Octane requirement?

PlayingWithTBI · Jul 1, 2022

Pinger said:
LPG has the highest octane of all the available fuels and is currently around one third the price of diesel.

Yeah, things have changed quite a bit through the years. Back in the 70s and 80s we had a couple trucks with dual fuel (LPG/Gas) in which we lost ~10% HP and mileage running on LPG, do you see the same? We did have a John Deere combine that ran on LPG so we had a 500 gallon tank on the ranch, we didn't have to pay highway taxes on it. :waytogo:

Then we bought a F*rd F250 4x4 RCLB diesel pickup in 83 and got better mileage plus, we ran on farm diesel without paying highway taxes again - even more savings! In 90 we bought another F*rd F250 SCLB 4X4, my daily driver. I put ~35K miles/year on that thing and didn't know/care what mileage I got. All of our ranches had at least one 550 gallon tank, I'd just pull up and fill 'er up!

Any more, I don't think I'd want a diesel! Now we try to combine trips to reduce fuel and maintenance expenses - no fun anymore

tayto · Jul 1, 2022

a lot of the bad wrap LPG got was because of the low compression of the 70s/ 80s V8s that were getting converted. apparently you want them around 11:1 or 12:1. I believe like E85, milage is expected to be lower with LPG compared to gasoline.always wanted to tinker with a modified propane setup but hasn't happened yet. have a buddy with a squarebody dump truck that is on propane. we have talked about putting a set of 305 heads he has off a low hour marine motor but neither of us has had the time.

Pinger · Jul 2, 2022

PlayingWithTBI said:
Yeah, things have changed quite a bit through the years. Back in the 70s and 80s we had a couple trucks with dual fuel (LPG/Gas) in which we lost ~10% HP and mileage running on LPG, do you see the same? We did have a John Deere combine that ran on LPG so we had a 500 gallon tank on the ranch, we didn't have to pay highway taxes on it.

For certain I'm losing mpg (inevitable as LPG has a lower calorific value than gasoline) and, due to my older system having a (venturi) 'mixer', I'm probably losing a bit of power. But only at the top end of the rev range - there's still all the torque lower down to tap into.

PlayingWithTBI said:
Then we bought a F*rd F250 4x4 RCLB diesel pickup in 83 and got better mileage plus, we ran on farm diesel without paying highway taxes again - even more savings! In 90 we bought another F*rd F250 SCLB 4X4, my daily driver. I put ~35K miles/year on that thing and didn't know/care what mileage I got. All of our ranches had at least one 550 gallon tank, I'd just pull up and fill 'er up!

Any more, I don't think I'd want a diesel! Now we try to combine trips to reduce fuel and maintenance expenses - no fun anymore

I use my Suburban for work deliveries and up until gasoline and diesel prices spiked, running something diesel would have looked the better choice - and my Suburban a frivolous one. Oh how things can change! I'm quids in now and will be for as long as gasoline and diesel prices remain high (or go higher still).

Pinger · Jul 2, 2022

tayto said:
a lot of the bad wrap LPG got was because of the low compression of the 70s/ 80s V8s that were getting converted. apparently you want them around 11:1 or 12:1. I believe like E85, milage is expected to be lower with LPG compared to gasoline.always wanted to tinker with a modified propane setup but hasn't happened yet. have a buddy with a squarebody dump truck that is on propane. we have talked about putting a set of 305 heads he has off a low hour marine motor but neither of us has had the time.

Similarly, I'm often tempted (when I read the guys here tweaking their ignition timing tables) to optimise my timing for LPG. What stops me - apart from needing a different PCM - is that LPG availability can be patchy and retaining the ability to run on gasoline as back-up matters.
You're right though, a motor optimised for LPG would be a good one - but not as straightforward as it first appears. A simpler 'mixer' system does hurt top end power (not a problem if chasing low/mid range torque and efficiency) but the newer injector systems require the engine be run to operating temp (or to circa 60% of) on gasoline. Thus, proper gasoline operation has to be preserved. That said, knock may not be too much of a concern with engine temp only 60% of full running temp.

The price of LPG vs gasoline/diesel is key. Has LPG remained low cost in the USA?

Schurkey · Jul 2, 2022

Pinger said:
The price of LPG vs gasoline/diesel is key. Has LPG remained low cost in the USA?

