Carb size

Hey guys

What effect does the size of the carb have on the engine

I know the answer to my question is probably not a simple one but a simple answer would help if one exists.

Cheers

Steve

So much is related to air density. The more dense the air, the larger the carb bore can be. At higher altitude, the air can be so thin that a large bore carb will not generate enough vacuum to properly draw fuel. So, on a hot day at high altitude, you might end up richening the needle in order to get enough fuel through the spray bar. This screws up a lot of people that take a boat run at low altitude to a race at high altitude.

A smaller bore carb that works at high altitudes will also work well at lower altitudes. But, the performance might be less than running a bigger bore carb when running at low altitudes.

I don't know if this response it what you were looking for, but you might find it interesting.

Al Hobbs

I agree with Al,

Smaller bore = higher pressure drop creates a stronger fuel signal at the spray bar / quicker throttle response aka strong out of the hole / easier to tune

Bigger bore = lower pressure drop at the spray bar creating a weaker fuel signal / slower throttle response aka slug out of the hole / harder to tune

To small of a bore and you greatly reduce the performance of your engine and I belive same to be true with to big of a bore.

I maybe wrong but this is how I see it. If I'm wrong, hopefully some more knowledgeable people on the subject will chime in and correct me.

Carb size is limited to how big the intake track is. Any bigger and it serves no purpose. Any smaller and it is not optimized.

As far as throttle response and fuel delivery that is all in the spray bar size,design and placement in the bore.

Look at a MAC.67 and a CMB1.01 same size carb.................SO what dose that tell ya?????????????????

A VERY GENERAL rule of thumb:

Smaller carb = better low end throttle response & torque

Larger carb = better top end & more rpms

It can be said without any question, that air which is forced around a 90* angled edge immediately prior to the carburetor's mouth, will effectively reduce the flow throughout the induction system. To maximize air flow potential, a radiused entry of some sort is essential, especially in single carburetor applications. Although many different lengths & shapes can be applied, the features that most influence flow potential are radius size & radius shape as well as the the overall bore area compared to the intake track's area. Large bore carburetors will require pressurized fuel systems (120 inches of water =4.336 psi approximately at WOT) & metered fuel supplies for a good throttle response over the entire range.

Jim Allen

Suggested Google reading: Blair_and_Cahoon)_Design_of_an_intake_bellmouth_Sept._2006.pdf

Thanks guys for the replies, to be honest I've never really taken much notice of the carb size over the years, and it only cropped up last week when my nephew was over and saw a few of my engines and asked me why the carbs are different in size, at the time my response was "because they have been designed that way" unfortunately that was not a good enough answer for him hence why I posted the question. These are the engines in question.

I think MOPARBARN reply is simple enough for me so should be simple enough for my nephew(I hope)...

Thanks guys

Really nice collection. And thanks to everyone for shearing information.

yes, thanks for all the comments here on carb bore ID size.

While on the same line of thought (sort of):

Should a tuned-pipe stinger end remain at least equal or smaller in size (Inside Diameter) than the carb bore ID?

David now for fun check the spray bare hole size on a all those carbs and see what you have.

David Bryant, on 08 Jan 2016 - 3:14 PM, said:

David Bryant said:

yes, thanks for all the comments here on carb bore ID size.

While on the same line of thought (sort of):

Should a tuned-pipe stinger end remain at least equal or smaller in size (Inside Diameter) than the carb bore ID?

Click to expand...

The ID of the stinger is not connected in any way to the ID of the carburetor's bore. However, it's ID is definitely going to be connected to the total volume within the tuned pipe. Did you ever notice how long the stinger on any high performance racing two cycle engine is compared to the stingers found on our toy engines? Builders of racing two stroke engines are using tuned pipes whose volumes are approximately 28 to 32 times the engines total displacement. Doing this will allow the stinger's ID to be small & the stinger's length will be 13.5 times it's ID. As an example, my 27 CC gas engines use a stinger that is .406" ID & 5.500" in length. Increasing the stinger's length has little effect on the engine's operating temperature. Decreasing the stinger's ID will have a pronounced effect on raising the engines's operating temperature.

Jim Allen

There is certainly a working relationship between carb ID AND STINGER ID when pertaining to nitro boats.

Most of the infamous papers only pertain to gas or alcohol two strokes............

Nitro is a whole different animal................................

The photo shows machined & rolled steel tuned pipes made for various sizes of nitro & gas engines that have been fully tested on my dynos & at the lake. Fuels tested include nitro mixes. oxygenated fuels, different octane gases & methanol alcohol gas mixes. The basic principles of tuned pipe construction do not change because the fuel changes. Actually, almost any chamber that looks like a tuned pipe can be made to work. No one pipe can satisfy every running condition!

Jim Allen

Jim Allen said:

The photo shows machined & rolled steel tuned pipes made for various sizes of nitro & gas engines that have been fully tested on my dynos & at the lake. Fuels tested include nitro mixes. oxygenated fuels, different octane gases & methanol alcohol gas mixes. The basic principles of tuned pipe construction do not change because the fuel changes. Actually, almost any chamber that looks like a tuned pipe can be made to work. No one pipe can satisfy every running condition!

Jim Allen

Click to expand...

Well then you are certainly not finding the right combination. It Is a matter of fuel delivery and on a "pressurized fuel system the size of the stinger very much matters. And in a nitro pressurize system it needs a lot more fuel.

There is a relation ship on the size of the spray bar and the boar of the carb. also the size of the fuel line as this controls the volume of fuel that is trying to be moved by the suction on the tube in the carb. the size of the line to the needle also has to be taken into consideration as the pressure supplied by the pipe has to move the volume in this line. This is referred to head pressure. think of it like a well and how much head you have. the size of the pipe determines the weight of the water you are trying to lift.

There is a balance that has to be taken it to consideration. The needle just changes this balance between sucking and pushing nothing more.

The spray bar size is the main jet in the system not the needle.

Most don't understand this. the area of the hole in the spray bar is what controls the suction and how and where it is in the air stream.

There are different ways to create this suction at the spray bar in its design.

My self a simple straw stuck in the air stream at the right spot is the simplest way to do it. Easy to make changes with just a straw change.

No muss no fuss just stick a smaller or bigger tube in the hole.

Large fuel lines will not always be beneficial. Just means you need to build more pressure to move the extra weight of fuel.

The least amount of pressure you need to maintain enough flow is ideal as it will not change as much and you can un choke the pipe to flow more ex gasses. you don't want to choke the eng to build heat. There are other ways to build heat in the eng.

Just what I understand at this point in my testing.