[Healeys] Charge Robbing

[Michael Salter]

There are lots of very smart people on this list and, as it has been very
quiet lately I thought I would throw this out for constructive feedback.

Dealing with the issue of “CHARGE robbing” with a Siamese port cylinder
head.

The cylinder head design of the Austin Healey 100 is over 80 years old and
incorporates Siamese ports, a configuration never used in later designs. In
this type of head, the forward and the rearward pairs of cylinders each
share an individual intake port and the inner pair of cylinders share one
exhaust port.

Additionally, in the interests of simplicity, these engines use a firing
order of 1-3-4-2 or, expressed differently, 2-1-3-4, with the result that
these ports “see” a port flow pattern of “flow, flow, wait, wait, flow,
flow, wait, wait’.

This design results in a phenomenon called “charge robbing” wherein a
cylinder served by an individual port which draws the incoming air/fuel
charge first gets “robbed” of part of this charge by the cylinder which
draws its charge immediately thereafter.

Reputedly, this design results in the inner two cylinders (2 &3) receiving
a “leaner” intake charge than the outer two cylinders (1 & 4) and we were
always told that exhaust valve failure, a frequent occurrence which was
invariably confined to cylinders #2 or #3 in these BMC “A” and “B” series
engines, was a consequence of this “charge robbing”.

My 55 years of experience has been limited almost exclusively to
carburetted engines of this type, both single and twin carburettor
versions, and during that time I have regularly used spark plug colour as
an indication of the air/fuel ratio being burned in any cylinder. I’m aware
that this test is nowhere near as accurate as modern UEGO systems but these
weren’t available at the time however, over the years this test has never
indicated that the inner cylinders (2 & 3) consistently run leaner.

Some decades ago, it was determined that in these engines the gas
temperature in the Siamese exhaust port of cylinders 2 & 3 can be as much
as 100° C higher than in the outer two ports and the resulting higher
operating temperature of the exhaust valves in those inner cylinders is a
much more likely explanation for their premature failure.

For “charge robbing” to affect the air/fuel ratio the “charge” must contain
fuel droplets. To express this differently if the incoming charge consists
entirely of an homogenized gas mixture of air and vaporized fuel, whatever
enters either port will have the same air/fuel ratio. This is almost
certainly the case with a carbureted engine wherein the fuel is introduced
into the very turbulent incoming air stream as microscopic droplets well
before that stream reaches the point in the intake port where the port
splits, which allows plenty of time for any droplets to entirely vaporize.
In such a situation the volume of the charge entering the inner cylinders
may be smaller than that entering the outer cylinders, but the air/fuel
ratio will be the same for both.

The same situation does not apply in modern multi-port fuel-injected
engines where each port has its own injector. To minimize emissions during
throttle transitions, an absolute minimum of fuel spray is allowed to hit
the port walls so the fuel is injected into the intake ports just upstream
of each cylinder's intake valve. As the intake valve opens backflow of
combustion gasses through it finalizes the vaporization of the fuel
droplets within the incoming charge. In the very unlikely event that such a
port injection system be used in an engine with Siamesed intake ports the
smaller volume of air delivered to the inner cylinders would result in the
mixture in cylinders 2 & 3 being richer unless some sort of specialized
tuning is used to decrease the pulse width for those two cylinders.

So, how does this all play out in the case of the EFI system I’m developing
for my 1956 Austin Healey 100?

The fuel injectors in this engine modification are installed inside the SU
carburettor bodies upstream of the throttle butterfly and are therefore a
significant distance from the point in the Siamesed intake port where the
port splits. As throttle transition emissions are of no concern with this
vehicle, this position is ideal in that it ensures that the fuel droplets
are completely vaporized long before the intake charge reaches the point
where the port splits.

For this reason, it is very unlikely that there will be any difference in
the air/fuel ratio of the charge received by any of the cylinders.


Michael S

April 2025
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