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Carburation - Initial considerations and manifolds |
22 Mar 2001 |
When
trying to improve your Min’s breathing efficiency, carburation and inlet
manifolds are open to as much speculation as other ‘performance enhancing’
components. A great deal of which falls into the b*llsh*t category. As is my
wont, I’ll try and outline some basic guidelines.
Many
facets of our lives are governed by one particular idiom - big is beautiful,
and it seems from what many folk are told carburettors are no exception. This
is all very well, but it can have drastic effects on how well the engine
performs over the broad spectrum of its use. For maximum power, i.e. foot
firmly buried in the carpet pile, the method of introducing air/fuel into the
engine could be carburettors, injection, man and garden hose, milk pail, or
whatever, as long as the air/fuel ratio is correct and the fuel is properly
atomised for a complete and efficient ‘burn’. I’m not going to get into a
discussion on air/fuel ratios and what’s best for power/economy here - a full subject
in itself - but the metering device. And that, mes amies, is the carburettor.
Then, having selected a suitable one, a decent inlet manifold is a definite
requirement to maximise its performance. To cover all eventualities would take
a tome, so we’re going to concentrate on road use in this article.
Sizing
Before
shelling out hard earned beer vouchers on another carb, a nanosecond's
contemplation as to why wouldn’t go amiss. Just why do you need to change it?
If it’s solely to come tops in the bar room b*llsh*t competition, or
aesthetics, then this article is not for you. If maximum performance - be that
economy or power - is the goal, read on.
Your
Mini engine is no less for its ancient design than any modern unit. They all
accomplish the same thing - turning heat into mechanical energy to propel the
four-wheeled wonder down the road. To do this the heat has to be developed from
somewhere, in our case by burning (that’s burning,
not exploding) a fuel with an
oxidant - petrol from a pump and oxygen from the air. To achieve our goal of
either maximum economy or power, or combination of both, these two parts need
adding in a certain ratio. Years of development and tests have come up with
certain well-defined limits as to which are relevant where. The carb is
designed to complete this duty with efficiency given its mechanical/cost
limitations. To whit, the vehicle manufacturers have carefully considered the
carb chosen for each engine application, of which size plays a big part.
We
fit modified cylinder heads to improve volumetric efficiency. Greater power
outputs are achieved through maximising air consumption. The carb therefore
needs to be sized so as not to be restrictive. However, the most common mistake
made is fitting a carb that is too big. What you need is a carb sized to
provide adequate airflow for the expected power output, NOT the size of the
engine. This is based upon the fact that a certain volume of air is needed for
a certain amount of power. To illustrate, air consumption of a 65 HP 998cc
engine is going to be relatively similar to a 65 HP 1275 - so the carb size
needed is going to be more or less the same within certain limits. To stress
the point - if the correctly sized carb is used, there is absolutely no more
power to be had from going bigger.
Bearing
this in mind, on a bigger capacity engine, fitting a carb a shade on the small
side will make the engine work a tad harder to draw air through it to achieve
that power output. It’ll mean a slight reduction in outright power, but the benefits
are worthwhile - greater fuel economy and greater torque. Torque is what
accelerates the car in 99% of road use, so optimising this is far more
beneficial than tuning for maximum power. Parting with a sizeable chunk of
money for a big carb is a waste, as its full potential isn’t being used and
bottom end flexibility is severely compromised. Check out the relevant chart
for sizing.
 |
Click of picture to enlarge |
The
solid coloured area shows the carbs strongest operating envelope. When getting
into the shaded area, consider going up on carb size - BUT carefully consider
the text when doing so. This is not hard and fast, but a good guide, and
presuming a decent flowing manifold is used. It is possible to get really good
performance out of a bigger than seemingly necessary carb on a smaller engine
(say HIF6 on a 998) - but the knowledge of how to do so is limited to the most
learned ‘wizards’!
How many?
Throwing
more carbs at an engine isn’t going to make one iota of difference to maximum
power output unless the engine was drastically under-carbed to start with. In
which case a bigger single carb would have the same effect.
In
the days of yore, multiple carbs were seen as automatic power improvers - a
legend that still lingers in the minds of the unknowing/misinformed. The main
reason for their apparent huge power output improvements was more to do with
manifolding than carb quantities. Modern technology has proven and developed
the art of maximising airflow/velocity. As can be witnessed by some of the
horrible/frightening aftermarket manifolds produced some years ago for the
single SU in comparison to those on the market today, but we’ll consider this
more later.
The
main benefit of running more carbs, twins in our case, has already been
intimated earlier - smaller carbs produce better drivability. So using a
smaller choke sized twin set-up instead of a large single with similar airflow
capabilities for the same engine power output will generally produce better
drivability. The downside is the extra cost (twice the price), setting up, and
manifold efficiency. However, contrary to popular opinion, twin carb set-ups do
not go out of tune/sync quicker than a single. Once correctly set up,
there’s no difference whatsoever!
