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FUELforTHOUGHT
by Dave Koehler
6/7/04
Hello,
I’m converting my VW 1300cc 4cyl. inline
car engine to mechanical injection, Bosch K-Jetronic.
As I browse through all the different VW parts,
I see many different butterfly sizes for the
different engines with injection. Now, I’m
aware of why you just can’t put on like
50mm carbs (50mm bore for each cylinder), to
keep up air-velocity at lower revs, signal to
the main jets and all that.
But, as we talk injection the runner size is
already given, as it’s a standard cast
VW injection manifold I’m using.(G40)
I know that a very large butterfly makes it
hard to control gas flow at low throttle openings,
and that is probably why many of the VW TB’s
have a small and a large butterfly, the small
one opens first, and then the big one.
But, in general, is it possible to make a TB
TOO big? I mean, you don’t have any “low-flow”
problems here, as with carbs, or as with too
big runners or anything.
I’m thinking of making a TB with a single
flat slide, of like 60mm in diameter, and that’s
it. Then maybe I can work out some kind of non-linear
opening system so the first like 1/3 of throttle
opening has a low ratio to make it easier to
control at low openings, and the last 2/3 at
high ratio as I have to consider accelerator-travel.
The late BMW’s have no butterflies at
all; there’s just a totally unrestricted
“hole” all the way down. The intake
is regulated by an electro-hydraulic system
that determines intake valve lift from 0.3mm
to 9mm, that’s it.
So isn’t it a matter of making the throttle
easy to control at low openings while also maintaining
the biggest, unrestricting bore at full throttle?
Thanks in advance.
Regards,
Arndt Baslund, Denmark
Arndt,
Bigger throttle blades make
the engine respond quicker. This is fine for
drag racing, but it can be a little touchy for
part throttle control because when you get way
too large on the butterfly versus your cylinder
head and intake air flow capacity you may have
given the engine all the air it can take when
the butterfly is open 40 percent or less. Personally
I like bigger but I do not turn corners at a
controlled pace. Certainly you will want larger
butterflies than your carbs had. I suggest going
to Hilborn Injection and Kinsler Injection.
See what size throttle bodies they offer for
the VW in a conventional mechanical injection.
This should give you a starting point.
http://www.hilborninjection.com/
http://www.kinsler.com/
Have a safe race,
Dave Koehler
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Previous
Stories
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Fuel
for Thought
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5/7/04
Dave Koehler answers your
nitrous & fuel-injection
related questions
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