Regular vs Premium Gasoline

[Suns_PSD]

As I already stated the octane is determined by the compression ratio of the gas engine and nothing else. Higher compression leads to more pre-ignition and with a diesel engine with it very high compression ratio even diesel fuel can be ignited inside the cylinders.

Altitude was a factor with carburators and some need different jets to function well where the air was thinner or needed superchargers. With fuel injection this is not the case as the fuel to air ratio is regulated.

As a teenage I bought and read Colin Campbell's book "The Sports Car: Its Design and Performance" and although it was first published in 1954 it is still quite informative. Today's cars have computer controlled fuel injection and electronic spark control but are otherwise unchanged over the past 100 years.

During WW II the Germans were surprised at the performance of British fighter planes until they were able to investigate a plane on the ground and learned that the engines were using 100 octane gas and so could use a higher compression ratio in the cylinders and produce more power. The Rolls-Royce supercharged engines could have a higher compression ratio thanks to the availability of 100 octane aviataion fuel.

For the past 30 years the use of turbo chargers is the best way to increase horsepower while having maximum fuel economy with smaller engines. A sedan can travel at 70 mph and needs only 15 horsepower to maintain its speed as with the Citroen cars. The trick is to be able to provide more power for acceleration when needed and that is where turbochargers or electric motor assist with a hybrid design, come into play.

I think you were trying to say that 'the REQUIRED octane is determined by the compression ratio ...and nothing else'.

However, this isn't remotely true and hasn't been for decades. The quick version is that you are discussing 'static compression ratio', a mathematical calculation based on the head and piston dish volume multiplied times the stroke. In reality engines first and foremost have what is called dynamic compression that is always changing based on how when the valves open and close which varies constantly due to adjustable valve timing.
Other factors that affect cylinder pressures are forced induction air is super-heated and ignition timing varies dramatically on modern motors with RPM, throttle position, altitude, etc.

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[Calson]

This is not true. We have variable cam timing and overhead cams, more valves per cylinder, exotic alloys in heads and engine blocks, catalytic converters, lead free fuel and ethanol in all pump gas, direct injection, O2 sensors, knock sensors, ethanol sensors, MUCH MUCH more efficient turbos, synthetic oil, higher compression ratios, many more gears in transmissions, higher speed limits etc.

Overhead cams have been around since the 1930's. Lead free fuel was the norm until GM invented tetraethyllead as a gasoline additive.

New alloys and the use of ceramic materials has improved engine durability as have better motor oils. Synthetic oils only provide better lubrication at very low viscosities needed for post 2017 engines in an effort to improve fuel economy and nothing more.

If anything the move has been to engines with lower compression ratios that can use lower octane gas as well as biofuels (pushed by the corn distributors).

More gears in AUTOMATIC transmissions to improve fuel economy with marginal drivers. Same is true for the trucking industry where the top drivers can get 9-10 mpg wheras average drivers with the same rig and same loads on same routes average 5-6 mpg.

None of this applies to octane ratings for gasoline. People equate a higher octane rated fuel as having a higher Btu and that is not the case. A gallon of gasoline provides 120,214 Btu as compared to a gallon of diesel fuel that provides 137,381 Btu or propane at 91,452. Anyone familiar with ICE powered by gas, propane, and diesel already knows this. More mpg with higher Btu and not higher octane fuels.

 

[HammaMan]

Overhead cams have been around since the 1930's. Lead free fuel was the norm until GM invented tetraethyllead as a gasoline additive.

New alloys and the use of ceramic materials has improved engine durability as have better motor oils. Synthetic oils only provide better lubrication at very low viscosities needed for post 2017 engines in an effort to improve fuel economy and nothing more.

Technology has improved. Engine durability is the result of computer modeling and iterative design, not alloys. Better design, better tolerances at mass scale. It's not just synthetic oils that have changed, ALL oils have changed. There's not a single oil on the market today that has the same chemistry as at the turn of the century. There's only 3 suppliers for the base oil stock. The only difference is the additive package.

 

[SonarChief]

Overhead cams have been around since the 1930's. Lead free fuel was the norm until GM invented tetraethyllead as a gasoline additive.

New alloys and the use of ceramic materials has improved engine durability as have better motor oils. Synthetic oils only provide better lubrication at very low viscosities needed for post 2017 engines in an effort to improve fuel economy and nothing more.

If anything the move has been to engines with lower compression ratios that can use lower octane gas as well as biofuels (pushed by the corn distributors).

More gears in AUTOMATIC transmissions to improve fuel economy with marginal drivers. Same is true for the trucking industry where the top drivers can get 9-10 mpg wheras average drivers with the same rig and same loads on same routes average 5-6 mpg.

None of this applies to octane ratings for gasoline. People equate a higher octane rated fuel as having a higher Btu and that is not the case. A gallon of gasoline provides 120,214 Btu as compared to a gallon of diesel fuel that provides 137,381 Btu or propane at 91,452. Anyone familiar with ICE powered by gas, propane, and diesel already knows this. More mpg with higher Btu and not higher octane fuels.

From Car and Driver

At 128.7 horsepower per liter, the F-150's high-output V-6 engine is more power dense than a Porsche 911 Carrera's twin-turbo flat-six. Naturally then, the Ford hauls ass as effortlessly as it hauls a half-ton of manure. When fed 93 octane, this 5594-pound, self-propelled wheelbarrow will crash 60 mph in 5.3 seconds.

