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jpd37 07-21-2004 11:01 AM

Method to determine carburator CFM?
I have a 600 CFM Edelbrock on my stock 307 SBC and had several people at the Right Coast Show last week in Syracuse tell me I should have a 500 CFM carb. I have had some recent discussion on this site regarding rich fuel mixtures and possible air leaks. But no body ever said anything about the size of the carb being too large for the motor. I suspect there is some trial & error experience telling me a 500 cfm is better than a 600 cfm for the stock motor and I'm willing to spend the $ to buy a new carb if I'm satisfied it will solve my rich fuel problem. Can any one tell me if there is a formula or reference table out there that will begin to qualify the claim? I have also considered restricting the 600 cfm carb and metering down the flow, but my intuition tells me that the modifications are not going to be as effective or as efficient as the 500 cfm carb. I have done the leak tests suggested and found none. What do you think?

Knobie 07-21-2004 11:13 AM

I copied this from Holley:

How To Calculate CFM:
Engine size (CID) x maximum RPM / 3456 = CFM
CFM @ 100% volumetric efficiency

(Example: 350 CID x 6000 RPM = 2,100,000 / 3456 = 608 CFM)

Approximately 608 CFM would be required for this engine. However, most Street engines are capable of achieving only about 80% VE; a modified street engine with ported heads, headers, intake and carburetor can achieve about 85% VE; a fully modified race engine can achieve 95% or greater VE. The CFM number arrived at with this formula must be factored by this percentage.

Next, you need to decide whether a vacuum secondary or a mechanical secondary carburetor will work best for you.

As a rule of thumb, vacuum secondary carburetors work best on:
Relatively heavy vehicles
Street gearing
Automatic transmission
Engines built more for low-end torque

Conversely, mechanical secondary carburetors seem to work best on:
Relatively light vehicles
Strip gearing (4.11 or numerically higher)
Manual transmission
Engines built more for top-end horsepower
Hope this helps

k-star 07-21-2004 11:13 AM

CID x max rpm

This is for 100 percent volumetric efficiency.

I am sure this will start a big debate but thats the formula


jpd37 07-21-2004 11:45 AM

Thanks guys! Now all I need to know is what the max rpm is for a stock 307 GM motor, 1972 vintage.

k-star 07-21-2004 12:10 PM


Knobie 07-21-2004 12:31 PM

check out

Stock valve springs are known to crap out under 4500 rpm!

jpd37 07-21-2004 12:31 PM

Knobie, Thanks again for the info. If I understand correctly, once I have calculated the VE, I should factor that value by 80% to arrive at the cfm of the carb? Or would I factor the value by 1.2 to compensate for the inefficiency?

I checked the Edelbrock web site and they recommend a 500 cfm Performer for 305 and smaller motors. My calc (307 x 4500) / 3456 = 399.8 cfm. If I factor it by .8, the result is 319.8 cfm. That is a long way from Edelbrock's recommendation of 500 cfm.

What makes more sense? It seems to me Edelbrock should not be very far off unless they are trying to satisfy small block motors with big cams and valves.

On the other hand, I have had several people tell me a 500 cfm is the right size for my motor.

Since I cannot seem to find parity with the numbers compared to recent discussions, I'm reluctant to make a decision. Maybe even 500 cfm is too much.

Knobie 07-21-2004 12:42 PM

It all depends on what the plans are for the 307. As I said, the stock springs are poor. What I would suggest is replace the 307 with a 350. Or build the 307 with bolt ons and eventually move all the parts to the 350 when you get it. I would invest into some good heads! depends what you want under the hood!

mstngjoe 07-21-2004 02:31 PM

That old Holley equation is fine for a basically stock grocery getter. The method will allow you to choose an adequate carb for your application.

But, for those with a street/high performance engine it is nothing more than a good starting point for choosing a carb for optimum/peak performance. There are many variables that must be considered.

Here's a good article by Barry Grant:

And if someone where trying to choose a DP style carb, Holley has a chart for that application all by itself:

If you take the cfm result you get from the old Holley equation and then use the DP chart (for same application), it's easy to see there is quite a range in the recommended carb cfm.

The 600 will work fine on the 305. Would a 500 work better? Maybe....but that doesn't mean you can't use the 600. Especially since Edelbrock secondary activation is determined by the engines need (read air-flow). 07-21-2004 02:40 PM

That 600 may be a little big but will be fine for your car. As long as it is vacuum secondary with the proper spring, jets, pump cam, power valve, it will work great. It will operate as a 300cfm 2-bbl the vast majority of the time. Guys get into real trouble when they put on a 750 double pumper on a small engine and get big stumbles off the line due to way too much carb opened at too low a speed. Bottom end performance is your main concern. If you can punch the throttle and the car responds strongly and smoothly, you are fine. If there is a bog or a feeling of a surge when you hit 1500-2000rpm then the secondary spring is opening too soon or too late. Being a 600, it will have more than enough top end on your engine.

