Letís do a quicky about what a carburetor needs to do. It needs to supply the engine with the proper fuel amount to provide the correct air/fuel mixture under different load and engine speeds. The method it uses to sense the engine needs is through various vacuum signals that provide a measure of load and engine speed.

The method of controlling engine load is by throttling the air entering the engine. This is done by the throttle plate(s) opening position, which is set by the driver through the throttle pedal.

The engine is an air pump. If 100% efficient, it would move air volume at full throttle by the following formula:

Engine volume (cubic feet per minute) = Engine RPM X engine displacement (cubic inches) / 2 volume per revolution / 1728 cubic inches per cubic foot.

For a 289 engine running at maximum RPM of 6500 RPM the formula is:

6500 X 289/2/1728 = 544 CFM (cubic feet per minute)

But the engine isnít 100% efficient. Called volumetric efficiency, a basic street motor may be 80%, a street high performance motor 85% and an all out race motor 95%. Letís use 90% for a good semi race motor:

544 CFM X .90 = 490 CFM. This is how carburetors are sized.

But all this is at sea level and we are at about 5000 feet, and our engine is pumping air of lesser density. Kind of a negative super charger.

The normal barometric pressure at sea level is 29.92 inches of mercury & 24.89 at 5000 feet altitude, as in Denver. This ratio is .832. With this pressure difference, at high altitude, the carburetor acts as if it were larger. Because delta pressure differential changes as the square of the area, the carb size effect is only equal to the square root of the pressure ratio or roughly a factor of X .912.

So, if we want to find the proper carb size for our engine at Denver 5000-foot elevation, we take 490 CFM X .912 or 447 CFM.

Following is a chart for those that donít like to calculate. It shows carb size (CFM) for different RPM & engine displacements (CID). It is figured for better than average street performance engines using 90% efficiency and 5000 altitude.

CID 3500 5000 5500 6000 6500 7000 7500 8000
289 241 344 378 413 447 481 516 550
302 252 359 395 432 467 503 539 575
331 277 396 435 475 514 553 593 633
351 293 418 459 502 543 584 627 668
427 356 508 559 610 661 711 763 813
460 383 546 601 657 710 765 820 875

Now letís look at a basic 225HP 289cid motor with juice cam and automatic transmission. It rarely gets above 3000RPM. So, with 80% efficiency it would want 163cfm carb in Denver and 178cfm at sea level. Wow, that makes the Ford shoe box & 390cfm Holley seem massive. So, you can see why a vacuum secondary carb is very desirable for an engine of this type. It attempts to change CFM rating as the engine pumping volume changes. It can run on the front barrels which are, in most Ford Holleys, half the flow of the full 4 barrel carb, then gradually open the secondarys as the load & RPM changes.

ďBut, gee Walt, isnít ďbigger is betterĒ applicable to carburetors? If I want to go faster, donít I have to have a big carb to move more air into my engine? I thought, that at high altitude, with lower atmospheric pressure, the big carb would let the air in easier.Ē Well, not really, Guy.

A carb is a trade off, like many auto items. We want max horse power at max RPM, but we also want the best possible throttle response out of the corners. Especially for road racing with lots of turns, the throttle response in the mid RPM range is of primary importance. And think of what percentage of the time that we spend at full throttle max RPM. A big carb has poorer vacuum signals than a small one, which equates to poorer throttle response.

So, what we want to do is use the smallest carb that provides max or near maximum HP at max RPM. Let me give you an example.

A while back we had a vintage race car in for dyno testing. It had a 428 CJ motor. The carb was a Holley 3310 4150 type 750 CFM with vacuum secondarys. After we had finished it and had improved it to itís best possible performance, we offered to run it with one of our Holley 4150 vacuum secondary 600 CFM carbs.

First, the mid range bog that had been a problem was completely gone. But then, without rejetting, we ran a power curve with the following results:
3000 RPM up 8 HP
4000 RPM up 8 HP
5000 RPM up 18HP
6000 RPM up 14 HP

We put that same carb on a 351 Pantera racecar at 2nd Creek with great improvement in drivability and performance.

Jay Lindley was telling me of a test between a Ford shoebox and a 715 Holley carb on a Mustang, where the shoebox won.

So, even if youíre not from Texas, bigger is not better.

So much for now. See you next month with some other ideas on things that have worked for us.

Walt Hane
Tech Tips-1

Bullet 1 Suspension
Bullet 2 Dry Nitrogen
Bullet 3 The Panhard Bar
Bullet 4 Loss of Oil Pressure
Bullet 5 Inlet Fuel/Air Systems
Bullet 6 Colorado Hot Fuel Problem