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Engine guides and mods

Forfour Earth Upgrade

Very popular with Ricers. Upgrading the engine earth can release more power and torque from your engine, make your stereo sound better and increase your MPG.

Modification Details

  Earth Straps


Electricity takes the route of least resistance back to the battery earth. If you have a poor earth connection, the current can pass back along other cables causing unusual problems to occur. I have managed to have the headlights come on with the brakes and even had the heater blower sucking air in. Poor earth connections will generally affect things that can run a reverse voltage like bulbs and motors.

As the quality of the earth connections deteriorate it can cause unusual temporary problems 
plus you may notice a drop in performance, lesser MPG and an increased electrical noise through 
the speakers. This can be made worse when an amp is installed, even though the amp is 
earthed the standard earth cable cannot handle the extra load.

Upgrading the quality of the earth cables and the connections allows the electrons to flow 
at their full potential. This makes the whole process more efficient and allows all the 
connected devices to work more efficiently as well. Upgrading can:

  • Reduce electrical noise
  • Increase the efficiency of the spark plugs
  • Increase the efficiency of the engine
  • Increase the MPG
Give more power

Unleashing The Full Potential Of Your Spark Plugs

The spark plug is a very simple device, it introduces a gap in a high voltage system to create a spark between two electrodes. This spark ignites the compressed fuel/air mix to create the combustion in the engine. Each spark plug is fed by an HT lead from a coil pack. What many people don’t realise is there is no cable taking the electric away from the plug. The engine block actually becomes a part of the circuit, creating a route for the power to return to the battery.

If the engine earthing lead is degraded or poorly connected it can restrict the flow and therefore lessening the spark at the plugs. A lesser spark means lesser combustion when you need it. Lesser combustion means a lower performance and higher fuel usage per mile. Simply by upgrading the earth cable and increasing the connection you can unleash a greater potential from your plugs.


For years now, owners of Japanese cars have been upgrading their earth cables, quite often being the first mod they do. It releases extra power from the engine, increases torque and mpg.

People who are serious about car audio also upgrade the earth leads to negate the 
electrical noise, the more surface area the cable has, the more power it can carry.
They also upgrade the battery earth and alternator earth. This increases the flow 
to and from the battery allowing more power to reach the audio equipment.
A good power cable is essential but a good earth cable is even more important, without 
it you get electrical noise over the speakers from the alternator and the coil packs.

Smart Forfour

This lead set is currently only tested on the Brabus Forfour but may fit others.

This is what you will be fitting, the long lead is the battery earth and the short is the engine earth.

The engine earth can be found over on the left (circled green below).

Remove the 10mm nuts from each end.

You can now compare the old and new.
Apply some of the electrical contact paste to each end and fit the new lead.

The battery lead can be found on the right side of the engine bay (circled green).

Remove the 2x 10mm nuts from each end.

Again, you can now compare the old and new.

Apply some electrical paste to the terminal and tighten the terminal in place.

Apply electrical paste to each end of the new cable and bolt in place.

Recheck the tightness of all of the fixings and start the car to check.

Where Do I Get These?

If you want to buy a set of these simply click the banner below.

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