Carburetor Formula

[minifridge1138]

Hello everyone,

On a lot of threads, I see people asking about what size carburetor to use, or bragging that their giant carburetor gives them more power. It seems that carburetor size is a very misunderstood subject. Hopefully, this thread can help answer some questions.

Fact: the carburetor does not force air/fuel into the engine.
Fact: the engine sucks air/fuel out of the carburetor.

What does this mean? It means that a bigger carburetor will not get extra air/fuel into the engine. All it means is that a small carburetor will provide too little air/fuel.

How do you know what size carburetor is right for your engine? That is a simple physics problem. To simply see the formula skip the following section. For an explanation of the formula continue reading.

1) Our engines are sized by displacement. Displacement is a measurement of the volume of air/fuel the engine can use in a single rotation of the crankshaft. A 350 cubic inch engine will suck in and push out 350 cubic inches of air/fuel in a single rotation of the crankshaft.
2) Engines (our engines) are measured in cubic inches. Carburetors are measured in cubic feet. To convert from cubic inches to cubic feet, multiply by 0.0005787.
3) Our engines are 4 stroke engines. This means that the piston only sucks
in air from the carburetor every other stroke. So we need to divide our
revolutions per minute by 2.
4) Our engines only rotate so many times per minute. We have a maximum rpm limit. Since our engines displace a certain amount of air/fuel per revolution, the number of revolutions per minute is important.
5) Very few engines actually operate at 100% volumetric efficiency. This means that even though an engine is rated at 350 cubic inches, it will actually displace slightly less. Most production engines operate between 70% and 85% volumetric efficiency. High performance camshafts, heads, pistons, etc can produce a higher volumetric efficiency. At best, they will produce 100%. The only way to go beyond is to force more air into the
engine. That is how turbo charges and super chargers produce extra power.

Putting these five facts together gives us this formula:
((A * 0.0005787 * B) / 2 ) * C = carburetor size
A = Engine displacement in cubic inches
B = Maximum RPM
C = Volumetric Efficiency as a percent (100% = 1. 50% = 0.50 ).

The result is the appropriate size of a carburetor in cubic feet per minute. As you can see, the carburetor size is the result of the engine's build. Simply adding a bigger carburetor will not increase the displacement or volumetric efficiency. It can increase your maximum rpm, but only if the engine is capable of rotating that quickly (personally, I don't want to cruise at 10,000 rpms).

Lets play with the formula.
Engine size = 350 cubic inches.
Max RPM = 5,500 rpm.
Volumetric Efficiency = 1
((350 * 0.0005787 * 5500) / 2 ) * 1 = 556.9987 cubic feet/minute
So if my 350 small block is running at 100% efficiency, and I want to reach 5,500 rpms,
then I need a 556 cfm carburetor.

Lets rev higher.
Engine Size = 350 cubic inches
Max RPM = 7,500 rpm. (well beyond anything a street car should reach)
Volumetric Efficiency = 1.
((350 * 0.0005787 * 7500) / 2 ) * 1 = 759.543 cubic feet/minute

And just for fun...
Engine size = 350 cubic inches.
Max RPM = 100,000 rpm.
Volumetric Efficiency = 1.
((350 * 0.0005787 * 100000) / 2 ) * 1 = 101,272 cubic feet/minute


What does all of this mean? It means that putting that 850 cfm double pumper on your street car is a waste of money. Speaking from personal experience, my Corvette had a stock 350 motor with a 750 cfm carb. I got 14 mpg on the highway. I replaced it with a 600 cfm caburetor (which is still too big for the rpm range I drive in) and I now get 22 mpg highway AND better throttle response.

Work smarter, not harder. And have fun.

 

[Red73BB]

thanks for sharing this info.

I have wondered what made up this formula and how to apply correctly. Now I understand with this thread .

 

[tat2ed]

Very interesting. Thanks for that info. So simple even I can understand it. Somewhat !!

