[Shop-talk] Electric String Trimmers

[Karl Vacek]

I should add that yes, as Bob Spidell stated, there’s no actual thing as “aviation unleaded” but at least theoretically the fuel suppliers control RVP, etc to the ASTM specs required by the FAA-issued STC’s for aircraft use.  In fact, the refineries and distributors are supposed to maintain specific trucks only to deliver car gas for aircraft use to avoid any cross-contamination.  It carries no road tax (a huge part of car gas prices) but the cost of the separate refinery streams and distribution costs still price it well over a buck a gallon more than car gas at a nearby gas station.  Our airport sells mogas for $4.30 per gallon.  Costco had 93 octane for $3.30  the other day.

 

In my limited experience our airport’s car gas has been very consistent.  But I don’t have an auto gas STC for my present plane because 100 LL works fine for me and there’s little price difference.  And I’m sure that 100 LL is pretty consistent across the country.  Not too sure what I’d get out of a mogas pump at some unfamiliar airport.

 

As they say, YMMV

 

 

From: Karl Vacek [mailto:Stearman809 at GMail.com] 
Sent: Monday, August 26, 2019 12:22 PM
To: shop-talk at autox.team.net
Subject: RE: [Shop-talk] Electric String Trimmers

 

The gas I buy at the airport for my lawn equipment is indeed aviation unleaded.  In the early 1980’s the EAA ran a project to prove that low-compression aircraft engines could safely run on unleaded car gas.  Peterson Aviation in Nebraska did a similar project.  Both sell STC’s to allow use of that specific fuel in applicable aircraft.  Basically a permit to allow the use of unleaded auto fuel a specific in aircraft under specified conditions.

 

The wide marketing of gasohol soon after the STC’s became available caused issues – the FAA permission is ONLY for straight unleaded – no alcohol.  Here in the Peoples Republic of Illinois the corn lobby has us inundated with gasohol, and there are only a handful of places in the state where you can fill up a car or truck with ethanol-free car gas.

 

For use in those planes so certificated, many airports now carry “aviation mogas” – regular unleaded without alcohol.  It’s not to be sold for on-road vehicles, just planes.  And airports can legally pump it into cans for small-engine use.

 

I have plenty of the blue stuff (100 LL) in the Stearman, but haven’t ever put it in any lawn equipment.  Almost the same price, so that’s not the issue, but my lawn equipment just doesn’t need lead.  Even my 3-wheel Harley (well, it’s a 2-cycle Harley Davidson golf cart) uses unleaded just fine.

 

Used to be better to use 80 octane (red) but that’s pretty much gone today.  That was good for mowers, etc. too.  As noted, 100 LL has 4x the lead of old 80 octane, so lead fouling can be an issue in aircraft engines.  Some guys who have the auto fuel STC even run 75% auto gas and 25% 100 LL to simulate old 80 octane.  When 80 octane went away and 80 octane engines that to switch to 100 LL, many pilots experienced lead-fouling issues, and many top overhauls were done.  Sometimes guys lapped the guides with the cylinder in place too.  But that’s largely over now, as most engines have appropriate valves and guides to allow regular use of 100 LL.  Personally I’ve not had an issue with 100 LL in my engine, which has stock WWII era valves and guides, and I have indeed had occasion to pull three cylinders over the years and disassemble the valves, with no lead fouling issues present.

 

As far as purple gas being available at Warbird events, I’ve not ever seen it, but my Stearman is at the lower end of the Warbird spectrum.  We do have a Grumman TBM Avenger on the field, and Tom uses 100 LL in that – it’s got a Wright 2600-20 (1900 HP).  And P-51’s regularly fill up with 100 LL.  Maybe the purple stuff is available sometimes, but not commonly, and as far as I know most Warbird engines run fine on 100 LL.

 

One of the things that allowed higher boost pressures during WWII was water injection.  That’s still an option.

 

Thanks !!

Karl

 

 

 

Subject: Re: [Shop-talk] Electric String Trimmers

 

Keep in mind that Low Lead Avgas, like 100LL, still has 4x the amount of lead that standard automotive gas contained.  The guys who run this stuff in race cars and the like often report issues with lead deposits on valve stems and seats.  I have personally see a valve stem so leaded up that it stuck open.  Use at your own risk.

 


It was probably blue, and it was probably 100LL.  Half the lead of most 
of the earlier grades, most everyone's pretty much still standardized on 
this.

Some places and applications (warbird events?) you may find more heavily 
leaded super-sauce for your high-boost blown motors.  The reason the 
Merlin went from 800HP to 1700+HP in production was all that sweet, 
sweet American leaded 130-octane gasoline.

John

 

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