PUSHING IT TOO FAR
This article details some of the problems associated with trying to squeeze too
much power out of a turbocharged engine and the common mistakes made by many
people. Reading of the following related articles on this site is suggested as a
foundation for this article:
Ignition Tuning Ideas for Turbos
Ignition and Combustion
Fuel Octane vs. Horsepower
Intelligent Engine Modifications
Detonation
Detonation is defined as a form of combustion which involves too rapid a rate of
energy release which produces excessive pressures and temperatures in the
combustion chambers. These high pressures and temperatures can damage or destroy
engine parts in short order. Detonation is often accompanied by an audible
rattling sound.
Pre-ignition
Pre-ignition is defined as a normal combustion process starting before the
ignition spark is initiated. This is usually caused by a local hot spot raising
the mixture temperature above its auto ignition point. As combustion has started
before it was intended to, peak cylinder pressure occurs too early in the cycle.
This leads to excessive pressures and temperatures, often while the piston is
still in an upwards motion with cylinder volume decreasing rather than
increasing. Pre-ignition effects can include piston and spark plug electrode
damage. Pre-ignition is usually not audible and can often lead to detonation.
Effects of pre-ignition and detonation on piston dome
Plug suffering from mild pre-ignition damage RIGHT, normal plug LEFT
Brake Mean Effective Pressure (BMEP)/ Peak Cylinder Pressures (PCP)
BMEP is defined as the average effective combustion pressure occurring in a
cycle. It can be calculated by using the formula:
792,000 X BHP divided by (engine displacement in cubic inches X RPM).
This figure is useful in comparing different engines operating on different
fuels and the highest figure occurs at torque peak. The average range for
engines is 200 to 400 psi.
Peak cylinder pressure (PCP) is the maximum chamber pressure achieved during the
combustion process. This figure would normally be in the 600 to 2000 psi range.
Thermal Efficiency
Thermal efficiency describes the amount of energy extracted to perform useful
work from the total energy contained in the fuel. TE is primarily affected by
the compression ratio and ignition advance in a given engine design. Most
engines are in the range of 25 to 35%. The lower the TE, the higher the exhaust
gas temperature. TE can be calculated with the following formula:
2545 X BHP divided by (Btu/lb X lb. fuel/hr).
Specific Power Output
This describes the amount of hp developed per unit displacement. It is usually
expressed in HP/liter or HP/cubic inch. This is useful in comparing different
engines and stress limits. Generally speaking, the higher the specific output,
the higher the stresses on the engine and the lower the engine life will be. It
can be calculated by:
HP divided by engine displacement
Performance Considerations and Tuning Effects
On a given fuel, the maximum and mean cylinder pressures that can be achieved
are limited to a certain figure. This is known as the knock limit. Trying to
achieve cylinder pressures above the knock limit WILL destroy the engine. At
wide open throttle, cylinder pressures can be altered by changing boost pressure
and ignition timing. If the knock limit on a given fuel occurs at 700 psi PCP,
this limit could be achieved by using 5 psi of boost with the timing set at 30
degrees BTDC or at 12 psi with the timing at 15 degrees BTDC. The engine will be
considerably more efficient running less boost and more timing and the thermal
stresses will be reduced as well.
As mentioned above, TE is affected by CR and ignition timing. As the timing is
retarded, PCP is developed later in the cycle. This allows more energy to be
lost through conduction into the water jackets because the piston is further
down the bore and the rod has a less advantageous angle on the crank pin to
deliver force to the crankshaft. Retarded timing also raises the exhaust gas
temperature considerably. This raises the thermal stress on the pistons, spark
plugs, valves, exhaust system and turbocharger. In severe cases of retarded
timing, the mixture is still burning when the exhaust valve opens. Because
turbochargers are driven by the energy in the exhaust stream, high EGTs caused
by retarded timing produce so much energy at the turbine that even a fully open
waste gate cannot control the boost pressure. All in all, retarded timing is
counterproductive to producing an efficient, powerful engine.
Most naturally aspirated engines require between 30 and 38 degrees of ignition
advance to achieve PCP at the correct crank pin position to make maximum power.
By compressing the mixture through turbo charging, the rate of flame front
progression increases and slightly less ignition advance is required to achieve
PCP at the correct moment. In most cases, less than 5 degrees of retard is
required however. We see many people throwing in 15 to 25 degrees of retard in a
vain attempt to stop detonation at very high boost pressures for the fuel and
compression ratios that they are running. It should be stressed that there are
no free rides here. If you plan to achieve high specific outputs on low octane
pump fuels for extended periods, you WILL have to reduce the CR. Truly high
specific outputs are only available when using high octane fuels or by injecting
anti-detonants. There are sound scientific reasons why there are no factory 10
to 1 CR turbocharged engines which produce specific outputs of 175 hp/L. In
fact, there is NO production, piston, automotive engine which I am aware of
which can achieve a specific output of this level on 92 octane pump fuel
anywhere. Despite this fact, many people try to do this with expensive results.
High compression ratios and high boost simply don't mix on pump fuel. If you try
this, you will either be unhappy with the results or blow up the engine. When I
say production engine, I mean one that you can buy off the showroom floor, no
modifications, with the factory warranty intact. HP to be tested on a proper
engine dyno, not on a chassis dyno with phantom flywheel correction factors
applied. If Toyota, Honda or Ford could do this with factory reliability, don't
you think that they would? As discussed in some of the reference articles above,
set reasonable hp goals and modify the internal components as required to obtain
these levels reliably.
Making it Live
Reducing the compression ratio or using higher octane fuel are the two best ways
to increase power on a turbocharged engine. If you drive on the street, you
pretty well have to use pump fuel. In this case, you may want to fit some lower
compression pistons. Pistons and spark plugs are often the first parts in the
engine to suffer from the effects of overpressure and over temperature
conditions. A high output engine should always be fitted with colder spark
plugs- a point often overlooked by amateur engine builders. Forged pistons and
turbo motors go together like jam and toast but there are wide variations
between forged pistons. On a turbocharged application, temperatures and
pressures will far exceed anything seen on a naturally aspirated engine. Because
the specific output is higher, the rate of energy release is higher. Piston dome
temperatures can run between 450 and 550 degrees F. Most aluminum alloys have
lost over half of their strength at 400F. Turbo pistons need to have thick upper
sections to be able to dissipate heat faster to the skirts and cylinder walls to
keep dome temperatures down to safe limits. High silicon pistons can be fitted
tighter because of their lower expansion rates for less rattling when cold but
because they are more brittle, they don't stand as much detonation as a low
silicon piston. Compression ratios for street use should generally be kept in
the 7.0 to 8.5 to 1 range.
High silicon forged piston for use on naturally aspirated and low output turbo
charging.
Note relatively thin dome and corner radii.
Cast factory piston left, custom designed low silicon, forged piston right
Note thick corner radii for
higher heat transfer rates
Most factory turbocharged engines are equipped with under- piston oil jets.
These are an especially good idea on engines with large bores where the center
of the piston dome is a long way away from the sides to be able to dissipate
heat efficiently and extra thickness can add excessive weight to the
reciprocating assembly.
Oil jets