 

[TMPJoe]

you will also find that on the Holley website and I believe on the Edelbrock website

of course no one gets 100% volumetric efficiency... and the upper RPM limits are rarely applicable... but that is an excellent discussion! Most people plainly go too big....

one of the nice features of the quadrajet (yes your STOCK) carb was its small primaries and unique air horn design, acted like a 2 barrel when cruising... then large secondaries acted like a much bigger carb at higher RPMS and WOT

 

[MikeMc71]

you will also find that on the Holley website and I believe on the Edelbrock website

of course no one gets 100% volumetric efficiency... and the upper RPM limits are rarely applicable... but that is an excellent discussion! Most people plainly go too big....

one of the nice features of the quadrajet (yes your STOCK) carb was its small primaries and unique air horn design, acted like a 2 barrel when cruising... then large secondaries acted like a much bigger carb at higher RPMS and WOT

I agree, the Quadrajet carb is very unique in that respect. It is probably one of the most unappreciated and misunderstood carbs out there. The thing is very complex, compared to some of the other options, but when it is set-up roght and working well, it is very hard to beat.

 

[Carver81]

So what is involved in changing out my stock carb?

So based on your application of the formula (A great piece of info BTW), if I have a 1981 with the L81 and the stock Rochester carb and the 3C computer controlled engine management system, what all do I have to change? Anyone have any experience with this? My understanding is that is not as simple as taking out the carb and replacing it with another because of the 3C system.

Carver81

 

[TMPJoe]

No its not simple for you. The 3C is a rudimentary first pass at computer controls. Just about the "first" of its kind I think. SOmeone can tell me more but it certainly is about as early as I remember seeing.

I haven't played with that but there are members here experienced. You are pretty much stuck with either staying pretty much stock or completely changing to a non stock set up. It depends on your laws up there for emissions and also your personal goal for the car.

If you choose to go all out with a high performance set up its a good time to just rebuild the entire engine and driveline.

To me it comes down to having a goal for your car. Is this a collector as is completely numbers matching original or is this going to be your personal toy?

 

[Carver81]

No its not simple for you. The 3C is a rudimentary first pass at computer controls. Just about the "first" of its kind I think. SOmeone can tell me more but it certainly is about as early as I remember seeing.

I haven't played with that but there are members here experienced. You are pretty much stuck with either staying pretty much stock or completely changing to a non stock set up. It depends on your laws up there for emissions and also your personal goal for the car.

If you choose to go all out with a high performance set up its a good time to just rebuild the entire engine and driveline.

To me it comes down to having a goal for your car. Is this a collector as is completely numbers matching original or is this going to be your personal toy?

I am trying to keep the car as stock as possible as it is now a numbers matching setup. Thanks so much for your comments.

 

[petes74ttop]

Hi Minifridge1138, that is great information!
22 miles to the gallon, now that's a swap I'd like to do. My carb # is 7044206. My 74 is stock and my car has no A.I.R., can you tell me what car I can get a 600 cfm from or the carb #, thanks PG

 

[minifridge1138]

I have a 600 cfm Holley 4 barrel with electric choke. Part number: 0-80457S. I paid $250 for it at OReillys Auto Parts.

I believe the same carburetor with a manual choke is part number 0-1850S (shiny finish) and 0-1850C (dichromate finish).

Personally I like Holley Carburetors because it is what my friends have and we can share parts. A lot of people like edelbrock (I switched from an edelbrock intake manifold to a holley intake manifold and I think it hurt my low end torque). Holley also makes a 570 cfm carb. The formula tells us that a 350 cubic inch motor can run up to 5500 rpms with 557 cubic feet of air, so a 570 should do the job, also.

I would also add that the Quadra-jet carburetor is a great carburetor. It has small primaries, so you use very little fuel at constant speed and large secondaries, so you get plenty of fuel when you hit the gas.

 

[petes74ttop]

Thanks, again, PG

 

[TMPJoe]

pete

Holley also makes a carb called a street avenger series.. very user friendly if you don't go with the stock quad... comes in 570,670 770 and 870 series... has a vacuum advance and electric choke... requires no tuning.. you can change springs without taking the carb off.. similar to the new edelbrock design.. I run a 770 on the street with my motor and run in the high 11's.. never touched it other than to install the lightest spring possible and adjust the idle speed mixes...

