Regardless of the overall goal of squeezing great performance out of the engine build, the main goal is to build a well-tuned combination that delivers strong performance throughout the rev range. And as every magazine story, blog post, and YouTube video will tell you, no single component will provide the catalyst for perfect performance.
However, certain parts contribute a great deal to the overall operation of the engine. The camshaft is obviously one of the largest, as is the carburetor. In general, many home engine builders tend to go overboard with their carburetor choice, assuming they can "jet down" during tuning to achieve the perfect match. This might work in some cases, but it's not necessarily the most efficient or effective method.
To arrive at the most suitable carburetor selection, there is a basic formula: Displacement multiplied by the maximum speed divided by 3,456. For example: A typical 355 cuin small block - a .030 rebuild - with a maximum engine speed of 6000 rpm would work well with a 616 cfm carburetor ((355 x 6000) 3456 = 616.32).
But wait - there's more! You also need to multiply the result by the engine's base volumetric efficiency - its ability to handle the air/fuel load. In a series engine, it is 80 percent. For slightly modified engines it is about 85 percent, for heavily modified engines with high compression it is about 95 percent.
So for this 355 engine the 616 cfm rating means a real world figure of 493 cfm in a stock combination; 524 cfm in a lightweight engine; and 585 cfm in a combination of high compression and high performance. And since carburetors are not sold with specific flow ratings, you would select the cabinet model above that rating, e.g. B. a 600 or 650 cfm carburetor.
Forgiveness. We should have warned you that math would play a role in this story.
Holley and Summit Racing have interactive carb selection calculators on their websites that take into account the various volumetric efficiency options, while Edelbrock's website offers a variety of volumetric efficiency formulas. However, none take into account other factors such as the intake manifold – low rise x high or single flat x twin flat, the airflow capacity of the cylinder heads and whether a power modifier is involved. Rear end, vehicle weight, and even shaft gear and torque converter RPM can affect your ideal carburetor choice.
When it comes to a Holley carburetor, there's also the issue of mechanical or vacuum-driven secondaries. For most street and street/strip engine combinations, you should use vacuum secondary engines to improve drivability. Mechanical secondary carburetors - the classic "twin pump" design - should be reserved for really powerful combinations and, as Holley recommends, lighter vehicles curb weight of 3,100 pounds or less.
Based on the classic Carter AFB design, Edelbrock's carburetors are all mechanical secondary carburetors, so the only real choice after determining the appropriate cfm rating is whether you want an electric choke or a manual choke. And you probably want an electric one. Edelbrock carburetors have a secondary air port, Performer series carburetors have a counterbalanced non-adjustable port below the secondary boosters, and Thunder series carburetors have a spring-loaded and adjustable secondary air port above the boosters. Designed for lighter or heavier vehicles, it allows early or late opening with a simple spring adjustment.
With any reservations about applications, we wanted to reduce the carburetor conundrum and take the easy way out: we enlisted experts Smitty Smith of Edelbrock and Blane Burnett of Holley to provide recommendations on optimal carburetor selection for five theoretical engine designs ranging from small-block to Big Blocks and the increasingly popular LS Swap. We assume that all will be used with automatic transmission.
Again this may vary depending on the vehicles the engines are fitted into but your information provides a good overview of carburetor choices which are mainly used on the road and occasionally on the track. Usual disclaimers apply: your mileage may vary, consult your doctor before using heavy machinery, void the Tennessee offer, and so on.
On the road or on the track, proper carburetor selection is essential for optimal performance. In addition to the basic formula for determining the correct cfm requirement – (displacement x max rpm) 3,456 – other factors include the degree to which the engine is designed for high performance (volumetric efficiency), the transmission and even the weight of the vehicle .
Volumetric efficiency is greatest in a high performance engine where the correct camshaft and ancillary components move air in and out better. High compression also has a large impact on volumetric efficiency, as the extra compression provides more power and therefore greater efficiency than a combination of lower compression and comparable size.
Of the primary carburettor manufacturers Holley and Edelbrock, only Holley offers models with mechanical secondary circuits. They would come from the legendary Double Pumper line. Holley's other carburetors feature vacuum operated secondaries recommended for general street use and moderate street/street engines.
The basic Holley carburetor four-cylinder family includes the smaller 4150 and 4160 series and the larger 4500 series, commonly known as the Dominator. The 4150 series shown here is the more performance-oriented version of the smaller carburetors, thanks to a secondary metering block with removable jets. The secondary metering plate type 4160 has no removable nozzles but can be converted to type 4150.
Edelbrock's four-barrel carburetors are based on the classic Carter AFB, which also gave rise to the Quadrajet carburetor found on numerous GM production models. Rated for up to 800 cfm, they include a feature called Adjustable Secondary Valve (AVS) that allows you to change the opening rate of the secondary valves with the simple turn of a screw.