Power at the wheels dropped from 380 to 360 horsepower with the change from 93 to 87 octane. That difference seemed to grow, and we could even feel it from the driver's seat at the test track. Compared with premium fuel, regular feed sapped the F-150's urgency both leaving the line and in the meat of the tach sweep. The rush to 60 mph softened to a still-blistering 5.9 seconds, and the quarter-mile stretched from 14.0 to 14.5 seconds, with trap speed falling 4 mph. Tapped into the Ford's CAN bus, we recorded a peak boost pressure roughly 1.9 psi lower during acceleration runs on regular gas, down more than 10 percent compared with the 18.1-psi peak on premium. The high-octane gas also helped when soft-pedaling the accelerator, elevating 75-mph fuel economy from 17.0 to 17.6 mpg. That won't make a financial case for running 93 octane, but then you didn't buy the expensive engine as a rational choice. You can think of this EcoBoost engine's more aggressive high-octane tune as a sort-of sport mode that can be switched on or off with every fill of its 36.0-gallon tank.….

https://www.caranddriver.com/features/a28565486/honda-cr-v-vs-bmw-m5-ford-f-150-dodge-charger/

 

[dochawk]

Copy that. Playing the ecoBoom! game of chicken. No thanks. 93 and no tune for my boring little PB.

Although I've seen claims (probably on this forum or a linked article) that the ecoboost has internal reference of 98 octane.

*If* that is true, you're playing chicken on any fuel short of aviation gasoline; it's jus a question of which rooster.

Once lead was removed from gasoline there was a gradual reduction in crime with fewer brain damaged children becoming adults.

Yet we still have enough to staff congress and 50 state legislatures!

More gears in AUTOMATIC transmissions to improve fuel economy with marginal drivers

Are you saying that a skilled driver can get the same mileage with a 3+O automatic as a 10 speed? ?

 

[Samson16]

the ecoboost has internal reference of 98 octane.

What am I to take from that? Are you saying that the combination of compression ratio, boost pressure, and various vacuum/load factors that the 3.5 EB engine actually requires 98 octane to run mid-scale on the tables?

 

[Calson]

The differences between the Ford 3.5L and the Porsche engine are many but in particular the torque curve for the two engines. I owned a 1957 Triumph roadster and its engine was originally designed for use in tractors and so it had far more torque at low rpms than the engines used by Porshe. Traveling along Highway 1 on the California coast I would encounter the Porsche cars and they had a higher top speed but could not keep up with the Triumph on the uphill grades.

"Power dense" is a meaningless term. The fuel determines the Btu's available to the engine. My last truck had a diesel engine and it produced double the lb ft of torque of the gas V-8 and it did so at half the rpms.

Also being ignored is the heat buildup with greater horsepower and the need to dissipate that heat as effectively as possible. The Duramax diesel could pull a heavy trailer up an 8% grade at 7,000 feet of elevation in 105 degree heat and the engine coolant stayed within it normal range. The same was true for the ATF of the Allison transmission.

My Mercedes CLK with its high compression engine got its best fuel economy with 93 octane gas. If I used 87-89 octane gas the mpg was reduced by 10%. I paid 10% more for the 93 octane and got 10% more mpg so it was a wash. That is by no means the case with the 3.5L engine if one is not towing a heavy trailer. The devil is in the details.

 

[dochawk]

Are you saying that the combination of compression ratio, boost pressure, and various vacuum/load factors that the 3.5 EB engine actually requires 98 octane to run mid-scale on the tables?

Not saying, but repeating another source.

It said the reference point (?) is 97, and it adjusts down from that based upon what it calculates the fuel in the tank to be. So boost in particular would keep going up for fuel up to 97, as well as, apparently to a smaller extent, timing adjustment.

 

[Samson16]

I see. That allows for @HammaMan et al to brew their E26.325 fuel cocktails. ?

 

[SALEEN961]

Not saying, but repeating another source.

It said the reference point (?) is 97, and it adjusts down from that based upon what it calculates the fuel in the tank to be. So boost in particular would keep going up for fuel up to 97, as well as, apparently to a smaller extent, timing adjustment.

The factory calibration on my 2021 3.5EB starts out with an inferred octane rating of 89.2 at 0.0 OAR after a KAM reset. When the knock sensors are very happy and the OAR adjusts to -1.0 the inferred octane rating is 98. I'm not sure what the low end of the spectrum for inferred octane is when the OAR reads 1.0.

 

[Samson16]

I'm not sure what the low end of the spectrum for inferred octane is when the OAR reads 1.0.

Isn’t that where an “87 tune” would have to work its magic?

 

[UGADawg96]

Isn’t that where an “87 tune” would have to work its magic?

Why did you change your avatar to a Jeep???

 

[Samson16]

Why did you change your avatar to a Jeep???

We bought one yesterday! Sahara with the 2.0T/ZF8 Mckinley seats and Alpine sound. I’ll switch it back to the PB in a bit. I was directed to the JLWrangler forum which is affiliated when I first posted about it and my account is good for both ?

 

[HammaMan]

We bought one yesterday! Sahara with the 2.0T/ZF8 Mckinley seats and Alpine sound. I’ll switch it back to the PB in a bit. I was directed to the JLWrangler forum which is affiliated when I first posted about it and my account is good for both ?

Blasphemy!

 

[SALEEN961]

Isn’t that where an “87 tune” would have to work its magic?

It depends on how the tune is structured. You could structure the tune to have a -1.0 OAR with 87oct if you wanted to. On the stock tune 87oct fuel should give you a positive OAR, but the reading is inferred and not measured. Things like IAT, ECT, MCT, driving style, and even rattling heatshields play a role in how the PCM infers the octane of the fuel being used.

I hope to never get a tank of gas bad enough to max it out at +1.0, but if -1.0 is 98oct, and 0.0 is 89.2oct, a reading of +1.0 might indicate something close to 80.4oct.

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