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Engine guides and mods

450 Fortwo 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.

Upgrading The Engine Earth

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 braket.
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.
Loosly attach one end of the earth cable...

...and loosly 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.

Snapped/Stripped Body Earth Stud

The body stud is only an M6 thread and a 10mm nut and this stud is spot welded to the car.
It is important that you don't over-do it when you tighten the nut up. The maximum suggested
torque for an M6 bolt (non high tensile) is about 4Nm which isn't very much at all.

If you do snap or strip the thread from this stud, you will have to drill a hole in the body next to the original stud and
place a new M6 bolt through the hole. I recommend using a high tensile steel bolt, bare the paintwork back to the
metal on the back of the hole and use a washer on the new bolt. Use another washer on the front, then the earth
cable lug, then the new nut. Tighten to about 5Nm and spray paint the head of the bolt to protect the bare paint.

Snapped Engine Earth Bolt

Marcus G had an issue where the bolt holding the earth cable to the engine snapped as it was being removed.
He said, if this bolt feels tight and you are worried about it snapping (leaving your engine unearthed) then try removing the bolt just below it first. Use penetrating fluid and use a wind-out/wind-in motion when removing it.

This lower point is ok to use for the replacement earth lead, however, now you know that one comes out and
can be used for earthing, you can use a little bit more force trying to remove the top bolt. Even if you can't,
the worst case scenario is you either have both earth leads fitted or snip the old one off at the base.

Oscar S snapped the engine earth bolt so used one of the larger gearbox bolts. He had to drill the earth
cable lug open so the bolt would fit through and use a washer so the head didn't interfere with the lug. 

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.

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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