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Colin Wood
Colin Wood Associate Editor
1/12/21 10:32 a.m.
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On paper, running aviation-grade fuel might sound like a great idea since it has a relatively high octane along with no ethanol.

In practice, however, it may not be the best idea, even if you do manage to get your car out onto the taxiway.

Learn more about avgas—and why you might not want to run it in your car—over on Classic Motorsports.

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alfadriver (Forum Supporter)
alfadriver (Forum Supporter) MegaDork
1/12/21 11:17 a.m.

If you want to poison your exhaust system with lead, it's not bad.

But if you don't want lead to coat your WB sensors, let alone the catalysts, it's a terrible idea.  

RichardSIA
RichardSIA HalfDork
1/12/21 11:33 a.m.

I've used it on and off for many years. Mixed about 50/50 with "Supreme" pump gas.

None of MY cars have a CAT or related sensors to worry about.

With race gas at $7.00 to $12.00 per gallon and more than one car at 10.75-1 or 12-1 so really needing the octane numbers, I will no doubt be doing so again.

This will not totally eliminate the corn syrup but will dilute it pretty well. 

914Driver
914Driver MegaDork
1/12/21 12:07 p.m.

I drained 10 gallons out of a plane I bought, put in in my carbed C-1500 mixed with pump gas.  Oh MY!  Mr. Truck liked it, don't get used to it Hon.

I give them a drink of Ethanol free 91 once in a while, but a carbureted race car, abaloobie. 

dean1484
dean1484 MegaDork
1/12/21 12:48 p.m.

Ok another question re Avgas.  Internet myth and legend is that it will cause pistons to burn or valves to burn.  I never had that problem.  I never have got a "real" answer on this.

alfadriver (Forum Supporter)
alfadriver (Forum Supporter) MegaDork
1/12/21 1:37 p.m.

In reply to dean1484 :

That myth makes no sense to me at all.  This fuel is specifically designed to be the most reliable fuel ever.  

Paul_VR6 (Forum Supporter)
Paul_VR6 (Forum Supporter) SuperDork
1/12/21 1:38 p.m.
alfadriver (Forum Supporter) said:

In reply to dean1484 :

That myth makes no sense to me at all.  This fuel is specifically designed to be the most reliable fuel ever.  

Lazy O2 sensors is really the only downside, IMO

RichardSIA
RichardSIA HalfDork
1/12/21 1:38 p.m.

In reply to dean1484 :

Back in the 80's I used to have a full-race engine in a FIAT 850 Spider street car.

100LL was the ONLY readily available suitable fuel I could get for it.

Engine ran very clean, I got to verify that every time I made the mistake of running too much regular pump gas as I would then break a ring or two. Never had any other issues.

APEowner
APEowner Dork
1/12/21 1:39 p.m.

I run 100LL in my Formula Ford.  It's easier for me to get than race race and it makes the exhaust smell great!  It does mean that I can't leave the O2 sensor for the data acquisition system in the exhaust all the time.

03Panther
03Panther SuperDork
1/12/21 1:43 p.m.

In reply to dean1484 :

That myth started because the general public thought that the higher the octane number, the “hotter” the fuel is, and gives a “motor” more power. 

Not how octane number work at all

03Panther
03Panther SuperDork
1/12/21 1:49 p.m.

I’ve never personally bought any Av gas; I assume there are also different grades of av gas available?

i also do not know what 100LL is. Have heard it mentioned before, but would not know where to get it. 

There used to be some places that had Cam2 at the pump, but it’s been many a year since I’ve seen that 

Paul_VR6 (Forum Supporter)
Paul_VR6 (Forum Supporter) SuperDork
1/12/21 1:56 p.m.

https://www.shell.com/business-customers/aviation/aviation-fuel/avgas.html

If you have a track or VP dealer closer than an airport, it's usually easier to just buy race fuel.

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) MegaDork
1/12/21 1:58 p.m.