I was paying triple+ for home-heating propane last February compared to Trump-era summer pricing. Some of that is the normal winter/summer price difference. (300 gallons/$1100)

My last fill was within the last month; "only" double the Trump-era pricing. (500 gallons/$1050)

My "1000 gallon" (they won't fill it above 750 gallons) tank does not have a liquid line; I can't refill my own bar-b-cue "20 gallon" tanks. Dirtbags.

I don't know how that compares to LPG.

alpinecrick · Jul 2, 2022

Supercharged111 said:
85 is normal at higher elevations. . . and it's stupid. EFI knows you're at altitude. Factory specs 87, so if timing is set to stock then 87 is what it calls for.

This^^^……mostly….
Octane rating is obtained in the lab under controlled conditions—barometric pressure, humidity, and ambient temp. Anything outside of those specific conditions changes the octane rating.

Here at altitude the lower air pressure creates smaller fuel molecules and raises the octane. HYPOTHETICALLY raising the octane rating. In other words, here at altitude, 85 octane is actually 87 octane—HYPOTHETICALLY. This is why in Colorado gas is 2 octane lower than the rest of the world.

In real life I get better fuel milage and more power with mid grade in my 5.7, 5.0, and 5.3 Vortecs, and also with the 5.7 TBI I used to own.

I have compared regular 85 octane to mid grade 87 octane dozens of times now, and I can tell a difference in power and see the difference in fuel milage in all my trucks and vans.

30 years ago mid grade was 5 cents more than regular and running mid grade paid for itself. Today mid grade is 30 cents more and doesn’t pay for itself, but I climb the pass a whole lot easier……

I run mid grade in all my gas trucks. Premium does not increase milage nor does it produce more power. I’ve tested that too.

L31MaxExpress · Jul 2, 2022

alpinecrick said:
This^^^……mostly….
Octane rating is obtained in the lab under controlled conditions—barometric pressure, humidity, and ambient temp. Anything outside of those specific conditions changes the octane rating.

Here at altitude the lower air pressure creates smaller fuel molecules and raises the octane. HYPOTHETICALLY raising the octane rating. In other words, here at altitude, 85 octane is actually 87 octane—HYPOTHETICALLY. This is why in Colorado gas is 2 octane lower than the rest of the world.

In real life I get better fuel milage and more power with mid grade in my 5.7, 5.0, and 5.3 Vortecs, and also with the 5.7 TBI I used to own.

I have compared regular 85 octane to mid grade 87 octane dozens of times now, and I can tell a difference in power and see the difference in fuel milage in all my trucks and vans.

30 years ago mid grade was 5 cents more than regular and running mid grade paid for itself. Today mid grade is 30 cents more and doesn’t pay for itself, but I climb the pass a whole lot easier……

I run mid grade in all my gas trucks. Premium does not increase milage nor does it produce more power. I’ve tested that too.

At lower elevation premium absolutely increases power when the timing is optimized for it.

alpinecrick · Jul 2, 2022

L31MaxExpress said:
At lower elevation premium absolutely increases power when the timing is optimized for it.

Yes, when tuned for premium. That’s why Black Bear asks if you want to tune for regular or premium.

It’s a tough call, a guy can spend money on parts and mods, or tune for premium, get more power and ostensibly more reliability.

alpinecrick · Jul 2, 2022

Another advantage is as we go up in octane the additive packages are mo’ better. One reason mid grade cost only 5 cents more 30 years ago was it had the same additive package as regular. Today it has mo’ better additives, and of course costs more.

The problem is the new EPA sulphur rules. Higher octane gas has less sulphur so refineries are blending it into low octane fuels to reduce sulphur, and in the process making mid grade and premium more scarce, thereby raising the price.

Supercharged111 · Jul 2, 2022

alpinecrick said:
Another advantage is as we go up in octane the additive packages are mo’ better. One reason mid grade cost only 5 cents more 30 years ago was it had the same additive package as regular. Today it has mo’ better additives, and of course costs more.

The problem is the new EPA sulphur rules. Higher octane gas has less sulphur so refineries are blending it into low octane fuels to reduce sulphur, and in the process making mid grade and premium more scarce, thereby raising the price.

Despite that, when I was in CA last year it was 10¢/gal difference between all 3 grades.

Octane requirement?

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