Is there a best fuel ???

Ok, below are some popular topics :
* 100 LL lubricates valve guides but also causes them stick. But, some report it is trouble free if you burn off the plugs at shutdown.
* Mogas burns clean and many users have thousands of trouble free hours operation.
* Some say MM oil keeps the rings loose and guides free, while others claim SNAKE OIL.
* A neighbor says he uses an additive from ACS that keeps the lead completely out of a borescope exam.

It appears there is no basis to decide what to. I am a recovering “Stuck exhaust valve” victim and would really appreciate some useful input to pursue ��

This is an endless "discussion" and I am happy to weigh in with my experience and the guidance I follow---- I am not an engine guy and count on the experts, even to assist with timing and compression at annual though many years of experimental ownership.

1. Revise your leaning practices, particularly on the ground. Likely to reduce stuck valve problems.
2. Consider running MMO in your fuel, at the amount recommended on the bottle. I cannot say why, but I was advised to do so by my A&Ps over time (at least saying it can't hurt and may help), and I have never had a stuck valve. (I run a lot of autogas with or without alcohol, but at 50% or less of tank contents. Usually to top off the tanks before a trip where I need 5-10 gallons total.) I skip MMO sometimes, but...

Leaning on the ground

Thanks Mike. I do what you suggest except I have not been using MMO. My engine builder suggested a rich mixture is indicated by the black plug body even though the insulators are clean.

Using 100% mogas I was not aware taxi leaning was required, although I have experimented with it. So I will be more diligent, and add MMO to the drill. As usual, if it works, I wont know for sure if the NEW valve and guide would never stick either. The bad valve had scoring on the stem like something harder than carbon was involved .

Larry DeCamp said:

Ok, below are some popular topics :
* 100 LL lubricates valve guides but also causes them stick. But, some report it is trouble free if you burn off the plugs at shutdown.

Click to expand...

Valve guides are fed oil through the pushrod and lead provides no additional lubrication benefit there.

Pre-70's, lead deposits would coat the valve face and valve seat and the lead coating prevented wear between those two surfaces, due to the constant pounding. Once the EPA banned lead, manufacturers started using hardened valves and seats and that became the acceptable substitute for lead in this application.

I assume Lyc is using hardened materials here, as they approve the use of unleaded fuel.

deposits in the guides is something of a mystery and you will find many conflicting reasons for it's presences. I will avoid sharing mine to keep your thread on track - PM me if you want it. I just did my wobble test at 750 hours and they are in spec, so it is not a universal issue.

Larry

Larry DeCamp said:

The bad valve had scoring on the stem like something harder than carbon was involved .

Click to expand...

Many reasons for scoring on a valve stem and you can't imply that it was caused by the build up. Most likely reason is bad geometry. Often a crankshaft will get a small groove worn into it at the main oil seal (rubber). This proves that softer materials can wear away harder materials in certain situations. plastic tie wraps eating into the 4130 engine mount tubes are another example.

Larry, are you asking just to generate some drama?

As far a cause and effect, on the inner workings of the engine, you are probably not ever going to be able to discern the effects due to fuel as they are mixed within the clutter of numerous effects from multiple causes already.

Historically there are some specific issues that have precipitated standard changes to the engines, but they are now - - well . . standard so can be discarded as "results".

The real operational effects of different fuels will come in the vapor pressure issues resulting from different fuel supply designs and operating techniques. This is not to be taken lightly, or casually, but can be addressed.

This is just an opinion, and limited to our fleet as it stands today. Also limited to mogas and 100LL, not some new fuel that can bring new issues.

Hi Larry and Larry,

LR172 Larry, do you know of any definitive test data, analysis or publication by Lycoming that says lead on the valve face prevents wear? I have been unable to find anything in all my searching. In fact, everything I can find seems to indicate the opposite, and that any buildup on the valves is not a good thing, be it from lead or coking oil. The reason is twofold, first is that the small amount of rotation (approximately 1 revolution per minute) that the valve performs is designed to keep it concentric and ensure that the wear pattern is even throughout it's life so it fits neatly into the valve seat to best dissipate heat. Any buildup may prevent the even rotation. The second is that the thermal conductivity of lead is 44% less than that of the alloy steel that the valve seat is made of, essentially providing an insulation layer if there is any such buildup. According to the documents below, the leading killer of valves is excessive heat, so the more effectively the valve can dissipate the heat into the valve seat and subsequently into the highly conductive aluminum head, the better.
https://www.aopa.org/training-and-safety/air-safety-institute/valve-safety
There is a good publication that discusses this (although it is with regard to helicopters) and stresses the importance of having a good surface area between the valve and the valve seat to enable the heat to be pulled away from the valve and prevent distortion. Page 16 of the following article has some great info on this:
https://austhia.com/PDfs/AHIA-piston-engine-durability-report.pdf
Going back to a 1977 publication from Lycoming on valves, they stress the importance of keeping heat away from the valves. They make no mention of the benefits of lead:
https://www.lycoming.com/sites/default/files/SB301B.pdf
In fact in all the documentation I have ever read published by Lycoming, I have never found anything that has stated that lead has any benefit other than providing detonation resistance and the benefits of the low vapor pressure in fuel systems not designed to handle the higher RVP of mogas. Everything else I have found from Lycoming regarding lead is generally a mitigation measure trying to deal with the secondary problems is causes, such as lead bromide corrosion related problems, plug fouling, hub fouling, high oil change interval requirements, etc etc.
I'm not alone in this quest to find concrete data saying lead is necessary for anything other than detonation resistance in NA engines with CR greater than 8.5:1 and for fuel systems inadequately designed to handle the higher RVP of mogas, take it from John Deakin:
https://www.avweb.com/features/pelicans-perch-55lead-in-the-hogwash/
If you are able to find anything worthy of review, but all means send it in my direction.

Regards,

Tom Mills
RV-7, IO-360 M1B, 8.5:1 CR, 2xPMAGS, than only drinks mogas.

tgmillso said:

. . . . take it from John Deakin:
https://www.avweb.com/features/pelicans-perch-55lead-in-the-hogwash/

Regards,

Tom Mills
RV-7, IO-360 M1B, 8.5:1 CR, 2xPMAGS, than only drinks mogas.

Click to expand...

Nicely done TOM!

This comment from the article above is interesting . . . and 20 yrs old.

"The old knowledge and “corporate memory” died, the marketing folks took over at Lycoming and Continental, and today we see the results. There may be a few real engineers left, but I don’t see much evidence of that at the factories."

There is a general rule in the industry, any industry - never let the engineers talk to the customer - - that is marketing and sales job.