Spark Plugs & Heat Ranges

[Tidybuoy]

I did not follow my own rule and I changed/fixed multiple items together - and now and now I don't know what caused the improvement.

I changed from Bosch WR4CC to WR-5-DC plugs. The previous plugs were new and only about a week installed but I ordered the wrong plugs. I also changed the fuel filter and now the car is running 100% better and the bucking at 2500k-3000k has almost gone away.

I have future CIS tuning work to do.

My question is: what is the benefit of switching to a hotter plug and should I have an even hotter plug. My car is a '74 that was built to '76 911s specs with '76 CIS components (not sure if that makes any difference).

Any advice on plugs would be great.

[Joe Bob]

Well, the way I've done it is.....WOT while driving and then shut it down. Do a plug read. A nice burn is obvious.

White, lean....black, rich.

[Flat Six]

Originally Posted by Joe Bob

White, lean....black, rich.

Joe Bob's concise, but if you want a little more detail and context on reading plugs:

http://www.boschautoparts.com/Technical%20Resources/Spark%20Plugs/SparkPlugFaces.pdf

[T77911S]

WR4 was too cold.

too cold a plug can foul when starting the engine or just from idling.
a spark plug operates best at certain temperatures, that is, the temp of the plug itself. if below optimum temp, the plug can not "clean" itself. too hot, and the plug can melt, get too hot and possibly cause preignition.
preignition and detonation are 2 different things. preignition happens before the plug fires and is much more damaging to an engine. detonation happens after the plug has fired.

if you could always drive your car on the hiway, never stopping to idle, you could put in a WR3 or 4 and never have anyproblems. its the rich idling that requires the need for the hotter plug.

i ran a Wr4 in my car as a test. around town is was black as midnight. but, on a 600 mile trip, i stopped and pulled some plugs, they were a nice clean white/grey color.

from the research i have done on plug reading, (mostly drag racing sites), there is a lot more involved than just looking at a plug with the naked eye. most say you need to be able to see down inside the plug to see the mixture. some say the mixture can be read from the ring around the electrode.
timing can also be read from the anealing of the grounding strap.

with todays tools, i dont think plug reading is the art it use to be. with dynos and analyzers, mixture and timing can be set for optimum power, then its just a matter of a cold enough plug that will not foul.

always start with a cold plug and work up to a hotter plug, but set timing then mixture first. a rich mixture can make the proper plug look too cold, and a hot plug can make a rich mixture look correct.

[Bob Kontak]

Tidybouy - It will help folks searching on similar problems if you close out (or stay within) your other thread leading up to these repairs with what helped.

Adjusting Mixture and What's the Stopper on CIS

Glad you got it running better.