Custom Carb manifold.

[Bass Man]

I thought I saw someone's build with a custom tubed-runner intake manifold, but I can't seem to find it.

What I can and want to do, is replace my stock carb intake with a custom made tube-runner intake. I can fab it fairly easily, but I just need to know if it would be with the trouble.

What I was thinking about doing is getting a couple pieces of 1 1/2" exhaust tube, have 2 of them bent (for 1 and 4) and just fab up a "box" where the carb would sit on top of, and all 4 runners would be connected.

I would either block off the coolant ports or run them together, but they wouldn't be part of the manifold. I could either build it to fit my stock A1 carb, or maybe find a 32-3# weber in the junkyard and work around it. Comments?? Can anyone find the pic of the guy with the tube runners?? I'ver been searching for 2 hours and couldn't find anything.

[2oodoor]

waste of time, you would be better off just using the stock intake and cutting out the separator under the carb. The one I have I could look into any hole in it and see out the other side no problem.
now if you want to build an intake like that, build it for dual sidedraft or even downdraft webers or either motorcycle carbs.
http://www.redpepperracing.com/forums/index.php?showtopic=38290
photo credit to preludepower

[cygnus x-1]

So you're talking about a 4-1 design instead of the 4-2-1 stock design? I think some of the older carbed Civics were like this. But they wouldn't be compatible due to the completely different port spacing. It would not be trivial to get the flow distribution right though.

Honestly though the manifold is not the problem, it's the carb. The stock carb is just wheezy. Everyone that does the conversion immediately notices the difference. A tuned runner setup with a Weber might get you somewhere but I agree with Roo. By the time you get to that point you may as well just go for bike carbs or DCOEs. That bike carb setup has some definite potential since bike carbs are cheap and plentiful.

C|

[Bass Man]

Well, my dad just said he can get some carbs for $30-40, and he runs one of them on his 1400cc Shovelhead.

If I got something like a 40-50mm, would I need 2 or 4?

[lostforawhile]

you can look on ebay all the time and find four carb mikuni setups for bikes, they have all the linkages and everything already with them. the best way to make a manifold,is to take a stock one, cut it off at the flange, then weld your four tubes to it. remember to get the proper angle, the engine is tilted forward quite a bit, if you welded pipes straight to it,they would want to stick out the hood. i can't remember the angle off the top of my head at the moment. i know when i milled the manifold part for my SUs ,i had to put quite a bit of angle on it. the only thjing you wouid probably need to do to those bike carbs is maby modify the linkages a bit,depending on how far apart they are spaced. also remember you need a balance tube between all the carbs.

[lostforawhile]

we have a guy on here who works in a cnc machine shop, if you got a set of carbs i'm sure you could send him a gasket for them,and he could make you four mounting flanges to go on the carb ends, it's crucial is the person,pm him and ask what he would charge and if he could do it for you.

[Bass Man]

Cool! Thanks lost.

It's at a 15* Angle.

[cubert]

like lost said and i posted in your other thread, the harley carbs are pretty useless for this app...find an inline 4 jap bike and grab the carbs...


and damn you roodoo for posting that picture...makes me want to take a spare set of carbs for my bike and find a rusted out honda to mess with them on

[cygnus x-1]

I have a secret project (until now) to fit up a set of Hayabusa TBs to my A20. I have a manifold design drawn up in CAD but I haven't had it made yet. It could be made to fit carbs, but spacing will be an issue. My design requires that the TB spacing (80mm on center) be modified to the same as the Honda head ports (90mm on center). It's also a 3 piece design where you have a head plate, some tubes for the runners, and a TB plate. The 3 piece design is so you can put in different length runners without having to make an entirely new manifold. You just swap in the new runners and go. It also means that you could potentially use different TBs or carbs by changing just the TB plate, and not have to remake the entire manifold. The major issue though is that the carbs or TBs have to be modified to have a 90mm spacing.

C|

[Bass Man]

Spacing as in fitting? such as tube to tube?

[cygnus x-1]

Spacing as in fitting? such as tube to tube?

Yeah, the ports on the head are spaced 90mm from each other so ideally you want the carbs/TBs to have the same spacing. The other option is to have the runners curve in a little so they can match up to whatever the spacing of the carbs/TBs is. Either way will work and both have pros/cons.

C|

[markmdz89hatch]

Can someone put a protractor on the damn head, while the car is on dead level ground and tell me the EXACT angle of the head perpendicular to the ground? I kinda can't do that since mine is immobile and no where near on level ground.

I'm working on a manifold fabrication now, and don't want to end up spending thousands of dollars on a manifold mocked up at the wrong angle.

Sorry for the urgency too, but my window of time for this info is nill. (Yeah I'm a prick, but if my plan works out and I get the exact angle, I promise there will be many more happy ppl on this board.)

::wink::

[markmdz89hatch]

I'm curious too, what's the need for the balance tube on that setup in the pic that roo posted? As long as the runner lengths and diameters (total volume) are all identical, as well as the carb sizes and jetting, wouldn't there be no need for the balance tube? Don't you lose a hell of a lot of air velocity and proper flow into the head when you put the tube in?

[2oodoor]

Ask FlaLude from preludepower... it's his set up and he says they are making more once it is bugged out, I wondering about that myself and thinking about a pull thru boost situation or nitrous or other auxilary fuel port, looking at the big end of that thing..
I dont have a fancy protractor like that or I would do that measurement.

