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AvGas or Unleaded?


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One of my cars requires 95 min fuel. If I run 91, it does a certain thing that I can feel happening at low revs. I filled up with Shell 98 in Albury the other week, and almost right away it was running like on 91. I genuinely believe I got 91 fuel. It was not a main street station. I normally run Caltex Vortex 95 in it, exclusively. I've been running my Jab 2200 on the same Caltex 95 for many months as a trial - no operational difference to Avgas. But I think I'll be going back to Avgas.

 

 

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 The octane rating method is different for Avgas and Mogas. Mogas is actually about 5 lower for the same numerical  figure. This must be borne in mind when choosing suitable fuels but it's not the only thing  (as explained in the article) The avgas  double figures are lean and rich readings. Rich mixtures give better knock resistance.  Piston engines take off power figures relate only to rich mixtures  which often show soot in the exhaust.. and are not supposed to be leaned out at over 75% power .  At very high altitudes they must be leaned for take off but you will be under the 75% unless the engine is supercharged  where it doesn't apply. Nev

 

 

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Note: The Jabiru recommendations for unleaded fuel storage are at odd with the advice from the fuel company technicians which in layman's terms say - store the fuel in an airtight (non vented) container that is a t least 75% full. Fuel stored this way has a long (12 month?) "shelf" life.

 

The fuel company advise makes more sense to me, as it is the volatile fractions which are most readily lost in a vented/open to atmosphere situation, with a resulting degradation of the fuels performance in certain conditions. By storing in an airtight container with a small air space (minimal oxygen ) the fuels volatiles are likely to be retained, thus preserving the fuels designed performance.

 

 

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What does the lead actually do? It increases the octane rating and helps lubricate the upper cylinder I think but don't know how.

 

So why is there no replacement already?

 

 

 

The chemical compound added to petrol is actually tetraethyl lead, which turns to a tan-coloured lead oxide upon combustion.

 

Lead oxide is a lubricant that lubes the valve stems and upper cylinder, and a layer of lead oxide coats the valve seats and faces, and reduces valve damage (valve seat recession) by cushioning the constant hammering against the seat.

 

Without tetraethyl lead in petrol, and using standard valves and non-hardened, cast-iron valve seats, you end up with recessed valve seats and burnt valves, particularly with constant high RPM operation.

 

You can counteract the lack of tetraethyl lead by utilising stellite faced valves and seats (inserts). Some hardened seat inserts are high chromium alloys, rather than stellite.

 

These components are sometimes nitrided as well to increase their durability. Nitriding introduces nitrogen into the surface of the metal, and it produces a similar result to case-hardening steel.

 

There is a lead replacement chemical, it is a potassium compound, and it is readily available as a fuel additive.

 

https://www.nulon.com.au/products/fuel-treatments/lead-substitute-valve-saver

 

 

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Jabiru certainly don't advise it. There's a lot of good gen in Jabiru's info, and they are pretty specific about Mogas risks and they are correct in my view. Mogas is tailored for modern universally fuel INJECTED motors. NO carburetter equipped motors are made for modern vehicles. and haven't been for 30 years. High under cowl temps can make the fuel in the bowl FIZZ like a popped champagne cork does and the float will sink and cause flooding if you aren't careful to control fuel line temps.  Nev

 

 

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Skippy, I can only surmise that the cost of the lead replacement additive, as well as a rapidly-declining number of unleaded engines, would be the reason for the potassium lead replacement compound not being added by the oil companies.

 

Interestingly, in the U.K., the oil companies did agree to add Potassium-based Lead replacement additive, to their unleaded petrol. The general opinion is that the Potassium additive is not as effective as Lead tetraethyl.

 

One has to remember that Lead tetraethyl was initially added to Avgas during WW2 as a desperately-needed octane booster - to get the Avgas of the day to 100 octane.

 

In fact, there's a story about a secret shipment from the U.S. to Britain of the brand new 100 octane, Lead tetraethyl boosted Avgas, that enabled the RAF aircraft to win a crucial air battle against the Luftwaffe.