Terje 07-21-2004 03:03 PM

Isn't the 600 CFM for his current carb a maximum capacity for that carb?

Even if the maximum volume his engine will ever suck at full throttle is 350 CFM, the proper air fuel mixture is still around 14 to 1 (I don't know the exact ratio. might be 14.2:1)

A good quality properly tuned carb should deliver the correct air fuel ratio at all air flows.

That formula calculates the minimum capacity you should use, not the maximum.

In other words, if you put a 350 CFM carb onto an engine capable of sucking down 700 CFM, then at full throttle, the engine will run fine, but with only half the capacity it is capable of.

A 350 CFM capable engine with a 700 CFM carb should run at full power at half throttle. Giving more throttle on such a setup should have no effect.

On my carb (Rochester Q-Jet) the secondaries are activated by the airflow through the carb, and thus the airflow determines the fuel flow maintaining the correct 14 to 1 air fuel ratio.

What should be happening is when the engine is at idle, and I suddenly floor it, the carb will gradually increase the throttle as the speed of the engine increases and thus the airflow gradually increases.

jpd37, I believe that if your carb is properly tuned, it should run fine on that engine.


Originally posted by
If there is a bog or a feeling of a surge when you hit 1500-2000rpm then the secondary spring is opening too soon or too late.
Willys, I've got a stock 350 with a Q-Jet in a 1977 C20 I just bought. I just got it to pass smog by replacing all the vacuum hoses and putting the carb together correctly.

Either the idiot before me or some mechanic of his put the choke lever on wrong so the choke was stuck, and also put the fast idle cam upside down.

Anways, at this time, when I'm driving slow and floor it, the power increases slowly at first, then there is a sudden increase in power.

I guess you're calling that, "a bog or a feeling of a surge when you hit 1500-2000rpm".

So, my "secondary spring is opening too soon or too late."

How do I go about figuring out which and adjusting it? There is a screw that I believe tunes the spring that holds the secondaries closed. I think maybe I should adjust that.

I have the a Rochester Q-Jet manual on request at the library, but it's taking forever for them to get it. 07-21-2004 03:12 PM

Terje; your observations are correct. Regardless of the carb size, the engine is a positive displacement air compressor and will demand a precise amount of air at any given speed. The problem for carb designers is in the large range of speed that an auto engine runs. A Dominator carb on a 500cuin engine at top speed is perfectly sized for the application. Friction losses are relatively low and throttle vacuum signals are high enough to induce proper and responsive gasoline flow. The problem with this combo come is when you want to idle the engine. The huge carb that was so good on the top end is WAY too big for the low air velocity at low speed and the pressure drop through the venturi is too small to signal gas flow. That is why manufacturers developed vacuum (or in the case of the QuadraJet, baffle) operated secondaries. With this modification, you have your cake and eat it too. At low speed only small primaries are in service so there is plenty of pressure drop in the venturis and responsive gasoline flow. However when the engine speeds up, the secondaries kick in and the engine still gets it's gas and air through the larger venturi area. The spread-bore Q-Jet is a great idea and wonderful ranging carb. Unfortunately, the maker never refined it to appeal to the performance market. Don't understand why.

Huge carbs with mechanical secondaries were developed for and work best on drag racing engines that are designed for wide-open-throttle (high engine speed) operation. Those don't need all the low speed jewelery associated w/ vacuum secondary carbs. They can be made to run on the street by increasing accelerator pump size and other such tricks at the expense of fuel economy and other operational niceties of the vacuum secondary units.

Terje 07-21-2004 03:20 PM

Here's a picture of my carb that shows that little screw I'm talking about:
(Click the picture to zoom in)

The screw I'm talking about is right next (on the left in the picture) to the shaft that holds the secondary baffles in place.

There appears to be a small allen screw that prevents the screw shown from turning unintentionally. I believe that I should loosen that allen screw and turn that screw one way or the other and maybe experiment a bit until my engine performs better.

If you guys believe that might improve my performance, I'll pull you my allen key and start messing with that screw. 07-21-2004 03:45 PM

OOPS! didn't see the question in your post. Yes that is the classic response of secondaries opening too soon. Signal is lost and it takes fuel flow a little time to catch up with the engine demand. I am not familiar with tuning hte Q-Jet so can't really comment on the details. Maybe a Q-Jet expert will chime in.

Terje 07-21-2004 03:57 PM

jpd37, is this your carb?

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