A good friend had a Cessna 310Q with IO-470 Connies. The 310 has 4 tanks - the tips, which are mains and hold 50 gallons each, and the inboard aux tanks at 31.5 each. He put premium unleaded pump gas in the aux tanks, and 100LL in the mains. Takeoff and climb to 7500' was on the mains, then 10 minutes of cruise to establish EGT, CHT and speed. Then he switched 1 engine at a time to the aux tank with pump gas. 5 minutes later there was a 6-7 knots increase in IAS and no appreciable changes in EGT and CHT. This is ONE data point, so take it for what it is worth (though I was present to witness the experiment). I strongly suspect that a very small timing change will allow pump gas with no loss of HP for normally aspirated 4 and 6 cylinder engines. I'm not qualified to speculate on what else might need to be controlled in a safe pump gas alternative, but I have no doubt it can be done. Our turbocharger friends may have a much harder time solving the problem. YMMV
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Unleaded Avgas - Thoughts?
- Thread starter bertschb
- Start date
Larry DeCamp
Well Known Member
Please define the procedure for setting timing for 93e10 with SDS ignition. If there are others like me, they just know to watch CHT. Watch it do what ? Adjust it to do what ?
Also for me, with no (above 10K) altitude activity, I plan to use a 100ll map and a 93e10 map with the switch so life is simple.
You have now given me a new goal - to use this phrase in my work environment.
Jjackh10
Well Known Member
Yea, gotta work that into the next steering team meeting. Extra points if I can claim it in relation to comments from the plant manager.You have now given me a new goal - to use this phrase in my work environment.
He'll just be impressed uneducated turnaround trash said it. Thanks guys! The one aspect that George and GAMI did which at least to me is under appreciated; and they do not really get any credit for it.
The original PAFI effort was trying to match the ATSM specification exactly for 100LL without TEL.
The key change that George made was to recognize that the ATSM specification for 100LL was created after the fuel and matches the chemical properties of the fuel. The actual requirement is a drop in replacement that matches the effective performance of the 100LL with requiring changes to the engine/fuel system.
EAGLE effectively adopted this change. Which makes EAGLE at least possible.
Tim
The original PAFI effort was trying to match the ATSM specification exactly for 100LL without TEL.
The key change that George made was to recognize that the ATSM specification for 100LL was created after the fuel and matches the chemical properties of the fuel. The actual requirement is a drop in replacement that matches the effective performance of the 100LL with requiring changes to the engine/fuel system.
EAGLE effectively adopted this change. Which makes EAGLE at least possible.
Tim
You have now given me a new goal - to use this phrase in my work environment.
Was able to suppress my white trash DNA long enough to almost sound eloquent, Who'd a thunk it?Yea, gotta work that into the next steering team meeting. Extra points if I can claim it in relation to comments from the plant manager.
sansoneservices
Well Known Member
3- timing.. not an issue unless the octane changes
5- fuel mixing.. still don't think that's a problem
1,2,4- paint and sealant.. these issues scare me
5- fuel mixing.. still don't think that's a problem
1,2,4- paint and sealant.. these issues scare me
There are instrumented test cells where all of this can be solved. Didn’t GAMI build an extensive test facility out of necessity?
There’s plenty of out of service RV fuel tanks with sealant and paint in the corner of hangars and insurance junk yards. Just need to put a few gallons of the test subject fuel in these and spill a bunch on the paint. Give them time and continually check the integrity of the sealant and paint finish. They’ve been at this fuel argument for years with what seems like political roadblocks preventing any progress.
The politics of successfully mass producing a new fuel for GA as a whole is going to drive Rotax sales to increase and some in the experimental crowd to become quite impatient.
You might be onto something with your last paragraph. Rotax, UL Power and I'm sure others are designing engines to run on auto gas. Engine manufacturers who can run auto gas will take more and more market share. Thus reduce the demand for 100LL. Reduced demand for 100LL will drive up price. More people will buy engines that burn auto gas, and it will become a positive feedback loop. 100LL and the engine manufacturers that refuse to adapt will become extinct.
It's not only about the lead either. 100LL is a lot more expensive than auto gas. Economics is also encouraging the change.
They do in fact list the IO-360-M series as compatible with "MOGAS", defined as automotive fuel by Lycoming. See table 3, and the requirements of the fuel in table 2. The problem is ensuring that the order for the fuel you bought meets the specs outlined in the document below.
Toobuilder
Well Known Member
Not in California. Sometimes 100LL is LESS expensive than the corner gas station.
