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post #1 of 34 (permalink) Old Jul 11th, 20, 7:40 PM Thread Starter
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Al
 
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Gas

I was out to the airport yesterday filling my 5 gallon containers with 100LL Avgas. I use this gas in a couple of 10+:1 compression ratio high compression small blocks. While there I gassed up my 65 Chevelle with a 9.6:1 CR GM crate SP383 motor. I have been running 91 octane non leaded - non ethanol pump gas without issues in it and other than it smells like **** it works well. After a few minutes my car fell on it face with this 100 LL Avgas. Started running rough and the idle RPM went from 850 to 400 and would barely run. Ran fine above 1000 RPM but not below. Nursed it home and got some gauges on it, all normal. After I increased the idle speed setting up to 850 RPM it runs better but not as well as it did with pump gas.
So help me understand this, 100 LL Avgas has lead in it. Lead changes the speed of ingition/combustion, is this going on in the idle circuit? I noticed that to get it to run right I have screwed in the idle speed screw to the point that the idle jets are out of the circuit. That is, I can screw the idle screws all the way in and it still runs. This means to me the butterfly blades are open the idle circuit is not in play.
My plan is to burn through this tank of gas and start using pump gas again, I'll have to reset the idle system but that is easy. For what it is worth my other cars love 100LL Avgas, maybe they are set up for it.
I've never seen how changing gas has made this difference is idle performance before.
Any thought out there?
Thanks... Al
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post #2 of 34 (permalink) Old Jul 11th, 20, 8:20 PM
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Beth
 
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Re: Gas

Higher octane gas burns slower than lower octane. "Pinging" is caused by the fuel exploding instead of burning smoothly.

If your engine isn't pinging, using a higher octane will lower the power output.

That said, I don't think your engine should run badly on 100 octane. Sounds to me there is trash or water in it.
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post #3 of 34 (permalink) Old Jul 11th, 20, 9:02 PM
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Re: Gas

Av gas has completely different vapor pressure. It's made to evaporate at high altitude. Runs like crap on the ground.

Try running it mixed with pump gas premium.

I tried it in a 10.5:1 big block Chevy once. Ran bad. Wouldn't idle.

Chas
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post #4 of 34 (permalink) Old Jul 11th, 20, 9:20 PM
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Re: Gas

https://www.fuelexpert.co.za/canirunavgas.php


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post #5 of 34 (permalink) Old Jul 11th, 20, 9:46 PM
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Eric
 
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Re: Gas

Quote:
Originally Posted by chasmalo View Post
Av gas has completely different vapor pressure. It's made to evaporate at high altitude. Runs like crap on the ground.

Try running it mixed with pump gas premium.

I tried it in a 10.5:1 big block Chevy once. Ran bad. Wouldn't idle.

Chas
This is the same BS spewed around the web about AvGas, and it has never been true. "made to evaporate and high altitude and runs like crap on the ground" Baloney.

When do you think aircraft engines need max power?? It's at takeoff, not at altitude. If it won't run at its best on the ground it'll never even get in the air.

There was a time when AvGas was the base stock for almost all race gasolines on the market.

Density and vapor pressure are different, it typically requires richer idle feed restrictions and main jetting compared to pump gas.

A little more truth for you...mixed in with a little of the hearsay/old wives tale BS still too:
https://www.yellowbullet.com/threads...ed-gas.566149/
https://www.yellowbullet.com/threads...#post-46953722
https://www.yellowbullet.com/threads.../#post-9724326
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post #6 of 34 (permalink) Old Jul 11th, 20, 10:06 PM
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Re: Gas

could the higher octane mean the motor needed more timing? jim

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post #7 of 34 (permalink) Old Jul 11th, 20, 10:17 PM
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Re: Gas

Experience...

It ran like crap in a 427 -400 bb Corvette in 1983.

Stalled leaving the airport.

Put a half tank pump gas whatever in it and it ran ok.

Chas
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post #8 of 34 (permalink) Old Jul 11th, 20, 10:28 PM
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Re: Gas

Ericknova72,

Have you ever tried Avgas? If so please share your experience.

Chas

Last edited by chasmalo; Jul 11th, 20 at 10:29 PM. Reason: Name
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post #9 of 34 (permalink) Old Jul 11th, 20, 11:12 PM
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Re: Gas

The engine needs to be tuned for it to run good. The lead build up is BS as these cars were designed to run with lead.

You're just jealous because you can't hear the voices!

