gas octane question for PB F150

[Gros Ventre]

You’re considering only static compression ratios and not dynamic compression ratios. The only way you would not change dynamic compression ratio is if the valves closed at BDC (assuming the cylinder reaches ambient air pressure at BDC) for both 87 and 93 octane. I don’t know the cam profiles of these trucks so I don’t know exactly where the valves close in relation to the piston position, but I can pretty much guarantee it isn’t at BDC. Using higher octane allows “advanced timing”. This means that the valves will close closer to (but still after) BDC when using higher octane fuels than when using lower octane fuels (aka retarded timing). Advancing the timing and closing the intake valves sooner means less air is pushed out of the cylinder giving a higher *effective* AKA dynamic compression ratio. It sounds complex, but really the theory is pretty simple

Variable timing is kind of a “pseudo” variable compression ratio

My purpose wasn't to give a Post-doctoral dissertation... rather to simply at a top level for those reading it to get the general idea. Thank you for your guarantee abut BDC... For those who've never operated or repaired a Fairbanks-Morse opposed piston, valve-less, two stroke diesel engine going through that level of stuff was just over the top... So write down that stuff you wrote above and submit it for your next academic degree submission... but can it for these kind of discussions. PS There's nothing pseudo about the compression ratio, it is what it is... or are you trying to explain the atkinson cycle engine? Hm-m-m...

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[Snakebitten]

Dolsen's post wasn't that difficult to understand. Lots of folks around here have a pretty good understanding of the substantial number of variables in play on the 3.5 Ecoboost VS yester-decades much less sophisticated normally aspirated motors.

I enjoy a good debate.
It's loses it's appeal when it draws personal insults.

 

[dolsen]

My purpose wasn't to give a Post-doctoral dissertation... rather to simply at a top level for those reading it to get the general idea. Thank you for your guarantee abut BDC... For those who've never operated or repaired a Fairbanks-Morse opposed piston, valve-less, two stroke diesel engine going through that level of stuff was just over the top... So write down that stuff you wrote above and submit it for your next academic degree submission... but can it for these kind of discussions. PS There's nothing pseudo about the compression ratio, it is what it is... or are you trying to explain the atkinson cycle engine? Hm-m-m...

I’m not sure why you’re upset with me, can you elaborate on that? I only ask because you aren’t the first person on this forum today that’s trying to go back and forth with me in an egotistical way, so it must be something I’m doing. If you don’t care to elaborate, and only wish to sling more insults, save your breath. I explained a seemingly complex process in a very simple way (because it is simple)

 

[Samson16]

The issue isn't cheap gas. The issue is the only thing octane rating does for you is tell you how well the engine will resist detonation or self ignition. It's a rating. Your engine is designed (eg compression ratio) for a certain minimum octane rating. So long as the engine sees that rating, it'll operate fine. You can use "cheap gas" or spend more on "Top Tier" gas as your budget allows. Top Tier fuel does bring some benefits via additives etc. But when you select the octane button... buying higher than needed is spewing money out the exhaust pipe.

I think they want the engines to run on 87 as a marketing ploy. So they retard the timing until the engine won't pre-detonate. Unless,of course, you use your truck as well, a truck, then they recommend you put some decent gas in it before you damage something lol.

 

[Jus Cruisin]

Ran 87 in my 2016 F150 King Ranch 3.5l Ecoboost except when I was towing my enclosed car trailer. I continued using 87 in the Powerboost that I never used to tow anything. My 6.2l does require 93 so that's all it gets.

 

[JExpedition07]

Ran 87 in my 2016 F150 King Ranch 3.5l Ecoboost except when I was towing my enclosed car trailer. I continued using 87 in the Powerboost that I never used to tow anything. My 6.2l does require 93 so that's all it gets.

This is why I run 93 in my 5.0, the GMC 6.2L static compression is 11.5:1 and requires 93. The Ford 5.0L compression is even higher at 12.0:1…ya detonation resistance is a good thing at 12.0. The thing is pretty much sitting at race car compression.

The 5.0L can run 87 more efficiently than the 6.2L due to the more advanced fueling system and Ti-VCT. The 5.0L relies mostly on the port injectors at low speed, the direct injection comes on with increased RPM. It’s actually the exact opposite of the 3.5 EcoBoost. The 3.5 relies primarily on its direct injection system and the port injection is supplementary. Dual injection in both cases allows for better metering and control, and Ti-VCT can alter cylinder load. This is why Fords 5.0L and 3.5L Eco can both run safely on 87, but they are doing so with less power than on 93. The GM pretty much needs 93 to run well in my experience, it is super gimped on regular as all it can do is retard ignition timing big time.

 

[Samson16]

You’re considering only static compression ratios and not dynamic compression ratios. The only way you would not change dynamic compression ratio is if the valves closed at BDC (assuming the cylinder reaches ambient air pressure at BDC) for both 87 and 93 octane. I don’t know the cam profiles of these trucks so I don’t know exactly where the valves close in relation to the piston position, but I can pretty much guarantee it isn’t at BDC. Using higher octane allows “advanced timing”. This means that the valves will close closer to (but still after) BDC when using higher octane fuels than when using lower octane fuels (aka retarded timing). Advancing the timing and closing the intake valves sooner means less air is pushed out of the cylinder giving a higher *effective* AKA dynamic compression ratio. It sounds complex, but really the theory is pretty simple

Variable timing is kind of a “pseudo” variable compression ratio

I appreciate your explanations. I've always thought of compression ratio as the difference between the max and min cylinder volume. Interesting that BDC is the reference point for timing advance/retard. By delaying the valves closing more of the air escapes reducing the volume of air to be compressed. In that sense it is varying the effective compression ratio. Thank you.

