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Engine guides and mods

Roadster Earth Upgrade

Upgrade the Roadster 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 Engine Earth - Fortwo and Roadster

This is the replacement earth lead that I made, 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. 

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

You will almost certainly find an old, tired, tarnished looking earth cable like this.

Using a ratchet and a 10mm socket, remove the nut and pull the earth cable off.

To undo the other end you will either need a flexidrive or a universal joint.

The other end of the earth strap is connected to the engine via the exhaust bracket.
You will need an E11 or E12 socket. Remove the bolt and remove the earth cable.

Here are the two cables together.

Dab each connection with a bit of copper grease, this will help maintain a better joint, 
will fight against corrosion and will seal the two parts together.
Loosely attach one end of the earth cable...

...and loosely attach the other end.
Routing of the cable is up to you but I found going under the TIK and 
over the crossmember worked well as it kept it clear of the exhaust.

When you are happy with the routing of the cable, 
tighten up both ends and go for a test drive.

Upgrading The Battery Earth

With the engine earth improved, the new bottleneck will now be the battery earth which is the same diameter as the original engine earth. Ideally the engine earth needs to be the thickest cable possible to reduce resistance so we will be replacing 19mm squared cable with 35mm squared cable. (That is suface area, it's not actually square).

This is what you get, a new battery terminal and a short, thick earth cable.
Like the engine earth, this cable is hex crimped and sealed with adhesive lined heat shrink.

Each end is slightly different, the terminal with the larger hole fits on the new battery connector stud. This terminal must be fitted upside down to angle correctly and so it doesn't touch the boot liner when it is replaced. Again use some electrical contact paste.

Remove the front boot liner, this information can be found here.

After removing the old battery lead, apply electrical contact paste and attach the new battery terminal to the battery. Ensure it is straight before you tighten it up fully. Apply more electrical contact paste to the connections both ends before routing the cable as seen below. Tighten down both ends so it secures the earth lead.

You may notice that the Roadster battery earth lead was designed to stay clear of pipework. Replace the boot liner and type in your stereo code (if applicable).

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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