Using higher than recommended octane fuel can result in incomplete combustion?
- Thread starter The Critic
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I think he needs to back off that crack pipe just a tad. If he said 87 to 100+ Sunoco race fuel it may be something to think about but I still cant see it. Not everyone posting on forums is getting full voltage to the bulb you know.
Now that you mention compression ratio, it's worth mentioning something that some here already know but others may not...
Compression ratio and cylinder firing pressure, while related, are 2 different things. Compression ratio is the ratio of the difference in cylinder volume between the piston at the top vs. bottom of the stroke. Cylinder pressure is the actual pressure in the cylinder, often expressed as an average or BMEP = brake mean effective pressure.
All else equal, a higher compression ratio produces higher BMEP and vice versa. But all else is not always equal. For example, an engine with an optimized well tuned intake can pack the cylinder fuller and increase BMEP without changing the compression ratio. An extreme example is forced induction, where a turbo or supercharger crams the cylinder full under pressure to achieve very high BMEP with a low compression ratio. Alternately, an engine may have a less efficient intake and use a high compression ratio to compensate for that. Ultimately it's BMEP that matters and compression ratio is just one way to get it.
BMEP is essentially torque per unit volume of displacement. Or, you can say that torque is BMEP times displacement. Most normally aspirated gasoline engines are in the ballpark of 1 ft. lb. per cubic inch displacement. If it has much more than that, it probably needs high octane gas. If it has less, it probably doesn't.
Of course there are many more factors that contribute to octane requirement: compression ratio, shape of pistons & combustion chamber, valve timing, exhaust and pulse timing, etc. Among all these, BMEP is one of the strongest contributing factors.
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HAHHAHA, love this:
BMW, sandbag? Say it ain't so!
That was wildly obvious if anyone watched that X5 video I posted recently.
I'll post the table from the article here though (good share!):
My personal observation.
Seen Corvette carboned up using premium fuel which I argued it was egr.
Oldsmobile with iron duke could run anything. Nothing mattered.
Saturn Vue 2.2l ran best on 89. 87 it would stall and hard start. It was carboned up and needed manual cleaning which I didn't want to do.
Caravan 91, 89, and 87 ran. Runs best on 87 however I get more miles between fill up on 89. I haven't ran anything but 87 for last 12k however since I found air intake leak I suspect 89 would run same as 87. 89 would ping under a load which is backwards. At 8.00 a tank more I am struggling to justify 89 however Dodge blames fuel for dropped valve seats stating engine was set up for higher octane. They claimed to change seat and valve design to handle lower octane but who knows.
Seen Corvette carboned up using premium fuel which I argued it was egr.
Oldsmobile with iron duke could run anything. Nothing mattered.
Saturn Vue 2.2l ran best on 89. 87 it would stall and hard start. It was carboned up and needed manual cleaning which I didn't want to do.
Caravan 91, 89, and 87 ran. Runs best on 87 however I get more miles between fill up on 89. I haven't ran anything but 87 for last 12k however since I found air intake leak I suspect 89 would run same as 87. 89 would ping under a load which is backwards. At 8.00 a tank more I am struggling to justify 89 however Dodge blames fuel for dropped valve seats stating engine was set up for higher octane. They claimed to change seat and valve design to handle lower octane but who knows.
HAHHAHA, love this:
BMW, sandbag? Say it ain't so!
That was wildly obvious if anyone watched that X5 video I posted recently.
I'll post the table from the article here though (good share!):
View attachment 98481
I would've liked to see them run the M5 on 87
Why? Your question would make (some) sense if gasoline would ignite by compression, which it (hopefully) doesn't. It's ignited by a spark (again, hopefully).
Actually higher octane fuel has marginally less fuel value (energy), which can result in slightly worse fuel economy (under low load conditions), however in many cases this is compensated by more advanced ignition (more so in a higher load scenario). Obviously this highly depends on the actual engine, load and other conditions (e.g. temperature).
It isn't worth losing sleep over it. The main difference is torque and, as a result, power.
