CHAPTER
XI
TUNING AND MODIFICATION
[Engine
Tuning] [Supercharging]
[Raising Rear Axle Ratio]
[Chassis Modifications]
[Modified Minor Club]
[Suppliers of Special Equipment]
It
is an encouraging thought that at a time when manufacturers
of popular cars are forced by economic conditions to
adopt an increasing degree of standardization, enthusiastic
owners are still prepared to spend a moderate amount
of money, and sometimes a considerable amount of time,
to ensure that their cars have a better performance
and still better handling qualities than unmodified
models from the same production line. If proof
were needed, one could point to the steadily increasing
interest in "do-it-yourself" tuning and modification
kits for popular cars, which has become evident in almost
every country of the world.
Part
of the explanation lies in the fact that the engine
of the modern "family" type of car possesses considerable
reserves of stamina; its power output can be increased
quite appreciably, without risk of unduly shortening
its life or incurring other expensive consequences.
In this respect the B.M.C. "A" type engine fitted to
the Minor 1000 is no exception to the general rule.
As
installed in the Minor, for example, it develops a maximum
power of 37 b.h.p. at 5,500 r.p.m., whereas the same
engine in more highly tuned form in the Austin-Healey
Sprite develops 43 b.h.p. This little power unit can,
in fact, be developed to give over 50 per cent more
power than in the standard form, without sacrificing
reliability, easy starting and tractability.
Engine
tuning is, of course, too complex a subject to be dealt
with in any detail in a short chapter. An owner who
proposes to carry out such work may find of interest
a specialized book on the subject. Pitman's Engine Tuning
for the Owner-Driver. The less experienced owner, on
the other hand, need not be debarred from improving
the performance and handling qualities of his car, since
his needs are well catered for by specialist firms that
produce bolt-on engine tuning and suspension modification
kits which are supplied either on an exchange basis,
or which can be fitted either by the manufacturer of
the kit or, in some cases, by local distributors. Road
testing and final adjustments are then carried out by
skilled mechanics. The addresses of some of the
firms specializing in modifications for the Minor are
given at the end of this chapter.
It
should, perhaps, be made clear at this stage that the
appeal of these modifications is not restricted to owners
who are anxious to try their hands at trials, rallies
or club racing, although modified Minors have acquitted
themselves well in such events. One of the chief attractions
of a moderately-tuned Minor is that it puts in the hands
of a family driver a small, comfortable car, compact
enough to fit into the modern pattern of congested traffic,
with inherently good road manners but now possessing
a very lively performance that enables it to hold its
own with cars of up to twice its engine capacity and
of far higher price, even when allowance is made for
the cost of tuning.
Engine
Tuning.
In conventional tuning, modifications
are made to the induction system, cylinder head and
valve gear that aim at improving the "breathing" of
the engine; that is, its ability to draw in an adequate
amount of air at high speeds, when the restriction caused
by the standard carburettor, induction system and valve
gear limits the maximum power that can be developed.
The
most usual modification, therefore, is to substitute
a twin-carburettor inlet manifold for the single-carburettor
layout. The increased flow of gas through the
engine then calls for an improved exhaust manifold and,
preferably, a straight-through type of silencer.
The reduction in exhaust gas back-pressure will not
only enable the maximum performance to be obtained,
but will also tend to prevent any possible overheating
and will improve the petrol consumption.
When
the engine is tuned to this extent, the maximum revolutions
obtainable in the gears will be limited by valve bounce;
that is to say, the valve springs will no longer be
able to keep the valves and tappets in contact with
the cams. When this happens the valves tend to
remain open and the engine speed is automatically limited.
The fitting of stronger valve springs overcomes this
difficulty and also improves the life of the valves
and seatings. In the latter respect, however, there
is a good deal to be said for fitting better-grade valves
than the standard parts if the engine is likely to be
driven hard.
In
addition to ensuring adequate filling at high speeds
it is usual to carry out a fair amount of work on the
cylinder head, both to ensure that the volumes of the
individual combustion chambers are matched as closely
as possible — an ideal that cannot be expected in the
standard engine, owing to the limitations of mass-production
casting techniques — and also to raise the compression
ratio to a moderate extent, in order to extract a greater
amount of heat energy from the burning charge.
