Best upgrade or modification

All, this may bring out some different opinions but what has been the best upgrades or modifications you have made to your cars?

cheers

Bilstein and Eibach suspension set up. Gives a nice lowered stance and very comfortable damping in the age of speed bumps and terrible roads. 

Kingy, Great start! I have Eibach springs in my shopping basket, hadn't decided on which shocks but a good recommendation 
Cheers

Cheapest and most effective:

Head Light Relays
Starter relays

4-2 manifold
Dual down exhaust.

 

Briano1234 said

Cheapest and most effective:

Head Light Relays
Starter relays

4-2 manifold
Dual down exhaust.

 

Reliability first

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Looks. Lowering.

Sounds. Exhaust

Usability. Relays on lights.

They have to be taken as a sum though. No one mod is any good by itself


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New Michelin, Continental or Pirelli tyres to replace the part worn 15yr old taiwanese ones on it that might as well be made of wood.

Lower strut brace makes a massive difference and well worth doing.

Thanks these things help me with my planning and buying before I get to start on it with the spanners again!

Interested to hear some more