[lostforawhile]

I'm curious too, what's the need for the balance tube on that setup in the pic that roo posted? As long as the runner lengths and diameters (total volume) are all identical, as well as the carb sizes and jetting, wouldn't there be no need for the balance tube? Don't you lose a hell of a lot of air velocity and proper flow into the head when you put the tube in?

the point is you can't get every cyl exactly the same,maby in theory you could, but all you can do is get them close. the tube is important in making the setup function right. the other reason for it,is you tap one end for your power brake booster. when you let off the throttle, the throttle plates close inducing a high vaccume under them,the balance tube is tapped right behind the carbs tapping this vaccume for the brake booster. the carbs have to have the balance tube to ensure the same mixture in all intake ports. an interesting point,is some ferraris had multiple carb setups and no balance tube,this is what led to people saying the carbs were possessed. it makes them very finicky,the mixture in one carb can go way off.

A twin carburetor of the variable-area choke type for use in multi-cylinder engine of a motor vehicle which twin carburetor includes a balance tube interconnecting a front side mixture passage and a rear side mixture passage at a point downstream of each throttle valve for balancing the pressures therein to equalize the amounts of air-fuel mixtures to be supplied to individual cylinders of the engine and an additional balance tube interconnecting the front side mixture passage and the rear side mixture passage at a point downstream of each choke for balancing the suctions or depressions acting on each air valve to provide the same effective cross-sectional area at each fuel jet for thereby producing air-fuel mixtures having equal air-fuel ratios in each mixture passage.

[markmdz89hatch]

Tim, thanks for that insight. Honestly now that you bring it up, it was always kinda a mystery to my as to how only one runner got the brake booster vac tap, which would automatically throw off the a/f ratio.

Now here's where I have to toss out a giant what-the. Almost all DCOE setups I have ever seen do not contain a balance tube. Is that just because of the pre-balance-tube train of thought? I suppose that's a good argument seeing as nearly every single ITB setup (based on the same premise as one carb per cyl) has a vacuum manifold which taps all runners. hmm..... something to chew on now....

[cygnus x-1]

For ITB setups you need a steady MAP signal for the injection controller to read. The typical way to do this is to tap each runner to a single line and feed that to the sensor. It's essentially the same thing as a balance tube on carb manifolds. I guess if your carbs were well matched you wouldn't need a balance tube?

C|

[lostforawhile]

For ITB setups you need a steady MAP signal for the injection controller to read. The typical way to do this is to tap each runner to a single line and feed that to the sensor. It's essentially the same thing as a balance tube on carb manifolds. I guess if your carbs were well matched you wouldn't need a balance tube?

C|

you can never get them perfect. there are too many variables in each cyl that cause the carbs to be off,

[2oodoor]

you can never get them perfect. there are too many variables in each cyl that cause the carbs to be off,

That is why you educate yourself on the principals, and apply foresight to the engineering of mulitple carb set ups. We are not re-inventing the wheel here, just making it roll better for longer.
I have not noticed the brake booster tap on other DCOE apps, something I need to look a little closer to. Since the brake booster uses vacuum only part time and also incorporates a check valve, I don't see it interferring with the mixture a whole hella lot.

[markmdz89hatch]

That is why you educate yourself on the principals, and apply foresight to the engineering of mulitple carb set ups. We are not re-inventing the wheel here, just making it roll better for longer.
I have not noticed the brake booster tap on other DCOE apps, something I need to look a little closer to. Since the brake booster uses vacuum only part time and also incorporates a check valve, I don't see it interferring with the mixture a whole hella lot.

That being said, I think I'm going to look into the principals of a 'balance' tube, and use them to incorporate a vacuum manifold from which to tap for the brake booster. However, I would end up using smaller ports and possibly angling the taps toward the head and soften all of the edges of the tap to minimize the turbulence in the runner so I get a consistent flow to the head. I dunno, just a thought.

[lostforawhile]

That is why you educate yourself on the principals, and apply foresight to the engineering of mulitple carb set ups. We are not re-inventing the wheel here, just making it roll better for longer.
I have not noticed the brake booster tap on other DCOE apps, something I need to look a little closer to. Since the brake booster uses vacuum only part time and also incorporates a check valve, I don't see it interferring with the mixture a whole hella lot.

it causes a large vaccume change whenever you press the brake pedal, if you tried to tap it off of one carb, it would unbalance that cyl,thats why all the carbs are tied together for a ,stronger vaccume signal. on the stock setup it's just tapped off of the manifold, but with a dual or more carb setup,there is really more then one manifold.

[FlaLude]

Dang what a nice setup, hey, thats my car! The balance tube was originally for the brake booster, but after using cygnus's idea about the vacuum canister I no longer needed it. There was too much fuel getting into the line that way so I switched it. Now its just plugged off. As a balance tube I don't know if its necessary or not, I am going to try another manifold without it to see if there is any difference.

[markmdz89hatch]

Dang what a nice setup, hey, thats my car! The balance tube was originally for the brake booster, but after using cygnus's idea about the vacuum canister I no longer needed it. There was too much fuel getting into the line that way so I switched it. Now its just plugged off. As a balance tube I don't know if its necessary or not, I am going to try another manifold without it to see if there is any difference.

What is this vacuum canister you speak of? Please elaborate, I'm extremely interested.

[lostforawhile]

[FlaLude]

Its just a storage tank for vacuum with an inlet and outlet. There was a stock one on the prelude that mounts on the firewall and I just used that. Works perfect, no drop in idle when pressing the brakes like I had before with the vacuum running to the booster from the crossover pipe.

What is this vacuum canister you speak of? Please elaborate, I'm extremely interested.