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Old 08-14-2017, 01:33 PM   #71
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Originally Posted by Dragonman View Post
Certainly.

While I'm not gonna pretend to know what's an optimal detonation point in the compression stage, I imagine it's at that point where the ratio achieves its designed 11.5:1, I do know that's what adjusting the timing does. Not sure if the LFX has variable valve timing or not but if the intake valves open to soon or remain open to long can also contribute to "knock", which is detonation of fuel/air outside the point it's supposed to be exploding and is not good for the motor. Pistons and valves are in the wrong place when it happens. The lower the octane rating of a fuel usually means it can be fired simply by compressing the mix, think diesel engines, the higher it is requires external stimuli to achieve explosion.

So, the car is designed to give it's full power within a single cylinder with a air fuel mix containing 93 octane when the piston has compressed it all to that 11.5:1 ratio. Putting 87 in it causes pockets of the A/F mix to detonate before or after the ratio is reached. When the computer senses it happening it adjusts the timing which will change the compression ratio the A/F mix gets to when fired. Either before, at or after the piston reaches top dead center (TDC). I don't know which way it adjusts but I'm sure someone here can tell us.

In my experience, however limited, retarding timing makes the engine output less power while advancing the timing has the opposite effect, up to a certain point. If the timing is advanced to far the spark will not hit the A/F mix during any amount of ignitable compression and the car won't run.

Just thinking about it, I wouldn't think you'd want the explosion to hit while the piston is traveling up. TDC probably isn't really all that good for the rods or crank either, but, it can't be very far past TDC on the down stroke or you lose the compression ratio needed.

Alright, I've probably rambled ignorantly on long enough.

Still about 4 1/2 months away from the end of my 6 month 93 octane study.
No rambling at all! I always thought advancing the timing would "increase" horsepower to a point.

11.5 to 1 is a high ratio, so by this logic alone - it should probably use higher octane.
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Old 08-14-2017, 03:39 PM   #72
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Advancing the timing does mean beginning the burn before TDC. So by the time the piston is passing TDC the majority of fuel is burning.

It isn't as if all the fuel explodes in an instant. One of the most important aspects in engine design is the burn pattern. Air and fuel are still being introduced to the chamber when ignition begins and the way it swirls determines the rate and efficiency of the burn. Higher octanes burn slower and thus ignition can begin earlier. This allows more of the fuel to burn, thus increasing power and MPGs.

Anyway, with direct injection, the computer controls when and how much fuel is introduced along with the spark timing. Thus detonation can be avoided by delaying the introduction of fuel. But this means there is less time to add fuel and still expect it to burn effectively. Resulting in lower power and lower MPGs.

The main way to control the burn and the way the air/fuel moves involves the shape and length of the intake and exhaust ports, the placement and size of the valves, the rate and height and time at which valves open and close, the size and shape of the combustion chamber, etc.

And that is a simple explanation

As you can see, it is quite involved and it used to be all trial and error. With the advent of super computers and dynamic equations much of the guess work can be avoided, but it still comes down to building and testing, tweaking, repeating.

Of course, cost and reliability are always factors affecting the final product. There comes a point of return on investment. The masses do not want to spend another $1,000 on a car for 0.1 more MPG or 1 more HP.

Thanks. I knew there would be someone nearby that knew more about this than I did. I reckon I never really thought about the "Direct Injection" part and still had it in my mind the air and fuel both were passing through the intake valve and what you said changed that perception. All that's passing the intake valve is air and the mixing of fuel with air occurs in the cylinder. Used to be timing only referred to ignition spark. Now it can refer to spark + fuel + valves and other things I'm unaware of.

Quote:
Originally Posted by rbenotti View Post
No rambling at all! I always thought advancing the timing would "increase" horsepower to a point.

11.5 to 1 is a high ratio, so by this logic alone - it should probably use higher octane.
Yeah, I probably should've adhered to the rule from my bike. It has 9.5:1 ratio and baldy states in the manual to put no less than 93 in it. If you can't get 93 the manual states to pour in octane booster. I'll look tonight That's what I get for trying to save a buck or two. I don't think I'll go back to putting 87 in it, even after the "study" is done.
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Old 08-14-2017, 04:58 PM   #73
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You increase the spark advance to get the peak cylinder pressure to hit at 14-20 degrees After Top Dead Center, that is where the connecting rod has mechanical leverage on the crank so it will push down. If you were to dyno an engine constantly and keep advancing the timing, eventually you will get to the point where you will not gain as much power anymore or lose power. That is considered the MBT and you use what octane you can to get to that timing.

