Taking Spark From Fuel-InjectionIssue

Question: In looking at new cars for the first time in several years, I have discovered that the reliable carburetor has been completely replaced by various types of fuel injection. Will you explain the difference between throttle-body and multiport injection systems? Are these injection systems necessitated by electronic controls? --R.C.H.

Answer: Fuel injection systems and carburetors perform the same important task of mixing the correct quantity of gasoline with the air being sucked into the engine.

Because the precise control of the fuel mixture is critical to performance, fuel economy and engine longevity, carburetors and fuel injection are among the most important systems in any car. While fuel injection is not a recent development, new computer controls have vastly increased their usage in imported and domestic cars.

All fuel injection systems work by mechanically squirting a small quantity of gasoline directly and at precisely the right instant into the air moving through the intake manifold. A multiport fuel injection system features individual fuel injection nozzles at each cylinder, typically mounted in the intake manifold just outside the intake valve. A throttle-body injection system has all of the injectors at one location.

When a motorist pushes down on the gas pedal, a so-called throttle plate increases the amount of air that is permitted to be sucked into the engine. The computer then commands the electronic fuel injection system to inject a certain quantity of gasoline into the manifold, based on sensors that transmit information on coolant temperature, atmospheric pressure, manifold vacuum and the oxygen content of the exhaust.

This is pretty much the same for either throttle-body or multiport injection systems. Multiport systems are newer, but not necessarily better. As a motorist, I doubt you would buy a car because it had one type of fuel injection rather than another. The type of injection system that is used is based on a variety of factors and not on one system being superior to the other.

Fuel injection is theoretically more precise than carburetion, but many owners have experienced costly problems with injection systems that become clogged by dirty gasoline. Carburetors, however, are not problem-free and can cost hundreds of dollars to rebuild or replace. The point is that buying a car because it features a specific mechanical system is not sound practice. Either--a fuel injection system or a carburetor--can cause major problems.

Q: I had two tires installed on my 1971 Lincoln Mark III. I thought I had bought 225s, but when I got home I discovered I had 235s. The 235s look like they are half an inch wider than the other two tires on the car, but they clear the tire wells and the ride does not seem affected. Are they OK?W.L.

A: The 235 refers to the width of the tire measured in millimeters, which is roughly 9.25 inches. It’s best to stick with the manufacturer’s recommendation, but the difference in your case is minute. The additional 10 millimeters is less than half an inch. I checked with experts at Ford, who say the tires are OK.

You should always consult your owner’s manual or the tire sticker on the car before buying tires, so you know what you want to buy. All too often, tire dealers will attempt to sell you a larger size tire at a higher cost or an odd tire size that may not be appropriate for your car. If you want to change tire sizes, then you should seek the advice of a top-notch tire expert.

Vartabedian cannot answer mail personally but will respond in this column to automotive questions of general interest. Do not telephone. Write to Your Wheels, You section, The Times, Times Mirror Square, Los Angeles 90053.