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Water…maybe but too much diesel makes them hard to startNever thought about it this much. I run premium in the Trans Am, Caprice and Malibu. So everything gets it. Keeps it simple.
Anything else in my LS powered cars and I can see it pull timing in my logs. The Malibu DI 2.0 turbo loves premium.
The only think I own that does not care is the 1997 K1500. I think it may run on water...
I used to take the gas people had put into their diesels and run it in my 99 K1500. I got a little heavy on the diesel/gasoline ratio once and cold starts were quite iffy
My engines ignition timing is too aggressive for 87, even with knock sensors it still pings on it. Putting 93 in gives me the best power and performance. Many modern ECUs will try to push the limit to find the best timing with acceptable knock, if your car does this you might get better power from 93, if you have a low compression (10:1 or lower) port injected NA engine it probably won't make a difference.
Actually a port-injected engine will knock at a lower compression ratio than a direct injected one.
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I’ve always been an advocate for using the octane called for by the OEM. My understanding higher octane = slower to ignite which can be beneficial in many(FI/Euro engine/small engine) cases. Basically, for higher compression. As the old tuner’s adage, lean is mean but low octane increases the change of blowing holes or cracking ring lands in your pistons once you add boost or compression.
The Japanese have always said higher octane for better performance but “regular” octane in the US is adequate. In the US, our super/premium is Europe’s regular.
The Japanese have always said higher octane for better performance but “regular” octane in the US is adequate. In the US, our super/premium is Europe’s regular.
This is a great link!
Even though I infer the opposite conclusions form the author, who was primarily looking at 0-60 and quarter mile times for "difference".
It is amazing what the measurable power difference where and that there were some as much as 10 hp in vehicles originally rated for 87 only!!
And that a mere 2 Octane would make a measurable difference in the M5 is also amazing.
And in the F150 it make a difference of a full half second in the quarter!
People sometimes spend thousands on intake/headers/exhaust to achieve a half second gain in the quarter!
And this wasnt even at sea level or in summer time wich would have been sure to increase the deltas! (I live near sea level in Florida).
This is the smoking gun!
PS:
I run 93 Octane in my 9.8 compression ratio V8. Nissan "recommends" premium but states regular acceptable.
This article make me feel vindicated... plus there is also the fact that mine is a higher mile engine at nearly 177,000 miles so despite my best efforts to keep the internals clean I am sure some deposits must be there. (And those can serve as a source for pre ignition so I believe hi mile engines can do with at least a step up in fuel class regardless.)
Only when I do gentle (interstate at 65mph) driving at altitude I go down to midrange.
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I'm late to this party but...
Higher octane pump gas (91/93 vs 87) does NOT burn slower. A .010 change in specific gravity will have a higher impact on the burn rate than a 10 point spread in octane. The composition of the fuel is what matters when it comes to burn rate. For example, Sunoco SR18 (118 octane) burns faster than 87 pump gas.
Higher octane pump gas (91/93 vs 87) does NOT burn slower. A .010 change in specific gravity will have a higher impact on the burn rate than a 10 point spread in octane. The composition of the fuel is what matters when it comes to burn rate. For example, Sunoco SR18 (118 octane) burns faster than 87 pump gas.
This is exactly how I understand it as well. Octane affects the anti-knock index only. The octane rating does not affect the burn time. Cylinder pressure, fuel mixture, and ignition timing are more directly correlated to burn time.
No it isn't
Research octane (EU) vs knock index (R+M/2, US)
Theoretically, but I've heard of more people having issues with DI engines knocking on regular, although this probably has to do with the fact most of them are turbocharged and high compression to begin with.
Close, but not exactly. BMEP is directly proportional to torque per unit displacement. The work performed by the torque through a complete cycle (i.e., two revolutions for a 4-stroke engine) equals BMEP times displacement. In other words, BMEP is the pressure which would hypothetically produce the same brake torque in an idealized engine with the same displacement, no friction or throttling losses, and constant pressure through the power stroke. For a 4-stroke engine that cranks out 1 foot-pound per cubic inch, that works out to 48×pi psi, or about 151 psi.
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