Also, higher octane does not burn slower, the same fuel types burn at the same rate. Octane rating is just how resistant the fuel is to self-igniting under heat/pressure. Alcohol based fuels burn slower and cooler than gasoline, so E85 will have a higher MBT than 100% gasoline.
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Old 08-15-2017, 05:38 PM   #74
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There are a lot of ways to spin this discussion but you can't dispute the laws of physics or mechanics. Just from a personal observation, for the 1st 2000 miles of ownership, I used 87 octane and was averaging 19.2 mpg. For the next 3200 miles, I used 93 octane and jumped up to 19.6 mpg and that was after installing a CAI and going with aluminum wheels so in that respect, it doesn't appear to be advantages to go with premium. However, it does feel a little more peppier and I'm noticing less "soot" buildup on the tail pipes. Unfortunately, most of my driving is 40/60% city/highway so, hopefully, YMMV. I would like to say the 93 burns cleaner and is more responsive but I really can't say ... maybe when gas prices go back up I'll drop down to 87 but for now, I'll stick with the 93 just because of the perceived performance increase.

It's a sports car dammit!
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Old 08-15-2017, 05:56 PM   #75
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Originally Posted by OlePhu View Post
There are a lot of ways to spin this discussion but you can't dispute the laws of physics or mechanics. Just from a personal observation, for the 1st 2000 miles of ownership, I used 87 octane and was averaging 19.2 mpg. For the next 3200 miles, I used 93 octane and jumped up to 19.6 mpg and that was after installing a CAI and going with aluminum wheels so in that respect, it doesn't appear to be advantages to go with premium. However, it does feel a little more peppier and I'm noticing less "soot" buildup on the tail pipes. Unfortunately, most of my driving is 40/60% city/highway so, hopefully, YMMV. I would like to say the 93 burns cleaner and is more responsive but I really can't say ... maybe when gas prices go back up I'll drop down to 87 but for now, I'll stick with the 93 just because of the perceived performance increase.

It's a sports car dammit!
Ahaha!! 4 tenths/gallon is nothing, but maybe it illustrates a point. If it isn't any peppier, and there isn't any damage with lower octane (and you are ok with it), higher octane doesn't make sense. Not downplaying the reducing timing aspect - the car is doing what it is ultimately designed to do.

That said - I put 91 (or higher) in my Harley after talking to a guy a couple years ago on a bike trip about higher compression. So I'll give it a try.

Of course - I'll likely be adding a Vitesse at the same time, so it will feel faster off the line anyway.
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Old 08-17-2017, 10:46 AM   #76
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compression ratio alone doesn't determine the octane requirement of the engine. .4MPG change, yet you put on a less restrictive intake and lighter wheels and attribute all changes to the wrong fuel?
I retired from GM, but I have somewhere in my boxes of crap I've got the air/fuel/timing map for the stock ECU. It can adapt from 86 to 93, but it's not going to result in more power with higher octane without the tune to go with it. The ECU has a very mild "aggression" when it comes to timing.
Use higher octane if you want, but in all likelihood you're causing carbon to build up faster in the combustion chambers. I've seen 100K mile LFX engines from Equinox owners who ran high octane because they believed marketing- it's not pretty in there. The LFX is known to have issues from carbon deposits already- so why add to the problem?
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Old 08-17-2017, 02:07 PM   #77
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What about those of us who have the LLT engine. Does it also have a tendency to have carbon deposit buildup too? What can we do about it?
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Old 08-17-2017, 02:15 PM   #78
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Quote:
Originally Posted by VTSummit View Post
compression ratio alone doesn't determine the octane requirement of the engine. .4MPG change, yet you put on a less restrictive intake and lighter wheels and attribute all changes to the wrong fuel?
I retired from GM, but I have somewhere in my boxes of crap I've got the air/fuel/timing map for the stock ECU. It can adapt from 86 to 93, but it's not going to result in more power with higher octane without the tune to go with it. The ECU has a very mild "aggression" when it comes to timing.
Use higher octane if you want, but in all likelihood you're causing carbon to build up faster in the combustion chambers. I've seen 100K mile LFX engines from Equinox owners who ran high octane because they believed marketing- it's not pretty in there. The LFX is known to have issues from carbon deposits already- so why add to the problem?
So basically it can learn and adjust, but with no real benefit, other than adapting to fuel (which I guess is important).