…and yes, in that case, economics is encouraging change
Clostermann
Member
You can follow progress on the testing of UL100E on FlyEagle.org. Testing was just completed on an IO-390-C3B6 with an 8.9:1 CR. Materials testing to date on ~100 materials including sealants, paints, elastomers, bladders and composites show that the fuel behaves like 100LL. Results also listed on the web site.Why did I start the thread?
As stated in my OP, to try to get some discussion started on this topic so we can all learn more.
My motivation for the use of non-leaded fuel?
1- 100LL is going away in four years so I'd like to learn more about the three UL fuels under consideration and how they may affect my engine/airplane.
2- I would like a cleaner engine and unleaded fuel should help in that regard.
I couldn't care less how much aviation fuel costs. Rounding error for me in the grand scheme of things.
<<Wanna abide to the manufacturer's recommendations? Wanna save a couple of $ flying 20 hours/year? Don't care about the environment? Never question anything you're being told? Assembled an "experimental" category craft but don't want to experiment? Afraid of loosing that precious and expensive new engine warranty?
Any doubt, then stay in the safe realms of history, and burn lead.>>
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Clostermann
Member
3 degrees retard is needed on the IO-550 series to maintain adequate detonation margin with no impact on take-off power. ETBE in the fuel makes up the power that would normally be lost by the change in ignition timing. Measured EGT increase was only 2%.
I didn't realize they made a version of the 390 with an 8.0:1 compression ratio. I actually asked Lycoming a couple weeks ago if they could do 8.0:1 on my 390 engine that I have on order and they said no. The only options available are 7.3, 8.9 (stock) and 10.1.
For us in the "experimental" world, this is a non-issue, but what about the certified aircraft (which I guess is where 99% of the IO-550s live?) Will they be able to just ask their A&P to change the timing? Will they then see lower power when going back to 100LL?
Clearly the future is a more intelligent fuel delivery and ignition systems like we have in even the least expensive automobile, but the path to that is not clear.
Clostermann
Member
Typo corrected. It's 8.9:1
I’m glad others are pushing towards a true 100LL replacement. While I appreciate GAMI trying to get ahead with G100UL, it’s clearly not compatible with rubber hoses, fuel bladders, o-rings or epoxy based paints, all of which are found on basically every aircraft ever made. I won’t be putting it in my airplane. Swift 100R at least acts like 100LL in the presence of these materials…
Did we ever get any conclusive analysis as to why the Lycoming IO-360 valves were having issues with UL94? Presumably Mogas and these 100LL replacement fuels are going to behave similarly with regard to valve seat protection.
Thanks! I appreciate the support.
Lycoming made this statement, and Swift has commented a couple times. One was in a recent video with Scott Perdue. UND was running the engines at peak EGT. I linked the video to start at the segment.
Michael
By definition, "octane" is the measure of "resistance to detonation". So, why would one type of fuel with a 100-octane rating be any different than another type of fuel with that same 100-octane rating? Valve seat recession is the big issue currently, which Swift 100R claims to have solved.
Goal for FAA and ASTM testing.
- Drop-in Capability: Swift 100R is aimed at being a direct replacement for 100LL without requiring engine modifications or timing changes.
- Ongoing Testing: FAA is analyzing performance and detonation margins for high-compression engines like the IO-550 to confirm that no adjustments are needed.
- Timing Philosophy: While some lower-octane unleaded fuels (like UL94) might require timing adjustments to prevent detonation, 100-octane unleaded replacements (100R and G100UL) are designed to avoid any changes.
Clostermann
Member
Many larger engines and aircraft were certificated using 100/130 and FBO 100LL gasoline with MON (motor octane number by ASTM D2700) of 103-106. No unleaded fuel meets this octane level. G100UL is specified at 99.1 MON and both the Swift and Lyondell/VP Racing fuels are specified at 99.6 minimum MON (100.0 aviation lean rating). The UL100E fuel is typically produced to 100.2 MON so it is a 100-octane fuel. The ASTM D910 spec for 100LL also calls for a minimum of 99.6 MON, but producers routinely produce fuel with MON in the 103-106 range.
Bottom line is this: unleaded fuels may be 100 octane but some engines and operating conditions need 106 MON. Most don't, but those that do may need some timing retard to stay out of detonation range under some conditions.
As for VSR, it is only a hot topic now because of the issues reported by UND with 94UL. As Mike Luvara stated, UND routinely run their engines at peak EGT which is not a recipe for valve seat longevity. Anti-VSR additives do work but they leave deposits and will likely not be needed unless you live at peak EGT.
