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Michael=E2=80=A6you obviously are bored!
Alan Wasserman
Alanwassermannc@gmail.com
732-887-0742
www.wassermanstudio.com
> On Apr 11, 2025, at 10:47=E2=80=AFAM, Michael Salter <michaelsalter@gmail.=
com> wrote:
>=20
> =EF=BB=BF
> There are lots of very smart people on this list and, as it has been very q=
uiet lately I thought I would throw this out for constructive feedback.
>=20
> DEALING WITH THE ISSUE OF =E2=80=9CCHARGE ROBBING=E2=80=9D WITH A SIAMESE P=
ORT CYLINDER HEAD.
>=20
> 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 sha=
re an individual intake port and the inner pair of cylinders share one exhau=
st port.
>=20
> Additionally, in the interests of simplicity, these engines use a firing o=
rder of 1-3-4-2 or, expressed differently, 2-1-3-4, with the result that the=
se ports =E2=80=9Csee=E2=80=9D a port flow pattern of =E2=80=9Cflow, flow, w=
ait, wait, flow, flow, wait, wait=E2=80=99.
>=20
> 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 ai=
r/fuel charge first gets =E2=80=9Crobbed=E2=80=9D of part of this charge by t=
he cylinder which draws its charge immediately thereafter.
>=20
> Reputedly, this design results in the inner two cylinders (2 &3) receiving=
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 w=
hich 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.
>=20
> My 55 years of experience has been limited almost exclusively to carburett=
ed engines of this type, both single and twin carburettor versions, and duri=
ng 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 te=
st is nowhere near as accurate as modern UEGO systems but these weren=E2=80=99=
t available at the time however, over the years this test has never indicate=
d that the inner cylinders (2 & 3) consistently run leaner.
>=20
> Some decades ago, it was determined that in these engines the gas temperat=
ure 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 higher operating tem=
perature of the exhaust valves in those inner cylinders is a much more likel=
y explanation for their premature failure.
>=20
> 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 i=
f 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 t=
he fuel is introduced into the very turbulent incoming air stream as microsc=
opic droplets well before that stream reaches the point in the intake port w=
here the port splits, which allows plenty of time for any droplets to entire=
ly 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 ai=
r/fuel ratio will be the same for both.
>=20
> The same situation does not apply in modern multi-port fuel-injected engin=
es where each port has its own injector. To minimize emissions during thrott=
le 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 c=
ylinder's intake valve. As the intake valve opens backflow of combustion gas=
ses through it finalizes the vaporization of the fuel droplets within the in=
coming charge. In the very unlikely event that such a port injection system b=
e used in an engine with Siamesed intake ports the smaller volume of air del=
ivered 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.
>=20
> So, how does this all play out in the case of the EFI system I=E2=80=99m d=
eveloping for my 1956 Austin Healey 100?
>=20
> The fuel injectors in this engine modification are installed inside the SU=
carburettor bodies upstream of the throttle butterfly and are therefore a s=
ignificant distance from the point in the Siamesed intake port where the por=
t splits. As throttle transition emissions are of no concern with this vehic=
le, this position is ideal in that it ensures that the fuel droplets are com=
pletely vaporized long before the intake charge reaches the point where the p=
ort splits.
>=20
> For this reason, it is very unlikely that there will be any difference in t=
he air/fuel ratio of the charge received by any of the cylinders.
