Spark Plugs

[Alex Goodhart]

Resistor plug

[Martin Benade]

If you can get the heat range you want a resistor will have no noticeable effect on how it runs.

[Alex Goodhart]

How to determine i

[C J Murray]

I'm not sure why you are not asking JW these question but I will take a shot at it.

In NGK terms you should use a 6 or 7 heat range for most 616 engines. Visual inspection is not as useful as it used to be because modern fuel doesn't color the plug as much and correct plugs can look like they are running hot or lean.

If you have a very hot ignition system then iridium plugs are fine but generally an old school copper plug works best with the only short coming being that it wears quicker. Fancy plugs in a stock 6 volt 356 can cause problems.

Do not use a plug that projects further into the chamber unless the engine builder assures you that it will not hit the piston. Sometimes there are also slight differences in thread length in various plug types that have the same thread diameter and pitch. Your piston, not shown, has a lump on it and how close it gets to the plugs I don't know.

Most ignition components don't care if you use resistor or non-resistor plugs.

[C J Murray]

spark plugs...

[Alex Goodhart]

is the solution

[Ron LaDow]

You can easily index the plugs by marking the ceramic for the electrode location, screwing it into a cylinder with the piston off TDC and checking the clocking.
Not good? Pick and mark another plug; they are not consistent.

[Alex Goodhart]

I have read different reports

[Ron LaDow]

one
I tried to delete an earlier post, and found no way to do so. As a result, I plugged the "one" into the field and was able to delete the remainder.
I'm sure there are those who find the steam-powered Registry web-site acceptable.
'Nuff said...

[Ron LaDow]

I did not know different plugs would point differently due to product variation.

Back when the Lusso got "indexed" plugs, they certainly did. You can prove it now by selecting random plugs and screwing them in to see if it is still true.
But in hot-rod buff-books years back, there were claims of power increases from clocking the plugs a certain way with reference to the intake valve or some such. No one ever offered dyno sheets to prove it. I do not deny that at the upper ends of IC engine work (NASCAR, F-1, Pro Drag Racing and the like) someone may have found some percentage which makes the effort worthwhile.
Allowing that, here is the only power gain I *know* from indexing plugs:
I bought the Lusso way back when you could buy leaded gas from the corner station, and that stuff fouled plugs on a regular basis. So the NGK "P" series plugs (or the equivalent from your fave supplier) was a good idea to keep the tip out there where it could be blown clear by the chamber turbulence, and that was what was fitted when I bought it,
But Ferrari didn't allow clearance for the ground electrode clocked at "6 o'clock" and it (at best) got 'nudged' closed, so there was no spark in that hole. Could'a been worse, but I caught it the day I brought it home, and thereafter "indexed" the plugs with the ground electrode at "12 o'clock".
A 12-cylinder 250 Ferrari makes much more power than a 9-cylinder Ferrari. Hence INDEXED PLUGS MAKE MORE POWER! Kinda like adjusting your throttle linkage for full throttle,

[C J Murray]

The reason that I cut apart the plugs pictured is to get a reading on the fuel mixture. It is quite hard to see what is happening at the deepest point of the ceramic and that is where you read mixture. Those plugs are out of my 2133cc and show very slight richness. If the dark ring was wider it would mean the mixture was richer yet. No dark ring means running very lean. The optimum is a narrow dark ring but a little more fuel doesn't hurt power and adds some safety margin.

Ignition timing is read by looking at the ground electrode to see where the line is between the light portion and the dark portion that starts at the base of the ground electrode. The line is supposed to be half way around the 90* bend.

The pictures that Alex posted show the added, lower, spark plug holes enter at quite an angle. I assume that the angle is needed to have access to change the plug. The angle appears to leave a sharp end to the edge of the hole in the chamber. If so I would think that the sharp edge would be susceptible to hot spots that could cause irregular detonation. I would think that area should be massaged.