[Healeys] Charge Robbing

[Alan Seigrist]

Michael -

I learned something new today!

I wonder if charge robbing can be addressed by offsetting the carburettors
a bit so that they are closer to  one cylinder vs another.

Cheers,

Alan

On Fri, Apr 11, 2025 at 11:42 AM Michael Salter <michaelsalter at gmail.com>
wrote:

> 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
>
> _______________________________________________
> Support Team.Net http://www.team.net/donate.html
> Suggested annual donation  $12.75
>
> Archive: http://www.team.net/pipermail/healeys
> http://autox.team.net/archive/healeys
>
> Healeys at autox.team.net
> http://autox.team.net/mailman/listinfo/healeys
>
> Unsubscribe/Manage:
> http://autox.team.net/mailman/options/healeys/healey.nut@gmail.com
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://autox.team.net/pipermail/healeys/attachments/20250501/3a5455c1/attachment.htm>