A turbo charger is basically an exhaust gas
driven air compressor and can be best understood if it is divided into its two
basic parts, the exhaust gas driven turbine and its housing, and the air
compressor and its housing. I did say divided didn't I. Well I should have said
like a set of Siamese twins because each of them perform different functions
but, because they are joined together at the hip via a common shaft, the
function of one impacts the function of the other. How? Take a perfectly set up
compressor section and mate it with an incorrect turbine section, or visa versa,
and you end up with with our Siamese twins trying to go in different directions.
The result is that our Siamese twins end up wasting all of their energy fighting
each other and go nowhere.
When considering a turbo charger most folks tend to look
at the maximum CFM rating of the compressor and ignore everything else under the
assumption that the compressor and the exhaust turbine are perfectly matched out
of the box. I will grant you that in stock factory applications that is probably
close to the truth but, in all out performance applications, nothing could be
further from the truth because of the extremes of operation in a performance
application.
The goal in a performance application is to get the
exhaust turbine up to speed as quickly as possible however, it must be mated to
a compressor wheel that will generate as much pressure as it can as soon as
possible. This is a contradiction because the exhaust turbine generates the
drive power and the compressor consumes that power. The larger the compressor
and the higher the pressure (boost) we want, the quicker the power from the
exhaust turbine is used up. Put in a larger exhaust turbine and it will take the
engine longer to develop enough hot expanding exhaust gas to spin it, slowing
down the compressor and causing turbo lag. At this point I am going to repeat
something stated earlier, do not think of a turbo charger as a bolt on piece of
equipment, think of it as a system.
The turbine is powered by hot expanding exhaust gas, a lot
of hot expanding exhaust gas, the more and the hotter the expanding exhaust gas
the better. I am sure many of you have seen pictures of turbo charged engines
with cherry red hot exhaust systems and turbo housings. The captions under most
of these types of pictures proclaim outstanding horse power numbers. What most
of the articles related to these pictures do not tell you is that the engine was
under an extreme load. A load so heavy that the engine was almost at its stall
point for a prolonged period of time. A condition that most turbo charged
engines will never see.
The real point I am trying to make is that the exhaust
turbine will not generate enough power to turn the air compressor fast enough
for it to work properly unless the engine is feeding the exhaust turbine a lot
of hot expanding exhaust gas, a condition that can only be created when the
engine is under a load. There is where the selection of transmission gear ratios
and the ring and pinion ratio play a critical part. The fact that the engine
must be under a load is the reason why, no matter how high you rev a turbo
charged engine with no load on it, you will not see the boost gauge move.
This is also where the term 'turbo lag' came from. Turbo
lag is basically the amount of time it takes from the time you place a load on
the engine (stomp the gas peddle to the floor and dump the clutch or, get full
converter lock up with your automatic trans) until the time the engine develops
enough hot expanding exhaust gas to spin the turbine fast enough for the
compressor to do its job.
Effectively, a turbo charged engine is a normally
aspirated engine until the turbine and compressor spin up. To minimize turbo
lag, it is imperative that the turbine and the compressor are properly matched
to the engine as well as the engine being properly matched to the transmission
gears, the ring and pinion gears, and the tires.
