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Weber 48IDA Downdraft Carburetors - Imagine That!

Bob McClurg Mar 1, 2001
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We live in a high-tech world. There's no question about that. From microwave ovens to cellular phones every aspect of our lives seemingly revolves around electronics. When it comes to our love of performance cars, here, too, we are inundated with microchips that control everything from fuel and spark delivery to traction control. But, despite the high-tech attributes of computer-controlled machines there still lies in most hot rodders the desire for the simplicity of raw muscle.

The Italian-manufactured Weber 48IDA downdraft carburetor installed atop a Chevrolet V-8 (be it a Rat or Mouse) is a natural performance combination. When bolted in 2 x 4 configuration on an individual runner (IR) intake manifold, the Weber downdraft provides peak torque and instant horsepower (as opposed to a conventional four-barrel carburetor and plenum-style intake manifold, which progressively builds torque and horsepower). Simply put, a 48IDA Weber downdraft induction system performs much like a mechanical fuel-injection system, yet gives you the driveability of a conventional four-barrel fuel meter.

There is, however, a mechanical downside to Weber carbs, and that is they are somewhat hard to tune. The carburetor's fidgety nature has been known to produce a potentially fiery phenomenon known as "fuel standoff," which can transform a race car into a smoldering pile of ash in short order. However, those racers who knew how to deal with the Weber 48IDA's eccentricities were quite successful. In the mid '60s, 48IDA Weber carburetor systems powered up the infamous Corvette Grand Sport endurance racers, and early McLaren Can Am cars (just to name a few) producing championship-winning results.

When it comes to the street machine set, the 48IDA's looks are the primary reason why these carburetors enjoy such overwhelming popularity. Look at it this way, anybody can bolt on a Holley, but when you slap a set of these sexy little Italian downdrafts onto that small-block-powered Camaro or a restified Tri-Five, you make a high-performance statement which cannot be ignored.

Here's the dilemma. These carburetors went out of production sometime in the '80s, and are in extremely short supply.

While it is true that NOS Webers can still be purchased through a very limited dealer network, their relative scarcity dictates a lofty selling price. On the other hand, used sets or rebuildable Weber cores also fetch a veritable king's ransom at swap meets.

In November of 1999, Bob Ream and partner Wes Henderson founded Imagine Fuel Injection. The Weber 48IDA carburetor was quickly disappearing from the performance aftermarket, and there were enough people out there who still wanted performance, economy, and the looks offered by the Weber 48IDA without having to scour the country and pay the lofty prices being commanded for these carburetors.

Ream and Henderson reasoned that if they could offer an IR-based, electronically-managed Weber-type throttle body fuel injection setup at an competitive price, they could effectively recapture, as well as revive, a waning market segment of the performance induction system's industry.

Today, most enthusiasts drive an electronically managed, fuel-injected car or truck. They drive them to work. They drive them to the racetrack. One thing which the EFI cars of the '90s have taught motorists is that a vehicle can perform at its best at all operational levels with this type of system. So why not install a setup similar to this on the car you drive for pleasure?

Using readily available, cast-aluminum Weber IR intake manifolds as the foundation (ie: Inglese Induction Systems, Moon, Blue Thunder, Ultra for Chevrolet small-block, or Inglese or Moon intakes for big-block Chevrolet applications), the guys from Imagine Fuel Injection designed a billet-aluminum down-draft throttle body, which essentially utilizes all the outer dimensions of the Weber 48IDA. "By doing it this way, we can use the Weber intake manifolds, their induction stacks, and even their air cleaners," said Bob Ream.

And these throttle bodies can be had in two stack sizes. Imagine's low-profile, 4-inch throttle body is absolutely perfect for Bow-Ties with low hood clearance problems. On the other hand, the 5-inch unit is perfect for everything else in between.

Both throttle bodies are manufactured from CNC-machined T-6, 6061 heat-treated billet-aluminum, and feature a 2-inch (50.8mm) standard throttle bore size. On "turnkey" Imagine Fuel Injection systems, these intake manifolds are blended to the IR intake runners to achieve maximum air/fuel flow and performance. Imagine Fuel Injection's billet-bodies can also be had in either a clear bright anodized finish, or an optional polished-aluminum show finish.

Due to the location of the electronic fuel injectors the system's bell crank-type throttle linkage had to be relocated 1-inch higher (than an original 48IDA Weber setup) on the throttle body for clearance. This modification was also necessary to maintain fuel injector-to-intake valve alignment, which is so critical with throttle body-type, multi-point fuel injection systems.