That’s
for SUs. The much-hallowed Weber is effectively a twin carb in itself - having
two ‘chokes’ in a common body. Deemed to be the BIG power producer - yet
another fallacy. Again we’re talking mainly about installation/manifold
efficiencies and more appropriate sizing than any super-natural capability.
Tuning a Weber is easier for most as calibration can be easily altered through
the use of a myriad of components even when the ‘crème de la crème’ is used -
‘split Webers’. Tuning an SU can be awkward because of the use of the tapered
needle, made more laborious when there are two of them. This split Weber
business is an abhorrent thing to do to such a well designed piece of equipment
and completely unnecessary on anything other than the absolute out-and-out
circuit racer. Not to mention the rude amount of money needed to do the job
properly. Getting the benefits out of a Weber on a Mini means bodywork surgery,
which is hassle and far from simple. It therefore isn’t really worth
considering for a road car, and would need article on its own - so will be left
for another time!
Manifolds - singles
Having
selected a carb suitable for the job, choosing a compatible manifold is of
great importance. After all, if the carb is right and a crap manifold used,
much of the sought-after power potential will be lost. It needs to meet a
number of criteria - carry the carb at the correct angle, allow fitment of a
suitable air filter, and provide uninhibited passage of the fuel/air mix to the
engine without compromising velocity. This last feature being the most important
for usable power, the others a definite bonus to avoid fitting hassles and
frustration.
There
are a variety of manifolds on the market, generally cast in aluminium-alloy.
Few understand why this is; generally believing it’s mainly down to ease of
production and is certainly one of the major considerations. Aluminium conducts
heat faster than iron, the benefits of this being two-fold. As far as maximum
power is concerned, the cooler the inlet charge, the better. Aluminium’s
ability to dissipate heat faster keeps the intake charge down in comparison to
an iron one. For economy, quicker warm up is essential, again aluminium's rapid
conduction of heat gets it up to running temperature quicker - hence Rover’s
introduction of an aluminium-alloy manifold on the MG Metro engine to utilise
these principles. The mistake they made was making it slightly too big in the
ports.
Port
size needs to be designed to achieve maximum airflow without compromising
velocity for the given space the manifold has to fit in. In a Mini this equates
to precious little! Big ports mean lower air speeds, and as a consequence
bottom end/mid range drivability suffers. In days gone by, there were only a
few alternative manifolds on the market for single SUs. The best ones being mainly
produced to suit one particular engine build type - namely the 1380cc
big-bores. The small-bore engines were considered the poor cousins. A couple of
the better manufacturers ended up making two manifolds to alleviate the
situation, but were based on the big bore item. The big problem here was the
fit - the big-bore block is 3/8” taller than the small-bore.
To
achieve maximum performance, manifolds were conceived to be as flat in side
plan as possible. Consequently on the big-bore engines, the jet tube was
perilously close to the bulkhead. I remember from the pained experiences of
others that if the engine wasn’t kept rock still, the jet tube invariably ended
up being whacked against the bulkhead, bending it and rendering it useless. A
situation made worse by using the choke!! When this design was used for the
small-bore engines, mayhem ensued as the fragile jet tube was practically
resting on the bulkhead before the engine was even started. One manufacturer
tried countering this problem by using a much steeper angle on the
carb-mounting flange. This worked after a fashion but necessitated the carb
float bowl angle to be re-set to avoid fuelling problems. Not many folk
realised or understood this, so ended up frustrated by apparently strange and
inconsistent engine performance! Nearly all manifolds needed finishing off by
hand to achieve acceptable airflow performance, so were not really consistent
‘off the shelf’.
One
of the unfortunate aspect of all this is that some ill-informed or
less-knowledgeable manifold suppliers/manufacturers today have copied these
manifolds exactly, and therefore all the inherent problems. There are only a
couple of really good, thoroughly investigated and developed manifolds for the
single SU on the market, the best by some margin being the Mini Spares/Mania
components. So when deciding on which one to go for, before you look at the
cost consider the design. It should fit easily, take a standard air-cleaner box
and hold the carb high enough to avoid bulkhead/speedo cable fouling. The ports
should taper nicely from manifold to carb mounting face and have a reasonable
cast finish. There should be provision for water heating, the tube size
compatible with the Minis heating pipes (1/2” bore as opposed to the 5/8” bore
of the Metro). As for port sizing, for road use on practically all engines
1.35” at the manifold face is good. For big engines, i.e. 1400cc plus or 1380cc
where maximum top end power is wanted, 1.4” at the manifold face is needed.
