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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=E2=80=AFAM Michael Salter <michaelsalter@gmai=
l.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 =E2=80=9CCHARGE robbing=E2=80=9D 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 =E2=80=9Csee=E2=80=9D a port flow pattern of =E2=80=9Cflow, f=
low, wait, wait, flow,
> flow, wait, wait=E2=80=99.
>
> This design results in a phenomenon called =E2=80=9Ccharge robbing=E2=80=
=9D wherein a
> cylinder served by an individual port which draws the incoming air/fuel
> charge first gets =E2=80=9Crobbed=E2=80=9D 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) receivin=
g
> a =E2=80=9Cleaner=E2=80=9D 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 =E2=80=9CA=E2=80=
=9D and =E2=80=9CB=E2=80=9D series
> engines, was a consequence of this =E2=80=9Ccharge robbing=E2=80=9D.
>
> 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=E2=80=
=99m aware
> that this test is nowhere near as accurate as modern UEGO systems but the=
se
> weren=E2=80=99t available at the time however, over the years this test h=
as 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=C2=B0 C higher than in the outer two ports and the resulting highe=
r
> operating temperature of the exhaust valves in those inner cylinders is a
> much more likely explanation for their premature failure.
>
> For =E2=80=9Ccharge robbing=E2=80=9D to affect the air/fuel ratio the =E2=
=80=9Ccharge=E2=80=9D 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 whe=
re
> 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 durin=
g
> 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 upstrea=
m
> 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 th=
e
> 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=E2=80=99m
> developing for my 1956 Austin Healey 100?
>
> The fuel injectors in this engine modification are installed inside the S=
U
> 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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>
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>
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<div dir=3D"ltr">Michael -<div><br></div><div>I learned something new today=
!<br><br>I wonder if charge robbing can be addressed by offsetting=C2=A0the=
carburettors a bit so that they are closer to=C2=A0 one cylinder vs anothe=
r.</div><div><br></div><div>Cheers,</div><div><br></div><div>Alan</div></di=
v><br><div class=3D"gmail_quote gmail_quote_container"><div dir=3D"ltr" cla=
ss=3D"gmail_attr">On Fri, Apr 11, 2025 at 11:42=E2=80=AFAM Michael Salter &=
lt;<a href=3D"mailto:michaelsalter@gmail.com";>michaelsalter@gmail.com</a>&g=
t; wrote:<br></div><blockquote class=3D"gmail_quote" style=3D"margin:0px 0p=
x 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div d=
ir=3D"ltr"><div class=3D"gmail_default" style=3D"font-family:arial,sans-ser=
if;font-size:small">There are lots of very smart people on this list and, a=
s it has been very quiet lately I thought I would throw this out for constr=
uctive feedback.</div><div class=3D"gmail_default" style=3D"font-family:ari=
al,sans-serif;font-size:small"><br></div><div class=3D"gmail_default" style=
=3D"font-family:arial,sans-serif;font-size:small">
<p class=3D"MsoNormal" align=3D"center" style=3D"text-align:center;margin:0=
in 0in 8pt;line-height:115%;font-size:12pt;font-family:Calibri,sans-serif">=
<span style=3D"text-transform:uppercase">Dealing with the issue of =E2=80=
=9CCHARGE robbing=E2=80=9D
with a Siamese port cylinder head.<span></span></span></p>
<p class=3D"MsoNormal" style=3D"margin:0in 0in 8pt;line-height:115%;font-si=
ze:12pt;font-family:Calibri,sans-serif"><span style=3D"text-transform:upper=
case">T</span>he 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 in=
take
port and the inner pair of cylinders share one exhaust port.<span></span></=
p>
<p class=3D"MsoNormal" style=3D"margin:0in 0in 8pt;line-height:115%;font-si=
ze:12pt;font-family:Calibri,sans-serif">Additionally, in the interests of s=
implicity, these engines
use a firing order of 1-3-4-2 or, expressed differently, 2-1-3-4, with the
result that these ports =E2=80=9Csee=E2=80=9D a port flow pattern of =E2=80=
=9Cflow, flow, wait, wait,
flow, flow, wait, wait=E2=80=99.<span></span></p>
<p class=3D"MsoNormal" style=3D"margin:0in 0in 8pt;line-height:115%;font-si=
ze:12pt;font-family:Calibri,sans-serif">This design results in a phenomenon=