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post #10 of 34 (permalink) Old Jul 11th, 20, 11:36 PM
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Eric
 
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Re: Gas

Used it for 10 years+, 11:1 406 SBC with iron heads with zero problems, and half my friends have used it too. It's not all that different than 108 octane race gas sold today. Has to be jetted a little richer and richer idle restrictions. Roughly 104 octane at the richness levels a performance automotive engine runs at. I've got a shoebox full of low 11/high 10 second 1/4 timeslips in a 3400 lb car using it. Some of those friends are spraying 250-275 Hp worth of nitrous on top of Avgas. no problems there either, it's all in the tuning.
I'm using E-85 now, in a 13.7:1compression aluminum head 406 simply because it works and is everywhere around me and easy to get, added plus it is even cheaper to purchase than pump gas and less than 1/2 the price of AvGas. If it wasn't around, I'd still be using Av

100LL is spec'd to be minimum 99.6 octane at full lean, and 130 octane at full rich for aircraft use.

If you get bad results in a car, it's your carb and timing tune-up, not the fact the fuel is "AvGas" that is the problem.

The "only works at altitude" crap was started by race fuel manufacturers, to steer guys into buying their product instead in the 1970's. Seems like this got stuck in some guy's heads, then repeated around the bench racing table, and now a ton of people believe it because it sounds plausible to believe "aircraft fly above you in the sky, therefore high altitude, therefore it don't work at ground level" because if fits with basic simple thinking.

One of the great all-time myths in the hotrod community, along with "high nickel" SBC stock blocks, "can't cruise a high stall converter on the street you'll burn up the trans", 'big cam engine don't make enough vacuum to open the secondary's on a vacuum secondary carb", Ford engines "need a little backpressure to run right", yada, yada, yada that shade tree hotrodders actually believe as gospel.
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post #11 of 34 (permalink) Old Jul 12th, 20, 2:05 AM
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Re: Gas

Amen to all that.
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post #12 of 34 (permalink) Old Jul 12th, 20, 2:15 AM
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Re: Gas

Thanks Erick. You've definitely got experience with avgas.

Chas

Last edited by chasmalo; Jul 12th, 20 at 2:15 AM. Reason: Sp
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post #13 of 34 (permalink) Old Jul 12th, 20, 6:20 AM
 
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Re: Gas

my granddad use to have a car dealership, and one of the mechanics use to have a dirt track car that he would work on on the weekends. he had 108 avgas in the car, and i would "lift" a few gallons of it every once

in a while. i would advance my distributor 4 degrees and proceed to tear up the town!! man that car became a whole other beast with that stuff in the tank!! this was in a '70 ls6 454 by the way...
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post #14 of 34 (permalink) Old Jul 13th, 20, 7:02 PM
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I’m just curious as to why you would want to run anything other than pump gas at all in a cruiser. Why limit yourself in such a way that you can never venture very far away? I guess if you never want to leave your zip code ok but why give yourself the headache you now have? We run 112 Sunoco in our race sleds, you put that in a stock rig and it won’t hardly even run, plug fouling fool, builds next to no heat. Just curious.
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post #15 of 34 (permalink) Old Jul 13th, 20, 8:02 PM Thread Starter
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Al
 
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Re: Gas

Quote:
Originally Posted by Chebell View Post
I’m just curious as to why you would want to run anything other than pump gas at all in a cruiser. Why limit yourself in such a way that you can never venture very far away? I guess if you never want to leave your zip code ok but why give yourself the headache you now have? We run 112 Sunoco in our race sleds, you put that in a stock rig and it won’t hardly even run, plug fouling fool, builds next to no heat. Just curious.

I stated running 100 LL Avgas in my 57 Rochester fuel injected sedan about 4 years ago. I did this as I'm familiar with the long tern storage capability of Avgas, in general it will not deteriorate like pump gas with ethanol does, it does not attack the fuel system as ethanol pump gas had done on this car. When I bough this car the entire fuel system needed to be purged of rust from ethanol gas. I made the decision to run only gas that I could trust in this old engine, so that is the history of my 57 Chev fuelie and Avgas. I recently bought a 61 Corvette, again a Rochester fuel injected car and so running it with Avgas as it is easy since I'm set-up to use this fuel in my other car(s).

The Chevelle runs fine on 91 octane but I try and keep the ethanol away from it for similar reasons. It was low on fuel while I was filling the 5 gallon cans so I top it off with Avgas for convenience reasons - I was there, the car needed gas so I filled it with Avgas.

Also I don't like the smell of pump gas exhaust in this Chevelle it is un-plesent at best and burns the eyes, Avgas does not seem to do this. Something to do with the camshaft overlaps I suspect.

Unleaded non ethanol pump gas is selling for about $3.90 a gallon around here and 100LL Avgas is at $4.10 a gallon so not much difference... Al
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