What octane gas??

[mduenas]

I feel absurd asking this, but I had a 351w carbureted and always used 91, now I have a 302 with fitech, should I use a lower octane?

 

[bmc69]

Using a higher octane than just what is required to avoid detonation is a waste of money. Did/do you have to run the 91 in the 351W to avoid detonation?

I had to run 93 + octane booster in the 10.5:1 408C I had in my '69. The stock 5.0 HO..perfectly happy with 89. Same with the 400 in my '78; I can even get away with 87 in that one if I knock the timing back a couple degrees.

 

[pcf_mark]

I'm cheap so I tune for 87 octane. I have a 302 HO and I am sure there is more power to be had by increasing the timing but not enough to warrant 89 or 91 octane gas at every fill up for power I will only see / feel when I floor it once in a while.

 

[blubuckaroo]

I feel absurd asking this, but I had a 351w carbureted and always used 91, now I have a 302 with fitech, should I use a lower octane?

Do you know what the compression ratio of your motor is?
Mine is 9.7 and I've never had a problem with using 87 octane.

 

[bronconut73]

I am also about 9.5 to one but I have to use 93/91 octane.
It could be a timing thing. I may have a little more advance than Blu.
Or it could be that our heads have slightly different characteristics.

 

[mpboxer]

I have to use 91 octane too. Don't know the specs about my motor, but I have a little dieseling when I shut it off using cheap gas. I've tried adjusting timing with no luck and I don't want to put on an anti-dieseling solenoid. Plus, literally a couple bucks each time I fill up for better gas is worth it.

 

[Steve83]

This contains several Ford TSBs about gas in general, octane, and various formulations:


https://www.supermotors.net/registry/media/484645

Another possible cause of knocking is combustion chamber deposits, which increase compression & create hotspots. They can be removed by dripping water slowly into the throttle body of a fully-warmed-up engine at medium RPM (~2K).

 

[rmk57]

I use 94 in my 170 six. Then I can have the ignition timing bumped up to around 12 degrees and that really wakes the little engine up, all 105 hp.;D

 

[No Hay]

I have to use 91 octane too. Don't know the specs about my motor, but I have a little dieseling when I shut it off using cheap gas. I've tried adjusting timing with no luck and I don't want to put on an anti-dieseling solenoid. Plus, literally a couple bucks each time I fill up for better gas is worth it.

Same with me and mine.

I've always wondered how much ping or knock is acceptable related to damage being caused. Is some light pinging a warning, or worse?

 

[Steve83]

Higher-octane gas is NOT "better". It contains less energy; it's harder to vaporize, which means it doesn't burn as well; and it costs more.

 

[mpboxer]

Higher-octane gas is NOT "better". It contains less energy; it's harder to vaporize, which means it doesn't burn as well; and it costs more.

What I meant by “better” was less volatile. I guess all the tracks in the nation shouldn’t carry high octane fuel. Or higher performance vehicles shouldn’t use it. I’ve always thought is keeps the engine running cooler. I know from personal experience also you get better fuel mileage too. A long time Ford mechanic family friend suggested it went we looked at the dieseling during shut down. It happened to fix it in my situation. I’m not an expert just my life observation.

 

[WheelHorse]

I feel absurd asking this, but I had a 351w carbureted and always used 91, now I have a 302 with fitech, should I use a lower octane?

Is there anything special done to the 302?

If it's a stock 302 and your base timing is at 10, your mechanical and vac advance isn't too aggressive, then 87 octane will be just fine at sea level.

 

[bmc69]

What I meant by “better” was less volatile. I guess all the tracks in the nation shouldn’t carry high octane fuel. Or higher performance vehicles shouldn’t use it. I’ve always thought is keeps the engine running cooler. I know from personal experience also you get better fuel mileage too. A long time Ford mechanic family friend suggested it went we looked at the dieseling during shut down. It happened to fix it in my situation. I’m not an expert just my life observation.

I'm getting to be an old fart...and an automotive machinist for 40 years and counting. Beyond that...I'm also a combustion engineer. So..in the very simplest of terms it is always the case that you want to run the absolute minimum octane your engine can accept while achieving its best efficiency. For the old 70s engines, you simply cannot tune them to get any benefit from more than 89 octane. They simply don't have the compression.

Modern engines take care of the issue with the knock sensors. Our Murano was a rocket ship on 93 (what was recommended on fuel door) and a turd on 87...but it's computer knew how to take care of the timing for each.

 

[Nothing Special]

What I meant by “better” was less volatile. I guess all the tracks in the nation shouldn’t carry high octane fuel. Or higher performance vehicles shouldn’t use it. I’ve always thought is keeps the engine running cooler. I know from personal experience also you get better fuel mileage too. A long time Ford mechanic family friend suggested it went we looked at the dieseling during shut down. It happened to fix it in my situation. I’m not an expert just my life observation.