The standard ratio of 8.3:1 can safely be increased
to 8.7:1; a ratio as high as 9:1 has, in fact, been
used without rendering the engine rough or lacking in
flexibility at low speeds, although super-premium fuels
must be used if pinking is to be avoided.
Before
leaving the subject of conventional tuning, it should
be mentioned that probably the least expensive way of
obtaining a really substantial improvement in engine
torque (pulling power) and smoothness, combined with
reduced fuel consumption, is to fit a G.M. Modifier
between the carburettor and the standard inlet manifold
in order to re-atomize the globules of petrol that condense
on the carburettor throttle plate, ensuring much more
even fuel distribution. Two modifiers can be fitted
when a twin-carburettor layout is used. While a single
modifier, fitted at the carburettor flange, will give
excellent results the principle can be taken a step
further by substituting a Mangoletsi-modified inlet
manifold for the standard article. These manifolds,
which are obtainable on an exchange basis from the G.M.
Carburettor Co., have a G.M. modifier fitted permanently
at each flange that mates with the inlet passages in
the cylinder head and also at the carburettor flange.
The induction passages are polished where necessary.
Owners have reported that the fitting of one of these
manifolds, retaining the standard single carburettor,
has given results that are closely comparable with those
obtained from some twin-carburettor modification sets.
Supercharging.
An entirely different approach
to the problem of increasing the output of the engine
is to draw the mixture from the carburettor through
a belt-driven supercharger and to feed it under pressure
to the cylinders, thus ensuring that a full charge is
drawn in under all normal running conditions, plus about
one-third of the nominal capacity of each cylinder when
the engine is operating at full power. For this reason
a supercharger can also be used to compensate for the
inevitable loss of power that is experienced when an
engine operates at high altitudes and is drawing in
air of reduced density.
Except
when used for racing, most supercharger installations
are of the low-pressure type: that is to say, the "boost"
does not exceed 2-8 lb/sq. in., which is unlikely to
overstress any modern engine in the family-car class.
This type of supercharger usually gives smoother running
since besides packing more mixture into the cylinders,
it also compensates for some of the deficiencies of
mass-produced induction systems.
Modern
superchargers are virtually trouble-free. They are lubricated
automatically from the engine and are quiet in operation,
apart from a slight whine which is evident with some
types while accelerating but which disappears when the
throttle is eased back. It is essential, however, to
obtain the advice of a specialist before deciding on
a suitable type and to consult your insurance company
regarding a possible increase in premium; some companies
do not view superchargers too kindly and impose a rather
heavy loading on the policy.
In
any event, before deciding on the most suitable method
of increasing engine power output it is advisable to
write to the specialist firms listed on pages 123-4;
these will provide a great deal of information, including,
in many cases, reprints of reports of road tests of
modified cars carried out by the leading motoring journals.
One is thus able to study comparative specifications,
performance figures and costs.
Raising
Rear Axle Ratio.
When the engine has been tuned
and normal driving conditions entail a fairly high proportion
of fast main-road cruising, it is worthwhile considering
raising the rear axle ratio from 4.5:1 to 4.2:1.
The higher ratio will reduce the engine revolutions
throughout the speed range, resulting in a lower noise
level in the car. reduced "fussiness" at high speeds
and improved fuel consumption. The maximum speed will
be slightly reduced but acceleration is not noticeably
affected.
Sets
of higher-ratio gears, complete with a recalibrated
speedometer, can be supplied, and fitted if required,
by tuning specialists.
Chassis
Modifications.
As has been stressed elsewhere
in this book, the Minor has excellent handling characteristics
and providing that it is not driven to the limit of
its performance, a tuned example will probably need
no further modification. If full advantage is taken
of the increased power available, however, two modifications
are well worthwhile. The first is an anti-roll bar,
which is quite easily fitted to the front suspension
and brings about a marked reduction in roll when cornering
fast. It also improves tyre adhesion.
