influence of high octane...?

[bluefoxamazone]

hello friends,

I know that a lot has been written on the "octane" subject, but I would like to throw something in this group.

I noticed that the Cranbrook was ideling a little "rough"... I checked the plugs and wires but couldn't detect anything wrong. The car starts and runs fine.

I took it for a ride and instead of putting 98 octane fuel in, I switched to 95 and filled up the tank. I guess the tank was half empty  or half full if you like, before I started to fill her up.

After driving some 160km (100miles) I felt that the engine was running much smoother and the rough idling was gone.

Now the question of one million, is this sudden change of behavior due to a different octane or just coincidence...?

 

[Plymouthy Adams]

higher octane...less flammable the gas..you paying for that extra chemicals to retard the flaming that is needed ONLY for the higher compression engines to prevent knocking...it is a SLOWER burning fuel...

[Andydodge]

Bluefox..........I'd even try something like 91 Octane but only if it doesn't have ethanol in it......I used to run the 41 Plymouth which had the original 201 engine on 91 Octane and it ran fine...the Dodge which has the 318 Poly running 9:5 to 1 pistons I run on 98 Octane normally but have sometimes put 95 octane in and you can feel the difference........I mentioned ethanol and I will not use it in anything..........alcohol is for drinking, not for my cars...............lol...............andyd     

[mmcdowel]

My Meadowbrook likes the low octane with no ethanol.  Makes us both happy!

 

[plymouthcranbrook]

I get the lowest octane level. Impossible to get gas around me without ethanol.  Car runs fine.  Only annoying thing is the fact that I have to fill the carb each time if the car sits for a few days by excessive cranking.

Edited June 26, 2018 by plymouthcranbrook

[bluefoxamazone]

6 minutes ago, plymouthcranbrook said:

I get the lowest octane level. Impossible to get gas around me without ethanol.  Car runs fine.  Only annoying thing is the fact that I have to fill the carb each time if the car sits for a few days by excessive cranking.

since I have installed an electric fuel pump next to the mechanical pump it starts right away. I used to have the same issues, even with a new carb. I turn the key now, let the electric one pump for  two seconds and start.

 

Edited June 26, 2018 by bluefoxamazone

[Jakub]

Many older engines have the same problem. My P15 is still under restoration, but I have some experience with my "Volga", which also is a little rough when idling. The engine was designed to use 72 octane gasoline.

What helps? Increasing spark plug gap, much above factory advised values - "Volga" should use ~~ 0.35 mm gap; Increasing that to ~~ 0.8 would help. But only a little.

What really helps, is simply driving car a lot. When I started using it as a daily driver - all idle problems were gone.

[DocHemi]

pure-gas.org ROCKS (listing of all stations with ethanol free gas)

Edited June 26, 2018 by DocHemi

[Hickory]

95 and 98 octane, wow. The highest near me is the 93

[kencombs]

31 minutes ago, Hickory said:

95 and 98 octane, wow. The highest near me is the 93

Likely different rating system.  Research Octane Number is used in some countries and is higher than Motor Octane Numbeer .  The USA standard is an average of the two.   People think our octane rating went down several years ago.  And it, did, but not nearly as much as it seems, because it changed to that average from Motor, which had been used for years.

[DrDoctor]

   A friend of mine has a ’54 Pontiac with a late-model Pontiac V8 engine. He’d been using premium fuel in it, but on one occasion, he couldn’t get it, so he used regular. The car actually runs better, and gets better mileage with the lower-octane fuel. It does sound counter to conventional wisdom, but if it works—go with the flow. Regards . . .

 

Hickory,

   Same hear . . .

Edited June 27, 2018 by DrDoctor

[Don Coatney]

Conventional wisdom is an oxymoron. 

[insaneradio]

13 hours ago, plymouthcranbrook said:

I get the lowest octane level. Impossible to get gas around me without ethanol.  Car runs fine.  Only annoying thing is the fact that I have to fill the carb each time if the car sits for a few days by excessive cranking.

My p-15 too requires excessive cranking after sitting for a couple days.  I am curious as to how you relate this to the fuel? Not a challenge but learning!

[austinsailor]

Our old cars have the fuel exposed to the atmosphere. Today's don't, with fuel injection and sealed systems.  Today's gas evaporates much easier when exposed to the atmosphere. Has nothing to do with octane level.

