Velocity Stacks, Belts and Door Handles

["C.M. Smith" <cmsmith@domain.elided>]

Colin Campbell's excellent and very readable books on Sports Car design are
good sources of info for ram effect as well as every other aspect of road
cars for sporting drivers, especially suspension design.

IMHO, or rather, as I am reliably informed, I don't think inertia has much
to do with it. Air movement in an engine's intake tracts, as well as gas
movement through the exhaust, is pretty well continuous, what changes are
the pressure waves moving up and down the intake and exhaust passages from
atmosphere in  to atmosphere out. What triggers the pressure waves are the
valve openings and closings and the abrupt changes in cross-sectional area
of the various pipes and passages. What causes the ram effects for intakes
and the extractor effects for exhausts is the length and shapes of the
pipes and passages. A lot of the research was done on two stroke cycle
motorcycle engines, leading to extractor exhausts of precisely engineered
shapes and lengths and individual intake velocity stacks for each cylinder
(sometimes one of course!).

The reason short stacks work well at high rpm but not at low rpm is the
higher resonant frequency of the intake tract (think organ pipes, or flutes
if of variable length as in Hondas or Porsches). Long stacks work at lower
rpm for the same reason , longer pipes have lower frequency resonance. If
the intake length is optimized for low rpm (as for street engines) a
relatively long intake tract is used. The concept is that the timing of the
intake valve opening coincides with the resonant reflection of a
compression wave in the intake tract arriving at the intake port. This wave
is, in turn, caused by the last time the intake valve opened, sending a
vacuum wave back up the intake tract, which is reflected at the open
atmosphere as a compression wave. Clearly, if the frequency of the
reflected wave is an even multiple of the length of the tract the
compression wave will reach the intake port as the valve opens and
"supercharge" the cylinder. The effect is relatively slight compared to
mechanical supercharging, but worth doing. Remember the waves move but the
air doesn't, as for ocean waves (or any other waves for that matter) a wave
merely compresses the air molecules at the "peaks" and decompresses them at
the "valleys" (rarefaction waves). Naturally, the air is always moving into
the engine and the exhaust is always moving out, but the waves move up and
down just as if the air and gas were stationary.

Exhausts work on similar principles as far as valve opening causing
pressure waves ( actually, it is the rarefaction waves that are exploited
by timing the reflected compression wave back up the pipes to reach the
exhaust valve as it opens) in the exhaust tracts, but a bigger effect is
due to the effect of the joins in the pipes. The now standard Y into Y of a
4 cylinder inline exemplifies this for street engines, and all modern Alfas
are very good, even with cast headers. This configuration allows each
cylinder to help the others, twice on each set of four combustion events
(two revs). For two strokes, individual intake stacks and separate
carefully engineered exhausts probably work best, also, the extraction of
the exhaust is often used to scavenge exhaust from the engine and draw
fresh charge in, even to the point of drawing unburned mixture through into
the exhaust with some spectacular effects on high output engines (lousy
fuel mileage!).

The V8 is a real challenge and stock bent crank layouts can benefit from
unequal velocity stack heights, and weird exhaust layouts. The engine
"sees" the entire length of the intake from atmosphere to intake valve, and
it's difficult to make these equal with same height stacks on a
conventional NA pushrod bent crank V8. Flat cranks use different firing
order and are amenable to individual velocity stacks with separate carbs or
injection, and especially Y into Y exhausts. Ferrari likes flat cranks for
their V8's and drivers put up with the imbalances that are unavoidable with
the "two four cylinder engines on a common crank" that this requires.

V6's are problematic, especially 90 degree layouts. Inline 6 and any 12 can
easily be optimized. Flat fours are tricky and flat 6's not much better.

Of course the ultimate for street engines is variable intake, and now
exhaust, tracts. These are not to be confused with variable valve timing
which has a different function. Porsche and Honda are very big on these,
and other manufacturers are seeing the benefits as attractive. By varying
the effective length of the tracts by crossover pipes etc, ram effect and
exhaust extraction can be optimized for several rpm ranges, which when
blended together yield tractable very high performance street engines with
high output at high rpm's high torque at low rpm's and very fat mid ranges.
Money for nothing and ch*cks for free as Dire Straits had it (sorry for the
s*xist quote).

Engine compartment air is hot, most air in the compartment comes through
the radiator. The temperature under the hood drops at higher speeds not
because the ambient temperature drops (of course it doesn't, driving fast
on a hot day doesn't make the air cooler) but because the radiator rejects
more heat to a higher volume of air passing through the rad core. The
engine runs cooler and the air in the engine compartment is cooler, but
probably not by much. The intake air is cooler yet, mainly because the air
entering the intake system from the front of the car has no time to pick up
heat from the engine compartment , not because the underhood temperature is
lower. That's why it's so important not to simply install a free flowing
filter (or god forbid, run with no filter, how often do you intend to
rebuild the engine when the bores are ground down by dust?) without also
installing ducting to keep the coolest air available flowing into the
engine. O'wise, much storm and noise signifying nothing is your only reward
for the new filters. Again, ALFA does a great job with its stock setup,
combining good flow with quiet operation (well relatively quiet!).

Rubber belt cam drives have several advantages. They are much lighter and
require no lubrication (eschew lubrication, or abhor lubrication?). They
are simple to repair ( I should say replace before repair!!!), compared to
chains. Service life is adequate and they can be examined easily for signs
of wear, although replacement at specified intervals is the only way to
sleep nights. They are much quieter, and cheaper to manufacture and
install. They don't stretch (much!!!). So, the only advantage chains have
is very long service life. Chains don't jump sprockets as belts have been
known to skip cogs, but a properly installed belt shouldn't skip a cog either.

164 door handles are subject to chronic failure. The design is ridiculous
and the materials inappropriate. The cast metal with a slender crank
extending from one end of the handle cracks, unsurprisingly, at the spot
where the crank flows into the handle. As it is a casting, trying to bend
it back is pointless. They break and must be replaced by another identical
part which will also break. Pathetic.

Sorry to be long winded but the ram effect keeps popping up with the same
dialogue following. The subject is fascinating for anyone interested in
high performance engines. Understanding that those polished chrome intake
runners are like finely tuned musical instruments adds to the pleasure of
driving an Alfa. Those marvelous sounds of intake and exhaust as you run up
and down through the gears are musical for a reason. Remember this when
contemplating intake or exhaust mods, those factory boys really know what
they're doing and improving Alfa's is difficult. You can get different
performance by changing things, but all around better performance is quite
difficult to achieve unless you are prepared to spend a lot of money, and
ultimately, put up with some compromises like lower torque at one end of
the rev range or the other, more noise, less flexibility, etc. "Tuning"
intakes and exhausts is very tricky.

Michael Smith
Calgary, Alberta
Canada
91 Alfa 164L, White, original owner 

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End of alfa-digest V7 #869
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