Engine management is often the biggest stumbling
block when adding a turbocharger to a naturally aspirated engine. You can bung
on an extra injector, fit an interceptor, fiddle the fuel system and play silly
buggers with the factory ECU or you can start from scratch with an aftermarket
management system - but even that has its drawbacks.
Arrrgggghhh, decisions!
In this two-part series we’ll investigate
different ways to tackle the engine management to ensure your engine runs
happily with a ‘charger bolted on the side. In the first part, we’ll focus on
no/low cost approaches...
Budget Approaches to Engine Management
Many experts poo-poo the idea of running a newly
turbocharged engine without major engine management changes. But truth is, if
the engine is not running lean or detonating there’s absolutely no reason why it
should fail (ignoring weaknesses in conrods, crankshafts, etc).
No Changes?
In some instances, you may be lucky enough to own
an engine that’ll accept forced induction without the necessity for engine
management mods. If you’re not chasing a whole lot of power and you’re thinking
of less than around 0.5 Bar (7 psi) boost, you may get away using an untouched
management system.
According to Lachlan Riddel, head of Gold Coast’s
ChipTorque, some Honda engines are equipped with a MAP sensor that’s rated to
above atmospheric pressure – around 1.5 Bar (22 psi) absolute. This means the
engine management system will recognise boost pressure and Mr Riddel says it’s
likely the ECU has the fuel and ignition mapping to cope with up to around 0.5
Bar (7 psi) boost. However, be aware that the MAP sensors in other vehicles will not continue to have a proportional output with any boost pressure.
In vehicles using airflow meters (rather than
MAP sensors) you’ll often find there’s headroom for a few psi of boost. This is
because the airflow meter will see the added turbocharger as merely a breathing
enhancement (similar to an upgrade exhaust or cam swap) and, within limits, it
can’t recognize that boost pressure is occurring. It’s for this reason that when adding a turbo, you’re more likely to have
success using a standard airflow metered system than one using a MAP
sensor.
Inevitably, the standard management will encounter
some type of cut or limp-home mode when the limit of ECU maps is met. Note that
this can occur at any point in the rev range – it isn’t necessarily max power
that you need to be concerned about. In fact, Mr Riddel claims some management
systems – such as Holden V6 systems – may enter a limp mode under deceleration.
Unfortunately, there’s no way of determining how the management system will cope
until you try it.
“The success of running a turbocharger with
standard management also depends a lot on the knock sensor and the ECU’s
learning,” Mr Riddel says. This is crucially important in avoiding detonation.
As you’ll discover, the correct ignition timing is extremely important in
achieving a reliable turbocharged engine.
In addition to these management issues, the
vehicle’s injectors and fuel system must have enough capacity to maintain
suitable mixtures at increased output. Where boost pressure is kept low, you are
more likely to avoid fuel system problems but if the engine starts to run lean,
there are several low-cost approaches to increase fuel flow. (These will be
discussed later.)
So, in short, you may be able to get by running a
newly turbocharged engine with standard management - but this depends on the
characteristics of the load sensor, the extent of ECU mapping and how much extra
performance you’re chasing. You really need to ‘suck it and see’.
Keep boost pressure to minimum and you’re in with
a chance.
Avoiding Cuts and Limp Home Modes
According to the proprietor of Sydney’s
Silverwater Automotive Services, David Alexander, most turbocharger fitments
that run more than around 0.5 Bar (7 psi) of boost will overshoot the standard
ECU tables.
“They
[the ECUs]
generally have only a small
amount of scope above standard,” he says.
When the management reaches the limit of its
mapping, or where the standard management system won’t accept any boost, there
are a few alternatives...
If you hope to keep the standard management system
but you’re encountering a cut or limp-home mode that occurs on boost, it is
possible to skirt around it with some simple changes. For example, this Suzuki
Baleno engine hit a cut whenever the standard MAP sensor received any more than
0.35 Bar (5 psi) boost - the solution was to install a one-way bleed valve to
relieve some pressure from the sensor. This eliminates the cut but results in
fuel and ignition that correspond with a slightly lighter load.
In the case of engines using an airflow meter, a
similar result can be achieved by installing a bigger airflow meter or a
relatively small bypass passage to the existing airflow meter. Again, the ECU
will be fooled into thinking there’s less load than actual – this will help
avoid any cut but, because the ECU thinks the engine is running relatively light
load, it’ll be more difficult to obtain ideal fuel mixtures and ignition timing.
Ignition Timing Control
“In most turbo fitments I find it’s a good idea
remap the ignition timing, especially in the area where the turbo starts to come
on boost,” Mr Riddel says. This achieves a smoother power curve and reduces the
chance of detonation.
Interestingly, the correct ignition timing is
often more important than running rich mixtures according to Simon Gischus of
Nizpro Turbocharging.
“I know that everyone learns that running an
engine lean will put holes in pistons but, really, it’s spark that f%&^s
engines,” Mr Gischus says.
“We’ve run modified Ford XR6 Turbo engines on the
dyno at full noise for two minutes running 15:1 air-fuel ratio with no worries.
People just can’t get their head around it. It doesn’t matter how much petrol
you put in an engine – the advance curve must be right before anything
else.”
The task of retarding ignition timing can be
tackled in a number of ways.
The cheapest approach is to turn the distributor
(where fitted) a few degrees. This will retard ignition timing across the rev
range – great for avoiding detonation but far from ideal for extracting maximum
performance and drivability.
