how come? what can you run your boost at without one?
Ok, given this topic comes up time and time again, I may as well make a full
article out of it and post it in the tech section of the web site...
Using Fuel Cut Defencers, AKA blowing up your engine.
Fuel Cut is a deliberate mechanism put into ECUs by car manufacturers as a
safety feature. It exists in order to prevent turbos being over-boosted and
the consequent effects on the engine from the increased intake heat,
mechanical load, and other such factors. In Toyota's case when fuel cut is
triggered the ECU hard chops fuel delivery to the engine for a fraction of a
second. This is enough to kill the boost and bring the engine back to
within normal operating parameters. Fuel cut is particularly important on
some vehicles, as raising the boost above stock level will almost invariably
cause the turbine shaft to snap due to the extra load: certain Subaru and
Mazda set-ups are particularly prone to this.
Fuel cut is triggered in various different ways. In general the trigger is
tied to the volume of air that is being injested by the engine, not boost
level. This explains why it is easier to hit fuel cut on cold nights than
it is on hot days - the denser cold air means a larger mass of air is
entering the engine for any given boost level.
The following general methods are used to trigger fuel cut:
- Flap air flow meters (1GGTE, 5MGE, early 3SGTE, etc): Either AFM reading
reaching a certain value and/or microswitch trigger when the flap is opened
a certain distance.
- Karman Vortex AFMs (7MGTE, 1UZFE): Threshold built into the ECU that
triggers when a certain flow is reached through the AFM
- MAP (1JZGTE, 2JZGTE, late 3SGTE, etc): Threshold built into the ECU that
triggers when a certain pressure is reached by the MAP sensor.
What Fuel Cut Defencers do:
FCDs monitor the signal from the AFM/MAP sensor, and when the output reaches
the level that will trigger fuel cut they begin to modify the output,
telling the ECU that no more air is entering the engine, regardless of how
much more air is actually being stuffed in there. This is *EXTREMELY*
dangerous on turbo engines, as it causes the air/fuel mixture to run lean.
A lean mixture under boost promotes detonation, which in turn causes a
variety of ailments ranging from blown head gaskets up to electrical faults
(caused by pistons and rods exiting the block and knocking off the
alternator on the way past).
Arguments people use to try to justify the use of FCDs:
"The engine runs rich anyway, there's plenty of fuel there, it doesn't
matter if it runs leaner".
Answer: Turbo engines must run rich by design. Compressing the air causes
the intake charge to heat up significantly, which is a prime cause of
detonation. The ECU runs the engine rich so that the excess fuel will cool
the intake charge and assist bringing it back down to a level that will
prevent detonation from occuring. An FCD will effectively remove this
safety buffer. Typically turbo engines will run richer than they absolutely
need to in order to ensure the safety margin is maintained, in which case
the FCD will 'get away' with leaning out the engine for a few extra pounds
boost. However that does not allow for any variation in fuel quality,
ambient temperature, etc, all of which contribute to eroding the safety
margin before the FCD even enters the picture.
"The ECU learns! Surely it'll use feedback to work out that it's running
lean and adjust to suit"
Nope. ECU learning is based on Exhaust Gas Oxygen sensor feedback. The
oxygen sensor gives the ECU a rough guide as to whether the engine is
running rich or lean, and the ECU will compensate to try to achieve the
correct mixture. However, this only applies when the engine is idling or
under light cruise - as soon as you put your foot down the O2 sensor
feedback goes right out the window and the ECU resorts to pre-programmed
fuel maps that are made to cope with normal conditions up to the point where
fuel cut occurs, and not beyond. Once that point is reach and passed the
ECU is in 'wildly inaccurate mode'.
"But plenty of people buy these things, they MUST work without harming my
Really? OK, in that case I've got some penis enlargement pills here you
might be interested in too, and a really good investment scheme in Nigeria
that will make you $500,000 (FIVE HUNDRED THOUSAND US DOLLARS).
Getting around fuel cut:
There are various ways to do it, some good, some not. The essential problem
is to make sure that the correct amount of additional fuel reaches the
cylinders once the engine goes beyond where fuel cut normally happens. Some
examples of methods to do achieve this:
- Additional injector controllers: Used to run a 7th injector to add fuel.
FCD also required to enable the engine to go beyond fuel cut. Not
particularly nice method due to fuel distribution and atomisation problems.
- Rising rate fuel pressure regulator: Cheap hack. Used to ramp up the fuel
pressure to the injectors as boost increases, thereby deliberately
overfueling the engine. FCD also required to make this work. Very
inaccurate method, and fuel injectors have limitations as to how much extra
pressure can be usefully delivered to them.
- Remapping daughter boards: The only equivilent to 'chipping' for Toyota
ECUs. Additional board that is added to the ECU to monitor and alter the
signals going to/from the ECU. Has the advantage over FCDs that it can also
be used to alter the fuel delivery and ignition timing. If done properly
then this is the next best thing to the last option...
- Complete replacement ECU: Link, Autronic, Motec, Haltec, etc etc.
Supra Club of New Zealand