Reading AC Delco spark plug heat range. - Hot Rod Forum : Hotrodders Bulletin Board
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Old 02-02-2010, 03:57 PM
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Reading AC Delco spark plug heat range.

Which is a hotter plug? R45T or R43T?
I am getting mixed answers from the internet.

Looks like all spark plug manufacturers does not follow the same convention. Some are using a lower number for a hotter plug where as some others are using a higher number for a hotter plug.

I am going to to stick with AC Delco and I would like to understand how their heat range numbering system works.

Their site does not say much on this topic
http://www.acdelco.com/parts/sparkpl...tification.jsp

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Old 02-02-2010, 05:06 PM
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The higher the number, the hotter the plug on AC delco's.
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Old 02-02-2010, 05:58 PM
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Quote:
Originally Posted by sqzbox
The higher the number, the hotter the plug on AC delco's.
Thanks

Spark plug heat range has nothing to do with engine heat... correct?
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Old 02-02-2010, 06:02 PM
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If you mean that if you run a hotter plug the engine will run hotter? NO.
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Old 02-02-2010, 07:15 PM
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A hotter plug is for an engine that is lightly loaded or cruised. A colder plug is for heavy acceleration or hard use.
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Old 02-02-2010, 07:49 PM
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Are you just curious about plugs, or do you have plans to change heat range on a modified engine?
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Old 02-02-2010, 09:22 PM
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No, I am not just curious.

The motor in my car calls for MR43T. The previous owner put R45T. The motor runs ok, but has a lot of starting and idling issues and such.

I am in the process of bringing the motor back to GM specs.

I was trying to find a logical reason why the previous owner decided to go with hotter plugs.
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Old 02-03-2010, 11:33 AM
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That is very strange, and I would question WHY, would a car require a plug with that ID number. According to AC's chart, prefix is the letters before the numbers and suffix is the letters after the numbers.
That would make the plugs (if AC),

M=marine
R=resistor
4=14mm thread
3=heat range
T=tapered seat shell design

That would make your recommended plugs a colder marine type plug designed for a boat engine.
What year car and engine are we talking about anyway?
The previous owner probably just took what the parts counter dude gave him.
I could see him putting in R43T's but not MR43T's.
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Old 02-03-2010, 03:05 PM
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The only reason I can think of for hotter plugs is a high-mileage engine that uses oil and has problems with plug fouling.
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