Octane rating is an empirical measurement of resistance to detonation under specific conditions; different measures of octane measure under different conditions (RON, MON, “aviation lean”, “aviation rich”, etc) and those conditions are not the same as your engine. So two 100 MON fuels will perform the same in the test cell under standard test conditions, but may have different RON ratings and different detonation margin in your engine.
Avgas 100LL (Low Lead) is primarily tested and specified based on its Motor Octane Number (MON), specifically designed for high-performance aircraft engines. Under ASTM D910 standards, 100LL must have a minimum MON of 99.6 (often referred to as 100).
Key details regarding the testing of 100LL:
- MON Focus: Avgas is tested to MON values (using the ASTM D2700 standard) because RON (Research Octane Number) is not relevant to the high-load, high-temperature nature of aircraft engine operations.
- Rich Rating (Performance Number): In addition to the lean MON of 99.6, 100LL is tested for a "rich" rating, often called the Supercharge Rating (ASTM D909), which is 130.0 minimum.
- Two-Number System: Historically, 100LL is denoted by these two MON-based numbers: 100/130 (Lean/Rich).
- No RON: Unlike automobile fuels (which use a combined RON/MON average called AKI), Aviation gasoline does not use the Research Octane Number (RON) in its rating or testing.
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glider_rider
Well Known Member
I keep my CHTs below 390F,
oil temp below 203F,
RPM 2400 - 2700,
Engine is IO-390 EXP 119, CR8.9
Ignition advance is a standard P-Mag curve starting at 20deg
IMHO neither MON nor RON testing conditions come even close to the operating parameters of my engine.
The core point of my post was that test cell conditions are not the same as engine conditions (they are chosen to attempt to be representative, but are not the same) and therefore test cell performance may not be the same as engine performance.
Unless your airplane has one of these bolted to the front.
Swift 100R fuel is currently testing and actively deploying their 100R unleaded avgas on actual aircraft.
Here are the details regarding the status of Swift 100R testing:
- Current Usage: As of September 2025, 100R is being used at five major flight schools in the US and five in Europe (Germany, Netherlands, Austria, and Belgium).
- Approved Aircraft: The fuel has received a Supplemental Type Certificate (STC) for use in specific Cessna 172R and 172S models equipped with Lycoming IO-360-L2A engines (same cylinder size as the IO-540).
- Flight Testing & Validation: Beyond initial testing, 100R has undergone over 350 hours of high-stress engine endurance testing, exceeding the FAA's 150-hour certification requirement. Flight testing was completed with FAA test pilots using Cessna 172s.
- Future Testing: Swift plans to expand testing to more complex "550-class" six-cylinder engines to cover the broader piston fleet.
- Safety Record: High-volume users, such as the San Carlos Flight Center, have reported no issues with the fuel's performance in training environments.
Knowing that unleaded AvFuel was inevitable during the lifetime of my RV-9A (22-years later now), I went with standard compression of 8.5:1 (160 HP) for exactly that reason. The IO-390 EXP has very large displacement cylinders compered to most other GA pistons and can be built with compression ratios ranging from 8.9:1 to 10:1. So personally, I don't fully understand using such a big cylinder displacement, requiring such high octane demands in such light airframes as the RV.
I do remember when they rapidly switched to 100LL, eliminating the higher leaded 100/130 and 80/87 fuels. During this transition, I was flying a Piper 140 (150HP) with 7:1 compression, and what a mess that was for lead build-up -- stuck valves and frequent sparkplug cleaning was the order of the day!
Clostermann
Member
Owners of certificated aircraft with 550 engines will need to file a form 337 (no cost) and can adjust timing during their annual. No power loss during TO with UL100E with 3 degrees retarded timing and only 1% TO power loss (3HP) with UL100 at the same timing.
Toobuilder
Well Known Member
I’ve said many times that there is no one size fits all approach to ignition timing, but I can point you in the right direction.
First big scary thing with lower octane fuel is detonation at high power. Fortunately, as my thread on timing sweeps on the dyno show, the PV Lycoming is very insensitive to timing when rich and high MAP with regards to power output. What this means is that you can retard timing significantly without a power penalty. Retarding timing at high MAP buys you detonation margin without hurting performance. The in situ test for this would be to load up with your low octane fuel of choice and back off the timing a couple degrees from data plate value. Perform a few timed runs from brake release to 1000 feet AGL to establish a performance baseline. Note the takeoff distance and time. Next, back off the timing in one degree increments and look for a drop in performance. My guess is that you will have the timing back 6 or 7 degrees from data plate values before you detect any performance loss. If you DO find the drop off, add in a degree or two of advance and call it good. That value now anchors your “TO and initial climb” portion of your ignition map.
