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Engine guides and mods

451 Fortwo Earth Upgrade

Upgrade the 451 Fortwo earth leads

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.

Upgrading The Battery Earth

Pull back the carpet on the passenger side and remove the polystyrene block.
The brown cable from the negative terminal to the body is the one we will replace.
Using a 10mm socket, remove each end of the lead from the car.

Clean up both connection points ready for the new lead.

Fit the battery terminal facing towards the seat and tighten down with a 13mm socket.
Attach the spade connector to the stud at each and and tighten down securely.

The battery connection should be orientated upside down.

Type in your radio code and replace the polystyrene foot block. Job done.

Upgrading The Engine Earth

This is the replacement earth lead that I mad
e, it’s flexible rubber covered multistrand cable with a 25mm square surface area. The lugs are crimped on with a proper hexagonal die using an 8 ton hydraulic crimper. Both ends are sealed with adhesive lined heatshrink and all joints are coated in electrical contact paste or copper grease. 

The original earth is connected to the body just inside the engine bay.

The first connection is a E12 bolt on the right side of the engine.

The second is up under the right side of the engine bay.

This is held in place with a standard 10mm hexagon nut.

Remove the original earth lead, apply the copper grease to each terminal of the new lead and fit.

Engine Earth Conclusion

Although I was sceptical I really did feel a difference, the engine note was ever so 
slightly different and after two tanks of petrol have seen an increase of 15 miles per 
tank under my 'normal' driving style. I have supplied a few engine earths and kits to 
people and they have all agreed that there seems to be an improvement in power 
delivery and smoothness, some even say the engine sounds different.

How To Buy

If you wish to buy one of these leads, take a look at our shop.

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