There are a number of things that can be done to a standard MGA to improve
its performance. Some of them are relatively cheap. Some arenít.
The main improvements fall within the following categories:
Wheels and tyres
Gearbox and differential
MG published advice on tuning the MGA for improved performance. The various
stages of tuning involved:
polishing the ports and combustion chamber and grinding the inlet and
replacing the camshaft and distributor with those from the Riley 1.5.
replacing the pistons to increase the compression.
changing the carburettor needles.
There are many other modifications that can be made. A popular one is to
replace the original engine with one from an MGB (1.8 to 2.0 litres). This is
unpopular amongst purists.
Probably the cheapest method for improving the handling of the MGA is to fit
an anti-sway bar.
Wheels and tyres
The MGA was originally sold with 5.60-15 tyres. The closest size you can get
today is a 165SR15 (this is the same as 165/80/15).
The first number (165) is the width of the tyre in millimeters.
The second number (80) is the "aspect ratio" (as a percent of the tyre's
width). In this case the aspect (height of the sidewall) is 80% of the width
(165 x 0.80= 132 mm).
The third number (15) is the size of the wheel that the tyre will fit (in
The standard tyre is fine for normal driving. However, for high speed
cornering a tyre with better traction and shorter sidewalls is to be preferred.
The car makes contact with the ground at 4 points (assuming you are driving
on 4 tyres and they are all touching the ground). All of the forces that affect
the car whilst it is being driven (eg acceleration, braking and cornering) are
transmitted through those 4 points. How well a tyre can handle those forces
depends on a number of factors including the tyreís traction and its ability to
retain its profile (resist rolling during cornering).
To improve the handling of your MGA you might consider wider wheels and
tyres with a lower aspect ratio. Assuming your car is currently fitted with
165/80/15 tyres, you could change to, for example, 185/65/15 or 195/60/15.
You need to keep in mind that any change in the diameter of your tire will
affect the engine speed (RPM) required to propel the car at a particular speed.
For example, a 185/65/15 is approximately 3.8% smaller than the "standard"
165/80/15. This means your speedometer will be affected, the engine RPM required
to travel at a given speed will increase by 3.8% and your top speed will be
reduced by the same percentage.
Changing to a 195/60/15 will have an even greater effect (approximately 5%).
This reduction in the size of the tyre may allow you to travel through
corners faster but it will reduce your top speed. It may assist in hillclimbs.
In short, it will affect the performance of the car in different ways. The trick
is to determine the best tyre having regard to your driving ability, the track,
the rate of climb at the hillclimb, your differential, etc.
As a rule of thumb, an increase of 10mm in width (eg from a 165 to a 175)
requires a reduction of 5 in the aspect ratio to retain roughly the same
diameter of tyre (ie to ensure your speedo retains its current level of
accuracy). In other words, the following are roughly the same diameter as the
Unfortunately, tyre manufacturers do not produce tyres in every size. And
high performance tyres using "racing" rubber compounds are typically not
produced in widths less than 195. And they are rarely made with an aspect ratio
higher than 60. If you want a "racing" tyre for an MGA you are likely to fit a
195/60/15 which is almost 5% smaller in diameter than the original tyre.
Before you rush out to purchase a wide racing tyre you must check the size
of your wheels. The standard MGA wheels had 4 inch rims. The general rule is
that the width of the rim should not be less than 70% of the width of the tyre.
Applying this rule, the maximum tyre widths for particular rims are as follows:
Rim Maximum tyre width
4 inch 165
4.5 inch 175
5 inch 185
5.5 inch 195
Some people do exceed the above limits. However, this may adversely affect
the performance of the tyre.
Gearbox and differential
The standard rear axle was 4.3:1 (on the 1500, 1600 Mk I and Twin Cam) or
4.1:1 (on the 1600 Mk II). A 4.55:1 was offered as a factory installed option.
For competition, a 3.909:1 was available. Whilst the Twin Cam was in
production, a 4.875:1 and a 5.125:1 was also offered.
Close ratio gears were (and still are) also popular amongst the competition
set. With a close ratio gearbox, the top gear is the same as in a standard box
whilst first gear in the close ratio box is closer to second gear in a standard
box. This means that the gears are "closer" together, resulting in less of a
"gap" between gears. This results in a lower loss of engine revs when changing
gears. However, it does make for a slower start off the line!