Carbon deposits? Are we back to the days of fuel cleaners (which I was under the impression weren't really needed anymore)? This is not good in my eyes.

The more things change, the more they stay the same.
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Old 08-17-2017, 07:22 PM   #79
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The only thing I will suggest (as stated by someone else too) is to use a top tier gasoline. There are many on the market. They put in more(than what's required by law) additives and detergents to keep our engines clean. I use Mobil,ONLY Mobil regular 87 octane in all of my vehicles. Once a year in my Camaro I run a bottle of AC Delco Fuel System Treatment. It's a little expensive if you purchase it from your local Chevrolet Dealer, so I usually try to buy it on line. No fuel issues!!!
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Old 08-18-2017, 01:19 AM   #80
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Originally Posted by OlePhu View Post
There are a lot of ways to spin this discussion but you can't dispute the laws of physics or mechanics. Just from a personal observation, for the 1st 2000 miles of ownership, I used 87 octane and was averaging 19.2 mpg. For the next 3200 miles, I used 93 octane and jumped up to 19.6 mpg and that was after installing a CAI and going with aluminum wheels so in that respect, it doesn't appear to be advantages to go with premium. However, it does feel a little more peppier and I'm noticing less "soot" buildup on the tail pipes. Unfortunately, most of my driving is 40/60% city/highway so, hopefully, YMMV. I would like to say the 93 burns cleaner and is more responsive but I really can't say ... maybe when gas prices go back up I'll drop down to 87 but for now, I'll stick with the 93 just because of the perceived performance increase.

It's a sports car dammit!
Quote:
Originally Posted by VTSummit View Post
compression ratio alone doesn't determine the octane requirement of the engine. .4MPG change, yet you put on a less restrictive intake and lighter wheels and attribute all changes to the wrong fuel?
I retired from GM, but I have somewhere in my boxes of crap I've got the air/fuel/timing map for the stock ECU. It can adapt from 86 to 93, but it's not going to result in more power with higher octane without the tune to go with it. The ECU has a very mild "aggression" when it comes to timing.
Use higher octane if you want, but in all likelihood you're causing carbon to build up faster in the combustion chambers. I've seen 100K mile LFX engines from Equinox owners who ran high octane because they believed marketing- it's not pretty in there. The LFX is known to have issues from carbon deposits already- so why add to the problem?
If OlePhu installed RS aluminum wheels and tires he saved 6.8 lbs. on the wheels but added 16 lbs. with the tires. For a net increase of 9.2 pounds. And the increase in weight, being in the tires, means it is as far away from the center of rotation as you can get. That is certainly a detriment.

As for a CAI, I haven't experimented on my Camaro, but from what I see there may be a gain in WOT (where everyone dyno tests) but I am willing to bet there is no gain at all, and perhaps even a loss in efficiency, when driving like a citizen.
By deleting the resonator in the stock intake you are introducing a high pressure area in your intake tube that limits airflow.

As for your Equinox owners.

Just how often do you think they take the machine out and blow the carbon out? If they are the typical drivers I see on the streets the answer is never.

I see idiots who slow because the vehicle in front of them is going to make a turn, and when the road is clear again, there is not one chance in hell that they will press the accelerator enough to make the vehicle downshift to regain their speed. They think they are going to blow up if they cause a downshift, "Oh My God! What is all that noise?"
While you and I say "Whoo Hoo!"

My ex used to putz back and forth to work and every other week I would take her car to refuel it and blow the carbon out. I could actually see in the rearview the crap spewing out the exhaust. Two or three WOTs and it was good to go for another couple of weeks.

When I get the chance at home I spark up the old stereo and burn the dust off. Crank it up and you can actually smell it.