The
second is a modification to the rear suspension designed
to reduce or eliminate rear wheel "skip" or "tramp",
which occurs when the full power of a modified engine
is transmitted through the lower gears while accelerating
away from a standstill. The modifications offered by
specialist firms to cure this trouble fall into two
categories: locating the rear axle by means of torque
arms, and the substitution of telescopic shock absorbers
for the standard type. In most cases the latter can
be mounted without the need for further modification.
The particular scheme recommended by the B.M.C. Competition
Department, however, calls for vertical mounting of
the dampers, when it is necessary to cut into the boot
and to weld in a sheet-metal box on each side to provide
upper fixing points. A racing or trials enthusiast would
consider the work involved (and the restriction of the
space in the boot) justified by the efficiency of this
particular modification, but one of the less elaborate
schemes will probably satisfy the average owner.
Finally,
we come to the all-important subject of brakes.
Again, for normal driving the standard system will probably
prove entirely adequate but if the brakes are used frequently
at high speeds they may show a tendency to "fade" when
they become overheated — that is, pedal travel will
increase and efficiency will fall off — although normal
braking will be restored when the shoes and drums have
been allowed to cool. Specialist tuning firms can supply
"hard" linings which give consistent braking but — as
is usual in such cases — these have a lower coefficient
of friction than the standard linings and call for a
correspondingly heavier pressure on the brake pedal
to produce a given braking power. This drawback
can in turn be overcome by installing a brake "booster"
in the hydraulic circuit; here again the tuning specialist
can offer equipment that can be fitted with the minimum
of trouble.
Modified
Minor Club.
Before leaving the subject of
modifications it may be mentioned that owners of modified
cars and those who are contemplating tuning and other
alterations can make contact with fellow enthusiasts
through the medium of the Modified Minor Club.
The address of the Secretary is 9 Blakesley Avenue,
Ealing, W.5. (remember,
this is the address from 1960, you'd be better to contact
the MMOC)
A
member who requires details of a particular modification
is put in touch with another who has already carried
it out. In addition, a great deal of first-hand practical
experience is, of course, exchanged during discussions
at club meetings.
SUPPLIERS
OF SPECIAL EQUIPMENT
(remember, these are addresses
as they were in 1960, they may not still exist!)
Engine
Conversions and Tuning Kits:
Alexander Engineering Co., Ltd.,
Thorne Road, Haddenham, Bucks.
Aquaplane Co., Ltd., Oulton
Broad, Suffolk.
V. W. Derrington, Ltd., 159-161
London Road, Kingston-on- Thames, Surrey.
Downton Engineering Works, Ltd.,
Headlands, Downton, Wilts.
Palace Gate Garage, 50 Queens
Gate Mews, London, S.W.7.
Performance Equipment Co., Tower
Building, Liverpool, 3.
Speedwell Performance Conversions,
Ltd., 763 Finchley Road, London, N.W.11.
Induction
Manifold Modifiers:
G.M. Carburettor Co., Ltd.,
Cranford Works, Malt Street, Knutsford, Cheshire.
Performance
Testing Meter:
Bowser, Monks & Whitehouse,
Ltd., Doncaster.
Superchargers:
Carburettors, Ltd., Grange Road,
Willesden Green, London, N.W.10.
North Down Engineering Co.,
Westway, Caterham, Surrey.
Shorrock Superchargers, Ltd.,
Empress Works, Walsall Road, Willenhall, Staffs.
Suspension
Conversions:
Postland Engineering & Trading
Co., Ltd., Crowland, Nr. Peterborough, Northants.
Palace Gate Garage, 50 Queens
Gate Mews, London, S.W.7.
Speedwell Performance Conversions,
Ltd., 763 Finchley Road, London, N.W.11.
Transmission
Modifications:
Alexander Engineering Co., Ltd.,
Thome Road, Haddenham, Bucks.,
and most of the other firms
listed above under Engine Conversions and Tuning Kits.
Of
course any companies or products mentioned in the text
may not exist today and any prices listed are of course
no longer valid, so take company and product refernces
with a grain of salt.