octane rating and engine performance

noteven wrote:

4x4ord wrote:

noteven wrote:
I drove a company F150 3.5 ecoboost that cost less money per mile for fuel running 91 vs 87.

This is from the owners maual for the ecoboost engine:

We recommend regular unleaded gasoline with a pump (R+M)/2 octane rating of 87. Some stations offer fuels posted as regular with an octane rating below 87, particularly in high altitude areas. We do not recommend fuels with an octane rating below 87.
To provide improved performance, we recommend premium fuel for severe duty usage, such as trailer tow.


Maybe the engine in your company F150 is all carboned up from using the wrong fuel. If the engine is healthy there is no way its going to be saving you money using premium.

So Shell V-power 91 carboned up a ecoboost during our economy test? Those dirty rats!

We interpreted the manual wording above to mean 87 is design minimum, don’t use 85, and engine performance can be optimized using 91. Seemed to work.

In today’s age of consumer protection a company would be quickly harassed for “requiring” a more expensive propellant when they can program an engine to run on “regular” so the consumer can choose.

It makes sense that to get peak performance (as in power output) out of a boosted engine higher octane fuel would be required... so long as the fuel and ignition timing maps are tuned to aim for the higher cylinder pressure that can be optained with higher octane fuel.

Detonation occurs after the spark has ignited the fuel mixture and the cylinder pressure rises to a point where fuel can ignite spontaniously ahead of the flame front. Higher octane fuel is able to resist detonation.

Doing a fuel economy test you are running down the road at 60 mph running part throttle conditions ... the spark advance and cylinder pressure are reletively low and high octane fuel will not be not be required to run the optimum timing advance. On an engine desugned to accept regular fuel there is no way I'm going to believe high octane fuel will return an increase in fuel economy that will offset the additional cost of the fuel. High octane fuel is more likely to cause poorer fuel economy, if it is not needed, due to its slower burn rate.

4x4ord wrote:

Sunnysidebeach wrote:
I have an older GMC Sierra, and even with its older technology it adjust the injection timing, valve timing, and ignition timing for different fuels. Under the manifold, is a knock sensor that detects the first sign of knocking and adjusts the timing to make that knocking stops?

My turned up 9.5 to 1 compression ratio 340 HP 5.3 Ltr motor becomes, a 240 HP motor, when pulling a load on 87 octane, I can just imagine what it would do on 85.

So while it will work at altitude, you will feel allot less power from your motor.

With a 9.5 compression ratio engine if you think you can get a 40% increase in power by using premium fuelover regular you're dreaming.

I know, but that's what dreams are made of.
I just put premium in my 5.3 truck (btw Sunnyside, LTR engines are 5.7 350s). Was hoping to gain that magical 100hp.......
And 340hp isn't really turned up. I hope you didn't rebuild it and get less than the new stock ones.....

4x4ord wrote:

noteven wrote:
I drove a company F150 3.5 ecoboost that cost less money per mile for fuel running 91 vs 87.

This is from the owners maual for the ecoboost engine:

We recommend regular unleaded gasoline with a pump (R+M)/2 octane rating of 87. Some stations offer fuels posted as regular with an octane rating below 87, particularly in high altitude areas. We do not recommend fuels with an octane rating below 87.
To provide improved performance, we recommend premium fuel for severe duty usage, such as trailer tow.


Maybe the engine in your company F150 is all carboned up from using the wrong fuel. If the engine is healthy there is no way its going to be saving you money using premium.

So Shell V-power 91 carboned up a ecoboost during our economy test? Those dirty rats!

We interpreted the manual wording above to mean 87 is design minimum, don’t use 85, and engine performance can be optimized using 91. Seemed to work.

In today’s age of consumer protection a company would be quickly harassed for “requiring” a more expensive propellant when they can program an engine to run on “regular” so the consumer can choose.

Sunnysidebeach wrote:
I have an older GMC Sierra, and even with its older technology it adjust the injection timing, valve timing, and ignition timing for different fuels. Under the manifold, is a knock sensor that detects the first sign of knocking and adjusts the timing to make that knocking stops?