What type of fuel injectors does Imagine use with these systems? "A lot of it has to do with the compression ratio of the engine, the rpm powerband intended for the engine, the cam profile, and horsepower rating. Traditionally, either a constant-pulse Lucas or Bosch-type electronic fuel injector is employed with these applications. For example, we would use a 43-pound-per- hour fuel injector for a 427/454 big-block Chevy, while we would choose perhaps a 30-pound-per-hour electronic fuel injector for a small-block."

Imagine Fuel Injection also manufactures their own CNC-machined, billet-aluminum fuel rails from raw bar stock. At this juncture, it should be noted that on turnkey Imagine IR throttle body systems the base of these injectors is threaded in order for the injector to fit more securely into the injector port opening in the Imagine throttle bodies.

Two types of fuel pumps are recommended with these systems: A standard 200-gph Bosch-type electronic fuel pump is recommended for engines producing 500 hp or less. However, when it comes to engines producing in excess of 500 ponies, Imagine recommends the Aeromotive billet-aluminum high-pressure fuel pump (rated @ 500 gph ) along with the Aeromotive fuel regulator and fuel filter.

So much for the mechanical aspects. Now for the "Star Wars" technology. Imagine suggests using either the DOS-based HalTech F9A or Electro-Motive distributor-less engine management systems, available in either DOS or Windows-based versions. The F9A is a well known unit with a 15-year track record for precision and reliability. This system is primarily used on applications featuring a conventional electronic-type distributor. It's very trouble-free, and in any type of motorsports that is the name of the game.

When it comes to high-tech, or distributorless applications, Imagine recommends the Electro Motive engine management system with the crank trigger feature. This setup utilizes either a DOS or Windows-based program.

How do you setup these
programs? "Electro Motive has a program called Wintech Wizard. You fill one page of information outlining the base perimeters of your engine. What is the Brake Specific Horsepower (BSFC)? What the rating is on the fuel injectors being used? What is the cam profile? What compression ratio, etc.?

Imagine Fuel Injection also builds a number of use "specific" wiring harnesses that interface with their electronic throttle body fuel injection systems. Wire looms are available for most popular small- and big- block applications."

Of course, you can talk all day about how great a product is, but how well does it hold up in real world situations? Experienced dyno man, Terry Kell, of Las Vegas, Nevada, says this system is one of the best thing he's ever laid hands on.

Testing at Kell's Las Vegas dyno shop was done using a Brand-X engine built by famed engine builder, and Bonneville land speed record holder Mike "260-mph" LeFevers. "We pitted our system against a Holley 850-equipped Edelbrock Victor dual- plane intake. We recorded torque, horsepower, and exhaust gas temperatures. At 4,500 rpm the Holley/Edelbrock setup produced its most significant amount of torque, 406.24 lb-ft while the Imagine Fuel Injection throttle body setup produced 427.19 lb-ft Optimum horsepower figures for the Holley came in at 5,500 rpm, registering 399.98 hp. On the other hand, the Imagine IR system recorded its top horsepower figures at 5,000 rpm, registering a solid 418.96 hp.

Naturally, with any IR type intake setup, the exhaust gas temperatures (EGTs) were also more closely matched between cylinders. The Imagine Injection IR system showed as much as a 400-degree temperature drop between engine cylinder temperatures. And, due to the very nature of the electronic engine management system itself, the engine ran extremely clean."

What about airflow numbers? "Due to the very nature of the IR design, airflow is measured quite differently than with a conventional carburetor. However, these systems will ideally-flow approximately 1,000 cfm.

"This is really one of the best intake systems to have for a Chevy street machine, street rod, or sportsman class race car," says Imagine's Wes Henderson. "It runs as smooth as a Mercedes, yet exhibits absolutely crisp throttle response while exhibiting a slight lope, which gives you an indication of the performance potential which lies beneath the hood."

How do you lay your hands on one of these killer IR setups? Should you already own a Weber setup or already have the intake manifold, any, or all of these components, can be purchased separately, or you can purchase "the whole ball of wax" for about the same money you'd hand over for a fully decked-out Weber 48IDA setup. The choice is yours-great looks, old technology, or same great looks with all the high-tech modern advancements.


Imagine Fuel Injection
Phoenix, AZ 85017

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