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Which carburetor to choose? This question confronts anybody who is in the market for a carburetor, whether it is for the street, the drag strip or circle track. Holley offers a number of different carburetors that, looked at in total, may at first seem confusing. There is, however, a logical way to select the best carburetor for you particular needs.
The obvious first place to start is with the application or vehicle on which the carburetor will be installed. If the vehicle and engine is factory stock then simply turn to the "Street Legal Carburetor" section and find the listing for your particular application. Sometimes there will be a choice of either a square flange or spread bore flange carburetor. Either style will work in such cases, but the square flange carburetor will require the purchase of an adaptor to bolt it on to a spread bore flange manifold, and vice versa.
A high performance street or street/strip application is the next scenario to consider. In this case you will have to reference the carburetor listing (by CFM) section of the catalog. The first thing to consider here is the CFM or amount of air flow that the engine will require. There is a simple formula available that will put you right in the CFM ball park. The formula is follows:
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
The type of choke would be the next decision to face. Most of the Holley universal performance carburetors come equipped with either a manual or automatic electric choke. Manual chokes can be converted over to automatic electric choke operation with the proper Holley kit, and vice versa.
Most automatic transmission kickdown linkage hookups will bolt directly to the carburetor throttle lever. Chrysler applications will require the purchase of bracket, P/N 20-7, if it's not already included with the carburetor. Those running the GM TH-700R4 automatic overdrive transmission will require the purchase of bracket, P/N 20-95, for this purpose.
HOLLEY CARBURETOR
SELECTION GUIDE
Engine Types
MILD: Basically stock motor with minor modifications such as headers, dual plane intake and very mild camshafts. Can use stock converter and rear end gears. Daily driver or mild bracket use.
HOT: Moderately built motors with single plane intakes, cams with aggressive profiles and increased compression ratios. Recommend mild stall converter and deeper rear-end gears or manual transmission. Hot street car or bracket race car.
WILD: Purpose built race engines. Recommend very aggressive roller or solid lifter cam, high-rise race intake manifold, ported or aftermarket heads and high compression ratios. Use only with hi-stall torque converter or manual transmission.
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Vacuum Secondary Carb Chart
Vacuum secondary carbs are typically used in street applications where performance and economy are important considerations or in race applications where a car has limited traction and needs a "hit" that can be tailored to the traction available.
ENGINE SIZE MILD HOT WILD
300 390 600 650
350 600 650 750
400 650 750 750
450 750 750 850
500 750 850 950
550 850 950 950
Double Pumper Carb Chart
Double Pumper carbs are typically used in any application where maximum performance is required and economy is less important. Double pumpers have progressive mechanical secondaries and are ideal for the hard core enthusiast.
ENGINE SIZE MILD HOT WILD
300 N/A 600 650
350 600 650 750
400 650 750 800
450 750 800 850
500 750 850 950
550 850 950 1000
Kv,
Eggert Kristjįnsson.
6602581
