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Re: [TR] 2" SU's vs Triple 1/34"??

To: spamiam@comcast.net, triumphs@autox.team.net,
Subject: Re: [TR] 2" SU's vs Triple 1/34"??
From: Dave1massey@cs.com
Date: Mon, 7 Aug 2006 08:13:56 EDT
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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