How can I improve the performance of my MGA?

Glen Renshaw in his competition winning MGA
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 aren’t.
The main improvements fall within the following categories:
MG published advice on tuning the MGA for improved performance. The various stages of tuning involved:
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.
Suspension
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 inches).
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 tyre’s 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 "standard" 165/80/15:
175/75/15
185/70/15
195/65/15
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!
Compiled Daniel Levy