CARB SIZE MATTERS: What size carburetor do I need?

Posted by Robert McJannett on

Will a bigger carb make more power?

What size carburetor is correct for my engine?

Performance Improvements stocks carbs from Holley Performance, Edelbrock and Proform Parts. Sometimes people either have a preconceived notion of what size carb they want or they need some direction about what to choose. We can help you choose the best carb that will work for your vehicle and how you plan on driving it. You can always contact us for answers to specific carb questions but read on to answer some of the most frequently asked questions about carburetors.

 

Carb Size: Is bigger always Better?

While we’ve all heard people say - “Bigger is Better” - is that always true? We know that more air and fuel equals more power, does that mean the biggest carburetor always makes more power? Unfortunately, too much carb is a common mistake that can actually hurt the overall performance of your engine.

The right idea would be to match the carburetor to the engine's breathing ability or volumetric potential. The engine's volumetric efficiency is a measurable value and with a correctly matched carburetor, you will improve the performance of your engine.

 

How to calculate Carburetor CFM

The formula for calculating how much CFM (cubic feet per minute) your engine requires is: CFM = Cubic Inches x RPM x Volumetric Efficiency ÷ 3456.

Any ordinary stock engine will have a volumetric efficiency of about 80%. Most rebuilt street engines with average bolt-ons have a volumetric efficiency of about 85%, while race engines can range from 95% up to 110%.

Example: Using a 355 CID engine x 5,500 max rpm = 1,952,500
Take 1,952,500 x .85 = 1,659,625
Then 1,659,625 ÷ 3456 = 480 CFM

 

Even with about a 10% cushion, a 500 CFM carburetor will handle this engine great. If you try this formula yourself, be honest with how much rpm the engine will see. You’d be better off with a carb smaller than needed rather than something oversized that can lead to poor drivability and performance.

 

Vacuum or Mechanical Secondaries?

A vacuum secondary carburetor is usually most fuel efficient when it's used on street driven cars that have automatic transmissions. Vacuum secondary carburetors have one accelerator pump and work off sensing the engine load, which progressively open the secondary butterflies as rpm increases.

A mechanical secondary carburetor uses mechanical linkage to open the secondary butterflies and most also have dual accelerator pumps (a.k.a. double pumper). Mechanical secondary carbs are best used on applications with more radical camshafts, high rpm racing situations, and manual transmissions.

 

Does Carb size matter?

Will a bigger carb make more power? Recently the Engine Masters show on the Motor Trend On Demand site did a comprehensive test to see just what differing sized carbs would produce. Here is what they learned.

Base Engine 383 stroker Chevy Small Block:

Edelbrock air gap dual plane intake.

210 cc heads

Comp Cam hydraulic roller cam 230/236 @ 50

 

A better than average 400 HP street performance engine. The test was four Holley XP series carbs, 650, 750, 850 and 950 CFM. Now this series of dyno pulls was on the Westech Performance dyno. Probably one of the best facilities around. Here is what they found. Remember these numbers were from wide open throttle.

Carb Size

Manifold Vac

Actual CFM

 Torque

Horsepower

Holley 650

1.8

600

479.6

470.4

Holley 750

1.5

606

482.1

477.15

Holley 850

0.5

615

483.8

474.3

Holley 950

0.4

618

480.6

471.5

 

So, will a bigger carb make more power? Looks like bigger is not always better. If this was to be a 327 or 350 engine street driven just for pleasure, the 650 will give the best all-around performance, excellent throttle response, good fuel mileage, great low and mid range performance. No one will notice the 3 or 4 HP missing. On this 383 engine, again for street driving the 750 would be the right choice. From there on if the car is being set up for maximum performance, drag racing etc the 850. The 950 needs more engine.

If you want to see the whole show go to Motor Trend On Demand, Engine Masters episode 44: CARB SIZE MATTERS.

 

On this episode of Engine Masters, they take the mystery out of carburetor selection. More air means more power, but is there such a thing as too big? And how do you know when you’ve passed good? David Freiburger, Steve Dulcich, Steve Brulé from Westech Performance, and the dyno get together to compare carbs from 650 to 950 cfm on Our Ultimate Average 383 to help you find all the power and all the driveability.

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