 

[RalpL]

This thread is an interesting read, but I have to go back to the original issue about octane ratings. Like the F150 manual recommending 91 when towing, etc, my wife's BMW also recommends the use of 91. My issue is I never see 91 at any gas stations near me - the choices are 87 regular, 89 midgrade, and 93 premium. I'm sure there must be some stations that have it (Sunoco?). Why are the manufacturers fixated on 91 when it seems to me it's not in the normal supply chain? I suppose if I really wanted 91 I could mix 89 and 93, but am I missing something? I can excuse BMW because 91 is quite common in Germany.

 

[dolsen]

This thread is an interesting read, but I have to go back to the original issue about octane ratings. Like the F150 manual recommending 91 when towing, etc, my wife's BMW also recommends the use of 91. My issue is I never see 91 at any gas stations near me - the choices are 87 regular, 89 midgrade, and 93 premium. I'm sure there must be some stations that have it (Sunoco?). Why are the manufacturers fixated on 91 when it seems to me it's not in the normal supply chain? I suppose if I really wanted 91 I could mix 89 and 93, but am I missing something? I can excuse BMW because 91 is quite common in Germany.

Because 93 isn’t available nationwide, but 91 or greater is. That’s, from what I understand, the only reason

 

[dolsen]

I appreciate your explanations. I've always thought of compression ratio as the difference between the max and min cylinder volume. Interesting that BDC is the reference point for timing advance/retard. By delaying the valves closing more of the air escapes reducing the volume of air to be compressed. In that sense it is varying the effective compression ratio. Thank you.

I’m not entirely certain if that’s the reference point, I’m not an expert on engines, I’ll be the first to admit that. BUT max cylinder volume does occur at BDC and therefore, theoretically, has the highest volume of air (I won’t get into the fluid dynamics of it, so for arguments sake, we’ll say the volume of air is in fact at max at BDC). Anything past BDC would have a smaller volume of air assuming the valves are still open past BDC because the decreasing cylinder volume would push out the air. The “timing” is in reference to intake valve position relative to the piston position…

anyways I think I’m getting too deep in the weeds, for the sake of this thread, I think this is enough.

The process of how internal combustion engines work is very simple, the complexity lies in the details behind the purpose and use of all of the individual components and how they relate to and interact with each other
EDIT: I started rambling and forgot what I was saying haha! Yes your synopsis is correct

 

[Gros Ventre]

I’m not sure why you’re upset with me, can you elaborate on that? I only ask because you aren’t the first person on this forum today that’s trying to go back and forth with me in an egotistical way, so it must be something I’m doing. If you don’t care to elaborate, and only wish to sling more insults, save your breath. I explained a seemingly complex process in a very simple way (because it is simple)

Compression ratio is a straightforward calculation: what is the change in volume of the cylinder. The fact that they manipulate things with variable valve timing and such does not change the compression ratio. The fundamental fact of the Otto cycle is that if you use high octane fuel in an engine that isn't designed for it, you don't get more power. The rest of the explanation you put forward, such as guaranteeing that BDC & etc... was inappropriate and personal. Your explanation seemed to try and lapse over into the Atkinson cycle where valve timing is used to improve MPG.

 

[RalpL]

Because 93 isn’t available nationwide, but 91 or greater is. That’s, from what I understand, the only reason

Maybe there is a regional aspect. I'm in SW FL now, having moved from N VA near DC, and 93 is available everywhere which is what I use in my wife's BMW. I've only been using 87 in the F150.

 

[dolsen]

Compression ratio is a straightforward calculation: what is the change in volume of the cylinder. The fact that they manipulate things with variable valve timing and such does not change the compression ratio. The fundamental fact of the Otto cycle is that if you use high octane fuel in an engine that isn't designed for it, you don't get more power. The rest of the expanation you put forward, such as guaranteeing that BDC & etc... was inappropriate and personal. Your explanation seemed to try and lapse over into the Atkinson cycle where valve timing is used to improve MPG.

I didn’t guarantee anything, reread my post. You are the only one taking anything personal.

also, reread my post… I said it was a “pseudo” variable compression by changing the volume of air. If changing the amount of air (and by definition, increasing fuel so you don’t create a lean situation) in a cylinder doesn’t create more power then please tell me how forced induction works to add power.

did I mention you probably should reread my post? Also, read up on static vs dynamic compression ratios

 

[Gros Ventre]

I didn’t guarantee anything, reread my post. You are the only one taking anything personal.

also, reread my post… I said it was a “pseudo” variable compression by changing the volume of air. If changing the amount of air (and by definition, increasing fuel so you don’t create a lean situation) in a cylinder doesn’t create more power then please tell me how forced induction works to add power.

did I mention you probably should reread my post? Also, read up on static vs dynamic compression ratios

I've read your post and find it misleading. It appears that you are so taken with all the add on gadgets such as variable cam timing that you don't understand the basic Otto Cycle.

 

[Samson16]

Speaking of Otto, has anyone seen that movie yet?

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