Mike
Well-Known Member
One of the most common mistakes people make when assembling an engine is not properly sizing the carburetor for the intended application. Some people figure if some is good, more has to be better and they dash out to buy the biggest carb they can afford. Only to later grumble about how the engine won't idle well and is lazy under hard acceleration.
There is a very simple formula for determining what size carburetor you need for your engine. Let's look at how it all comes together.
For our example, let's use a 350 cubic inch engine, equipped with a mild hydraulic cam, to go into a T-Bucket.
The first part of the formula is to determine engine displacement in cubic inches. We know we have a 350.
Now we need to determine what our maximum RPM will be for this engine. Realistically, with the use of a hydraulic cam, maximum RPM is going to be in the 5500 - 6000 RPM range. Let's use 6,000 for our example. We now want to multiply our total displacement by maximum RPM. 350 X 6,000 = 2,100,000
We now want to divide that number by 3,456. 2,100,000 / 3,456 = 608 (rounded up to a whole number)
The 608 figure is how large your carburetor should be, assuming 100% volumetric efficiency. Assuming this engine is naturally-aspirated, you're not going to get anywhere close to that kind of VE number. More realistically, your VE numbers are going to fall into the 75% - 80% range. Let's assume 80% for this example. We now want to determine 80% of the total calculated CFM, which was 608. 608 X .80 = 486.40 actual CFM.
Your 350 will be capable of flowing about 487 CFM of air at 6,000 RPM, assuming 80% volumetric efficiency.
ENGINE DISPLACEMENT X MAXIMUM RPM / 3,456 = CALCULATED CFM AT 100% VOLUMETRIC EFFICIENCY
CALCULATED CFM X VOLUMETRIC EFFICIENCY = ACTUAL REQUIRED CFM
350 X 6,000 / 3,456 = 608
608 X .80 = 486.40
I would look at bolting a 600 CFM vacuum secondary carb onto this engine. It will idle really well and have excellent throttle response.
Already, I can hear the murmurs from the back of the room. "Well, I used an 800 CFM double-pumper on my 350, and it ran just fine!"
That all sounds real good in the drive-in parking lot, but a 350 cubic inch engine at 80% volumetric efficiency would have to be running at 10,000 RPM before that 800 CFM carb will ever be required. We all know a Dominator is going to attract more gearheads at the local car show than a 450 CFM vacuum secondary carb ever will, right. So now, all you need do is determine if you want your engine to run really well, or to look really well.
Just to show you how rapidly this number can change, let's assume you have a 350 cubic inch engine that will only manage 75% volumetric efficiency and that you do not intend to ever rev it beyond 4500 RPM.
350 X 4,500 / 3,456 = 456
456 X .75 = 342
In this case, a 350 CFM carb is all that would ever be required. A 570 CFM vacuum secondary carb would work for an application like this. It will provide good idle characteristics and will also have good throttle response, without ever needing to pull the secondaries open, unless you were in an extreme situation.
When you use this formula, try to be as realistic as possible with the VE number and the maximum RPM number. If 6,000 RPM is going to be the absolute maximum number, don't try fudging the number to 8,000. Doing that is only going to cost in terms of idle quality and in throttle response. As you can see, small changes can change flow requirements by a considerable amount.
Stick to this simple formula and you'll never find yourself needing to get outside the tuning window of your carb, trying to get it to idle and take throttle.
There is a very simple formula for determining what size carburetor you need for your engine. Let's look at how it all comes together.
For our example, let's use a 350 cubic inch engine, equipped with a mild hydraulic cam, to go into a T-Bucket.
The first part of the formula is to determine engine displacement in cubic inches. We know we have a 350.
Now we need to determine what our maximum RPM will be for this engine. Realistically, with the use of a hydraulic cam, maximum RPM is going to be in the 5500 - 6000 RPM range. Let's use 6,000 for our example. We now want to multiply our total displacement by maximum RPM. 350 X 6,000 = 2,100,000
We now want to divide that number by 3,456. 2,100,000 / 3,456 = 608 (rounded up to a whole number)
The 608 figure is how large your carburetor should be, assuming 100% volumetric efficiency. Assuming this engine is naturally-aspirated, you're not going to get anywhere close to that kind of VE number. More realistically, your VE numbers are going to fall into the 75% - 80% range. Let's assume 80% for this example. We now want to determine 80% of the total calculated CFM, which was 608. 608 X .80 = 486.40 actual CFM.
Your 350 will be capable of flowing about 487 CFM of air at 6,000 RPM, assuming 80% volumetric efficiency.
ENGINE DISPLACEMENT X MAXIMUM RPM / 3,456 = CALCULATED CFM AT 100% VOLUMETRIC EFFICIENCY
CALCULATED CFM X VOLUMETRIC EFFICIENCY = ACTUAL REQUIRED CFM
350 X 6,000 / 3,456 = 608
608 X .80 = 486.40
I would look at bolting a 600 CFM vacuum secondary carb onto this engine. It will idle really well and have excellent throttle response.
Already, I can hear the murmurs from the back of the room. "Well, I used an 800 CFM double-pumper on my 350, and it ran just fine!"
That all sounds real good in the drive-in parking lot, but a 350 cubic inch engine at 80% volumetric efficiency would have to be running at 10,000 RPM before that 800 CFM carb will ever be required. We all know a Dominator is going to attract more gearheads at the local car show than a 450 CFM vacuum secondary carb ever will, right. So now, all you need do is determine if you want your engine to run really well, or to look really well.
Just to show you how rapidly this number can change, let's assume you have a 350 cubic inch engine that will only manage 75% volumetric efficiency and that you do not intend to ever rev it beyond 4500 RPM.
350 X 4,500 / 3,456 = 456
456 X .75 = 342
In this case, a 350 CFM carb is all that would ever be required. A 570 CFM vacuum secondary carb would work for an application like this. It will provide good idle characteristics and will also have good throttle response, without ever needing to pull the secondaries open, unless you were in an extreme situation.
When you use this formula, try to be as realistic as possible with the VE number and the maximum RPM number. If 6,000 RPM is going to be the absolute maximum number, don't try fudging the number to 8,000. Doing that is only going to cost in terms of idle quality and in throttle response. As you can see, small changes can change flow requirements by a considerable amount.
Stick to this simple formula and you'll never find yourself needing to get outside the tuning window of your carb, trying to get it to idle and take throttle.