Gentlemen.
Need thoughts on gap of new plugs (R45TS were removed) in a 350, 4 bbl, mild cam.
thinking I’d just mimic the existing set gap, but there are views that hotter might be more in line with a moderate cam, or no?
thanks in advance.

DW

 

In general:
Hotter plugs are best in low-compression, low-performance engines to burn off combustion deposits.
Cooler plugs are best in high-performance, high-compression engines.

 

Hotter is heat range of the plug. Not gap.
gap has more to do with the distributor/coil your running than anything else.
Heat range is affected by a number of variables. Your mild cam would be very low on that list.
Run the recommended plugs for your engine if it's all pretty close to standard.

 

Agreed: gap and heat are two independent things.

dkwrnr, the "heat" rating of the plug has nothing to do with how "hot" the spark is – that'll be the same no matter the plug's heat rating or gap. The correct gap is the widest space which allows the coil's energy to jump across the electrodes reliably so as to expose as much spark opportunity to the vaporized fuel as possible. You want a wide gap to readily ignite the fuel, but not so wide as to misfire because the spark didn't make the jump. Points & coil don't have the energy that HEI has, so are usually 0.035" while HEI can easily operate at 0.045". Experimenting with the gap is something that most of us don't do, but it's possible to maximize operation of your components.

The heat rating of the plug is how well the white porcelain around the center electrode holds heat to burn off combustion deposits as well as transfers the combustion heat to the head. Too "cold" and the plug can foul from build-up; too "hot" and the plug will experience shorter electrode life and could become a source for pre-ignition.

Most recommendations for today's unleaded, often ethanol-juiced, fuels are the AC Delco 5 range (often mistakenly cited as 45) and NGK 4 or 5. [The NGK rating is opposite AC-D with 4 being hotter than 5.] This heat range is good for most everyday common driving while sustained high speed driving could tolerate a colder plug. A properly chosen plug will have a slight tan/gray appearance; not white and not black.

 

I have found that you can call some companies and give them your engine specs and usage and they can help. I called NGK technical support. They were helpful.

 

Thank you all.
I found that factory plugs were R43TS, but prev owner installed R45TS, so I’ll keep that the same and gap .045 for the HEI.
Great forum here guys, I’ll post progress….

DW

 

Very few small blocks used a R43TS, or any 43 heat range plug, from the factory. Exactly what engine is in your 1980?

 

I kinda thought the same thing – that by 1980 they had spec'd hotter plugs, so... Out of curiosity I looked up the '80 Corvette specs from the GM Heritage site to see what plug(s) were called for:

 

The stock gap of .045 should be fine w/ HEI ignition.

Some, namely SWCDuke, have posted in favor of R45TS plugs for typical around town and highway driving to avoid potential fouling. I run R45TS but w/ a narrow gap (I don't have HEI).

 

Never would have thought that after so many years of using 45 heat range they would have gone back to 43, thanks.

 

Cant remeber where now, but I found a document from Chevy, I think, that specified the R43, but yet in the GM archive, and installed in the engine, were the R45’s…
Engine is a factory 350, mild cam, edelbrock manifold and Holly 4brl, and headers.

 

Would changing from iron to aluminum heads make any difference on plug selection? Currently swapping from stock '77 L48 heads and cam to Trickflow DH175 and Comp Cams 218/218.