called =E2=80=9Ccharge robbing=E2=80=9D
wherein a cylinder served by an individual port which draws the incoming
air/fuel charge first gets =E2=80=9Crobbed=E2=80=9D of part of this charge =
by the cylinder
which draws its charge immediately thereafter. <span></span></p>
<p class=3D"MsoNormal" style=3D"margin:0in 0in 8pt;line-height:115%;font-si=
ze:12pt;font-family:Calibri,sans-serif">Reputedly, this design results in t=
he inner two cylinders (2
&3) receiving a =E2=80=9Cleaner=E2=80=9D intake charge than the outer t=
wo 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=
=E2=80=9CA=E2=80=9D
and =E2=80=9CB=E2=80=9D series engines, was a consequence of this =E2=80=9C=
charge robbing=E2=80=9D. <span></span></p>
<p class=3D"MsoNormal" style=3D"margin:0in 0in 8pt;line-height:115%;font-si=
ze:12pt;font-family:Calibri,sans-serif">My 55 years of experience has been =
limited almost
exclusively to carburetted engines of this type, both single and twin carbu=
rettor
versions, and during that time I have regularly used spark plug colour as a=
n
indication of the air/fuel ratio being burned in any cylinder. I=E2=80=99m =
aware that
this test is nowhere near as accurate as modern UEGO systems but these were=
n=E2=80=99t available
at the time however, over the years this test has never indicated that the
inner cylinders (2 & 3) consistently run leaner. <span></span></p>
<p class=3D"MsoNormal" style=3D"margin:0in 0in 8pt;line-height:115%;font-si=
ze:12pt;font-family:Calibri,sans-serif">Some decades ago, it was determined=
that in these engines the
gas temperature in the Siamese exhaust port of cylinders 2 & 3 can be a=
s
much as 100<span>=C2=B0</span>
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. <span></span></p>
<p class=3D"MsoNormal" style=3D"margin:0in 0in 8pt;line-height:115%;font-si=
ze:12pt;font-family:Calibri,sans-serif">For =E2=80=9Ccharge robbing=E2=80=
=9D to affect the air/fuel ratio the
=E2=80=9Ccharge=E2=80=9D must contain fuel droplets. To express this differ=
ently 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 rat=
io.
This is almost certainly the case with a carbureted engine wherein the fuel=
is
introduced into the very turbulent incoming air stream as microscopic dropl=
ets
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.<span></span></p>
<p class=3D"MsoNormal" style=3D"margin:0in 0in 8pt;line-height:115%;font-si=
ze:12pt;font-family:Calibri,sans-serif">The same situation does not apply i=
n 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 t=
he
port walls so the fuel is injected into the intake ports just upstream of e=
ach
cylinder's intake valve. As the intake valve opens backflow of combusti=
on
gasses through it finalizes the vaporization of the fuel droplets within th=
e
incoming charge. In the very unlikely event that such a port injection syst=
em
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 decr=
ease
the pulse width for those two cylinders.<span></span></p>
<p class=3D"MsoNormal" style=3D"margin:0in 0in 8pt;line-height:115%;font-si=
ze:12pt;font-family:Calibri,sans-serif">So, how does this all play out in t=
he case of the EFI system
I=E2=80=99m developing for my 1956 Austin Healey 100?<span></span></p>
<p class=3D"MsoNormal" style=3D"margin:0in 0in 8pt;line-height:115%;font-si=
ze:12pt;font-family:Calibri,sans-serif">The fuel injectors in this engine m=
odification 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 w=
ith
this vehicle, this position is ideal in that it ensures that the fuel dropl=
ets
are completely vaporized long before the intake charge reaches the point wh=
ere
the port splits. <span></span></p>
<p class=3D"MsoNormal" style=3D"margin:0in 0in 8pt;line-height:115%;font-si=
ze:12pt;font-family:Calibri,sans-serif">For this reason, it is very unlikel=
y that there will be any
difference in the air/fuel ratio of the charge received by any of the
cylinders.</p><p class=3D"MsoNormal" style=3D"margin:0in 0in 8pt;line-heigh=
t:115%;font-size:12pt;font-family:Calibri,sans-serif"><br></p><p class=3D"M=
soNormal" style=3D"margin:0in 0in 8pt;line-height:115%;font-size:12pt;font-=
family:Calibri,sans-serif">Michael S</p><p class=3D"MsoNormal" style=3D"mar=
gin:0in 0in 8pt;line-height:115%;font-size:12pt;font-family:Calibri,sans-se=
rif">April 2025<span></span></p>
<br></div></div>
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</blockquote></div>
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