I knew this would catch the attention of somebody! Its really counter
intuitive!
Quoting Mike Pittinaro <mechimike@xxxxxxxxx>:
> > compression. An engine that looses compression is
> > much harder to crank
> Au contraire! Ever take the spark plug out of a
> lawnmower and pull the cord? It'll spin like crazy!
> Same thing goes if you pull the plugs out of a 440!
> Lost compression makes the engine easier to crank.
Mike, you are talking about complete loss of compression. I am talking about
partial loss of compression. When you crank an engine, a lot of the starter
energy is spent to compress the gases. However, assuming no firing and no
significant compression loss, almost all of this energy comes back during the
expansion or "decompression" part of the cycle (minus some lost on piston ring
friction and heat loss). When you crank a hot engine with all the oil close to
operating temperature, the heat transfer loss and friction loss are minimized.
In this case, the effects of bad compression will be amplified. If you have a
weak starter on top of that, the increased electrical resistance at high
temperature will make the worn engine much harder to crank and start. When the
engine finally starts firing, the combustion energy adds to the "decompression"
energy, and then the engine starts speeding up till it reaches idle.
> Why do you think people drive winter-beaters with
> 200,000 miles on them? the lost compression makes
> them easier to start in the cold.
The reason why such an engine may start easier is because the clearances in the
bearings and piston skirt are higher, minimizing the viscous drag when the oil
is cold. Remember, cold oil at say 20 degrees is well over 15 times more
viscous that at operating temperature. In these very cold temperatures, the
oil drag dominates.
>
> A rebuilt engine could conceivably have less
> compression than an older, high mile engine...the
> carbon builds up in the cylinder on the piston and
> increases compression.
Now, you are talking about compression ratio. If the starter has enough torque
to overcome the top dead center drag due to the high compression ratio,
the "decompression" energy will again return back to the system. Diesels for
example that are much tougher to cold start than gas engines due to their
different combustion characteristics start easier cold when the compression
ratio is high. Of course, they have tougher starters to overcome the initial
compression struggle, as well as the higher viscous drag due to larger bearings
and piston skirts. Regardless, a diesel with 20:1 compression starts easier
than one with 14:1.
>
> Hey...I will be going junkyard hopping this
> w/e...should I watch out for any parts for you, Dimitrios?
>
Yes, you know what I am looking for. A driver side window switch with power
vents and locks, and if I am lucky a cheap dueal snorkel!
D^2, 2x68's, both apparently with good compression.
> =====
> --Mike Pittinaro
>
> My girlfriend left me
> My trusty Imperial
> Remains more faithful
>
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