It is firmly in my mind that, just like people, machines need exercise or they will wither. My LS3 cries at me if I don't go WOT often enough.
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Old 08-18-2017, 04:50 PM   #81
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FYI for those claiming Top Tier fuel or additives have any effect. your engine is direct fuel injection. No fuel touches any internal part so nothing is "kept clean" by doing so. Watch this video (yes, a Ford, but excellent explanation of port injection where top tier fuel had a definite benefit) as there are illustrations and more so all can understand there engines hare very little in relation to the old port injection engines. And so much misinformation is spread via dealer, etc.





Just sharing to help owners understand their engines and the special care these need. ONLY use a full synthetic oil. The 5w20 is ONLY to allow the automaker to meet Cafe fuel standards. You want a 10w30 for normal driving and consider like Amsoils 5w50 for hard driving.


91-93 Octane and your engine will run more efficiently as the 11.5:1 CR is capable of using even higher octane. It does no harm to use 87, just power and fuel economy is lower due to timing retard.


Any in tank additive is pretty much throwing away $ as is any top tier fuel. Only use a fuel injector cleaner at most every 20-25k miles. These injectors operate at 2000-3000 PSI.


NEVER use a solvent based engine running upper induction cleaner such as Seafoam or BG, etc. if over 15k miles. They do little to remove the coking deposits but what is loosened is a very hard abrasive nature and smaller particles get forced between the piston and cylinder wall causing scouring.


GDI engine experience many times the raw fuel washdown as the old port injection engines so your motor oil is under severe stress from this and the other contaminants that enter the crankcase. So a proper truly effective air/oil (catchcan) crankcase evacuation system will remove most of these before they can mix with the engine oil. If not, changing oil at shorter intervals is a good idea.


Hope this helps clarify as so much misinformation is spread out there.


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Old 08-19-2017, 02:47 PM   #82
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If OlePhu installed RS aluminum wheels and tires he saved 6.8 lbs. on the wheels but added 16 lbs. with the tires. For a net increase of 9.2 pounds. And the increase in weight, being in the tires, means it is as far away from the center of rotation as you can get. That is certainly a detriment.
You are correct, sir, which is one of the reasons I stuck with the 18" aluminum instead of going up to the 20"... well one of the reasons:

18 x 7.5" / steel 14.95Kg/32.9lbs each. = 59.8Kg/131.5lbs Total (P245/55R18 all-season)
18 x 7.5" / aluminum 10.47Kg/23lbs each = 41.88Kg/92.1lbs Total (P245/55R18 all-season)
Wheel weight difference is 39.4lbs lighter ... Tire weight remains the same

Quote:
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It is firmly in my mind that, just like people, machines need exercise or they will wither. My LS3 cries at me if I don't go WOT often enough.
As for the CAI, I can't say how much of an advantage there is to it and maybe there's a real detriment to mpg, especially in city driving. I am learning to control the urge to punch it when in 2nd or 3rd just because it reminds me of the old Holly 4 barrel kicking in ... maybe someday, I'll grow up and drive like a citizen ... but I doubt it.
As for 87 vs 93 octane:
Best mpg with 87 (179 miles) was 24.1 and that was straight highway with cruise control at 75 before lighter wheels
with temperature in the mid 80's.
This week with 93 (398 miles), lighter wheels, mostly highway with cruise control between 75-80 and some city stop and go, I averaged 25.5 mpg
with temperature in the mid 90's.

Now, I admit, I don't know how much to attribute the increased mpg to the lighter wheels or to the 93 octane but my driving style hasn't changed all that much. I only run Shell or Chevron just because that's what I've always run for whatever reason. Just out of curiosity, I may go back to 87 and see how it affects mpg and performance now that I've done a few mods that weren't in place before and see if there's any difference though.

Regardless, I'm just enjoyin' the ride ... hope you are too.

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Old 10-19-2017, 04:46 PM   #83
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I'll add my two cents for what it's not worth...

Shell VPOWER premium octane made a noticeable difference with my LFX. It suddenly became much more responsive... It felt similar to the first day I got my CAI installed.

Anyone else noticed this with Shell v power?
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Old 10-19-2017, 05:35 PM   #84
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For the first 3 months I had my car I only ran 87 in it and averaged 25.1 across that entire timeframe with about 75/25 highway. For the last 7 or 8 tanks I have been using 89 and I am averaging 22.5 with the same drive. I am switching back to 87 when I fill up tonight to see if it goes back up.
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