28-36 TechFeatEFICarbs 3/20/14 10:18 AM Page 30
Fuel System Feature
When building a performance engine, limitations as to which fuel system you use includes how much your customer can afford to spend, and any rules that would prohibit the use of an EFI system in a particular racing class.
adjustment screws. What's more, the idle mixture can also be affected by the height of the float(s) inside the fuel bowl(s). When the petal goes to the metal, a carburetor will bog unless the accelerator pump provides an extra squirt of fuel. This too requires fiddling to get just the right shot of fuel so it's not too rich or too lean. A power valve inside the carburetor also allows more fuel flow through the main metering circuit when the engine is pulling hard under load. This is
necessary to provide maximum power (a rich air/fuel mixture) and to prevent the air/fuel mixture from going lean, which could result in preignition or
detonation. Carburetor tuning is often a trialand-error process that requires a lot of fiddling until the engine runs best – and even then, it won't be perfect once the ambient temperature changes. Other changes and add-ons can also mess up a carburetor tune. Opening up the exhaust headers, installing different mufflers, even changing the final drive axle ratio or tire size can all affect a carburetor. So why are many racers still using carburetors? Because they are familiar with carburetors and because carburetors seem simpler (no electronics). Carburetors also have a cost advantage compared to EFI. The typical bolt-on TBI fuel injection system will cost around $2500 to $2700, which may or may not include an electric fuel pump and additional plumbing. By comparison, you can buy a brand new street performance carburetor from $300 to $800, or a big bore racing carburetor for $800 to $1400 or more. Add extra $$ if you want the carburetor hand built, reworked, power coated, plated or modified for methanol or E85. Or, if you like to take chances, you could buy a used carburetor at a swap meet or online for maybe a couple hundred bucks. Used carburetors are always a gamble because it's hard to tell what the previous owner may have done to it. A used carburetor might work fine or it might be so screwed up you'll never get it to run right.
Bolt-On TBI Compared to a carburetor, a bolt-on TBI system is like day and night. For one thing, fuel injection depends much less on intake vacuum than a carburetor. A wild cam is great for high RPM power, but it also kills intake vacuum and low speed torque and throttle response. Consequently, if an engine has a long duration cam, that bumpty-bumpty-bump idle that sounds so cool may make the vehicle tricky to drive on the street -- especially when the engine is cold. TBI fuel injection sprays fuel into the throttle body whereas a carburetor Circle 30 for more information 30 March 2014 | High Performance Buyers Guide