What Carb Do You Really Need


Before we go any further let me give you the Gospel According to Bob. When you are shopping your parts and trying to figure out what you need- Be Real! You may want to go to the track but will you really? That kind of stuff. Don’t let your buddies set you up with a setup that just won’t work. There are no universal setups. Each and every situation is different. If all else doesn’t seem to make sense talk to the manufacturer direct. I said my piece now we can continue.

The Questions

One of the first things to determine is what size carburetor do you need? A number of factors come into play here.

  • What cubic inch size is your motor?
  • What do you do with it? Race, street use, towing, street use with occasional trips to the track?
  • What type of intake manifold do you have, split plenum, open plenum, tunnel ram, individual runner?
  • How fast do you spin the motor?
  • What is the volumetric efficiency of the motor?
  • Do you have a manual or auto transmission?
  • What is the rear gear ratio?
  • Do you want to get the best gas mileage possible or
    do you want to develop the most power possible?

Damn! Didn’t think it was that complicated did you? Keep in mind that a carburetor is just one part of the engine combination. All of the parts need to work together. And putting a larger carburetor on you current daily driver is NOT going to immediately give you 100 more horsepower. The carb needs to work with all the other parts you have chosen and how you use the vehicle.

And if you are putting this on your daily driver this next section is a reality check and going to really disappoint you! Fair Warning!

The Math

The main element to determine your CFM requirements is a pretty simple math formula. Here it is: CARB CFM = (Cubic Inches) times (Max RPM) divided by 3456

It goes like this: You need to know the CUBIC INCHES of your motor. You then need to figure out the maximum RPMs the motor will be spun to. Be reasonable! 5000 is a reasonable street number. Dude, your daily driver will grenade at 7000 RPM if it could even get there! Finally you also need the VOLUMETRIC EFFICIENCY PERCENTAGE (VE%) of the engine. The first two items (CUBIC INCHES and RPMs), are relatively easy to determine. The engine VE% is another matter. If an engine could use all of the air it ingested, it would have a VE% of 100%. Many performance engines reach this level. Certain race engines can actually exceed this and reach a VE% of over 100% at certain points in their RPM range. Most production engines and most street performance engines have VE levels below 100%. In fact,
stock, production, low performance motors will fall around 75%-85% volumetric efficiency.

Other Considerations

With all that said we come up with this. A stock, low performance production motor of 350 cubic inches and you wanted to spin it to 5000 rpms max and it had a VE% of 80%, the formula would determine a required carb CFM of 405 CFM.

If you had a warmed over street performance motor of the same size, better heads, camshaft, headers and a performance intake that raised the VE% to 95% and it was capable of 7000 rpms, the formula would give you a minimum required carb CFM size of 673 CFM.

This formula will get you in the ballpark and there are some adjustments to this formula. For the street a split plenum intake is standard fair. It splits up the carb input left and right to keep the air speed up. A vacuum secondary a carb is also a great choice because it will only bring in the secondaries when needed. This is a super street combo. Great throttle response and fuel milage.

Additional Notes

If you have a longer duration performance or race cam you may need to change the power valves to handle reduced engine vacuum signal to the carb.

When you buy a used carb always check the jetting. Most of the time they have been tinkered with in their life and with the info supplied by Holley set it back to stock. Once you have the baseline setup you can make the changes to improve the carb’s performance on YOUR motor. Don’t believe the cool story you were told! Trust no one!

Holley has a comprehensive listing for every performance and race carb they make and the individual factory specifications for each carb. Stuff like list no’s, CFM and stock jetting specifications. For the most part Holley carbs have this number stamped on the front, driver’s side of the choke horn. It varies on other product lines and the location can be


2 thoughts on “What Carb Do You Really Need

  1. good stuff. Am building a 351clevor for my 69f100 ranger. I am looking for a dual plane air gap intake. Have had heads ported, polished and bowl cut. will put anice little cam etc. every idiot I know says go with a tunnel ram blah blah. what a bunch of bs that is. I like your comment and the reality of real formula

    • Thanx! One thing that a lot of so-called engine guys don’t seem to get is that bigger is not always better.Plus every intake has a working range and a dual plane is perfect for the street. Good to 5500 rpm and even if it is not optimal for the top end it is primo for a quick street engine. The other twist is that carbs work the same way get too big and you are not in the working range of the carb. And no amount of tweedling will ever get it to run right!

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