[Diy_efi] RE: Timing and dyno pulls

[Oolan Zimmer]

> > Your well-maintaned cooling sys has almost zero air
> > correct?
>
> Not necessarily.  Some rely on it.  Not many these
> days, but still, it's not a given either way.

And some rely having no air in the pressurized system.

I've seen two different styles of coolant system on passenger cars.  The
first has a pressurized surge tank with an air gap on the top;  the surge
tank is designed to be at the highest point in the system so air collects
there.  The surge tank has a pressure cap on it that leaks to atmosphere if
the pressure is exceeded.  There are a number of permutations of this, and
this is by far the most common system I've seen in econoboxes.

The other has a pressure cap that is designed to let go at just above the
operating pressure of the system.  The pressurized part of the system is not
supposed to have any air in it and technicians have to follow a long bleed
procedure when changing coolant.  The pressure cap does not vent to
atmosphere;  it vents to a sealed surge tank.  When the user adds coolant to
the system, they add it to the surge tank;  they do not remove the pressure
cap.  The pressure cap is not necessarily located at the top of the system,
either.

I'm sure there are other styles and someone can correct any mistakes I've
made.

I agree with a previous poster that the state of the cooling system at WOT
and the state of the cooling system at idle/cruise can be very different.
Because the cooling system has to be smaller than the cooling requirements
of the engine, as the driver stays at WOT temperature and pressure rise.
Then, when they get off WOT the engine is producing less heat and the
cooling system brings temperature/pressure down.  So, even though the
radiator is undersized for the engine, it still supports a duty cycle of WOT
and decel/cruise.  On the street, almost all the time is spent off full
throttle.  On almost every race track, there is a mix of accel and decel.
On the banked ovals where it's all WOT and limited by aerodynamics, it's a
matter of horses for courses.

Oolan Zimmer
o_zimmer at soltec.net


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