I`m currently have the fr5ls ac delcos.From what I can tell from looking at my plugs I need to go to a colder plug.In searching I`m not for sure finding the part number for the colder plugs.Is it the fr3ls for the 86 and 87 alluminum heads?I`ve heard if you run a msd ignition box to run a colder plug as well.Should I maybe run 2 steps colder.Is the mr43lts a step colder?Or does it just step down the number like fr4ls and fr3ls for colder plugs.

 

I'm not sure of the AC Delco heat range numbers. However, running Autolites the 3926 is the stock plug for the Corvette with the '113 heads. Can go colder to a 3924, and colder still to the 3922. Both of which are typically in stock as other popular vehicles use them.

RBob.

 

I`d like to stay with the ac`s.Anyone else?

 

What the heck part numbers are those? Is that like a platinum plug for something?
I'm used to delco's being
R42T

Starts with R, then 4, then the heat range (in this example, 2), then the addons. T, S, etc etc....

Are you reading your plugs to tell you need a colder plug? ie. the heat line on the ground strap is getting close to the threads?

 

AC changed the part number on the FR3LS. It now has a 41-**** type number. I was using the FR3LS and swapped to a Champion RC12YC. I like this plug, plus you can read the ground strap easier.

 

Fr5ls is the number on the ac delco plug I have been running in my zz4 heads.Its the part number 88 and up alluminum heads in my haynes manual.The fr3ls is listd for 86 and 87 alluminum heads.Both of these in the book show an ac before those numbers.I`m guessing thats the long part number.Yes.The heat line is way down on the ground strap.I`m wondering if this is causing some knock counts/retard.Or it may be too much timing.I`m also rich.So I can change the heat range quicker than sending my chip back to be reburned....again.Which Iwill do after I get a little more info from a colder plug and more dattalogging.
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Is the fr3ls a colder plug than the fr5ls?Or is that champion a colder than stock plug?

 

The AC Delco heat ranges are all the same, regardless of the letters. So FR3LS is COLDER than the FR5LS by 2 steps. Its not any different than switching from a R45TS to R43TS, if you use those types of plugs. Most of the ranges GM uses are between 2 and 5, although there are plugs available outside that range. I bolded in the heat range numbers for you.

Autolite does the same, decreasing the number (last one in their case) is colder. Same for Champion. NGK's are backwards, the higher numbers are colder. They also dont translate from one brand to another so a 5 heat range in AC Delco isnt the same as a 5 heat range in Champion.

 

Yeah, the FR3LS is colder than the FR5LS. The RC12YC is champions version of the FR3LS. So the Champion would be colder than the one you have now.

 

thanks to you all.Thats what I needed to know.