To operate properly an engine needs the correct amount of fuel at
all times and the correct ignition timing. Getting these factors right is
essential after any tuning modifications have been done and is called
calibration.
The power generated by any petrol engine comes from the fuel
burned inside the cylinders. Any fuel needs oxygen to burn though and that
comes from the air. We have seen in previous articles that an engine is really
an air pump. The amount of power produced is directly related to the amount of
air the engine can process per minute. Tuning modifications are designed to
enable the engine to flow more air - just squirting in more fuel without
improving the air flow does nothing for power. Every molecule of fuel needs to
combine with exactly the right number of oxygen molecules if it is to burn
completely and release its energy. For best power the ratio of the weight of
air to fuel to achieve this is about 12.6 to one. So for every 12.6 lbs of air
the engine processes we can burn 1 lb of petrol. We call that an air/fuel ratio
of 12.6 to 1. For best economy the ratio is weaker - modern cars are set up to use
an A/F ratio of about 15 to 1 at part throttle for good cruise economy. At full
throttle the mixture is richened to maximize power output.
If the A/F ratio is weaker than 12.6 then power drops because the
engine could be burning more fuel with that amount of air. If the ratio is
richer than 12.6 then power also drops - the excess fuel can't burn because
there is not enough oxygen present and just gets pumped out again along with
the rest of the exhaust gases. Also this excess fuel displaces some of the air
that the engine could otherwise have processed. Whether an engine has carbs or
fuel injection the calibration must be correct at all rpms and throttle
positions.
Carbs are calibrated by changing the size of the various fuel
jets. Bigger jets let more fuel through for a given amount of air. The standard
car will have been calibrated by the manufacturer but if the engine is modified
in any way then the fuel mixture may no longer be correct. The solution is to
take the car to a rolling road dyno where the A/F ratio can be measured and
altered with different jets if necessary. In principle a fuel injected car is
no different. The ECU stores on a chip a map of how much fuel the engine needs
at different speeds and throttle positions to achieve the correct mixture.
Signals from the crank sensor and throttle sensor tell the ECU what is
happening. The ECU then looks up those positions in its internal map and
triggers the injectors for exactly the right amount of time.
It takes one or two milliseconds from the time the spark occurs
until all the fuel/air mixture in the cylinder is fully alight and expanding.
The spark plugs therefore need to be fired a little while before the piston
reaches Top Dead Centre so as to get the fuel mixture burning at the right time
to push the piston down and generate power. When measured in crank degrees
rather than seconds this time delay is called ignition advance. The perfect
time to trigger the spark depends again on engine speed and throttle position.
Cars used to use a mechanical distributor to set the spark timing. Nowadays it
is normally done by the ECU in a similar way to how the fuel mixture is
controlled. The ECU stores another map on its chip of how much ignition advance
is required which operates just like the fueling map.
The amount of ignition advance required depends on the engine
design. Average figures would be between about 10 crank degrees at idle to
about 30 degrees at peak rpm. The required advance usually increases with rpm
up to about 3,000 to 4,000 rpm and then stays fairly constant. It also needs to
increase at low throttle openings and reduce again at full throttle. If the
spark is fired too early (over advanced) then the mixture starts to burn too
soon and tries to push the piston backwards down the way it came before it
reaches TDC - very bad for power and a major cause of engine damage. If the
spark is fired too late (retarded) the piston has already gone part of the way
down the bore on the power stroke before the mixture is alight and much of the
effectiveness of the energy released is lost.
If I had £1 for every person who thinks that more ignition advance
is a good thing in its own right I'd be a rich man. Like most other things,
more advance is only good if there isn't enough to start with. Excessive
advance is just as detrimental to power output as insufficient advance but it's
also potentially much more harmful to the engine. In fact the better the engine
design the less advance is required and other things being equal, an engine that
requires less advance will produce more power.
Well that depends on your point of view. The OE manufacturers have
a number of criteria other than just maximising power. They need to retain
reliability, good fuel economy, allow for poor fuel, hot and cold operating
conditions and what happens to the engine as it wears. Setting the fuel mixture
to exactly 12.6 and the ignition timing to the optimum for best power is all
well and good if everything else stays perfect. But if the engine overheats or
you fill up with a bad tank of fuel those settings might cause detonation and
consequent engine damage. Using a fuel/air ratio of 13 or 13.5 instead of
12.6 might lose only 2% power but gain 5% economy. Using a couple of
degrees less than the optimum ignition advance allows a safety margin for low
octane fuel or engine overheating with again only a minor loss of power. The
standard calibration settings are what they feel is the best balance of
reliability, economy and power. In my own opinion, most OE engine calibration
settings are a very good compromise and not worth messing around with.
