Bob Blake carefully ports...
Bob Blake carefully ports away just enough material from each of the carburetors throats so that each one flows exactly the same amount.
Blake uses a series of gauges...
Blake uses a series of gauges to check his progress as he slowly opens up the venturis.
Randy Huffman performs a preliminary...
Randy Huffman performs a preliminary flow check on Blake Carburetions homemade flow bench. Once the carburetor is assembled, the real test is to see how well it flows connected to an engine on the dyno.
Blake prepares to dyno another...
Blake prepares to dyno another carburetor. If any tuning problems are noticed during the run, that carburetor is pulled off the engine, fixed immediately and run again. The dyno does not see another carburetor until that one is either passed or rejected.
To ensure that idle is smooth...
To ensure that idle is smooth and steady at around 1,500 rpm, Blake Carburetion drills two holes in the butterflies. This allows a minimal amount of air to flow through the butterflies while they are closed. If not, vacuum created by the engine can crack open the butterflies and disrupt the idle circuit.
A series to illustrate the...
A series to illustrate the evolution of the carburetor. At left is an old-style Holley. To get it into racing trim, Blake Carburetion spent just under two hours on the carburetor block (center). Now that Holleys HP series is available for racers (right), a lot of the carb tuners preliminary work has been eliminated.
The problem with racingat least from a promoters point of viewis 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 youre 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 cant get oxygen into the combustion chambers, everything you do is going to be limited. It doesnt 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. Its 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 engines potential, and races become more a factor of momentum than raw racing horsepower.
This is what makes the carb tuner can make a world of difference in how an engine runs, which is why these days its 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 its even improved over the years, Blake says. But Holley and Demon are in the business of mass production. Thats 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 thats where carb tuners like myself come in.
Blake stresses his business isnt about cheating up carbs. Instead, he says, he concentrates on making carburetors work exactly as designed and then improving them from there. Manufacturers cant 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 wont flow air, you cant do too much with it, so you worry about that before you start worrying about fuel mixing and everything else.
Although Blake Carburetion has several employees, Blake still does almost all of the venturi porting and polishing himself. Its a delicate art thats 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 carbs 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 carburetors throats is also vital for good response off the corners.
Whether its 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 carbs 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 doesnt care much what the flow bench tells him.
This deal with quoting flow numbers has gotten out of hand, he says. Its 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 Im working on flows as good or better, then I know its 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 Blakes specs. Every variable is charted both from the customers information and Blakes 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 its a 350 two-barrel or a 750 four-barrel double-pumper. A carburetor is great at adapting to the motor its on, Blake explains. People ask how in the world I can build a carburetor for their particular motor when Im testing it on a 750hp Chevy. Its the consistency thats important. When I put on a little two-barrel for NASCAR Late Models, this motor gets choked off and suddenly its got only 350 hp. I know what the air/fuel ratio should be no matter what engine its 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, thats 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 cant 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.