Carburetor Tuning - Deep Breathing

The problem with racing—at least from a promoter’s point of view—is racers have gotten good at it. The efficiency that smart racers and engine builders have been able to squeeze out of the standard 350ci pushrod V-8 has grown steadily over the years. Handling packages have also improved, and what you’re left with are cars running awfully fast on Saturday-night bullrings.

Often the solution is very simple: restrict the carburetor. An engine has to have air and fuel to make power, and if you can’t get oxygen into the combustion chambers, everything you do is going to be limited. It doesn’t matter how much fuel you flow. The optimum air/fuel ratio for a race engine is around 14:1. If the fuel flow exceeds this ratio, it can be blown out the exhaust valve and wasted (or burnt in the header pipes).

The perfect example of a division with restricted airflow is the NASCAR Late Model Stock division that races at tracks across the country. To control horsepower, the class is required to use a 350-cfm two-barrel carb on a 350 V-8. It’s akin to telling a marathon runner he has to breathe through a swizzle stick. The motor is choked off, horsepower is cut by more than a third of the engine’s potential, and races become more a factor of momentum than raw racing horsepower.

Tuner Aid

This is what makes the carb tuner can make a world of difference in how an engine runs, which is why these days it’s rare to see a racer in the upper divisions run a carb right out of the box.

Bob Blake is the founder of Blake Carburetion and has built more winning carburetors than he can count. He says carb tuners are important not because carburetor manufacturers like Holley and Demon do not make quality products, but because they have to make so many of them.

“The carb manufacturers make good stuff, and it’s even improved over the years,” Blake says. “But Holley and Demon are in the business of mass production. That’s fine if you are bolting a carburetor on your street car, but if you are racing, you are going to want every bit of performance you can get, and that’s where carb tuners like myself come in.”

Blake stresses his business isn’t about cheating up carbs. Instead, he says, he concentrates on making carburetors work exactly as designed and then improving them from there. “Manufacturers can’t economically build every carburetor by hand and spend all the time required to make sure every one performs exactly the same. They have to have some give in their tolerances. The first thing I do with a carburetor is make sure every nut, bolt, hole and part is blueprinted and exactly to spec. Then I concentrate on flow. If a carburetor won’t flow air, you can’t do too much with it, so you worry about that before you start worrying about fuel mixing and everything else.”

Venturi Matching

Although Blake Carburetion has several employees, Blake still does almost all of the venturi porting and polishing himself. It’s a delicate art that’s as much about experience and feel as it is about process. When racing in a series where boring is allowed, the process is to open up the carb’s venturis to a predetermined size to maximize both flow and velocity. You can get all the airflow you want through a carburetor simply by enlarging the barrels, but it will also reduce velocity, which can kill your metering signal. Good velocity of the air through the carburetor’s throats is also vital for good response off the corners.

Whether it’s a two- or four-barrel, every venturi on the carburetor must flow the same amount of air. Otherwise it will be nearly impossible to dial in the carb’s settings. Blake uses a set of gauges when porting the venturis but depends upon his homemade flow bench to make the final determination on equal flow. Like many carb specialists these days, though, Blake doesn’t care much what the flow bench tells him.

“This deal with quoting flow numbers has gotten out of hand,” he says. “It’s generally a bad idea to compare flow numbers from one shop to another. Were both flow benches at the same settings? What was the weather like that day? All we use our flow bench for is to compare it with our reference standard. We have one carburetor of each model we do that we know works really well. I know how it flowed on this bench at a specific setting. If a carb I’m working on flows as good or better, then I know it’s good. If it flows worse, I know I need to keep working on it or junk it. Who cares what the numbers are?”

Assemble And Test

From there, the rest of the carburetor is assembled to Blake’s specs. Every variable is charted both from the customer’s information and Blake’s records. Then the fun starts. Every carburetor that leaves Blake Carburetion has been tested on the dyno. Interestingly, the same test motor is used, no matter if it’s a 350 two-barrel or a 750 four-barrel double-pumper. “A carburetor is great at adapting to the motor it’s on,” Blake explains. “People ask how in the world I can build a carburetor for their particular motor when I’m testing it on a 750hp Chevy. It’s the consistency that’s important. When I put on a little two-barrel for NASCAR Late Models, this motor gets choked off and suddenly it’s got only 350 hp. I know what the air/fuel ratio should be no matter what engine it’s on.”

While the carburetor is on the motor, Blake checks how it controls the engine at idle all the way through redline. For circle-trackers, how the carburetors idle is particularly important because being able to handle low- to high-rpm surges is critical to being competitive.

“I also do a lot of carburetors for drag racers,” Blake says. “As long as it runs at full throttle, that’s all they care about. The stock car guys are a lot more finicky, and they should be. A circle-track carburetor has to have enough velocity and be tuned correctly to handle the extra flow when the driver mashes it on the floor. If the carburetor can’t handle all that fuel being dumped in there, the car is just going to fall on its face on restarts and coming out of the corners.”