- One of the most critical clearances inside any engine is the bearing clearances.
These must be tightly controlled if we want any chance of long term engine reliability.
Of course the crankshaft doesn't run directly on the bearing surfaces but instead we rely
on a thin tightly controlled film of oil to support the crankshaft and protect both
the crankshaft journals and the bearing surfaces.
Now in our more mundane road engines, what we tend to do as we build an engine
to produce more power and more torque than factory, or alternatively to rev much
further than the factory rev limit intended, is that we build the engine with
looser clearances.
Now the reason for this is that in a production engine block and a production crankshaft,
these components are actually relatively flexible, they don't have the kind of
rigidity that we need for an absolute performance race application.
What this then means is that at high RPM and under high load, all of these components
tend to flex.
So the reason we build the engine with slightly looser clearances is to make sure
that we're not going to have any problems with metal to metal contact.
A rough rule of thumb that we usually work to is that we need approximately one thou
of bearing clearance per inch of journal diameter.
This works for both our main bearings as well as our conrod big end bearings.
In the levels of professional motorsport however, things are very different.
When bespoke engines are produced purely for motorsport use such as the
Ford Cosworth engines in the Benetton F1 car behind me, these engines can be
designed from the outset to be incredibly rigid, making sure that under the high RPM,
in this case around about 12,000 RPM, and the 600 horsepower that these engines
produce, that the block is going to remain rigid and it's not going to move around
and the same could be said for the crankshaft.
What this allows is a thinner weight of oil to be used without sacrificing protection.
Using that thinner weight oil means that there's less frictional loss inside the engine
and less power is sapped.
And of course in the levels of F1, every last horsepower is critical.
Now this does present a problem though when it comes to starting the engine
for the first time.
When the engine is completely cold, the clearances are incredibly tight.
To the point that it's difficult to turn the engine over, we certainly can't start it.
What the engine engineers have done is taken into account the fact that the
alloy used for the engine block is going to grow or expand as it comes up to
normal operating temperature.
So all of the clearances are designed with the normal operating temperature of that
alloy block in mind and of course at cold temperatures before the engine has run,
all of those clearances shrink down.
So this is why we see in the likes of F1, the engines are pre heated before they
can be started up.
This uses an external heater system that is connected into the cooling system
via dry break connectors.
This circulates the coolant through an external heater and brings it up to
around about 65-70°C.
Once the water temperature is up at that level, then it's safe to start the engine for
the first time and the engine can go through a relatively normal warmup procedure.
In some instances we also see the oil in the dry sump reservoir being warmed up
as well as the oil.
What this does mean of course is that running and F1 car is a little bit different
to taking your normal club level car out for a weekend of racing.
It's a little bit more involved to start the car and get it up to temperature
and this is why we normally see a team of engineers looking after F1 cars
at a race meeting.
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