>Questions for those of you who have upgraded the carburation of your TR6's:
>
>Did any of you who opted for the Goodparts triple setup investigate 2" SU's
as a potential replacement? And conversely, did any who opted >for the 2" SU's
compare them to the triples?
>
>Just curious as to how some of you reached your decisions as to which way to
go.
>
>Thanks much,
Interesting question. All else being equal, more choke area is better IF YOU
NEED IT. 3x1.75" diameter = 7.21sq. in. and 2x2.0" diameter = 6.28 sq. in.
So, 3x HS-6 carbs can flow better than 2 of the HS-8.
Two "buts" here.
1) But, how much carb does the engine need?
Answer: Hard to say.
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?
Here are my thoughts on this subject, and I have thought about it quite a bit
over the last couple of years.
1) Fact: There is a certain minimum flow speed across the carb bridge to
achieve adequate atomization.
2) A guess: The factory sized the springs properly to the carb to minimize
loss of efficiency due to excessive pumping losses (I.E. creating too much
vacuum behind the carb piston). IF that is true, then you can't really change
the spring rate and improve the function of that individual carb because it is
already as good as it gets.
3) Fact: As you modify the engine you need to increase your choke area
proportionally to your increase in horsepower. If you stay with the stock
carbs, then they will be opening up fully before the engine has reached
maximum power. Then you start increasing your pumping losses because the
carbs offer progressively more resistance to flow as the air flow increases
(while the pistons were still lifting they did not increase their resistance,
they just opened up more!)
4) Conclusion: More carb area will be beneficial when you exceed the maximum
flow rate of the stock carbs. As long as the new carbs (3 of the same size,
or 2 of a larger size) have springs PROPERLYsized to the carbs, then you do
not need to mess with the springs at all!
5) Observation: The same carb bodies were fitted with different springs in
different engines!
6) Conclusion regarding #5 pertaining to #2. The springs installed by the
factory did NOT give MAXIMUM efficiency while maintaining adequate
atomization.
7) Final conclusion: It is somewhat unpredictable what springs you need.
Put more/larger carbs on the engine. Buy a selection of springs. Dyno the
engine with different springs and see which ones give best power. Be prepared
to change the needles too. Each spring will have a needle that gives the best
mixture throughout the desired operating range on that particular engine. You
want to see the HP generated by each of the 3 or 4 springs with its preferred
needle. I bet that you would find slight differences between the springs,
with the weaker ones more likely to give better performance. The weakest
spring might just be on the far side of the curve, though.
-Tony
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