 

[petes74ttop]

Hi, the 570 sounds like what I would be interested in.
Questions: I have a divorced choke-the kind with the spring on the manifold, can I still use that?
Can I keep the same manifold?
How about the throttle linkage-will that hook up or will I need something else?

My 1/4 mil times: 15.6 @ 92.3 PG:L

 

[minifridge1138]

I don't know about the choke/manifold issues.
I managed to re-use all of the same throttle linkages. I had to drill a few holes in the mounting bracket, but everything fit in the end.

 

[TMPJoe]

pete no that choke is for the quadrajet on the car orginally

you can block it off but once you start doing this one thing begets another..

if you want to stay stock just get a rebuilt or have yours rebuilt.... ecklers has quadrajets that fit your car.. they have a decent tech line that will help you get the right parts to do the job...

otherwise you might want to change manifolds ... sumitt and jegs carry the throttle linkage you need.. they sell the plates and throttle springs etc as a kit to hook it all up with a turbo style automatic tranny or 4 speed if you have it

 

[petes74ttop]

pete no that choke is for the quadrajet on the car orginally

you can block it off but once you start doing this one thing begets another..

You are so correct-one thing begets another!
I saw the post where one member changed from aQ-jet to a holly and got a big increase in mileage, It costs about $2,000 in gas, if that cost could be reduced to $1,600 that would seem to make good sence.
So that's what I'm asking, what else needs to be changed, replaced or monified?
Thanks for the help, PG

 

[TMPJoe]

Pete

Well getting good gas mileage with a 1974 Corvette is a challenge to be sure!

I think either you misread that comment as I recall reading the opposite in some posts, or the subject car most likely got better mileage from being overhauled in general.

On a stock mid-70's to 1980 corvette I would think a properly functioning quad would perform with the best overall mileage, but maybe someone else has a better opinion.

If you use the street avenger series Holley it is built around the sam kind of principles as the old quads as far as cruising versus WOT operation.

What is the condition of this motor and car in general? What have you done to the car so far? How many miles on the motor since it was rebuilt? etc.

those are important also... is the car running rough? surging? stalling?

why do you think you need to work on the carb?

 

[petes74ttop]

Hi TMPJoe, when Minifridge1138 posted that great info about cfm's and said he got 30% better mileage, that made me consider doing the same.
My goal 6 years ago was to do a ground up restoration and make my 74ttop a dependable daily driver. I have a replacement GM 350 crate engine, stock manifold w/ Q-jet, original cast iron headers a rebuilt th400 and chambered exhaust. I never go past 4,000 rpm. I had no problems in the 30,000 miles I've driven, ( except for the time with the fan belt, I now carry an extra set )
This was a good post and I learned something, Thanks to everyone, PG

.

 

[Vettehead Mikey]

Pete

Well getting good gas mileage with a 1974 Corvette is a challenge to be sure!
...............
On a stock mid-70's to 1980 corvette I would think a properly functioning quad would perform with the best overall mileage, but maybe someone else has a better opinion.

I regularly get at least 20mpg in my '73 - with stock manifold and carb. Same set up as a '74.

 

[TMPJoe]

I regularly get at least 20mpg in my '73 - with stock manifold and carb. Same set up as a '74.

Driving like its icy out will do that....

I only put a 1000 a season on mine if I am lucky.. (this year being laid up almost 0!)

so mileage is not the point of my rig....

but seriously that is why he should stick with the quad and a pretty tame stock tune.... just clean up what he has though..

most of the time I see people who don't have the car in excellent running condition... timing issues, carbs in need of rebuilding etc

the other thing is people who mismatch heads, cams, intake and carb.... this whole thread is a good way for a lot of guys to learn that bigger is not better.. right sizing is critical

how many times do you see people bolt on a set of heads thinking now I rocking.... they don't know what pistons they have, no clue as to the cam set up... they buy the intake that looks best... and then they toss their friends favorite carb on....

i get poor mileage on my car cruising because it has no lock up and a very aggressive TCI torque converter.. so its not a mileage queen.. like I said.. it wasn't my goal

personally if I wanted excellent cruising... i would use an EFI set up and an overdrive transmission....