It's really doubtful that the spark plugs affected the idle unless the old plugs were fouled.

There will be lots of opinions on spark plug brand, and basically no opinion is wrong there - though we all have our preferences.

What you do need to get right is the thread length and sealing style correct. That's determined by what heads you are using. Then you need to select the correct heat range.

 

I was unfamiliar with either of the plugs you mentioned so I looked them up.

Your NGK R5671A number is incomplete. There is also a dash number (like -7, -8, -9) that follows the A. The dash number is the heat range.

Champion 792 crosses over to NGK 5672A-9. (Not 5671A.) -9 is a rather cold plug in NGK. I never even ran that cold a plug in a strictly drag race engine, we ran -8. I think a -7 or -6 heat range in NGK would be much more appropriate for a performance street engine.

Both plugs are 14mm x 1.25 thread, 3/4" reach, with a gasket seal and a 5/8" hex.

 

Thanks Rick, this is good information.

I edited my original post to reflect the correct heat range (-7).

Offhand do we know the difference between the NGK 5672A and 5671A?

 

I did find that the Champion 792 and the NGK 5671A are non-resistor plugs. Don't know about 5672A or what the real difference is.

You can run a resistor plug with an MSD box. A resistor plug would be better for radio/stereo and also for EFI. But that's a side issue.

Also saw on the Summit listing for the Champion 792 plug is that's what Blueprint Engines recommends for whatever that's worth.

Were the plugs you replaced -7 or -9?

 

Rick,

The plugs I replaced were -7.

 

Made me check

My head manufacturer (Brodix} recommends Champion or Autolite ) for street use but the NGK 5671A-8 for race gas. Since I don't trust Champion after blowing the porcelain out of two plugs in one set in the 70's . Race team I worked on always used Autolite in Alcohol engines. Autolites I use are 3924. They are a copper electrode pug available at Wal Mart.

 

I'am new tonight, thanks for having me. I do seem to have the best results with NGK, R5671A-7 in almost all, all other plugs i'am sure they work just as well, i do notice when i "index" them they make a difference, idle and all. The -6 plug on crate engines also works well.

 

***FOLLOW-UP***

No vacuum leaks found.

I was able to resolve the idle issue by adjusting the idle mixture. I am at a steady 13-14 inches at 1000 RPM (which based on my research is normal for an engine of my specifications).

I am still curious about Blueprint's spark plug recommendation. As Rick mentioned earlier, based on the 10:1 compression and non-competition usage, most authorities recommend a heat range or two hotter than the Champion 792. I have sent them a query about it, looking forward to what they have to say. I'll post their response.

 

Here is the response I received today from Blueprint Engines:

We recommend that plug for use because according to Champion, given our specs for the engine they advised this heat range. I do think if mainly what you do is street driving and rarely get above 4,000 rpms then it can be a bit cold for the street use. The Autolite 3924 seems to be a good street plug and is approved for use in our engines if street driving only.

Rick: This seems to confirm your analysis about heat range.

Now I need to look up the specs on the Autolite 3924, and how it compares to the Champion 792.