Can you improve a car's engine by driving the car fast?

Question

Many car enthusiasts claim the following:

1. take 2 identical cars (equal horse power, torque etc).
2. give one car to an old person who only drives it slowly
3. give the other car to the car enthusiast making this claim

If you put both cars on a test bench you will see that car 1 has less power/torque than car 2.

The claim goes further:

1. Give car 1 to the car enthusiast, and by driving it "in a sportive way" the engine will improve again.

Is this really true? If so, how come?

Answer 1

In some cases (1, 2, 3) it's true for cars with diesel engines with diesel particulate filter (DPF).

DPF has to be regenerated (cleaned) now and then:

How do they work?

Diesel Particulate filters (DPF) or 'traps' do just that, they catch bits of soot in the exhaust.

As with any filter (think of the bag in your vacuum cleaner) they have to be emptied regularly to maintain performance. For a DPF this process is called 'regeneration' – the collected soot is burnt off at high temperature to leave only a tiny ash residue.

Regeneration may be either passive or active.

Passive regeneration

Passive regeneration takes place automatically on motorway-type runs when the exhaust temperature is high. Because many cars don't get this sort of use car manufacturers have to design-in 'active' regeneration where the engine management computer (ECU) takes control of the process.

Active regeneration

When the soot loading in the filter reaches a set limit (about 45%) the ECU can make small adjustments to the fuel injection timing to increase the exhaust temperature and initiate regeneration. If the journey is a bit stop/start the regeneration may not complete and the warning light will come on to show that the filter is partially blocked.

It should be possible to start a complete regeneration and clear the warning light by driving for 10 minutes or so at speeds greater than 40mph.

So, "old-person-driving" such a car (short distances, low speed) can cause serious problems:

people using their vehicle for short stop start journeys are suffering with fault lights appearing, and in some cases the vehicle going into what is called “limp home mode” restricting the performance dramatically.

Automobile Association advises:

• Occasional harder driving in lower gears should be sufficient to burn off the soot in such cases.
• If you're buying a new car and plan to use it mainly for town-based, stop/start driving it would be wise to avoid a diesel car fitted with a Diesel Particulate Filter (DPF)

Answer 2

Foka answer is spot-on for diesel engines with DPF. The same reasoning, to a lesser degree, apply to gasoline engines too.

You won't gain power or torque, not in a perceptible way. But driving a car in a "spirited" way make it run smoother. How, and how much this is relevant, depends on the age of the car.

Of course, as Glen Jaron pointed out, the car needs to be "ready", which means run-in AND also ready for hard driving (which means that the OIL must be warm; warm coolant is not enough).

A car not running as it should may be partly due to carbon deposits, that will be cleared and burned by the higher temperatures reached when running at higher revs

One of the reasons (probably, the prominent reason) of carbon deposits is linked to the stoichiometric ratio, especially in older cars. In fact, this gets more important as the car gets older.

The stoichiometric ratio is the ideal mixture of fuel to air link, wikipedia

Carburetors on older cars (and ECUs in newer injection systems, with or without lambda sensors) have the purpose of reaching this mix in which there is just enough air present in order to burn off all of the fuel, although it’s never 100% achieved.

The typical stoichiometric mixture ratio a carburetor (or fuel injection) has to achieve when mixing fuel and air is 14.7 of air mass to 1 of fuel mass – 14.7 : 1.

If the stoichiometric mixture is lower then this means the engine will be running fuel rich which means fuel is wasted, engine emissions will be higher and "dirt" (carbon residues) will accumulate.

If the stoichiometric ratio is higher then this means that the engine will be running lean and will struggle to give a smoother running performance, struggle to maintain combustion and possibly (in extreme cases) permanent damage to the piston's heads. (Lean mixture -> detonation -> Chamber too hot -> hole in piston

When the engine is "cold" (at start, or if it run very conservatively) the mixture is richer, to prevent engine hesitation and damage (for multiple reasons: the oxygen sensor (MAF, IAF, or whichever sensor you have) operates in a good reliable way only when temperatures are high, a richer mixture helps protect the engine, etc.)

In older ECUs this is quite important (see for example the Bosch Motronic); in modern ECUs this is far less.

It is believed (by ECUs reverse engineers) that ECUs do not learn the driving style. However, they have still to prepare the right mix, and they do learn "fuel quality".

Finally, modern engines also suffer from carbon deposit due to a different injection mechanism direct injection. Getting the combustion chamber hot (higher revs) helps in this situation as well.

Answer 3

Well it all depends on the initial stages of the car. An engine that is well run in will be more efficient comparably to the one which is driven slow all the time. Even rash and high RPMs are not advised on a new engine. Careful running in procedures ensure better performance as well as longevity of an engine.

My Source - http://www.automotix.net/autorepair/engine_repair_guide.html