100LL is 100 octane, low-lead.  As the article points out, the octane number is not derived as R+M/2 like automotive gasoline, 100LL is actually a bit higher in octane than 100 by automotive standards.  100LL is typically called Avgas, sold at airports for ICE aircraft.

Avgas is closer to white gas than gasoline (think coleman fuel).  It has a higher vapor pressure so it doesn't boil at higher altitudes, and on the average lower BTU content than gasoline - approximately 112kbtu/gal for Avgas vs an average of about 120kbtu/gal for gasoline.  (typical range of 115k-125k.  E15 is about 114k)

So, yes, Avgas has higher octane, but it isn't what I would call a performance fuel

ShawnG
ShawnG UltimaDork
1/12/21 2:07 p.m.

It's designed to burn s.l.o.w. because of large displacement engines that don't need to turn very fast.

The opposite of sporty car engines.

03Panther
03Panther SuperDork
1/12/21 2:25 p.m.

In reply to Curtis73 (Forum Supporter) :

Thanks.

Dumb question, but your 2nd paragraph go into some discription; are you still talking about the 100LL?

Alfa’s first post assumed we all only own newer cars, (most probably do) and warns against lead fouling up emissions equipment. Does the 100LL still have enough lead in it to do that? If so why does it have lead in it?

03Panther
03Panther SuperDork
1/12/21 2:28 p.m.
ShawnG said:

It's designed to burn s.l.o.w. because of large displacement engines that don't need to turn very fast.

The opposite of sporty car engines.

I thought higher octane fuel burn slower and colder, allowing higer compression without detonation problems. I never heard it was for slower rpm’s

rslifkin
rslifkin UberDork
1/12/21 2:32 p.m.
03Panther said:

Alfa’s first post assumed we all only own newer cars, (most probably do) and warns against lead fouling up emissions equipment. Does the 100LL still have enough lead in it to do that? If so why does it have lead in it?

100LL is low lead compared to other avgas grades (0.56 grams per liter of lead).  As a comparison, 100/130 avgas has 1.12 g/L.  From what I can find, leaded automotive gas historically had somewhere around 0.53 - 0.79 g/L.  So 100LL isn't all that low lead in that comparison. 

ShawnG
ShawnG UltimaDork
1/12/21 2:51 p.m.

In reply to 03Panther :

The burning slower part helps because you need a longer "push" in a big engine to take advantage of the large bore and long stroke.

In most aero engines, everything is over by about 2500rpm because engine rpm is limited by propeller blade tip speed. If the engine turns faster, there is usually a reduction gearbox to reduce the output rpm.

300 cubes is small in aero engines and that's spread over 6 cylinders. 

alfadriver (Forum Supporter)
alfadriver (Forum Supporter) MegaDork
1/12/21 4:20 p.m.
03Panther said:
ShawnG said:

It's designed to burn s.l.o.w. because of large displacement engines that don't need to turn very fast.

The opposite of sporty car engines.

I thought higher octane fuel burn slower and colder, allowing higer compression without detonation problems. I never heard it was for slower rpm’s

It does not burn slower, high octane means that it's more resistant to spontaneously breaking down and changing the type of combustion from a flame front more related to the turbulence to a flame front traveling at the speed of sound.

The part Curtis is alluding to is more the base chemistry to match what it's supposed to do- massive range of barometric pressure, huge temperature range, and pretty steady speed- all with essentially perfect reliability.

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) MegaDork
1/12/21 5:06 p.m.
03Panther said:

Dumb question, but your 2nd paragraph go into some discription; are you still talking about the 100LL?

Alfa’s first post assumed we all only own newer cars, (most probably do) and warns against lead fouling up emissions equipment. Does the 100LL still have enough lead in it to do that? If so why does it have lead in it?

Not a dumb question.  I was talking about Avgas (100LL).  It has a higher vapor pressure.  If you were to take regular gasoline up to high altitudes on a warmer day with the high UV of the thin atmosphere blasting on the wing tanks, there is a risk of the gasoline boiling. I think the confusion is that I used "higher" when I should have said lower.  Avgas has a lower vapor pressure... or  a higher resistance to boiling in low pressure.