The chip is where the fuel and ignition maps are stored in the ECU
of a modern engine. The aim of non standard chips is to take advantage of any
compromises the OE manufacturer has made to the standard calibration settings
which reduce power in favour of economy or reliability. The scope for
improvements is usually very small though. The best that can normally be
achieved is to remove any flat spots in the power curve and find a couple of
% extra power by richening the mixture up to 12.6 and losing any safety
margin in the ignition timing settings. The penalty is often significantly
worse fuel consumption, unreliability, poor starting and the power increase is
often not even noticeable. It takes a day or less of dyno time to establish the
new map settings for a particular vehicle and each chip costs a couple of
pounds. Total development cost perhaps a few hundred pounds. The selling price
of £200 to £400 from then on for a chip costing £2 means a huge amount of
profit for both the chip company and the fitting agent. To keep those sales
rolling along nicely it isn't surprising that the power claims tend to be
somewhat inflated.
Why people are prepared to spend so much on a 'performance' chip
is beyond me. Everyone nowadays is familiar with how much computer components
cost. A PC motherboard is maybe £60 and a complex piece of software that took
millions to develop might be £30. For a map that took a day to develop when
each new chip itself costs £2 it strikes me that to pay £200 or more is
madness. Still it's your money I guess.
On a turbocharged engine the chip might also control the boost
pressure. There are genuine possibilities for good power increases in this case
although it isn't really any more complex than adjusting a mechanical
wastegate. The penalty for excessive boost pressure is detonation and engine
life measured in weeks though. On a normally aspirated engine the chip can't
make any difference to how the engine physically operates and can't increase
the airflow potential. You therefore can't just "bolt on power" with
a chip swap - it is purely a calibration device, not a tuning device. No
different in principle to getting a carb jetted properly. Claims of 30% extra
power from chip tuning is purest nonsense - 3% is more like it. If the OE
manufacturers were that bad at calibrating their cars considering the millions
they spend on doing it they'd be out of business in weeks.
Any time the airflow potential of an engine is substantially
modified - by that I mean ported cylinder head, exhaust system, different carb
or manifold, longer duration camshaft etc - the fuel and ignition requirements
also change. Whether the engine has carbs and a distributor, or ECU controlled
fuel and ignition, the principles of calibration are the same. The best place
to get this sort of work done is on a rolling road dyno or engine dyno.
The engine is operated under load and the fuel/air ratio and power
are measured. Adjustments are then made to bring the mixture back to the
optimum settings. On a carb by changing the jets and in an ECU system by
changing the internal map (or chip) that the ECU works from. The ignition
advance can then also be altered a couple of degrees at a time to see if power
goes up or down at different rpm.
An engine can be modified in an infinite number of different ways.
Even similar sounding specs might work very differently. For instance a ported
head might increase airflow by nothing at all if it has been done badly or 30%
if it has been done well. The settings from someone else's similar sounding
engine might be nothing like right for your own. By the same token a
"performance chip" designed to squeeze a couple of % extra power out
of a standard engine is useless for a modified engine if the map settings it
contains are not what the modified engine now wants. Sadly it seems to be
commonplace for people to believe that a chip is a performance item in its own
right and that by fitting one it will magically make any possible combination
of cam, exhaust and head mods work properly together. Nothing could be further
from the truth.
So far I haven't seen a single definitive test where the
acceleration of a "chipped" standard normally aspirated car actually
improved. I've seen plenty of rolling road tests showing supposed increases in
power and comments about improved "driveability" during road testing.
Bring out the stopwatch though and these improvements seem to be rather harder
to pin down. Several years ago one of the car magazines did a reasonably
scientific test on a BMW as I recall. Performance chips from two different
manufacturers were tested against the standard item in a 0-60 sprint at a test
track. 5 runs were performed for each chip and the times averaged to remove any
bias. Within a tenth of a second they stayed the same in all cases despite the
extra horsepower claims. If anyone knows of a properly conducted test that
shows the opposite I'd be interested to read it.