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June 25, 2004
Yep....
You got it big guy!!!!
Of course, this assumes a reasonably
healthy engine pulling decent vacuum.
TWO CAVEATS....
1) If the engine's clapped out and is pullin'
lousy vacuum, the pistons will never
"top out", spring tension and fully open
butterflies notwithstanding
and thus....it follows, that....
2) the entire induction system has got to be "air tight" as well.
You don't need a midget under the hood.
You need an SU carb tool kit!!! It comes
with a little jet centering tool along with
"rods" you place within the piston tubes
which temporarily take the place of the
dampers if you choose to do this
driving down the road without the bonnet.
(Paint a wide red stripe at the appropriate
spot on the rod sticking out from the
carb. in place of the damper so you can
"see" the piston "top out" from the cockpit).
To do this "statically"
under no load, you have to temporarily
"load" the motor with a rapid "full" shot of the throttle to redline so
as to "bury"
a vacuum gauge if you happen to have
one handy and hooked up.
It takes skill
and normally two people to do this, for
obvious reasons. Engines don't like to
be treated this way for obvious reasons!
Understand....
It doesn't have to be exact for a road car
wherein you ain't gonna be driving to
redline most of the time.
The idea is you don't want the piston
"getting there" early in the game. Better
a little late than too early.
Now this is the hard way of doing things.
You can see why
sorting the foregoing along with the
mixture strength on a chassis
dyno is the easiest way to go!!
(And, while your on the dyno you can at
least sort the total timing if not the
entire curve).
Alternatively, with respect to mixture
strength, an air/fuel ratio set-up is the
next best way to go as I would think
engine health over time (and not absolutely max HP) would be the
priority in a daily driver road application.
The idea is......
1) you don't want the mixture
"drying out" (going lean) at top end
under load (for any reason)....and...
2) you want the engine to be able to
deliver full power at, or as it comes
up on it's redline and in order to do
this, the piston has got to be as far
from "obstructing" the throat as
mechanically possible.
Again...
as this is a road application and service
life is more important than obtaining
that last available HP (as in "winning"),
one might choose #1 as their priority in all of this.
It's not a racecar. "Ballpark top" is OK
for the road, as long as the piston
is not getting there too early
so as to louse up ("lean out") the
"intended" position of the needle within
the jet (which determines mixture ratio
throughout the rpm range, aided by
"correct" piston rise-rate as you put your foot well into it) toward top
end.
In terms of our daily driver machines,
this operation should be thought of
as crucial to engine service life (and not max HP), IMHO.
Cap'n. Bob
'60 :{)
PilotRob@... 32764
Posted on June 25, 2004 12:03 PM
Posted to category(s) Body | Carburetor | Engine (external) | Engine (internal) | Exterior | Fuel System | Gauges | Intake System | Interior | Restoration | Suspension & Steering | Technical Info | Transmission | Tuning
