[T2] where to inject the water

[Robert Mann]

      For the tiny fraction of ICE users who realise water-injection is
highly cost-effective, I point out some recent clarifications about where
to inject the water.
      For carbs with a throttle butterfly, the general rule which emerged 3
decades ago persists: a tiny hole just outside the butterfly is the water
injection site.  As summarised in my article, on the over-run or at idle
the vacuum here is negligible (just when no water is wanted).  As the
butterfly opens, vacuum builds to 1/3 atm, and at fast cruising is 1/6 atm,
suctions so strong that the water feed to that hole must be choked by a
needle in the range used for blood sampling (c. 0.5 – 0.7mm).
      The tiny hole in the venturi connects to the nozzle providing vacuum
for retarding the distributor.  The medical needle is stabbed into the
rubber connector just outside this nozzle, so as to position the needle
point inside the nozzle.
      For carbs with a throttle slide instead of a butterfly, the water is
injected downstream of the main fuel jet but still inside the throttle
slide.  I have done this in Amal carbs (on Norton & Jawa motors).
      Slapping the water needle in *after* the carb, for convenience in the
carb/intake flange joint, works OK for motors which are flat-out most of
the time e.g my Jawa 50.  The suction is presumably maximal on the
over-run, which is not desirable (tho' if some trace of oil can be
emulsified into the water, it may then help to prevent the piston seizure
so notorious when hot strokers go onto the over-run at the end of a
straight).  One racing stroker trying this position for the needle sucked
up the whole contents of the 150ml water bottle *fast *!  So I fear you do
have to drill into your Amal  ...
      The different vacuum connections on VW carbs remain somewhat
mysterious, to me at least.  My carb happens to be the excellent Brosol
'Brasilian Weber' 30.  The vac connection for distributor retard is OK for
water.  I had to get rebuilt the diaphragm can on my distribr, and suspect
corrosion may have been caused in that can by water coming down that vac
tube from the carb.  This potential is a good reason to slap the needle in
so near the carb that the needle point is actually well inside the metal
nozzle.
      For other carbs, the strict way to identify where to slap in the
needle is to read the vacuum regime at any given takeoff spot by an
ordinary vac guage on the dash.  Cheap 4mm ID flexible tubing is fine for
this approx 11' connection to the dash guage.  After you've identified the
carb nozzle which has the right vac regime, transfer the tube to the intake
horn so that the guage becomes a conventional intake manifold vac guage, an
estimable asset.  At worst, you'll have to drill a small hole into the
intake horn below the carb.

     I have far less experience with the newfangled systems termed *fuel
injection*, but the general idea is to slap in at least 1, perhaps 2 large
stainless needles (used to drain c. 1 pint from a vein) in the narrowest
accessible part of the air intake *after* the filter.

Robt Mann
'73 1600dp Devon camper
various Jawa strokers
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