In a message dated 8/6/2006 11:02:06 PM Central Standard Time,
spamiam@comcast.net writes:
> 2) But, how much actual choke area is going to GET?
> Answer: Depends! It depends on the springs. Assuming a roughly linear
> increase in choke area for a given engine flow rate (i.e. obeys the
> "constant
> depression" principle), then adding another identical choke will simply
> spread
> the same flow across 3 carbs instead of 2 and each one will open up 2/3 as
> much for a given engine flow rate. Is this a bad thing? Do the springs
> need
> to be changed?
>
One big hole in this theory is the presumption that air flow through the
carbs is constant. At WOT it is not. Air only flows when a cylinder is on its
intake stroke. Since a carburettor in a twin set up feeds three cylinders it
will see three pulses of intake charge for every two engine revolutions.
Looking at the firing order you will see that these charges are equally spaced
and
180 (crank) degrees in duration and evenly spaced over 720 (crank) degrees of
rotation. Peak air flow is whatever any single cylinder requires during its
intake stroke.
Now add a third carburettor. All we have done is shifted on cylinder from
each carburettor to another one. Now instead of seeing three intake pulses per
two revolutions, each carb will see only two. Instead of flowing for 540
(crank) degrees per every 720 for a 75% duty cycle, the carbs will now see only
two pulses for 360 (crank) degrees per two revolutions for a 50% duty cycle.
Gas flow through the jet is a function of instantaneous flow rates but piston
location is a function of average air flow so changing duty cycle from 75% to
50% changes everything. Twin carbs on a four cylinder will see 50% duty
cycle so perhaps some four cylinder parts are the ticket here.
So, what do you get with a triple carb setup? You get a less serpentine
intake arrangement which will benefit air flow dynamics and less air flow
restrictions mean more HP. And besides, it looks really neat. But you're back
to
square one with setting them up with needles and springs. The Triumph
engineers
spent many hours on the dynamometer developing the current arrangement and it
is no trivial matter. But we have the advantage of not caring too much about
emissions or fuel economy so we only have to get in the ball park.
At the end of the day, adding 33% more carburettors does not mean we can
deliver 33% more air/fuel to the cylinders. These cars were not carbuettor
limited anyway. But the sure look neat.
Cheers
Dave
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