High octane fuel is good for preventing knock or "dieseling" (pre-ignition). It will not give better mileage EXCEPT that it will let you run more ignition timing advance, and more advance will give better mileage. So especially modern cars with knock sensors can be set to run too much advance for 87 octane. When run on 87 octane the computer will retard the timing, preventing knock and hurting mileage. So in a case like that higher octane will allow the computer to keep the advanced timing that gives better mileage.

If you are going to run high octane on a stock-ish older engine you should set the timing with high octane to put you just below knock. That way to can get some benefit from the higher octane. If the timing is set for low octane and you run high you are wasting money at best. It might even foul your combustion chambers due to the lower temps (although most high octane fuels are also high detergent, so that might not be an issue).

 

[pbwcr]

What I meant by “better” was less volatile.

=====
You are misinformed regarding Volatility. The gasoline at the pumps all have the same vapor pressure. Which means none are more or less volatile.
As you now have more knowledge from several correct messages above you should be using the lowest octane that does not cause detonation.
Of Course if you have a Coyote motor the above is not applicable because higher octane will give better performance for that motor.
PaulW

 

[mpboxer]

Lots of good info here, thanks for the lesson.

High octane fuel is good for preventing knock or "dieseling" (pre-ignition).

Great, that's what helped me.

It will not give better mileage EXCEPT that it will let you run more ignition timing advance, and more advance will give better mileage...So in a case like that higher octane will allow the computer to keep the advanced timing that gives better mileage.

Good, so in my modern cars with fuel injection I'm not going crazy and do get better fuel mileage.

If you are going to run high octane on a stock-ish older engine you should set the timing with high octane to put you just below knock.

For sure and that's how I set my timing.

 

[mpboxer]

=====
You are misinformed regarding Volatility. The gasoline at the pumps all have the same vapor pressure. Which means none are more or less volatile.
As you now have more knowledge from several correct messages above you should be using the lowest octane that does not cause detonation.
Of Course if you have a Coyote motor the above is not applicable because higher octane will give better performance for that motor.
PaulW

Sorry if I didn't use the correct words/verbage. The point I was making was if the fuel is igniting prematurely wouldn't higher octane help that. And to a layman if something is igniting prematurely/sooner/quicker it is more flammable and more volatile.

 

[edmedlin]

Sorry if I didn't use the correct words/verbage. The point I was making was if the fuel is igniting prematurely wouldn't higher octane help that. And to a layman if something is igniting prematurely/sooner/quicker it is more flammable and more volatile.

Yes. Since higher octane fuel combusts at a higher pressure/temperature, it would help with pre-detonation.

 

[Steve83]

What I meant by “better” was less volatile.

The main function of a carburetor or fuel injector is to vaporize the gasoline. And volatility is the tendency or ability of a liquid to vaporize. So low-volatility gas is "worse".

I guess all the tracks in the nation shouldn’t carry high octane fuel. Or higher performance vehicles shouldn’t use it.

That's exactly backward. High-performance (high-compression) engines need fuel that's hard to burn so it doesn't pre-ignite (before the ignition spark). And yes, there are vehicles built that way. But your eB isn't.

I’ve always thought is keeps the engine running cooler.

Right, which is bad for power & economy. The hotter an engine runs, the more-efficient it is. That's why jet engines are so much more-efficient than piston engines. So by making your engine run cooler, you're making less power & burning more gas to go the same distance (lower economy). It's just like putting water or other contaminants into the gas. In fact, contaminating gas with ethanol is one of the cheapest ways to raise its octane.

I’m not an expert just my life observation.

I don't claim to be an expert, either. But my life experiences include college chemistry, physics, & thermodynamics; being the son of a professional petrochemical engineer; working in the auto repair industry for over a decade; and reading several Ford TSBs on this subject. You can read them, too, at the link I posted above.

The gasoline at the pumps all have the same vapor pressure. Which means none are more or less volatile.

Not according to Ford & the EPA. Winter-blend fuels are more-volatile than summer-blends.

Good, so in my modern cars with fuel injection I'm not going crazy and do get better fuel mileage.

Only if they're built to use it, and are labelled for high-octane gas. Most modern EFIs are built for 87 or lower (E10-80).

And to a layman if something is igniting prematurely/sooner/quicker it is more flammable and more volatile.

No, flammability is not directly related to volatility. You're confusing terms you don't seem to understand. Each word has its own meaning - you can't just lump them all together.

Since higher octane fuel combusts at a higher pressure/temperature, it would help with pre-detonation.

That's somewhat misleading. It burns at roughly the same temperature, which results in roughly the same pressure-rise. But it IGNITES at a higher flash point (harder to ignite), and it's commonly used in engines with higher compression (pressure). The higher flash point & lower volatility are why it's suited to high-compression engines.

 

[mpboxer]

You can read them, too, at the link I posted above....You're confusing terms you don't seem to understand...

And you're a condescending jerk %). Over the past several years I've noticed this. You seem like a very unhappy person. Hope you have a better new year!

As for others in this thread and on the board, thank you for making this site a great knowledge base and a welcoming educational resource.

Mike