1985 K20 Suburban/Spark Plugs

[Terlingueno]

Now that I have the time to really take the deepest of deep dives into my '85 K20 Suburban that I purchased in '94, I have some questions, this one involves spark plugs. The OEM specs per the owners manual states R44T. The original engine was a "Code M" heavy duty emissions, which according to the owners manual required 89 octane leaded fuel. When leaded finally disappeared, I started using unleaded. Truck ran great, miles piled up, in 2007 at 325K, I had a GM 10067353 350 crate installed. Now we are approaching 450K miles. It is and has been my only vehicle since '94, and need it to last as long as I do.

The specs on the crate motor state 87 octane gas and R45TS plugs. Been using both since '07. Truck runs good, but different than the OEM engine.

From what I can find on the interwebs, the original engine had 8.3:1 compression. The advertised specs of the crate motor state 8.5:1.

I am wondering why the heat range difference between the old engine plugs and the crate's. Pretty sure I know why the extended tip plugs are called for. The Sub is heavy, appx 5700lbs. I think I can I use R44TS plugs? But do I need to? Maybe someone can splain this difference to me?

 

[Grit dog]

You put 125k miles on that engine and that heat range and you want to drop it by 1 heat range why?
Honestly it won’t really matter either way but sounds like a solution searching for a problem.
I’d be surprised if it called for leaded fuel in 1985. But even if it did, you proved it doesn’t need it if you got that many miles without top end rebuild due to no lead. I’m sure they had hardened valve seats by 85.
Keep driving it and don’t worry about spark plug heat range.

 

[Sad Sack]

Use what crate motor calls for and call it a win.

 

[Grit dog]

You’re not helping combustion efficiency to any measurable amount in an old low power engine imo and if it isn’t fouling plugs then you’re fine although using S plugs already.
There’s no deep dive here to take. If it starts and runs reliably from A to B that’s all you can expect. Imo

 

[Terlingueno]

Understood. I was unable to get leaded fuel after January of '96. Ran unleaded after that. When the original motor hit 325K in '07, the crate engine went in. The octane rating on the crate is 87, as opposed to the original which was 89 and I don't know why it dropped. Here is the 1985 owners manual that came with the Suburban and when I bought the truck, I followed what it said...

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[Grit dog]

^Thats great info, however pretty basic and well, at anywhere between almost 20 and 40 years and 125k to potentially 450k miles on certain components, if you feel it’s lacking in power there are things to look at besides spark plugs. Carb efficiency or age, ignition components age/use, timing advance and curves, exhaust (cat converter), vacuum signals…