 

On another note relative to this thread - a few years ago I listened to a chemical engineer explain a lot of things to do with chemicals. Most was boring except his talk on gas and the different octane levels.  As mentioned elsewhere in this trread, higher octane burns slower. Or more accurately, takes more to make it explode rather than burn evenly. When this guy explained it at the molecule level, it made sense. The higher the octane, the more even and rounded the molecule is. The lower the octane, the more jagged and odd shaped the molecule. So when it burns, the higher octane molecule has less surface exposed to the flame or heat, so it doesn't ignite as quickly. Sort of like trying to burn a stack of newspapers(high octane)  or a crumpled up newspaper (low octane). The crumpled up one ignites nearly all at once, the stack slowly burns from the edges. In the fuel, the low octane starts burning all over, actually makes an explosion if hot enough, where the high octane doesn't have enough surface exposed to burn to the core at once unless it's really hot (higher compression). 

 

So, there it is, octane for dummies, something even I could understand!

Edited June 27, 2018 by austinsailor

[51_Meadowbrook]

15 hours ago, mmcdowel said:

My Meadowbrook likes the low octane with no ethanol.  Makes us both happy!

 

Does your spit gas out when it reaches full? Since I got my ‘51 5 years ago whenever I fill it up it spits gas down the side of the car just after the gas pump kicks off. 

[51_Meadowbrook]

I’ve heard that racing fuel is used by a lot of people. What exactly is racing fuel in comparison to regular gas? I’ve put 87 in my Meadowbrook since I got it with a lead additive if I remember and it runs pretty great and idles beautifully. 

[Plymouthy Adams]

racing fuel, depends on your local track as to allowable rating...most tracks require you buy their fuel to compete so to level the field.....many consider any octane rating over 100 as a racing fuel with many running 110-114...back in the day you could get 104 at the pump...

[bluefoxamazone]

good discussion... anyway from what I have learned so far... Our old engines are not designed for the higher octane fuels and they can't handle these very good.

 

So the moral... "keep it cheap and the octane low".... :-)

[51_Meadowbrook]

I know there are things that they recommend changing when running an old engine with unleaded gas, but I don’t know what they are. Would these things, when changed, affect the octane rating required for the best running engine?

[Plymouthy Adams]

CR is the prime reason for selecting the proper octane fuel....

[Andydodge]

I'm only going by the advertised Octane ratings that the two servos I go to state.....Shell V-Power is advertised as 98 likewise the BP servo advertise their highest as 98 Octane...........I have sometimes used their 95 Octane fuel and could notice that the car seemed not to run as well.....but could be my mind playing tricks.......lol..........andyd  

[austinsailor]

My 40 dodge required something like 70 octane, according to the manual.

[mmcdowel]

Yes!  There is something about the shape of the tank fill line that the pumps don't kick off in time (I'm guessing here).  I've learned to listen for a bit of a gurgling sound and cut the pump off then.  Took some practice.  

 

[plymouthcranbrook]

10 hours ago, austinsailor said:

Our old cars have the fuel exposed to the atmosphere. Today's don't, with fuel injection and sealed systems.  Today's gas evaporates much easier when exposed to the atmosphere. Has nothing to do with octane level.

 

On another note relative to this thread - a few years ago I listened to a chemical engineer explain a lot of things to do with chemicals. Most was boring except his talk on gas and the different octane levels.  As mentioned elsewhere in this trread, higher octane burns slower. Or more accurately, takes more to make it explode rather than burn evenly. When this guy explained it at the molecule level, it made sense. The higher the octane, the more even and rounded the molecule is. The lower the octane, the more jagged and odd shaped the molecule. So when it burns, the higher octane molecule has less surface exposed to the flame or heat, so it doesn't ignite as quickly. Sort of like trying to burn a stack of newspapers(high octane)  or a crumpled up newspaper (low octane). The crumpled up one ignites nearly all at once, the stack slowly burns from the edges. In the fuel, the low octane starts burning all over, actually makes an explosion if hot enough, where the high octane doesn't have enough surface exposed to burn to the core at once unless it's really hot (higher compression). 

 

So, there it is, octane for dummies, something even I could understand!

What he said