- Higher air density means more power.
- More heat means lower air density and power loss.
- The tuner can optimise the engine for maximum air flow with minimum
Air density is never constant. At sea level the
air is 35% more dense than at 10000ft. A 3 litre engine will fill its cylinders with 3 litres of air at 10000ft just as
it does at sea level. However, at altitude, the air contains less oxygen (26%
less) so it will produce much less power. As a rule of thumb, power will
decrease by 3% for every 1000ft.
Heat also affects air density. Air at 40deg Celsius is less dense than at 5deg.
Another rule of thumb is that you lose 1% of power for each 7deg Celsius
increase in air temp.
Another important area to consider is the air density within the cylinders. Two
identical engines, operating in the exact same environment can have different
air densities within the cylinders because they employ different induction and
exhausts and cooling methods.
One engine may have its ancillaries working against it. It is drawing air which
has been heated by the radiator and the exhaust manifold. Its inlet manifold
receives no cooling air flow. The water temp is regulated at 95deg so the air
flowing into the cylinder collects heat from the inlet ports and expands on its
way through. The ignition timing is retarded which raises the temp of the
exhaust valve and the piston crown. Therefore, as the intake air discharges into
the cylinders it expands rapidly and the cylinder volume is quickly with what is
now low oxygen air.
In addition, the cat has overheated and the internal honeycomb ceramic has
melted, blocking the exhaust gases from fully escaping, causing excessive back
pressure. The remaining gases mingle with the fresh inlet air and upsets lambda
ratio and quickly heat the inlet mixture.
On the inlet side, the air filter is over restrictive and blocked with built up
dust and grime. It is designed for minimum noise rather than optimum flow, and
uses various ribs and baffles to keep the noise low which causes terrible inlet
flow. The vacuum reading is 1psi, so the cylinders only receive 13.4psi pressure
differential instead of the optimum 14.7psi.
Other inlet restrictions also exist, the gaskets overlap, there is a build-up of
carbon on the back of the inlet valves, which blocks flow until the valves are
50% off their seats.
An identical engine with everything designed with power and efficiency in mind
will have a much more favourable cylinder air density.
1. inlet system free of restrictions
2. air intake in front of the engine compartment collecting air at ambient.
3. water temp regulated at 85deg
4. free flow exhaust with performance cat (or no cat)
5. ignition timing optimised for max power
The point is that a lot of factors can be changed to increase air charge density
in the cylinders which lead to greatly increased engine responsiveness and
The engine tuner can control these factors to optimise air and temp within the
Always remember that anytime heat is added to the air, charge density goes down.