The next step is to fly blocks of altitude at WOT and mixture set for best power and adjust your timing to find best IAS. With a rich mixture, the peak in speed will be hard to detect, but an over advance will show up as higher/climbing CHT with no speed increase or slowing down. Going from a position of low advance, you are looking to add timing, look for an IAS increase, add more timing, look for IAS increase…. The behavior you are looking for is more and more speed, a few degrees of no change, then a decline in speed. If you see that, back off the timing to the lowest timing of the speed plateau, note the MAP, and that becomes your ignition value for that MAP setting. Rinse and repeat for the remaining altitude blocks up to your max altitude.
Couple notes: keep an eye on CHT during this process. More power will drive an increase in temperature, but an over advance condition will show as hot with a performance loss. Plenty of PMag owners live this life every day but don’t know it.
Don’t be concerned if you end up with a very low initial timing value for the TO condition. If the performance is still there, the number does not matter much. My Rocket is an 8.5 PV with a data plate value of 25 degrees, yet my ignition is set to 17 at TO. I run this setting even with 100LL.
As stated, the best power mixture is rich enough that finding a peak in speed with timing adjustment is going to be tough. The peak is so subtle it’s hard to detect. Don’t stray too far from known values trying to find the peak. That said, the higher/leaner you go in altitude, the “peakier” it gets, and the LOP condition is quite distinct.
I use MAP as my ignition tuning signal in flight. I have a CS prop, so the RPM value is “all in” and remains stable above 2000 RPM in my program just because it make life easier. If you have a FP and are using RPM values in the flight range to advance, then watch for the possibility of an increased IAS driving more RPM which may then drive an advance in the RPM program - all while you are focusing on the MAP value. Not a safety issue, but be aware of this “do loop” possibility.
Finally - take advantage of the LOP function as a “flight test” switch. Understanding that the LOP function is locked out above a certain MAP, at altitude it’s VERY useful to instantly add and remove ignition timing for test. Example, say you are on condition at altitude and you have 26 degrees of timing but want to see how the airplane behaves at incrementally more and more advance - navigate to the LOP function and plug in 1 degree. Note your current IAS and flip the switch. This will add 1 degree to your previous 26 and give you 27. Monitor the CHT and IAS for 60 seconds and see if there is a performance change. If it stays the same or gets better, add another degree to the LOP window and monitor. Rinse and repeat until the speed declines. If your initial flip of the switch results in a speed drop or increased CHT with no speed increase, you are ALREADY too far advanced and need to back the timing down 5 degrees and try again. And keep in mind that if things don’t look right during this testing, simply turn off the LOP switch and restore the prior “safe” condition instantly.
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It really is a question of the compression ratio. If the 390 is a high compression engine, no. If it is a low compression engine, yes.
Stock compression for the 390 is 8.9:1
Like all things experimental, you need to investigate the entire advantage of using mogas. I can get 92 Clear at the gas pumps here in Idaho. Works great with mags and carb engine. Would I run that in EI and FI engine? I would have to test some conditions. Like running on one tank of 100LL and one of of Clear. Then make sure all my hoses are covered, fuel pump shroud is in, possibly have a recirc line for vapor lock conditions. Then run it and monitor conditions with your 4 cylinder monitor. Check Peak settings on both fuels and also see if there is any indication of possible detonation. I've often considered doing some engine mods (port, polish, flow match) and then putting lower compression pistons (7.5/1) in to see if you can run straight clear premium. In know several other RV's out there that did that and never looked back. On a O-360, I doubt you would lose more than 5 hp off the original listed HP. Most engines with those mods with higher compression pistons are putting out 190 hp or so. So if you settled on 175, are you really out much? On a 215hp, 390, maybe you lose 10. If you think you need those 10 extra HP to make it over the trees in a white knuckle experience to get out of a strip somewhere, you probably should not fly that day. I run MM oil in mine to help with lubrication and fuel stabilization. I run 100L in both tanks about every 10/12 tanks worth to get some lead back in the system. One good item for Clear fuel is that you won't be fouling plugs much. Food for thought!
