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.

Fuel Mixture

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.

Ignition Timing

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. In fact it is directly linked to how fast the fuel/air mixture burns. The faster the burn obviously the less ignition advance is required. 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 because the mixture in partially filled cylinders burns more slowly. 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 can create detonation which is 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 so again engine power drops.

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 because its mixture burns faster will produce more power. A fast burn is obtained by using compact combustion chambers and plenty of swirl and turbulence in the fuel/air mixture.

High Octane Fuel

High octane fuel does NOT give an engine more power in its own right. The fuel itself doesn't contain or release more energy than low octane fuel. In fact it often has less. What it does do is resist detonation better which allows the engine designer to use a higher compression ratio without having to retard the ignition timing. It's the higher compression ratio that produces the extra power. If you build an engine with too much compression ratio for the fuel octane being used then the engine will detonate or 'pink' at high throttle openings. To stop this happening the ignition advance has to be reduced below what the engine would ideally like for best power. This stops the detonation but loses more power than the extra compression ratio was giving in the first place. In nearly all cases when detonation is present you'll get more power for a given fuel octane by reducing the compression ratio and advancing the timing again. Obviously the ideal is to use fuel with a higher octane value, add compression ratio and still be able to leave the ignition adavance alone.

Many modern high performance engines have knock sensors built in to detect detonation. When they detect this happening they reduce the ignition advance until the detonation stops. In this way they can adjust for poor fuel without the engine suffering damage but the power drops. They will therefore perform best on higher octane fuels. For basic engines without knock sensors if there is no detonation with the ignition timing correctly set then you'll get no more power by using high octane fuel because the engine design doesn't need it. You also won't get more power by increasing the ignition advance because again the engine doesn't need it. Only by increasing the compression ratio will the higher octane fuel become of any benefit.

Why Don't The OE Manufacturers Get It Right?

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.

Chip Tuning

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.

Calibrating A Modified Engine

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.

Performance Increases From Modified Chips

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.

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