This is a long post, so I apologize up front, but I wanted to throw in all the pertinent info up front.
I believe that manufacture plug recommendations are based off utilizing 100LL. 100LL has a tendency to leave lead deposits on the plugs, but with MOGAS (unleaded fuel), this is almost a non-factor. Plugs have different heat ratings. The higher compression engines tend to use RH(E)M38x series plugs and I use RH(E)M40x series (O-470-R). The 38 series plugs run 'cooler' than the 40 series. In other words, the 40 series retains heat longer, whereas the 38's dissipate the heat more quickly back to the cylinder housing. Why are the different plugs needed? Since the O-470 has a relatively low compression cylinder, the internal cylinder temps are ultimately lower which means lead may not fully scavenge off the plug, so the plug manufacturers recommend a plug that needs to remain hotter to burn the lead off the plug. Well, lead is no longer a factor when utilizing MOGAS.
A cooler burning cylinder is a happier, longer lasting cylinder. Plus, a cooler burning
cylinder is much less likely to detonate. I attached a link about a good article from Sacs regarding plugs.
http://www.sacskyranch.com/faqsparkplug.htm

Also, one last thing...87 octane MOGAS has a 'hotter' explosion (puts
our more BTU’s) than 100LL, and 87 has a lower flash point (it will
detonate in a hot cylinder quicker that 100LL...hence pre-detonation
occurs). So, the case for a 38, in this example, would be that it
will keep that cylinder temp lower and offset the higher therms of the
87 and prevent potential pre-detonation.
Am I just blowing smoke here? Just a thought...
BTW, I am going to put the 38’s in at my next oil change and I will write an update.

I see a slightly hotter temp when I burn my 1:3 100ll to 92 oct mogas mix than if I burn only 100 ll. Maybe only 50-75* on the JPI. I get alot less led on the plane tho.

Glidergeek wrote:I see a slightly hotter temp when I burn my 1:3 100ll to 92 oct mogas mix than if I burn only 100 ll. Maybe only 50-75* on the JPI. I get alot less led on the plane tho.

Where do you get legal mogas in Riverside?

Tim

qmdv wrote:

Glidergeek wrote:I see a slightly hotter temp when I burn my 1:3 100ll to 92 oct mogas mix than if I burn only 100 ll. Maybe only 50-75* on the JPI. I get alot less led on the plane tho.

Where do you get legal mogas in Riverside?

Tim

Not sure in Riverside. But maybe he's talking about back when he came up to Idaho and we dumped in no eth. mogas.