A quick followup on the balance pipe question. I did some digging last
night just to be sure of this. This comes from several books I have on
sports car design, and one particular book on intake and exhaust systems
published in 1962 that focused almost entirely on British practice of
that time (which was then state-of-the-art-as-they-knew-it), so it is
particularly appropriate for LBC's. (I bought this book for $3 at a
used-book store on Cape Cod...)
As I wrote yesterday, the problem is indeed that a typical firing order
of 1-2-4-3 gives uneven timing between the cylinders that share a carb.
Intake valves generally close well after the piston has started up, on
the premise that the momentum of the air column will still force more
air into the cylinder. When the second intake valve opens, it robs the
first of some of that air, if the ports are siamesed. If the ports are
separate, the problem is diminished a little but there is still a big
difference in the air feeding the two.
The balance pipe provides a second source of air. Its flow constantly
changes direction as the forward then rearward pair of cylinders draw air.
Presumably it would matter whether the balance pipe was connected between
the runners to the 2nd and 3rd cylinders or between the common section
upstream of the split. In the former case, presumably everything would
behave differently if the firing order were reversed, 2-1-3-4. Much of
this was worked out by experimentation, and if there were any systematic
studies of every possible option, they weren't mentioned. Some things,
like valve timings, cam lobe shapes, etc., seem to have been done by default.
A second problem is that of mixture and atomization. The more explosive
the pressure pulse is when it hits the carb, the richer the mixture will
be and the better the atomization will be. You'd like the two cylinders
to have the same. The balance pipe even this out a bit, but this turns
out not to be a significant factor with a typical 4-cylinder anyway.
The pipe lengths of the intake system are generally so short that except
for the primary robbing of air (i.e. the direct impact of a low-pressure
wave from one valve impinging on another that is already open), there is
no ram-tuning effect. Longer intake lengths would lower the ram-tuning
rpm to where it might be useful, as well as provide more momentum to the
air column so as to improve cylinder filling after the piston starts up,
but that would harm throttle response, and provide more manifold surface
area for gas condensation to occur and for more heating of the air column,
thus reducing the volumetric efficiency. In addition, it would add weight.
Thus, intake systems are not lengthened intentionally.
For a 6-cylinder, the issues are a bit different, and more interesting (i.e.
complicated). On first glance, it would appear that three paired carbs
would be better. With a timing order of 1-4-2-6-3-5 or typical variants,
the intake valves on each carb are never open simultaneously, so there is
no problem with one cylinder robbing another. However the cylinders of
each end pair are separated in time by one other cylinder, while those of
the middle pair are separated by two and thus open symmetrically. Since
uneven pulsing of the air column gives a richer mixture, the end carbs are
typically jetted (needled?) to be slightly leaner. Also, the 2st and 5th
cylinders will still see the effects of the air movement from the 1st and
6th, so their intake lengths are designed to be a bit different. I think
I remember 1 and 6 as being longer, and if so, the reasoning would probably
be thus: For 2 and 5, the air column is already moving from 1 and 6, which
would give a slightly smoother, thus leaner mixture. To compensate, you
would shorten the intake for 2 and 5, making the pressure wave slightly
more explosive as it reaches the carb, so they too would be enriched.
The impact of all this is that you Jaguar 6-cylinder owners don't want to
swap the end carbs for the middle one unless you keep the needles in place.
The other way to do a 6 is to use two carbs. With manifolds like:
| |___| |___| | | |___| |___| |
\_____ _____/ \_____ _____/
| | | |
the center carb would get too much of the gas, especially when cold, because
momentum would carry any unevaporated into its tube where it could settle
until its intake valve opened. The usual solution is to offset the carb to
one side, typically to the inside:
| |___| |___| | | |___| |___| |
\________ __/ \__ ________/
| | | |
This results in some back-and-forth movement in the sections feeding 1 and 6,
but that is a small effect. The good thing is that the relative timing for
each cylinder on each carb is even, 1-*-2-*-3-* for the leading carb, and
*-4-*-5-*-6 for the back carb.
With any independent-venturi-and-manifold-path-per-cylinder setup, like Weber
conversions or the Solexes on my Porsche 912, etc., none of this matters.
Jim Muller
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