we left off with high pressure air leaving the compressor outlet. Unfortunately, physics
has worked against us this time, and the act of doing work to our inlet air to compress it
has raised its temperature.
This is bad.
Not only are we reducing density, we're increasing the
possibility of the great bugaboo - detonation. Remember, the onset of detonation is
usually the limiting factor on the amount of power a given engine can produce, and that
increased intake temperature (as measured at the intake valve) increases the chance of
So we have to cool the air back down again, without losing
That's the job of the intercooler, basically a "air
radiator" placed in the flow stream between the turbo compressor outlet and the
intake manifold. There's really not much else to say about them, except:
1) The more you can cool the air flow, the better. This
_normally_ means the bigger the intercooler, the better. (There are some smaller coolers
that are better designed than the lower-end "big" coolers though, so size does
not necessarily indicate effectiveness.
2) The cooler must be placed in a location where ambient air
can flow through it. This means that your cooler must have an intake path and an _exhaust_
path. Mounting a cooler flush against a plate does no good!
3) There's always a pressure drop across a cooler. How much
depends on the cooler design.
A turbo is a positive-feedback device. The more boost you
make, the more exhaust you make, which makes more exhaust, which makes more boost... in a
vicious circle. So we have to have some way of limiting boost.
What we _really_ want is a way of keeping the turbine
operating at a constant speed (see yesterday's post) so that we can maximise the
compressor efficiency - remember that turbines like to run at a single speed. However, as
measuring turbo RPM is not practical, and as boost level is directly related to turbo
speed, keeping the boost constant is the waste gate's
The waste gate is just a valve that opens when we have
exceeded our desired boost level, and allows exhaust to flow around the turbine, instead
of through it. This lowers the pressure differential across the turbine, less work is
done, and the turbo slows down.
The only "gotcha" with the waste gate
is that it
must be able to flow enough gas to let the turbo slow down. If it can't, then you get
"boost creep" where boost levels slowly grow as the car remains under
Everybody likes BOV's because of the nifty sneeze sound they
make. However, a BOV is an evil device. It's taking your precious boost and venting it to
someplace else. Bad!
Unfortunately, it's a necessary evil, and we have to live
with it. Here's why: You're under boost, the turbo is fully spooled, and life is good -
then you shift. That means your foot comes off the gas - and the throttle plate slams
shut. Suddenly, instead of flowing in a continuous stream through the engine, the intake
air smacks into the closed throttle plate. The turbo, which is still spinning and
producing boost because if it's rotational inertia keeps producing pressure, and the
intake stream, caught in between a rock and a hard place, jumps in pressure. In fact, you
get a high-pressure shockwave that travels from the throttle plate back to the compressor
vanes, that once it gets there, is a little like poking a stick into the spokes of a bike
The repeated shock is hard on the compressor vanes and the
shaft bearings, and in any case acts like a brake, slowing the turbo, and requiring it to
be spooled up again.
The BOV sits in between the turbo and the throttle plate,
and if it detects the shockwave created by a shift, vents it elsewhere - either to
atmosphere, or back to the inlet side of the turbo.
So, we lost boost pressure, but we kept the turbo spooled...
tough to say without a dyno if that was a fair trade on a race vehicle. On a street
vehicle, it was definitely a good idea, because we spared our expensive turbo a mechanical