The AI answer:
Lycoming Engine Compression Ratios and Fuel Octane Requirements
Lycoming engines have specific fuel requirements based on their compression ratios. The higher the compression ratio, the higher the octane rating required to prevent engine knocking. Below is an overview of typical compression ratios and their corresponding fuel octane requirements.Low Compression Engines
- Compression Ratio: 6.0:1 to 7.0:1
- Fuel Requirement: 80/87 (MoGas or Avgas)
- Compression Ratio: 7.0:1 to 8.5:1
- Fuel Requirement: 87 AKI (R+M)/2 or 100LL
Medium Compression Engines
- Compression Ratio: 8.5:1 to 9.0:1
- Fuel Requirement: 100LL minimum recommended
- Compression Ratio: 9.0:1 to 10.0:1
- Fuel Requirement: 100LL avgas
High Compression Engines
- Compression Ratio: 10.0:1 to 10.5:1
- Fuel Requirement: 100LL avgas
- Compression Ratio greater than 10.5:1
- Fuel Requirement: 100LL avgas or equivalent high-octane fuel
Conclusion
Selecting the appropriate fuel based on the engine's compression ratio is crucial for optimal performance and engine longevity. It's always advisable to consult the specific engine's operating manual for exact requirements, as variations may exist depending on the specific model and configuration.
Toobuilder
Well Known Member
So AI “thinks” that 100LL is required for engines from 8.5 to greater than 10.5. That’s some solid info right there!
This is almost no help in figuring out how to work with alternative fuels.. If I understand correctly, 100LL is 100 Mon and therefore approximately 105 AKI. They tell us anything below 8.5:1 is ok on 87 AKI and anything over 8.5:1 requires 105 AKI.
The AI has 8.5 compression in both Low and Medium category. smh.
I understand the general consensus is as follows:
7:1 needs 87 octane mogas
8.5:1 needs 91 octane mogas
8.9: or higher needs 100 octane (avgas)
Talk to your engine shop, or fuel system provider (SDS, EFII, Airflow). They know more than strangers on the internet, like me.
7:1 needs 87 octane mogas
8.5:1 needs 91 octane mogas
8.9: or higher needs 100 octane (avgas)
Talk to your engine shop, or fuel system provider (SDS, EFII, Airflow). They know more than strangers on the internet, like me.
My Parallel valve IO360 with 8.5 compression (narrow deck cylinders, if that matters) has been tested (by me, my installation) on both 91 and 93 auto pump gas, with ethanol blending. I monitored CHT and EGT and played with the timing until I was able to induce mild detonation (or convinced myself that I had induced it, at any rate, by the CHT rising 1-2 degrees per second) and then backed off the timing 3 degrees from that point for my detonation margin. My home field is at 2700' ASL so my WOT testing was done at 4000', so take that for what it's worth.
I found that for the 91, in my setup, I wanted 22 degrees timing down low at full power, and for the 93 I could run 24 degrees. My SDS timing map advances it as I climb to 25 degrees above 5500', and above 7000' when I go LOP I can run both fuels at 29 degrees with no issue. I routinely do a "cruise-climb" where I take off, pull throttle to 24" manifold pressure, go 50LOP and 29 degrees, and add throttle as I climb at 90 knots into the mid-upper teens, without any heat issues and without any step-climbing. I stopped buying the 91 octane about 6 years ago when the local Walmart began carrying 93.
Just one data point, and it should be noted that I'm running ethanol laced auto pump gas, not aviation MOGAS. 1983 hours and 10 years doing that, the engine is way happy - compressions are all 79 and 78, borescope on the cylinders is beautiful, burns a quart in about 15 hours, strong power all across the board.
I found that for the 91, in my setup, I wanted 22 degrees timing down low at full power, and for the 93 I could run 24 degrees. My SDS timing map advances it as I climb to 25 degrees above 5500', and above 7000' when I go LOP I can run both fuels at 29 degrees with no issue. I routinely do a "cruise-climb" where I take off, pull throttle to 24" manifold pressure, go 50LOP and 29 degrees, and add throttle as I climb at 90 knots into the mid-upper teens, without any heat issues and without any step-climbing. I stopped buying the 91 octane about 6 years ago when the local Walmart began carrying 93.
Just one data point, and it should be noted that I'm running ethanol laced auto pump gas, not aviation MOGAS. 1983 hours and 10 years doing that, the engine is way happy - compressions are all 79 and 78, borescope on the cylinders is beautiful, burns a quart in about 15 hours, strong power all across the board.
