Carburetor Sizing and Manifold Modifications

[CFS Carbs]

One of the most significant factors determining the air flow capability of a carburetor, is its throttle bore size, its diameter. Once the desired carb flow , for a particular engine, is determined, that pretty much dictates the size of the throttle bore of a suitable carb. On an engine used in competition, to get the full potential of what the carb can feed the engine at full throttle, it is important that the bore of the intake manifold, at the flange, be the same size and shape as the carb's throttle bore, and that the two bores are well centered on each other. A mismatch in size or shape, or poor alignment of the two bores, causes turbulence in the flow, which reduces flow, which reduces the power of the engine. So once the throttle bore size has been determined, the next important step is to get intake manifold bore to match that in size, shape , and to achieve good alignment of the two.

Being that the major intent, in most cases, is to get more power from a pulling tractor engine, the size of the carb's throttle bore will typically be larger than in the carb that the tractor factory used, back in the day. For engines where the original intake manifold will be used, enlarging the manifold's bore at the flange, and at least an inch or two up in, will be necessary to allow the engine to take advantage of the carb's flow capability. Depending on the thickness of the intake manifold's material, at and above the flange, that may or may not be possible. If increasing the manifold bore diameter doesn't quite get it to the carb's throttle bore diameter, then it may become necessary to use an adapter plate with a tapered bore, between the carb and manifold flange, to help deal with that mismatch in sizes. This isn't the optimal situation, but does help reduce some of the turbulence and increase flow.
For the guys that decide to use a fabricated intake("header"), there are some dimensions and orientations that he should specify to the header fabricator.

1) bore size(inside diameter), same as the carb's throttle bore size 2) center to center distance of the two mounting bolt holes, and bolt hole size, smooth or threaded, 5/16" or 3/8". 3) orientation of the bolt holes of a two hole bolt flange, to get the carb's throttle shaft pointing in the right direction, or a flange with four bolt holes. 4) flange thickness, which determines the length of the bolts or studs used to mount the carb. 3/8" is most common 5) WARNING: slotted flange holes often give poor alignment.
Back in the day, there was little standardization of the bolt hole center to center distance, in relation to the throttle bore size of the carb. Zenith made an attempt at it, but did allow a lot of variations as dictated by the tractor factories. For the vintage pulling tractors being used today, the three most common center to center distances are : 2 11/16"(2.687"), 2 15/16"(2.937"), and 3 5/16"(3.312"). For the 250 cfm carb that I build, which has a 1 3/4" throttle bore, I usually make the bolt hole center to center 2 15/16", unless a customer specifies something different. The bolt holes in the throttle body are threaded 5/16", and I usually use 5/16" carb studs . That way, the bolt holes in the intake manifold flange can be a smooth 5/16", or threaded 3/8", for a slip fit of the 5/16" studs through those holes. For 3/8" smooth flange holes, I have some short thin wall sleeves that take up the gap between the 5/16" studs and the 3/8" holes.

Assuming that the manifold bore is midway between the bolt holes, this arrangement of stud size and bolt hole size pretty well centers the carb's throttle bore on the manifold's bore. The gasket between them centers on the carb studs, so that it doesn't move around during the installation of the carb onto the intake flange. All this alignment is beneficial to the operation of the carb and the power of the engine. Efforts to achieve it, will pay off.

Norm Schenck/ Comp Fuel Systems/Birch Run MI/
520-241-2787/ normanschenck@juno.com

 

[VicS]

Thanks I'm impressed.
 

[CFS Carbs]

Thanks Vic. My aim is to help pullers be more successful, with good carbs and technical advise. Norm
 

[VicS]

Right now I'm a diesel puller. But glad to
know something better is available.
 

[CFS Carbs]

I ve been dealing with carbureted gas and methanol engines for over 50 years, so I don t know enough about diesel fuel systems to help you. But what is being done with diesels in the pulling world now, is remarkable
 

[DrAllis]

Enjoy reading things like this very much !! Now, I have a theoretical question along the same lines with an updraft tractor type carburetor. Lets
compare two very similar carbs but different throttle bores. Would it be fair to say that a 2 inch throttle bore will flow more than an 1 3/4 throttle
bore when both carbs are using the SAME sized venturi ??? I'm assuming yes ?? And I'm sure at some point the actual air cleaner connection (carb air
inlet diameter) starts changing things too. Thanks for any replies !!!
 

[CFS Carbs]

The air flow though an intake system encounters a series of places that resist the flow. It's like a DC electric circuit with a number of resistors connected in series. Efforts to reduce or eliminate the resistance to flow, of any one of those "resistors", results in more flow at the end of the "circuit".
In the case of a carb, the venturi, depending on its size and shape, and the type of venturi booster being used, may or may not be a significant "resistor" in the series of resistances. Depending on their relative ability to flow, of the venturi and throttle bore areas, increasing only the throttle bore size may or may not cause a significant increase in flow. If the venturi flow is already more restrictive than the throttle bore area, before the increase in throttle bore size, then the bigger throttle bore may give a minimal increase in flow. If the throttle bore was the more major restriction before the change, then the bigger throttle bore will cause a larger increase in flow. Also significant, are the shape of the throttle shaft and butterfly. So there are lots and lots of variables in the whole flow path.
I pay attention to the relative sizes of the venturi and throttle bore, because that is important to the part throttle drivability of the carb and engine. For an engine that needs to run well over a wide rpm band, and at part throttle as well as full throttle, this relative size becomes as important as the sizes themselves. It's all a balancing act, getting good part throttle drivability and good full throttle flow.
Then there is the really major factor of efficient fuel metering. If a carb doesn't meter fuel well, then it doesn't matter what air flow capacity it has. If fuel metering has been sacrificed to get more air flow, then the carb has moved in the direction of becoming useless. So the balancing act of sizes, and the efficiency of flow due to shapes of the venturi, the booster, the throttle bore, the throttle shaft, the butterfly, and flow passages before and after the carb, becomes a complicated interaction.


This post was edited by CFS Carbs on 11/26/2023 at 06:58 am.