Ferrari Life Forum banner
1 - 5 of 5 Posts

102 Posts

This is a Weber 45DCOE Carburetor

Basically a Carburetor is a device that controls the fuel/air ratio. When you put your foot down, the Carb "opens up" letting more fuel and more air into it. This is usually achieved by a butterfly valve and jet. This is how most cars were pre 1980s

This is a Bosch fuel injector. This goes right at the Inlet valve in an engine cylinder

A fuel injection system is totally electronically controlled. Basically it works by utilising a variety of engine sensors.Fuel injection systems are much more accurate in producing the exact ratio of fuel to air. This makes for a much more environmentally friendly engine - complete combusion can occur with the use of a Catalytic Convertor.

Engine Sensors
In order to provide the correct amount of fuel for every operating condition, the engine control unit (ECU) has to monitor a huge number of input sensors. Here are just a few:
Mass airflow sensor - Tells the ECU the mass of air entering the engine

Oxygen sensor(s) - Monitors the amount of oxygen in the exhaust so the ECU can determine how rich or lean the fuel mixture is and make adjustments accordingly

Throttle position sensor - Monitors the throttle valve position (which determines how much air goes into the engine) so the ECU can respond quickly to changes, increasing or decreasing the fuel rate as necessary

Coolant temperature sensor - Allows the ECU to determine when the engine has reached its proper operating temperature

Voltage sensor - Monitors the system voltage in the car so the ECU can raise the idle speed if voltage is dropping (which would indicate a high electrical load)

Manifold absolute pressure sensor - Monitors the pressure of the air in the intake manifold
The amount of air being drawn into the engine is a good indication of how much power it is producing; and the more air that goes into the engine, the lower the manifold pressure, so this reading is used to gauge how much power is being produced.

Engine speed sensor - Monitors engine speed, which is one of the factors used to calculate the pulse width

Hope this helps.

I believe the Carbed cars are more powerful than the Fuel Injected Cars (Denoted by an "i" e.g. GTSi or GTBi).

476 Posts
The 308 series from 1980 on uses the Bosh K Jetronic series injection system described here:

Originally they had Webber carbs. Because of increased legislation to reduce car emissions they were forced to dump the carbs and go to injectors (denoted on the cars by the "i" in the model GTSi or GTBi) with air/smog pumps for the exhaust and CATS. This robbed a good amount of HP and were so bad I believe there was almost a large lawsuit brought against the Ferrari factory by owners. The later four valve 308s (QuatroValve or QV- 308 GTS QV or 308 GTB QV) had an updated version of the injectors and no air/smog pumps but, still with CATS.

Upgrades are available for these cars though if you don't mind moving away from stock.

Norwood Performance
Redline Weber

those and some modern High Flow cats will get the power back.

If I were to look at a GTBi/GTSi I'd do it in a heart beat.

749 Posts
Love the photos backindauk!

Basically ...

1. Engines make power by burning fuel. The fuel has to burn in the presence of a lot of oxygen, and so the engine must "breathe" in a mixture of both air & fuel. The ratio of air : fuel varies but is around 15 parts air to 1 part fuel.

2. With normally-aspirated (ie non-turbo) engines, the air is drawn into the cylinders by suction caused by each piston moving "down" and drawing air in through the inlet valves.

3. In carburettored engines, the fuel required is "sucked" into the intake airstream as the air passes through the carburettors. A carb is basically a "profiled" restriction in the air intake piping to create a venturi shape, where the intake reduces in diameter and then increases again. As the air is squeezed through the venturi it creates a pressure drop & higher suction. Openings (jets) in the sides of the venturi lead to the fuel system, and so as air rushes past the jet the fuel gets sucked out & mixed with the air. The more air going past, the more suction & therefore the more fuel drawn into the engine - simple!

4. Unfortunately, there are lots & lots of variables to be considered to get exactly the right amount of fuel for any situation, (ambient temp, cold starting, acceleration, altitude, manifold pressure vs rpm, etc), and it is extremely hard to build all these variables into a "passive" (ie suction-based) carburettored system. There are some great carbs around, incl the Webers, but they are just too expensive for manufacturers to get setup accurately enough to meet modern emission controls in the long term.

5. Enter fuel injection, where the fuel is actively "injected" into the air intake system rather than just being sucked into it. With computerised sensing systems, all the variables can be determined & the computerised fuel delivery hardware can be programmed to inject just that amount & no more. With the big drop in computing & electronics costs over the past couple of decades it has become a "no-brainer" to switch most cars to FI.

6. FI by itself shoudn't reduce power, in fact it can often increase it. It depends on the sophistication of the hardware incl how much restriction is added to the airflow by the various metering devices used. I expect with the GTBi, etc that the main power-drop was caused by other stuff, such as added air-pumps, changed compression & valve-timing, etc. Someone else could confirm this?
1 - 5 of 5 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.