My turned up 9.5 to 1 compression ratio 340 HP 5.3 Ltr motor becomes, a 240 HP motor, when pulling a load on 87 octane, I can just imagine what it would do on 85.

So while it will work at altitude, you will feel allot less power from your motor.

With a 9.5 compression ratio engine if you think you can get a 40% increase in power by using premium fuelover regular you're dreaming.

I have an older GMC Sierra, and even with its older technology it adjust the injection timing, valve timing, and ignition timing for different fuels. Under the manifold, is a knock sensor that detects the first sign of knocking and adjusts the timing to make that knocking stops?

My turned up 9.5 to 1 compression ratio 340 HP 5.3 Ltr motor becomes, a 240 HP motor, when pulling a load on 87 octane, I can just imagine what it would do on 85.

So while it will work at altitude, you will feel allot less power from your motor.

noteven wrote:
I drove a company F150 3.5 ecoboost that cost less money per mile for fuel running 91 vs 87.

This is from the owners maual for the ecoboost engine:

We recommend regular unleaded gasoline with a pump (R+M)/2 octane rating of 87. Some stations offer fuels posted as regular with an octane rating below 87, particularly in high altitude areas. We do not recommend fuels with an octane rating below 87.
To provide improved performance, we recommend premium fuel for severe duty usage, such as trailer tow.


Maybe the engine in your company F150 is all carboned up from using the wrong fuel. If the engine is healthy there is no way its going to be saving you money using premium.

I drove a company F150 3.5 ecoboost that cost less money per mile for fuel running 91 vs 87.

I always ran the cheapest grade gas I could get in my Acura Tsx which was designed for high octane fuel. It ran fine and got excellent fuel economy. My 2007 Victory Kingpin is a high compression engine designed for high octane fuel as well. It only gets low grade (87 octane). When I ride with others my bike always uses less fuel than anyone else (and they all follow the manufacturer's recomendation of premium fuel) even when I've got a passenger and the others ride solo. I've never heard it knock. I'm at about 3000 feet elevation. Anyone who burns high octane fuel in an engine designed for regular fuel is wasting their money.


Edit: I said the Victory has a high compression engine. It really doesn't but the owners manual calls for 92 octane or higher. The compression ratio of the Victory is 9.8:1
Acura TSX is 10.5:1

patperry2766 wrote:
For clarification's sake, I did buy the 85 octane. I could tell a difference but it's hard to differentiate exactly how much of a power loss because of the altitude vs fuel. I did do a considerable amount of driving without the trailer and I really couldn't tell much of a difference in either fuel economy or performance that couldn't be attributed to the elevation.

However, once I got back to Texas and out of the mountains and 87 fuel in Amarillo, I could tell a significant improvement in performance. I've made multiple trips to the mountains of CO and SD in the past and always bought the cheapest gas available without paying much attention. I honestly don't know why I noticed it this time, but I've got over 100K miles on my truck and have had no issues what's so ever based on my fuel.

I guess for my own peace of mind, next year, I'll try the 87 in the mountains to see if I can tell a difference.

It was altitude, not the fuel. 85 in CO runs the same as 87 in Armadillo.
SD doesn't have mountains lol..... I wouldn't be thinking 85 down there, but then again I don't think they sell 85 in SD, although I've only bought diesel there in the last 15-20 years.
For an engine (we're talking newer NA, engines that adjust timing based on multiple parameters) designed to run on 87, I'd keep with that for max performance up to around 5000' min. Above that, 85 will do the trick.
Depends how the vehicle is calibrated though. This isn't 1 size fits all generality.
For example, the 6.2 fords I had, couldn't tell the difference. But our old 06 SRT 8, near sea level is a turd on 87 and a rocket on premium. Road trip down the freeway, she drinks 87. Anywhere I want to use the throttle, gets 91.

shelbyfv wrote:
I'm not at all worried about following the manufacturers recommendation. One problem of autodidacts (most of us in most subjects) is we don't know what we don't know.

Very true. Sometimes a little knowledge is a dangerous thing.