 

But after using the Lead tetraethyl boosted fuel for a period, the mechanics and engineers noted that valve and valve seat recession problems had virtually disappeared - and it was then that they figured it was the Lead oxide coating that was lengthening the life of the valves and valve seats under high RPM, high temperatures, and high engine output conditions.

 

There are other Lead-replacement compounds used besides Potassium - Sodium, Phosphorous and Manganese compounds are also utilised by a number of aftermarket suppliers.

 

Sodium-based - https://www.ebay.com.au/itm/RED-LINE-OIL-LEAD-SUBSTITUTE-12OZ-BOTTLE-RED60202-/303257644635?_trksid=p2349526.m4383.l4275.c10#viTabs_0

 

Phosphorous-based - https://www.fastphaseclassics.com.au/product/castrol-valvemaster-lead-replacement-additive/

 

Flash Lube, with their Valve Saver product, refuse to state what chemicals they utilise, even in the MSDS. The MSDS merely says "non-hazardous chemicals - 100%".

 

There are reports that the Potassium and Phosphorous based products can make for sticky valves and sooty engines. The Sodium based products are reputedly damaging to turbochargers.

 

The bottom line is that, a few years after ULP introduction, and many people running their cars on ULP instead of Leaded fuel, the reports of VSR were only minimal, and not common, as predicted.

 

There would probably be two reasons for this - the number of cast-iron seat heads in use was declining, many engines were already using hardened seats and valves - and many motorists do not do the "high-speed, long-distance" running, where VSR promptly raises its ugly head, when ULP is used with cast-iron valve seats.

 

 

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................................................

 

One has to remember that Lead tetraethyl was initially added to Avgas during WW2 as a desperately-needed octane booster - to get the Avgas of the day to 100 octane.

 

In fact, there's a story about a secret shipment from the U.S. to Britain of the brand new 100 octane, Lead tetraethyl boosted Avgas, that enabled the RAF aircraft to win a crucial air battle against the Luftwaffe.

 

.........................................

 

Informative reply Onetrack.

 

One small point that I might add - In piston aircraft engines, I always understood (?) that high octane fuels is required at higher altitudes (somewhere above 10,000 ft) tp prevent "knocking" in  the low oxygen environment and of course allow for the development  higher compression ratio (more powerful) engines.

 

All this combined gives a tactical advantage (as you stated) by allowing allied aircraft to fly higher and faster, probably further (on a Gal of AvGas)

 

 

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I found a 1978 NY Times article that outlines the development of the new Avgas during WW2 - it was actually a superior blend of 100 octane, made to British Air Ministry specs, so the Americans referred to it as 100/130 octane.

 

https://www.nytimes.com/1978/12/03/archives/new-jersey-weekly-4-who-helped-win-battle-of-britain.html

 

 

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 It was probably the 130 /145 rated fuel. Tetra ethyl lead was first introduced in the late 20's in the USA for aircraft  mainly to stop hot engines destroying them selves trying to get off the ground. Better ways of directing the air around the motors hadn't been developed then .Good finning  townend rings and cowls .Nev

 

 

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I have run 95 BP Mogas in my Tecnam Sierra for the best part of 750 hours with not one issue, I regularly check it for ethanol and have never found it. Only time my plane used avgas was on a trip last year from my home in Sale, Victoria to Ayers Rock & return as unleaded was harder to source, will give it a shandy of avgas these days only if I need a top up at an out of town airport. 

 

Other than that she is strictly on a  Pulp diet.

 

 

 

I have run 95 BP Mogas in my Tecnam Sierra for the best part of 750 hours with not one issue, I regularly check it for ethanol and have never found it. Only time my plane used avgas was on a trip last year from my home in Sale, Victoria to Ayers Rock & return as unleaded was harder to source, will give it a shandy of avgas these days only if I need a top up at an out of town airport. 

 

Other than that she is strictly on a  Pulp diet.

 

Excuse my ignorance, but please , how do you check for ethanol content in our fuel

 

 

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