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Discussion Starter · #1 ·
Anyone do this? Is it even necessary on the 900, or does the plug always end up in the right orientation just by torqueing it correctly?
 

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uh if the spark plug is in the hole the car runs...why would you need to index them?they are after all about right in the middle of the cylender...
 

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GearHead said:
uh if the spark plug is in the hole the car runs...why would you need to index them?they are after all about right in the middle of the cylender...
The theory goes that the flame front from the plug isn't even, because of the electrode sticking in the way. So you make sure that the plug is orientated "correctly", with reference to the valves, to ensure correct burning of the charge.

What the theory seems to forget is that the ignition takes place when both inlet and exhaust valves have been closed for a while, and the charge has been compressed to about a tenth of it's volume - and that the burning charge then expands again back to that original volume before the exhaust opens.

Since the cylinder tends to be round, it strikes me as snake-oil. I'd rather the plugs weren't left loose or over-tightened.
 

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One of the worst feelings in the world is a spark plug thread stripping out of an aluminium head due to over-tightening. I did it on a BMW bike engine (faulty torque wrench) - don't repeat it by trying to clock your plug electrodes...
 

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A lot of people who race old porsche's that have MFI and such index their plugs all the time. I have no idea if this works or not, but these old dudes swear buy it. I run the bosch 4 electrode plugs on my moded porsche. The plug that is a fav. for that car (stock) is an ngk-go figure, but with the cams, recalibrated MFI pump and everything else I have, anything but the bosch 4's runs rich. I would not waste my time indexing plugs on my saab or my porsche.
 

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BTW everyone you index plugs using special copper washers of different thicknesses, not by "Leaving them loose" or "Over tightening them" and personally I think indexing is of little importance in a 16valve Saab,

and I think Bosch +4 plugs are a gimmick and this is coming from a guy who gets $2 from Bosch for every set I sell

2 Truths related to multiple tip plugs
1. Electricity only follows one path at a time

2. The more "Stuff" you have in a cylinder the greater the chances you have for pre-detonation, that's why boats, F1 cars, and Top Fuel dragsters use surface gap plugs that have no electrode protuding into the cylinder (and why other racers and tuners "dull" or smooth out their ground electrodes)
 
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