Variable length inlet tracts: Italian content

["Mark Battley" <m.battley@domain.elided>]

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<In some of the more recent hot cars on the market (the Ford SHO is one)
there 
<are actually solenoid controlled flappers that open and close changing the=
 
<length of the runners depending on engine speed, throttle position, etc...=
 
I 
<think that there are actually 2 different length runners going to each
intake 
<and the flapper determines which one is being used.
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<By the way, this is the theory behind several modern German cars which
<have begun using variable-length induction systems; they use longer
<columns of air at low RPM to take advantage of the inertial ram effect
<provided by the long intake tube, but then the intakes shorten as the
<RPM level rises so as to take advantage of the lower inertia in the
<smaller column of air.  
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The following info comes from:
http://www.fortunecity.com/silverstone/lancia/58/tech_engine_2.htm#Variable=
%
20Intake%20Manifold:

Variable Intake Manifold technology appeared as late as the early 90's. The
earliest applicants including Audi's first 2.8 V6 and Opel's 3 litres 24
valves straight six. Many car makers applied it for the benifit of low spee=
d
torque to compensate the growing weight of their cars. Fewer sports cars us=
e
it because their original manifold design are already enhanced for top end
power. 
Who use it : 
=A0 
* BMW 1.9 litres sohc inline-4
* Daewoo 1.6 litres dohc inline-4
* Ford Duratec 2.5 litres dohc V6
* Honda 1.8 litres dohc inline-4
* Mercedes V6 series
* Nissan 2.5 litres dohc inline-6
* Toyota 2.0 litres dohc inline-4
* Volkswagen 1.6 litres sohc inline-6
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How about some token obscure Italian content:

This technology has become quite common in the 1990s, however Fiat actually
used it in the mid-late 1980s in the Croma CHT (Controlled High Turbulence)
which was Fiat's (last?) attempt to see how efficient they could make a
carbed car (the CHT was sold alongside injected and turbo versions). As I
understand it from its external appearance and advertising pictures (I
haven't had occasion to pull it apart..) it is basically the ubiquitous
1995cc DOHC with an inlet manifold which has two inlet tracts of different
lengths, curvature and diameters per cylinder, with butterfly valves that
are controlled by throttle position. On small throttle openings the longer,
smaller tract is used, both changing the inertia of the air column, and
increasing turbulence to improve the actual combustion. As I understand it,
lack of turbulence within the combustion chamber is one of the reasons why
hemispherical combustion chambers such as in the Alfa and Fiat twin cams
have relatively high emissions: they flow lots of air/fuel, but don't
actually burn it very completely.

The better mixing allowed Fiat to set up the (single) progressive twin chok=
e
carb very lean on the first choke, which does make the CHT very economical
for a relatively large carbed car. On paper the engine doesn't seem very
impressive: claimed max power is only 90hp, which is very low for any 2L
motor. In practice the car performs better than you would expect (although
it is not, and never was intended to be, a performance car): it has more
torque and a wider torque curve that the (approx 115hp?) injected version,
meaning that the CHT is actually quicker than the injected versions in lowe=
r
rpm acceleration increments. The injected version is only quicker if you us=
e
plenty of revs. What is most noticeable with the CHT is the good amount of
torque it has at lower revs and small throttle openings. Downside of the
carb setup is that it runs badly on the first choke until it is warm.. it
must be very lean.

Carbs were on borrowed time by the end of the 1980s: the CHT was dropped
from the range in the early 1990s (?), and for the last few years of
production the petrol engined versions of the Croma (yes, in Europe you
could get diesels) used 8 valve injected, 16 valve injected, and 8 valve
turbo injected.

Me, I'd sooner have the Turbo version, but my wife is happy with the CHT:
and that means that I get to drive the GTV....  (token Alfa content!)

Mark Battley
Auckland, New Zealand.

1974 Alfa Romeo 2000 GT Veloce     (Mine)
1989 Fiat Croma CHT                        (Hers)

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