FW: Bored engine in Rally car?

[Scott Fisher]

--- "Livolsi, Stephane"
<Stephane.Livolsi at investorsgroup.com> wrote:

> But, the main point that I was trying to make is
> that you won't get
> something for nothing.  If you want to increase your
> torque by increasing
> your displacement, you are probably giving something
> up in longevity.

Stephane,

While any increase in performance has some cost in
longevity because you're putting more stress on the
equipment than it was originally designed to handle,
it turns out that in general, increasing displacement
has the least negative effect of all readily available
hot-rod techniques.  In addition, increased
displacement offers an increase in power across the
entire rev range, unlike almost any other way of
upping the output.

An engine is (and this is a vast oversimplification,
but it's a starting point) basically an air pump that
relies on the expansion of fuel by heat to produce
torque at the crank.  To increase the torque, you can
increase the air, the fuel, or the heat.  Note that if
you increase the air, you need to increase the fuel
proportionally, and vice versa -- a nitrous-oxide
system basically increases the amount of available
oxygen in the engine, and you need to increase the
fuel to maximize it.

Increasing compression ratio increases the heat --
which produces more power but also puts more thermal
load on the engine, the oil, the cooling system, the
gaskets, etc.  Turbocharging does something similar
but even more radically, which is why it's such a
great way to make really dramatic improvements in
power output.

Aggressive cams increase the airflow AT CERTAIN RPMs
by changing the time, relative to the piston's
position in the cylinder, at which the air is allowed
in (and the exhaust allowed out).  It's a very complex
subject, but fundamentally it's like a tuned musical
instrument: for a cam to work at its best, it needs to
be "tuned" to work with the valve and port sizes the
way a column of air in a pipe organ makes a whole room
resonate; in turn, the ports need to be tuned to the
cylinder volumes, and as with the pipe organ it all
works best at a particular speed and volume of air
flow.  However, as another lister mentions,
high-performance cams put an extra strain on valve
springs, as well as on valves, and there's a risk of
catastrophic failure if the timing belt slips or
stretches -- a high-lift cam that loses its timing can
cause valves to meet pistons, with disastrous results.
 And bigger valves usually mean there's less metal in
the cylinder head, which can lead to cracking the
seats between valves if they overheat.  Plus bigger
valves tend to work mainly at higher RPM and can
actually cause low-RPM power to be less.

Conversely, increasing the displacement of an engine
simply means there's more air in each cylinder every
time the piston drops.  (There is also a very small
increase in compression if you increase displacement,
because you're now squeezing a larger cylinder into
the same size combustion chamber.)  If you increase
the amount of fuel proportionally (or if your
induction system adjusts automatically for volume),
this adds less stress than any other comparable
high-performance modification.  And finally, because
each stroke of the piston makes more power regardless
of RPM, it's a power enhancement you can feel from
idle to redline.

My guess about why it isn't used more is that the
gains aren't NEARLY so dramatic as with turbocharging,
and the turbo path is so easy and (relatively) cheap
on Audi because there are so many turbo I-5s out there
in junkyards waiting to have the engine or plumbing
dropped into a CGT or a 4KQ.  Power improvements tend
not to be quite linear with displacement -- meaning a
10% increase in displacement will yield less than a
10% improvement in power output.  So when your options
are a 7-8% increase in power at the cost of tearing
your engine apart, or a 60-100% improvement in power
by bolting on some pieces to the induction and
exhaust, it's a no-brainer.

> This
> is based on what I see in motorsports - these guys
> run huge HP/huge torque
> modified motors for  1 or 2 races, then they have to
> rebuild them.  It gives
> them what they want, but they sacrifice something
> for it.

Again, you've got it somewhat backwards.  A full-race
engine most likely has a limit to how much they can
change the displacement (or a maximum displacement for
the class, depending on whether it's production-based
or a purpose-built class like Formula 1) but will have
wild, vast increases in compression, camshaft design,
valve size, port shaping, and other changes.
Durability in a race engine depends on the class and
the team's budget; it's common for top-notch teams to
replace engines after every race, as you say, but the
replacement is usually to compensate for worn valves
and piston rings, not because of a change to
displacement.

There are two other topics that touch on your initial
question of why the factory engineers chose the bore
and stroke for our cars, and I'll mention them only
briefly: first, the bore-to-stroke ratio, and second,
unshrouding valves.

Bore and stroke ratio have an effect on WHERE in the
RPM band the engine makes maximum torque.  In general,
long-stroke engines make torque at a lower RPM than
short-stroke engines, in part because the stroke makes
for a longer lever arm on the crankshaft -- think
about trying to get off a stuck lug nut (whoops, lug
BOLT, this is the Audi list :-) with a 6"-long lug
wrench, then think about how much more torque you can
apply with a two-foot cheater bar.

You get a similar torque multiplier with a longer
stroke -- given the same force on top of the piston,
the longer stroke engine will have more torque at the
crank.  However, a shorter-stroke engine revs much
higher (or can) because the piston doesn't move as far
-- now you're using your little shorty 2" ratchet to
tighten down the lug bolt by making a lot of little
short movements very close together.  There isn't a
lot of force on any of the individual tugs on the
ratchet handle, but the bolt goes back on quickly.  So
for a car driven mostly on the street, a longer stroke
has an advantage because it acts like a longer lever
on a fulcrum and can get a heavy car moving more
easily at low RPM.

As for unshrouding valves: imagine two cylinders of
the same volume, one that's 3" wide and another that's
4" wide.  Obviously, you can fit bigger valves across
the top of the 4" cylinder than the 3" cylinder, so
you can get more air through the engine with the wider
bore (a reason why most full-race engines use wider
bores and shorter strokes to meet the displacement
limit for the class).  So there is a compromise
between how long the stroke is (for good low-end
torque with our heavy cars) versus how wide the bore
is (for good breathing at higher RPM).  While you're
correct in assuming that changing one of these will
change its relationship to the other, it turns out
that if you bore the 3" engine to, say, 3.1", that
extra tenth of an inch around the perimeter of the
cylinder will allow the valves to flow more openly
because there's less metal up near the valves --
they're said to be unshrouded.  We found this to be a
big benefit when my friends and I were building MGB
race engines a decade or so ago; the B-series engines
were heavily shrouded due to the cylinder head design
(otherwise quite good in many respects), and boring
the engines 0.030" oversize helped airflow through the
valves quite a bit.

Lots to digest.  My favorite book on high-performance
concepts in general (though it's specific to a
particular marque) is David Vizard's "Tuning the A
Series Engine," about the engines used in countless
Sprite, Midget, and Mini race cars for the past 45
years or so.  But above and beyond the engine-specific
tips, Vizard does a phenomenal job of explaining the
practical applications of concepts like cam timing,
exhaust-system design, port shape and the effect of
velocity and momentum on intake and exhaust charges,
and much more.  Any serious gearhead would enjoy
reading it, even if the closest you ever get to a Mini
is the skirt. :-)

Best,

--Scott Fisher
  Tualatin, Oregon



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