Tetra-ethyl lead was used as one of the primary additives in fuel as a way of raising detonation tolerance in the fuel.  It's important to note that Octane ratings don't necessarily correlate to the concentration of actual octane in the fuel.  Octane is a hydrocarbon present in gasoline, but it's not the most stable molecule.  It can break down quickly.  For that reason, manufacturers came up with other additives (like TEL) to increase the fuel's ability to resist self-igniting, and the confusingly call it an "octane rating".  Since TEL has the dreaded L in it, it was banned from use in automotive fuel.

It is still used in aviation fuel because its use is primarily way up there where we ground-dwellers don't breathe it and it also comprises a much smaller use.  The octane rating of fuels doesn't change the rate of the flame speed, it just prevents higher compression engines from self-igniting during the compression stroke.  In a car, you have a water cooling system while planes have air-cooled engines (for the most part).  Water is a much more reliable and efficient means of moving heat.  In an airplane with nothing but the thin air to keep the engine cool, keeping a reliable means of preventing detonation is much more important.  In a car if you get detonation and break a piston, you swear as you pull over to the side of the road.  In a plane if you break a piston, you soil yourself, start bargaining with God, and fall from the sky on a somewhat unpredictable trajectory.  Reliability.  It prevents soiled Hanes at 8000 ft.

Did some digging to jog my memory.  Automotive gasoline is primarily comprised of carbon chains between C4-C12 which includes alkenes, octane, butane, toluene and others.  You can alter the detonation tolerance of gasolines by skewing the composition of the hydrocarbons you leave in the finished product.  Butane for instance is very light and easy to ignite and higher concentrations of it can lower octane ratings of fuels.  Since TEL was taken out of auto fuels, it was replaced with MTBE which has the ability to increase octane ratings, but not as well as TEL.  MTBE works great up to certain concentrations, but you can't add enough MTBE to get the octane rating required for aviation engines. Hence, they allowed TEL to remain in certain quantities in Avgas.

Avgas (the most common of which is 100LL blue) narrows the range of hydrocarbons a bit, starting with small concentrations of C6 (toluene) and continuing up through C10 (Cumene or Isopropyl benzene) in very small quantities.  It is a much tighter band of hydrocarbon chains, hence the lower BTU content. (bigger hydrocarbon chains = more chemical bonds = more stored energy)  It is very similar to Coleman fuel or White Gas which contains chains ranging primarily from C5-C9.  If you've ever smelled coleman fuel, Avgas smells almost identical.

03Panther
03Panther SuperDork
1/13/21 3:28 a.m.

In reply to Curtis73 (Forum Supporter) :

Thanks to you and Alfa... I knew bits and peices of that; enough to wonder about the statement that it was made for large slow turning engines.

I remember as we shifted away from lead in fuel, a lot of folks were worried about their valves. I found out, before we got good and inexpensive at installing hardened seats, that switching a well broken in engine did not hurt it; a side benefit of the lead is it assisted in work hardening new cut seats. If ya cut new seats into a soft metal, no lead fuel would allow the valves to wear into the head, without the work hardening process the lead helped with.

ShawnG
ShawnG UltimaDork
1/13/21 12:42 p.m.

I learned something today.

The "burning slower" is what was explained to me in tech school.

Robbie (Forum Supporter)
Robbie (Forum Supporter) MegaDork
1/13/21 12:56 p.m.

I guess my question is "but why?"

Haven't we had grm articles that show that high octane non-ethanol fuels make hardly any more power than regular pump 93 while e85 can actually make more power than both?

And if you're changing to a higher compression ratio to leverage the high octane fuel, e85 should be better for that too no?

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) MegaDork
1/13/21 1:09 p.m.

In reply to Robbie (Forum Supporter) :

It's not so much that E85 can tolerate more compression, it's that it needs it.