>=20
>=20
>=20
> Michael S
>=20
> April 2025
>=20
>=20
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<html><head><meta http-equiv=3D"content-type" content=3D"text/html; charset=3D=
utf-8"></head><body dir=3D"auto">Michael=E2=80=A6you obviously are bored!<br=
id=3D"lineBreakAtBeginningOfSignature"><div dir=3D"ltr">Alan Wasserman<div>=
Alanwassermannc@gmail.com</div><div>732-887-0742</div><div>www.wassermanstud=
io.com</div></div><div dir=3D"ltr"><br><blockquote type=3D"cite">On Apr 11, 2=
025, at 10:47=E2=80=AFAM, Michael Salter <michaelsalter@gmail.com> wro=
te:<br><br></blockquote></div><blockquote type=3D"cite"><div dir=3D"ltr">=EF=
=BB=BF<div dir=3D"ltr"><div class=3D"gmail_default" style=3D"font-family:ari=
al,sans-serif;font-size:small">There are lots of very smart people on this l=
ist and, as it has been very quiet lately I thought I would throw this out f=
or constructive feedback.</div><div class=3D"gmail_default" style=3D"font-fa=
mily:arial,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:0i=
n 0in 8pt;line-height:115%;font-size:12pt;font-family:"Calibri",sa=
ns-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-siz=
e:12pt;font-family:"Calibri",sans-serif"><span style=3D"text-trans=
form:uppercase">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 h=
ead,
the forward and the rearward pairs of cylinders each share an individual int=
ake
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-siz=
e:12pt;font-family:"Calibri",sans-serif">Additionally, in the inte=
rests 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=9C=
flow, 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-siz=
e:12pt;font-family:"Calibri",sans-serif">This design results in a p=
henomenon 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 b=
y 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-siz=
e:12pt;font-family:"Calibri",sans-serif">Reputedly, this design re=
sults in the inner two cylinders (2
&3) receiving a =E2=80=9Cleaner=E2=80=9D intake charge than the outer tw=
o 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=9Cc=
harge robbing=E2=80=9D. <span></span></p>
<p class=3D"MsoNormal" style=3D"margin:0in 0in 8pt;line-height:115%;font-siz=
e: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 carbur=
ettor
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 a=
ware that
this test is nowhere near as accurate as modern UEGO systems but these weren=
=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-siz=
e:12pt;font-family:"Calibri",sans-serif">Some decades ago, it was d=
etermined that in these engines the
gas temperature in the Siamese exhaust port of cylinders 2 & 3 can be as=
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-siz=
e:12pt;font-family:"Calibri",sans-serif">For =E2=80=9Ccharge robbi=
ng=E2=80=9D to affect the air/fuel ratio the
=E2=80=9Ccharge=E2=80=9D must contain fuel droplets. To express this differe=
ntly 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 rati=
o.
This is almost certainly the case with a carbureted engine wherein the fuel i=
s
introduced into the very turbulent incoming air stream as microscopic drople=
ts
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. I=
n
such a situation the volume of the charge entering the inner cylinders may b=
e smaller
than that entering the outer cylinders, but the air/fuel ratio will be the s=
ame
for both.<span></span></p>
<p class=3D"MsoNormal" style=3D"margin:0in 0in 8pt;line-height:115%;font-siz=
e:12pt;font-family:"Calibri",sans-serif">The same situation does n=
ot 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 th=
e
port walls so the fuel is injected into the intake ports just upstream of ea=
ch
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 syste=
m
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 decre=
ase
the pulse width for those two cylinders.<span></span></p>
<p class=3D"MsoNormal" style=3D"margin:0in 0in 8pt;line-height:115%;font-siz=
e:12pt;font-family:"Calibri",sans-serif">So, how does this all pla=
y out in the 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-siz=
e:12pt;font-family:"Calibri",sans-serif">The fuel injectors in thi=
s 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 wi=
th
this vehicle, this position is ideal in that it ensures that the fuel drople=
ts
are completely vaporized long before the intake charge reaches the point whe=
re
the port splits. <span></span></p>
<p class=3D"MsoNormal" style=3D"margin:0in 0in 8pt;line-height:115%;font-siz=
e:12pt;font-family:"Calibri",sans-serif">For this reason, it is ve=
ry unlikely 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-height=
:115%;font-size:12pt;font-family:"Calibri",sans-serif"><br></p><p c=
lass=3D"MsoNormal" style=3D"margin:0in 0in 8pt;line-height:115%;font-size:12=
pt;font-family:"Calibri",sans-serif">Michael S</p><p class=3D"MsoN=
ormal" style=3D"margin:0in 0in 8pt;line-height:115%;font-size:12pt;font-fami=
ly:"Calibri",sans-serif">April 2025<span></span></p>
<br></div></div>
</div></blockquote></body></html>=
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