Oval pistons (was Re: s4 rough acceleration)

[Scott Fisher <sefisher@cisco.com>]

Rich Andrews posts:

> Ok....
> I remember reading that you are NOT supposed to use full throttle
> or exceed 4000RPM until oil temp is 80 degrees Celcius???
> Correct? Wrong?

That advice, or something pretty close, is in most reference works on
most cars I've ever owned, worked on, or read about; that exact ratio
(4k RPM/80C), for example, is quoted in the reference book for my 1967
Alfa Romeo.  (Well, not quite; in the Alfa, the book quotes 80 C and
"halfway to redline," which on the 1300 works out to 4000 RPM.)  I
believe that in my '83 CGT, the manual says "until the oil temperature
needle comes off the peg" or words to that effect.  Close enough, and 4K
is right at 12:00 on the tach, so it's what I look for.

Note that the oil temperature itself is not the primary reason for
waiting till the oil temperature is 80 C, though that -- mainly for
viscosity reasons -- is part of it.  Oil at 80 C is a general indicator
that the internals of the engine have reached a stable and proper
operating temperature.

Your book may also warn you about not using more than about
half-throttle as well; if not, this is something else to bear in mind,
for the following reasons.

> I also remember a thread (i'm not trying to start another one) 
> about the pistons being oval until the motor warms up... Then it 
> becomes round..
> Or was it the bore that was oval... Oh well... 

Um, well, not quite; I'm sure the factory made them both as close to
perfectly round as possible, regardless of temperature.

Here's the real story.  Aluminum (used in pistons) and iron (used in
blocks) expand at different rates when heated.  To account for this,
engines are typically built with cold clearances that are measurably
larger than they are expected to be once the engine warms up.

Furthermore, blocks have cooling passages through which a constant
stream of liquid removes excess heat, while pistons are lucky to have a
spray of oil on their undersides, so pistons typically run at a higher
temperature than the block, especially in turbocharged engines. 
Therefore, because a piston runs hotter than the block AND it expands
more quickly when heated, pistons are made noticeably smaller than the
cylinder bore while cold.  The gap between them is therefore typically
much larger (well, relatively speaking -- on the order of a few
thousandths of an inch, we're not talking centimeters here) when cold
than it is once the car heats up.

The bit about oval pistons/bores is a description of what happens if you
abuse an engine while cold.  A too-small piston in a too-large bore will
rock back and forth (or side to side) under load, and this can wear
either the piston, the bore, or both.  By the time one or the other is
oval, the damage has already been done.

There's more than just the piston/bore wear at stake: clearances
throughout the engine change during warm-up, including camshaft,
crank-to-rod, crank-to-block, valve-in-guide, etc. etc. etc.  But the
piston wear is most noticeable because of the different expansion rates
of aluminum and iron.

> Point being I don't 
> think it is a good idea to get on the accelerator hard until the 
> car has fully warmed up... Then have FUN!!

Exactly so.  And now, I trust you have a better idea of why!

--Scott Fisher