the engine, a vacuum will form in the ventury sucking harder on the idle jets and main jets and so
make the cranking mixture rich enough for starting. The moment the engine starts, a vacuum
canister will force the choke butterfly open slightly causing air to flow freely through the ventury. As
the engine heats up the choke butterfly will open completely also releasing the bottom throttle to
its normal position. The ECU does this by opening the idle valve to accommodate for more air. It
will enrich the fuel using the temperature compensation map. It will squirt a set amount of fuel
(Start Prime Pulse) the moment the engine starts to crank. When the engine starts, it will enrich
the mix (Start Enrichment) for a few seconds fading it out. As the engine heats up, the ECU will
lean out the mix on the water temp compensation map and reduce the idling RPM to the normal
setting.
Furthermore the ECU can compensate for air temperature correction where hot intake air is
thinner requiring a leaner mixture. It can also compensate for battery voltage deviation which
would influence injector opening times causing mixture deviations. Electric idle valves help the
engine not to pre ignite after the ignition is switched off. This is due to petrol spontaneously
combust on hot carbons in the engine. With the ECU fuel is cut off immediately during power off.
So no pre ignition here. On steep inclines the carburetor always have a problem where the bowls
overflow or the main jets are exposed to air casing the engine to loose power or flood. This is no
problem for the ECU as the petrol is in a closed pressure loop and the injectors can work upside
down. Another problem of the carburetor is peculation due to engine compartment heat. The fuel
simply starts to boil and then flood the carburetor causing the engine to stall. With fuel injection
again the system is under pressure resulting in a much higher temperature for peculation. Also the
fuel is circulated in the tank causing it to cool down.
On the timing side, the normal distributor had 2 dimensions for timing alterations. Firstly it had
weights that did a single advance with RPM curve till a certain RPM and then it was a flat curve.
This same curve was also programmed into later firing modules. For this dynamic timing the ECU
can alter the timing in degree divisions every 100 RPM’s. It can go up and down which is of great
advantage to top end camshafts.
For vacuum timing the mechanical vacuum canister was the norm. Some had an adjustable spring
tension but they always followed a fixed curve in one direction. The ECU uses the Map sensor
signal and the vacuum map to add or subtract timing especially for turbo engines. Again any
combination in degree divisions can be adjusted making this a nice feature to add power and
economy. It also helps to keep the plugs clean.
On the coil side, the ECU is far better than the old point condenser system. It uses variable dwell
timing which means that the coils is always charged the same time to ensure that spark energy is
at a maximum. Some systems may not have enough time at high RPM’s to achieve this. However
faster coils may be used. The ECU will calculate when to start charging the coil to ensure it is fully
charged before it will be discharged via the spark plugs. This method will prevent wasted energy
which heat up the modules and also ensure a proper spark every time, even during cranking when
battery voltage is low.
Now comparing the carburetor to a fuel management system we can clearly see the advantages.
There are lots of other features like fuel cutoff during deceleration, idle control for automatic cars,
and air conditioners, launch control etc. To successfully tune the engine yourself may take a bit of
trial and error. Two conditions to always be careful of. Never go too lean on WOT mixtures and be
careful of too much advance in the timing department. Your ear will tell you most of the time but it
is not always possible to hear pinging. A good lambda sensor however will indicate if you are on a
lean mixture.
2. Safety Precautions
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