Although Holley and Edelbrock performance carburetors are supplied practically ready to use, some adjustment to the fuel curve may be required. Power valves and jets can be interchanged on Holleys (see here), while metering rods with jets perform the same function on Edelbrock carburetors. There is a wider range of tuning options with Holley carburetors, but the Edelbrock design is generally easier to work with and adjust.
Supercharged induction engines have unique carburetor requirements. In a naturally aspirated engine, engine vacuum drops to nearly zero under wide open throttle (WOT). This in turn opens the power valve to enrich the mixture. This is not the case with boost pressure and the result can be destructive engine knock as the power valve circuit is closed and the engine revs catastrophically. "Fan" carburetors are modified to provide positive pressure at WOT, eliminating the possibility of the power valve closing.
Engine No. 1
Type: 350 small blocks
Dynamic target: 325 hp
We started with an entry-level model that is adopted by countless enthusiasts every year. The rebuild of the classic 350 on budget but aiming to offer more than the factory offered in a 1970s or 1980s smog-choked package with vented cylinder heads and a smooth camshaft. Nothing exotic or expensive - just a no-fuss, affordable addition to a second generation Camaro, C10 or G-Body.
Blane Burnett (Holley):A 650-670 cfm Ultra Street Avenger carburetor would be appropriate here.
Forge Blacksmith (Edelbrock):For a common small driver block like this, a 600cfm Performer Series carburetor is recommended - PN 1405 with manual choke or PN 1406 with electric choke.
Engine No. 2
Type: 383 Small Block
Dynamic target: 450 hp
Climbing the small-block ladder, this engine is a decidedly powerful street engine based on the classic lift combination. Since it's primarily used on the street, with a few forays into the drag strip annually, we'll be going with an open entry to optimize street/track ability. Ported aluminum heads and an aggressive .525+ inch lift camshaft with a comparatively wide cam separation angle complement low-rpm torque capability with high-rpm power.
Burnett (Holley):Depending on a few other factors for the vehicle you're getting into, a 670-770 cfm Ultra Street Avenger carburetor would fill the bill. This would also be a good choice for Holley's Terminator EFI system, which supports up to 600 horsepower.
Schmied (Edelbrock):This would be a perfect match for one of our 800cfm Thunder series carburetors - PN 1812 for manual choke or PN 1813 for electric choke.
Engine No. 3
Typ: 454 Big-Block
Dynamic target: 425 hp
Similar to our small entry-level Block 350, this would be a large block with an inexpensive pump that would be used almost exclusively on the road. It would use something like Chevrolet Performance's affordable rectangular iron port heads with large 325cc intake ports and one of its hydraulic cams holding 211/230 degrees and lifting .510/.540 inches at a 112 degree LSA. With a single plane entry, it would make a great truck, Chevelle, or early Monte Carlo tire turner.
Burnett (Holley): the classic 750 Double Pumper is the right choice here. It's made for a combination like this - even if the vehicle weighs over 3,100 pounds.
Schmied (Edelbrock):One of Edelbrock's 800 cfm Performer series carburetors is perfect for a smooth big block. Use PN 1412 for manual choke or PN 1413 for electric choke.
Engine No. 4
Typ: 572 Big-Block
Dynamic target: 600 hp
This is our wannabe street outlaws big block combo intended for use with a supercharged or turbocharged system. We're not talking the worst here, money is no object here, but a track-oriented combo with big heads, a big cam and a valvetrain that can handle boost-generated forays at high rpm. The thinking here is 10 or even 9 seconds e.t. is in the right vehicle with somewhat limited road time for cruise nights etc.
Burnett (Holley): An 850 cfm Demon Burst carburetor would work best here. Specially developed for power-induced engines, it amplifies the vacuum signal under boost to enrich the mixture and prevent knocking.
Schmied (Edelbrock):To be honest this combo would probably top Edelbrock's carb range, but on a big block 572 using a tall block (10.2 inch) I would use our Super Victor PN 2927 intake manifold with, say, a 4500 series and the power adder of your choice.
Engine No. 5
Typ: LS3 LS-Swap
Dynamic target: 500 hp
Replacing LS engines is becoming increasingly popular, and with good reason. The LS family is compact and lightweight, and the factory heads offer exceptional airflow capability that translates into great performance. With the right camshaft, a naturally aspirated 6.2-litre LS3 will pass the 500hp mark with a rear-buckled connecting rod. Our theoretical combo would be simple: a stock bottom end, a Comp Cams XFI cam with .566/.576-inch lift and a 113-degree LSA, and that's about it.
Burnett (Holley):A 770 cfm Street Avenger runs very well with a deep-breathing, carbureted LS engine. This is what Chevrolet Performance recommends for its LS boxer engines.
Schmied (Edelbrock):Edelbrock's Thunder Series 800 cfm carburetor - PN 1812 for manual choke or PN 1813 for electric choke - is the way to go. Plus, our E-Street EFI system is a great alternative to a carburetor here. It can withstand up to 600 hp.