A better but relatively unpopular approach that’s
relevant to single coil ignition systems is to use an aftermarket boost retard
unit. MSD currently sell a Universal Boost Timing Master which allows you to
retard timing 1 to 3 degrees for each pound of boost. A maximum of 20 degrees
retard is possible. Australian company, Jacobs, also used to sell a similar
product - these can occasionally be found second-hand at bargain prices.
A much more flexible approach is to install an
interceptor. Second-hand A’PEXi ITC (Ignition Timing Control) units are
available quite cheaply and give +/- timing adjustment in several preset rpm
zones. This is a relatively crude system that will probably get the job done but
a more sophisticated interceptor – one that can be programmed on the basis of
rpm and engine load – is preferable. The Xede, UniChip and Haltech units are the
most popular interceptors in Australia and are well suited to tailoring the
ignition curve. Of course, these units also provide programmable fuelling
(important for tuning out flat-spots), load signal clamping and a host of other
useful functions. Second-hand examples can be picked up from around AUD$400.
If expense is no concern you can look at the
option of custom remapping the factory ECU or a programmable aftermarket ECU.
We’ll discuss these in the second part of the series.
Fuel System
If you’re chasing a modest power gain, there’s a
chance the standard fuel system will suffice. However, in many instances, you’ll
need to do something to increase fuel flow.
If you find that the injectors are running
virtually flat-out at maximum power and the mixtures are dangerously lean, the
simplest way of enhancing fuel delivery is to bump up fuel pressure. This is
often seen as a quick fix but keep in mind that’s exactly what Ford Australia
did when adding a turbocharger to their 4-litre six...
The easiest approach is to restrict the fuel
return line to the tank (in cars fitted with a return-type fuel system). This
can be achieved by plumbing a 12V solenoid and adjustable ball valve in parallel
between the pressure regulator and fuel tank. When the solenoid closes at a
predetermined engine load, fuel is forced to flow through the ball valve. The
adjusted position of the ball valve determines the amount of extra fuel pressure
you achieve. For more on this technique, see Electronic Fuel Pressure Increase
A similar outcome can be achieved using a
different - or additional - fuel pressure regulator. Several manufacturers offer
rising rate fuel pressure regulators that increase fuel pressure exponentially
as boost pressure increases. Rising rate regulators are fed a manifold pressure
signal and, when on boost, the regulator diaphragm is compressed and fuel
pressure increases. A rising rate fuel pressure regulator is quite a popular
approach but some tuners, such as Mr Riddel, aren’t fans. This is because they
tend to flood the engine with fuel at the rpm where full boost is achieved
(typically in the mid-range).
“Certainly, rising rate fuel pressure regulators
have a purpose but, to me, the way they work is more suited to something like a
centrifugal blower where boost is progressive,” Mr Riddel says.
Mr Alexander from Silverwater Automotive Services
has a similar opinion.
“A rising rate reg is not a great approach from a
tuner’s point of view. With a rising rate regulator, the engine usually drowns
in fuel because fuel pressure is raised exponentially and the ignition timing is
usually left as it is,” he says.
"Then, with higher rail pressure, you also receive
lower pump flow - you’ll often need to upgrade the fuel pump to cope.”
You may also need to increase fuel system flow.
The easiest way to enhance fuel pump flow is to
increase its supply voltage. There are several aftermarket units on the market
that boost pump voltage while the engine is on boost. We haven’t seen any test
results of these products, but it appears they achieve a considerable increase
in fuel flow. The MSD Fuel Pump Booster is adjustable up to 22V. Interestingly,
we’ve also heard that re-wiring the factory pump with heavy gauge cable can
maximize voltage at the pump and slightly enhance output - certainly worth
investigating.
Other approaches include fitting bigger injectors
and/or upgrading the fuel pump. We’ll cover these in Part Two.
Extra
Injectors?
A
once-popular way to increase fuelling was to fit an extra injector (as seen in
the inside bend of this induction elbow). The extra injector was typically
mounted in the induction plumbing a short distance before the throttle – the
injector was activated at high load and the extra fuel passed through the
throttle, into the intake manifold and inside the cylinders.
Unfortunately,
this approach delivered unknown cylinder to cylinder fuel distribution –
some cylinders might have run rich while others ran lean. And, more often than not,
extra injectors were triggered purely on the basis of boost pressure – this was
guaranteed to provide over-fuelling when max boost was reached in the mid-range
and relatively lean mixtures toward the top-end.
Not
surprisingly, extra injectors have gone the way of the dinosaurs. Today, their
only application is where extra fuel is required and there’s a beneficial
cooling and rotor sealing effect inside positive displacement type superchargers
(where the extra injector is mounted ahead of the blower).
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Anti-Detonation Strategies
In addition to taking care of fuel and ignition
control, there are some other steps to ensure happy turbo operation.
One of the most important additions is an
intercooler. This is essential to help avoid detonation (especially if you’re
turbocharging with a standard management system) while also improving
performance. Similarly, a water injection system can be added. Water injection
is a highly effective anti-detonant and you may find slightly improved
performance when using a well designed system.
Other modifications to reduce the chance of
detonation include switching to colder spark plugs and ensuring the fuel tank is
always filled with the highest available octane. It’s a good idea to carry a
bottle of octane booster for those occasions where you can’t find high-grade
fuel.
So these are the most cost-effective ways to take
care of engine management when adding a turbocharger. In the next part of this
series we investigate spare-no-expense approaches – are they really worth the
extra?
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