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Writeup on ground wires for E34
You may choose different grounding points in the car. I happen to use the ones listed here because they were the ones that I found. The main principle is to use 5 or 6 lengths of multi-stranded, high grade copper in a heat resistant shielding with quality connectors. You want connections in a daisy chain fashion.This means all the wires are ultimately connected physically to each other. Here are the ideal locations. You may wish to experiment once you obtain a baseline.
WHERE TO GROUND
1. Right front fender
2. Left front fender
3. Intake manifold and\or throttle body
4. Engine block
5. Transmission housing (I could not find a transmission grounding point that was easily accessible on my car so I ground to the engine block twice).
Undo the two plastic screws holding the black cover located next to the ECU box by the right shock tower. This exposes a snake of brown wires from various devices all grounded to a chassis post as a grounding point.
In my setup, one wire goes from the grounding post to the right front bolt which holds the air filter box to the fender. From there another wire from the air filter box goes to the engine block.
Another wire originates from the grounding post on the chassis to one of two conveniently located threaded holes on the Alpinized intake manifold. From there, another wires to the the second location on the engine block and another goes to the right front fender whose location is the mounting point for the diagnostic port (it has a plastic bolt). Finding avalaible locations was not easy on my Bi-Turbo.
Now zip tie everything for safety.
START THE CAR
Turn on the ignition. You should notice an improvement in the immediacy of the engine catching life. Upon catching ignition, your idle should now be stable and consistent instead of a lumpy idle and the ECU "hunting" for a stability in the idle. Some people notice brighter headlights (I can say I have).
RESULTS YOU CAN EXPECT
Next, upon easing on the throttle, instead of a jerky somewhat hesitant acceleration, it should now accelerate smoother and have a more silky feel to the power delivery. Down shifts which are accompanied with a blipping of the throttle will respond quite correctly with a smoothness not experienced prior to the ground wires. In fact, the engine will seem more eager to rev freer and higher. Wide open throttle will have different effects for different cars. Some will just run smoother all the way and while others which used to hesitate or cough under WOT will now run with no hesitation. Crisper, smoother, more willing. If you refrain from accelerating harder under normal driving, you should actually expect slightly better fuel economy from the car due to a better spark and cleaner combustion.
Some tuners have dynoed the before and after results of ground wires. Just google it. They come JDM spec. for such cars as the Mitsubishi, Subaru, 350Z and a few others. Some people are skeptical that the product works because they don't understand why it should work. I haven't read an opinion where someone has correctly installed a quality grounding kit who has said it hasn't made a positive difference in their car.
WHY DOES IT WORK?
I am certainly not a licensed technician but offer my thoughts as a layman. Many cars use a generic chassis ground which ultimately connects back to the negative terminal of the battery to complete the circuit. However, there are many things to degrade the quality of chassis grounding such as rubber parts, layers of paint, etc. By using dedicated grounding which connects to the cable running to the negative terminal, you are in effect building a dedicated , direct highway from one part of the city to another part of city which bypasses all the city traffic which is congested by side streets and traffic lights.
This reduces the systemic losses inherent in traditional chassis grounding and improves the electrical efficiency of the vehicles components that depends on an electrical circuit to function.