Thanks for sharing the details of this testing. Very helpfull for those of us considering that option.
Airplanes with Lycoming engines with ratios under 8 have been running 87 octane for years before it disappeared. This list doesn't seem to factor that in.
glider_rider
Well Known Member
Very interesting stress testing. How did you set the threshold of the mild detonation? How did you measure that?
The way I do it is to look at the piston crowns at every oil change and check if they are all completely covered with black soot.
Toobuilder
Well Known Member
From Greg’s post:
“ I was able to induce mild detonation (or convinced myself that I had induced it, at any rate, by the CHT rising 1-2 degrees per second)”
Just a reminder about testing Unleaded Avgas
Excellent video and very revealing. Sounds like the Gami fuel is a no go but the Swift fuel looks promising.
Any thoughts on this thread now that SWIFT has expanded their 100R STC.
FAA expands Swift 100R unleaded approval
The FAA on March 11 increased to 840 the number of engines approved to use a 100-octane unleaded blend from Swift Fuels.
aopa.org
Besides expected $/gal?Any thoughts on this thread now that SWIFT has expanded their 100R STC.
FAA expands Swift 100R unleaded approval
The FAA on March 11 increased to 840 the number of engines approved to use a 100-octane unleaded blend from Swift Fuels.aopa.org
I am catching up with this thread rather late. This issue is of particular significance at my home airport (KRHV) because only UL94 is available. My RV-10 had a 9:1 compression engine when I got it 4 years ago but when I had to send it in for a premature engine overhaul last year, I decided to switch to 8.5:1 compression since this Lycoming SI (https://www.lycoming.com/sites/default/files/attachments/SI1070AB%20Specified%20Fuels.pdf) indicates that my IO-540-C4B5 with 8.5:1 compression is compatible with UL94.
I also reached out to Lycoming Technical Support to get their opinion and this is what I got back: "UL94 fuel would be acceptable for use in a IO-540-C4B5. Use of unleaded fuels requires LW-16702 additive or oil containing an equivalent additive which is generally not recommended for break-in as it can slow the break-in process. If possible, I recommend initial break in with 100LL and a non-dispersant mineral oil then switch over to unleaded with the LW-16702 additive/ oil with the additive after initial break-in is completed for best results."
The requirement for LW-16702 seems a bit strange - I don't see how that can help with preventing detonation.
The overhauled engine is getting close to the of the break-in (for which I have been mostly running 100LL). I mixed in small amounts of UL94 when I absolutely needed to (up to about 20%) but used it in just one tank and only for cruise. Once the break-in is complete, I would like to explore using higher amounts of UL94.
I recently called into the Ask the A&P's podcast and spoke with Mike Busch and his crew and I asked them the question about UL94 with this kind of an engine. You can listen to the podcast for his full answer (it should be getting released soon) but the net of it is that he suggested backing off the timing by 3-4 degrees if I want to exclusively use UL94. My plan (hope?) is to be able to run up to 50% UL94 so I will start playing around with the timing once the break-in is done.
I also reached out to Lycoming Technical Support to get their opinion and this is what I got back: "UL94 fuel would be acceptable for use in a IO-540-C4B5. Use of unleaded fuels requires LW-16702 additive or oil containing an equivalent additive which is generally not recommended for break-in as it can slow the break-in process. If possible, I recommend initial break in with 100LL and a non-dispersant mineral oil then switch over to unleaded with the LW-16702 additive/ oil with the additive after initial break-in is completed for best results."
The requirement for LW-16702 seems a bit strange - I don't see how that can help with preventing detonation.
The overhauled engine is getting close to the of the break-in (for which I have been mostly running 100LL). I mixed in small amounts of UL94 when I absolutely needed to (up to about 20%) but used it in just one tank and only for cruise. Once the break-in is complete, I would like to explore using higher amounts of UL94.
I recently called into the Ask the A&P's podcast and spoke with Mike Busch and his crew and I asked them the question about UL94 with this kind of an engine. You can listen to the podcast for his full answer (it should be getting released soon) but the net of it is that he suggested backing off the timing by 3-4 degrees if I want to exclusively use UL94. My plan (hope?) is to be able to run up to 50% UL94 so I will start playing around with the timing once the break-in is done.
Lead acts as a lubricant. Take that out, the additional additives act as the replacement lubricant, that is my understanding.
I’m based at E16 and have used the UL94 in my RV-7. I ran one tank through and filled up with 100LL but plan to use both fuels eventually switching to UL exclusively in the future