In general, Ethanol needs 30% ish more fuel to be stoich, but it also contains 30% ish less energy.  In an old-school wheezer small block with a carb, even if you re-jet for alcohol, your power production would be either a wash or a net loss because the compression, chamber design, and other factors are optimized for gasoline.  In a modern, higher compression vehicle with flex-fuel capability, you can see an increase in power on alcohol, but that is primarily because the engine is shooting the middle.  On gasoline, it retards timing and uses knock sensors - effectively crutching the tune and taking it outside the optimal operating parameters.  Running on E85, it can use all the timing it needs, but compression is a bit low for E85.  They've done a good job of making an engine that can adapt to both fuels, but in truth, neither extreme is optimal.  When building an engine for gasoline, compression ratios usually fall between about 8.5 to 11.  When building for straight Ethanol, those ratios are more like between 11.5 and 14.  Flex fuel engines are therefore often times engineered for the 10.5-ish range which is on the high side for emissions on gasoline, and on the low side for optimal cylinder pressures on Ethanol.

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) MegaDork
1/13/21 1:36 p.m.
ShawnG said:

I learned something today.

The "burning slower" is what was explained to me in tech school.

The speed of the burn is primarly set by the composition of the fuel.

A term used in chemistry is Activation energy, usually noted as Ea.  When you look at a graph of an exothermic reaction (like fuel burning), it looks something like this:

See the source image

In the case of a spark ignition, the Ea comes from two sources.  1) the compression stroke condenses the heat energy and raises the temperature to a point just before it would ignite, and 2) the spark plug sets things off at just the right time.  For a given fuel composition, the downslope (expulsion of energy) after ignition is relatively fixed.

In the graph above, if you increase compression, you are adding to the temperature and concentration of heat energy before the spark and you risk adding enough Ea to ignite the fuel before the spark happens.  So, if you can picture grabbing the very top peak of that graph and lifting it slightly, that is what increased octane rating does.  It reduces the risk of the compression stroke adding enough energy to the mixture that it ignites before the spark.

In truth, that isn't exactly how it works, but that was the easiest way I could think to describe it.  Upping the octane rating effectively prevents the surrounding heat energy from causing ignition before it's supposed to.

Now, using that same graph, imagine grabbing the far right end of the curve and dragging it further right to lessen the steepness of the slope.  That would be Avgas and represents a slower burning fuel.  In this case, you still get the same amount of energy from the reaction, it just burns slower.  Now grab that right side and drag it out even further, AND grab the line that says "potential energy" and raise it up a little.  That might vaguely represent diesel.  More energy content and an even slower burn.

The slower burning Avgas works with slower-spinning engines like aircraft.  As you spin an engine faster, you have less time for the fuel to do its work.  If you spun an airplane engine to 6000 rpms, half of the fuel would still be unburnt when the exhaust valve opens because it's taking its sweet time.  The other added benefit to slower burning fuels is (if properly engineered) greater torque production.  If you think about it, the cylinder pressure has its greatest mechanical advantage on the crank when the rod is at a 90 degree angle to the crankshaft.  Depending on the rod/stroke ratio, that usually happens somewhere between 20-30 degrees ATDC.  If you match a fuel burn rate to the intended RPM range of the engine, you can tailor when cylinder pressures peak and therefore effect the greatest torque on the crankshaft.

That's also why you don't see many 10,000 rpm diesels :)

So, back to the airplane vs car thing.  There are so many things happening with the heat during the compression stroke.  The one we're going to focus on is how much heat energy escapes through the chamber surfaces and makes its way to the cooling system.  If you have liquid coolant in the head, a lot of heat gets wicked away from the compressed charge.  If you have an air cooled airplane engine, the air's ability to remove heat is reduced by a LOT compared to a liquid coolant.  Far more heat energy is retained inside the combustion chamber which makes it far more likely to pre-ignite in an airplane engine compared to a car engine.  This is also why aluminum heads on cars can tolerate (read: require) higher compression to make the same power.  The aluminum is more efficient than iron at absorbing and releasing heat energy, therefore more heat energy "leaks" out before ignition.

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