Fuel gurus. 100LL AVgas mix with pump fuel??

[dieselgeek]

Couldn't think of a better place to ask this question.

As a tuner, I've been tuning a lot of guys to run on e85 lately.  It seems to be a good fuel for forced induction engines, relatively inexpensive for the local streetcar crowd around where I live.  As long as you can find a place that blends it consistently, and watch AFRs, it seems to be a good gas for the money.  With lower BTU, it's not exactly "mileage friendly" though.

There's a few guys however, who run 13B turbo Rotary engines, that are telling me they want to mix AvGas 100LL roughly 25% with pump gas (91-93 octane) to achieve a slight increase in octane.  These guys can't run e85 because, with only 4 injectors, they can't get enough injector (without adding supplementary injectors) to support e85 under the kind of manifold pressure they like to run.


I've always been told by sources that I trust, that AvGas is a bad idea due to "different properties."   I've tried reading up on fuels and understanding the properties, but it's taking me longer to grasp most of these than I have time to make recommendations to some of these guys.


Can anyone here tell me if I am being overly paranoid?  SO, my actual question is:   Assuming it's mixed consistently, is 100 Low Lead a good idea to mix with pump fuel (which I know is very inconsistent) for an increase in knock resistance?

[Stainless1]

Johnboy (SpeedLimit 1000) is probably still running AvLL100 in his scooter, he had a source for clean drained fuel.  It is low lead, not no lead.  So if you have things like a wideband O2 in your FI you may shorten its life a little. 
Can you consistently blend fuel...
Not unless you are doing it in a lab.... so tuning for inconsistent fuel yours or theirs is tough.
What octane do you need to achieve? 

[dieselgeek]

In reading about it, the "Low Lead" is actually very HIGH lead by gasoline standards.   2g per gallon...  which ranks up there with leaded race gas.  My experience has been, leaded fuel isn't THAT bad on O2 sensors so long as you have the tune in check.   I.e., if I'm just starting up for the first time, I make sure the sensor isn't in the exhaust stream until I have a good idle and startup tuned "by feel"


In this particular rotary guy's case, he wants somewhere around 100 octane.  I keep telling him that it's just a bad idea, I am *guessing* that AvGas is slower burning (high EGTs would be bad for his case).  For street use, I always tune them on the very safe side anyways.  Reading on the internet, we see some guys saying DON'T DO IT!  and many others saying "I've run it for years with no problems"

I'm hoping some of you guys can give better chemical, physical, logical reasons than what I've not been able to find thus far.  For me and any *serious* racing applications I tune, I always get a recommendation from Rick Gold or someone of similar qualifications.  but in the off season, I'm slammed with street car tuning requests and wish I was more knowledgable about fuels in general.


Thanks Stainless,

-scott

[Dean Los Angeles]

There is no reason to think that avgas burns slower. Aviation doesn't have the restrictions that automotive does.

There are any number of ways of getting higher octane numbers. The only reason for doing this is if you are having "knock" problems.

The other reason is horsepower. Horsepower and octane have very little to do with each other, unless knock limits what you can do.

E85 has an octane of about 100. If you are thinking about running avgas, why not just run racing fuel? Don't tell me cost is a factor? There are any number of "octane boosters" out there too.

As a side note, possession and use of leaded gasoline in a regular on-road vehicle now carries a maximum $10,000 fine in the United States.

I can't see any reason that mixing fuels would cause you a problem.

The basic chemistry is hydrogen and carbon. That's why we call them hydrocarbons. Different combinations make different gasolines and ethanols. The additives for anti-knock properties are either tetra-ethel lead, as in avgas, or oxygenates, mainly methanol-derived MTBE and ethanol-derived ETBE. Both hydrocarbons. There are other agents used in other countries.

More info here
http://www.landracing.com/forum/index.php/topic,2308.0.html

[RichFox]

i only know that I used to use 100LL in my Lotus 907 powered G/GR. I was running at 139mph on a 140mph record. Over and over again. I decided i heeded to go 5 richer on the pill. I showed Rick my plugs and told him the story but asked if I should take into account that EGR 110 has a higher Specific gravity and leave the pill alone. Rick said leave it. The roadster wend down at 141 with the old plugs and came back at 142 with new plugs. No other changes. So the 100LL was OK just not as good.

[46champ]

The biggest problem I've ran into running 100LL in motorcycles seems to be an atomization problem in constant velocity carbs.  This is probably due to the Reed vaporization numbers that av gas is brued to so it won't boil off at altitude.  I have never had problems it a float carb.  Sorry I can't help with fuel injection problems.

[38Chevy454]

Well, put me in the "used it for years without problems" category.  Not in a race car, but my 68 GTO with stock 10.75 compression.  It will knock on the 91 octane wanna-be gas super unleaded we have here in CA.  I mix it 5 gal per tank (about 25%) and it runs great.  I figure it bumps me 2-3 octane points.  enough it does not knock and runs better.  The added lead does not hurt since I do not have hardened valve seats.  The biggest problem is actually buying it.  The airport guys do not like to sell it to me "for my boat".  I do not understand the reasoning, but they act like I have to put it into an airplane and not 5 gal jeep cans.  You certainly can not just put directly into your tank.  The problem is that avgas does not have road tax.  There is airport tax, but that is a different story.

100LL mixes just fine with pump gas.  It really is no different than race gas, has similar smell.  My best understanding is a difference in vapor pressure so it does not vapor lock when used in an airplane.  Not an issue for an automtive engine.

[dieselgeek]

that's helpful guys - thanks for the replies!!

the "road tax" issue is good enough for me, really.