Single
SU inlet manifold flow test comparison
|
96.50 cfm |
MG Metro standard alloy manifold as
cast. |
|
99.00 cfm |
Howley 1.75" - previously the
best available - this one was quite heavily modified to achieve this flow
figure. |
|
105.00 cfm |
Titan Motorsport 1.75" as cast. |
|
108.10 cfm |
Mini Spares C-AHT770/A small-port
1.5/1.75" as cast. |
|
112.11 cfm |
Mini Spares C-AHT771 large-port
1.75" as cast. |
The
higher the cfm (cubic feet per minute) figure, the more power potential. All
manifolds tested on a known-performer cylinder head with a manifold-less
maximum flow 138cfm.
The
manifold to head mounting flange thickness is also an area where most have
fallen down. Some have copied the MG Metro manifold that has a raised lump
where the retaining washers sit up against to be level with the cast iron
exhaust manifold used only on the MG Metro. Others have gone for a thickness
that suits the budget exhaust manifolds they sell where the flange thickness
varies depending on what material was cheapest at the time of manufacture. This
mish-mash of flange sizing causes manifold gasket sealing problems. Either the
exhaust is blowing or the intake is leaking in air, causing erratic running.
The better ones therefore have a flange thickness compatible with the more popular
and generally higher quality exhaust manifolds - that's currently 8mm thick as
per Maniflow and Janspeed.
Manifolds - twins
We
already know the benefits of twin carbs, and have already commented on the fact
that the manifold type was responsible for the early horsepower gains. You only
have to look at it to see why - the carbs are almost running a ‘straight shot’
into the intake ports; far more efficient than the early single carb
counterparts despite pretty awful castings - especially internally - that
really were not at all as efficient as they should have been.
Apart
from the factory fitted manifolds of the Coopers and S, there are only a couple
of other options. Those fitted to the Sprite/Midget, MG1100 and
For
maximum performance out of a twin carb set-up, the best manifold is
manufactured in steel by those manifold fabrication wizards at Maniflow.
Unfortunately it is made to favour the bigger carbs, so isn't really suitable
for twin inch and a quarters - but probably works better than the original cast
aluminium ones! BUT - there is a big price penalty. If you decide to run twin
carbs, then this steel manifold is a must to maximum performance potential.
In conclusion
Apart
from the foregoing and stressing yet again that big is not necessarily best, I
would like to point out that if you’re considering a carb change, the best SU
to go for is the HIF variety. Space precludes me from depicting why. Suffice to
say much development was put into it - making it far more effective than the
older HS versions. I know I always bang on about this one particular point, but
it really makes a big difference in driving pleasure - ALWAYS, ALWAYS, be
honest about your car’s main use and how you mostly drive before deciding what
components you buy.
Useful
part numbers:
|
C-AHT770 |
Mini
Spares inlet manifold for 1.5" SU. Can be re-worked for 1.75" SU at
carb mounting flange/hole if/when required. Has 5/8"UNF threaded hole
for servo take-off adaptor
and 1/2" water-heating facility. Suitable for all road applications up
to 1380cc. |
|
C-AHT770A |
Mini
Spares inlet manifold for 1.75 SU. Has 5/8"UNF threaded hole for servo take-off adaptor and 1/2" water-
heating facility. Suitability as above. |
|
C-AHT771 |
Mini
Spares large port manifold for 1.75" SU on engines With a greater
capacity than 1380cc and serious cams. |
|
C-AHT770 |
SPRITE Titan Motorsport 1.5"/1.75"
manifold with lower carb height for MG Midget & Austin Healey Sprite to avoid
bonnet fouling. |
|
12H1405 |
Servo
take-off adaptor |
|
C-AEG488 |
Mini
Spares cast ally inlet manifold for twin SU - takes 1.25" H/HS2 or
1.5" H4 carbs (vertical stud pattern). |
|
C-AEG489 |
Mini
spares cast ally inlet manifold for twin SU -takes 1.5" or 1.75"
HS4/HS6 carbs. |
|
C-AEG490 |
Maniflow
fabricated steel inlet manifold for twin SU - Comes with all stud patterns to
take all SUs. |
|
MSSK1005 |
Twin
HS2 heat shield & return spring kit - stainless steel |
|
MSSK1008 |
Twin
HS2 heat shield & return spring kit - black |
|
MSSK1006 |
Twin
HS4 heat shield & return spring kit - stainless steel |
|
MSSK1004 |
Twin
HS4 heat shield & return spring kit - black |
|
MSSK9 |
Twin
card linkage kit includes accelerator cable manifold bracket, 2 cross bars
with linkage, choke and throttle cable trunnions. |
Note:
It is
entirely feasible to use an alternative servo take-off as per the injection
Minis and all Metros. This was simply the oil transfer pipe banjo bolt from the
engine block and the transfer pipe cut short.