 compared to a set of 44's. (I got 28+ mpg driving 40's on an L24 from LA to Phoenix!)
It's the WOT where they will fall short. 

IF you are running a stock cam, remember the L28 tops out around 5300 rpms, with 6,500 only a realistic shift point in first gear, and maybe second. Third (and maybe second) benefit from short shifting to a great extent.

Inlet ports can be done up with Devcon Machinable Epoxy (*Aluminum) you can basically do the whole intake port, it won't get appreciably above 220 - 250 F at most. You would do well to raise the Port Floor to give a more favourable short-side radius and by making the port smaller increase velocity to give the 30's the best chance for mixing a homgenous intake charge and getting it swirling into the combustion chamber.

There is a nice writeup by the C230 Deviant Aussies on 'small port L28' using the 30mm runner L20A head on the larger engine to see what would happen. It was a pretty good experiment... Damn I'm drawing a blank on Mark Amrovic's Screen Name though... It was a really torquey engine as I recall reading...which is good.

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Hey guys- I appreciate your help. I'll be looking at ozconnection's build, since I'm curious, but I'll be sticking with the stock head.


Now for the good news! I managed to find some 40mm mikuni carbs from a ski doo snowmobile that were pretty cheap! They're even the flat slide type which have the added novelty factor and more linkage to work with. Now onto the adapters!


Sorry for not posting any pictures lately- but since I bought the new carbs I have to wait until they arrive to determine the spacing of the manifold. The carbs come attached in sets of two so I'll have to follow the existing spacing.

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You did mention closing up the runners...Devcon aluminum putty is machinable and allows you to raise the floor of the port so in effect you get the same effect of porting for a better short-side radius... You can take that raised port floor into account when you make your manifold as well...

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Sorry for the confusion. I'm considering machining inserts for the manifold runners to run a taper from the end of the manifold up to the head. Given that these are pretty small, it wouldn't be a huge ordeal for me to machine them. I currently don't have the capability to machine the heads.


Carbs should arrive tomorrow!

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