Spark PLug Heat Range Question

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260
Location
Newtown, PA
I'm wondering if there is any reason, be it emissions, fuel economy, or whatever, that an auto manufacturer would recommend a hotter plug than is ideal for performance. Here's my case, I have a Saab 9-5 aero and on the recommendation of other saab owners I tried a plug that is one heat range colder than the stock plug. The car seems to run a lot better now. It doesn't seem to go any faster, just a lot smoother than before, even more refined. So what do you guys think, is there a reason they wouldn't recommend the colder plug right off?
 
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23,591
Well, I drive my Audi pretty hard (by my standards) and when I switched to colder spark plugs I too noticed smoother idle and slightly increased performance. Yet when I checked the spark plugs I decided I was better off with the hotter plugs, which remained cleaner. I didn't want to risk excessive carbon buildup and the hotter plugs showed no signs of overheating anyway. If I were to drive the car only at high speed, I'd go with the colder spark plugs. Alas, I do probably half my driving in the city at this time, so the factory-recommended heat range works out best.
 
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2,513
Location
Richmond, VA
I use cold plugs in my Dodge Ram 1500, 3.9 V-6 with the 5 speed manual because of light pinging with the factory recommended plugs. If I used 89 octane the pinging would stop with the factory plugs. I switched over to Autolite 3923 plugs but for this reason only. If the car runs OK with the stock plugs then I would use them.
 
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1,187
Location
Southern Vermont
Plug fouling is much less of a problem today than it was in the days of carburetors and heavily leaded gas. How often are you guys changing plugs these days? I am going 100K or more with platinums and iridiums. I hardly ever think about plugs any more.
 
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1,680
Location
CT
everyone seems to think that the info in the owners manual or service manual is gospel and 100% true and big trouble will happen if you don't adhere to it. When in reality the person who edited it probably has no technical knowledge, the person who wrote the info got their numbers from someone who is passing on the numbers who probably doesn't care what gets printed. Kinda like if you call technical assistance and their answers are just read out of the manual you already have, and when you ask the million dollar question "why?" they pass the buck. Or the recommended plug won't last as long, make the car run crappier sooner, which they hope causes you to bring it in for service so they can take your money.
 
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1,357
Location
California, USA
I'm changing plugs at 30,000 mile intervals on the BMW, dual ground electrode copper. They come out the proper color, so heat range is perfect, but gap growth is significant. Platinums are standard on most new cars, but not ones before the late 1990's.
 
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2,187
Location
Arizona
Raven; Don't know how much swapping/experimenting you've done, but most of the 9-3 and 9-5 owners I talked to (who cared about such things), including me with my Abbott computer, found the car ran subjectively better with copper plugs than platinum. When you went to the colder plug, did you make the switch to copper at the same time? Perhaps a switch back to the stock heat range with coppers will let you keep the smoothness you've found. Just an idea... I agree with and recommend your 10K change interval.
 

Raven18940

Thread starter
Messages
260
Location
Newtown, PA
Yeah I did switch to coppers from platinum (the aero can only take platinums my butt). I'll pull them out next oil change (5K) and see how they look. I drive the car pretty hard so I'm inclined to lean toward the colder plug. Plus I ran 2 tenths of a second better at the drag strip last night. 15.1 @ 97 mph [Big Grin] Though that might have had something to do with the weather being about 15 degrees cooler. :?
 
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23,591
quote:
I think it's odd that the idle wold be better with colder plugs.
I agree, but colder plugs improved my idle, too. However, I went from a tri-electrode NGK plug to a BERU four-elctrode plug, so it's not like comparing plugs of the same brand with different heat ranges. The NGK was the hotter plug, though. After all the trying out I've done, I've come to the conclusion that the all-over best choice for my engine are the OEM NGKs. They appear to be the most robust and work well under all conditions in my engine. Can't really ask for more.
 

Raven18940

Thread starter
Messages
260
Location
Newtown, PA
Hmmmmmm, so the risk is more fouling. Alrighty then I'll stick with the colder plugs. Reasons 1. I like to track my car once in a while so the extra performance/resistance to detonation is nice. 2. I switched to copper plugs from platinum because spark plugs in a Saab are best changed and gapped often. I'll probably be changing out every 10K.
 
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36,471
Location
ME
Some people have surmised that between the factory and final consumer the cars are started 50 times in 10 miles what with shuttling on and off the boats, car carriers, and dealership car-wash. Somewhat warm plugs tolerate this better. The average consumer who actually gets the car up to temp on a long commute could tolerate a cooler plug. Just a hypothesis I'm repeating.
 
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2,387
Location
Chicago area
There is a very wide temp range that a plug will fire correctly in, without fouling or overheating. About 400 deg F. Most plugs now are an extended tip design, which helps cool a hotter plug significantly.
 

Raven18940

Thread starter
Messages
260
Location
Newtown, PA
Do you think using a hotter plug has something to do with the super long intervals most plugs are running now a days. You know, to keep them from fouling.
 
Messages
1,001
Location
Baltimore
quote:
Raven18940: Do you think using a hotter plug has something to do with the super long intervals most plugs are running now a days. You know, to keep them from fouling.
The longer plug lives come from the fact that most current plugs are platinum from the factory and the fact that the current crop of electronic ignitions will fire plugs that the old ignition systems could never dream of firing. They provide extremely sharp rise times, high peak voltages, and fire under extremely adverse conditions with little maintainence. .
 
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