Very interesting academic exercise. It looks like the ECM performed as expected on the two fuel types.

What is missing is why we care about the difference and why higher octane fuel requires more advance to achieve the same economy or performance and an explanation of my we advance the timing at all.

As we lean out the mixture the fuel burns slower. As we increase Octane of the fuel is also burns slower. So the reason for advancing the timing is to make up for the above two phenomena. Also as the engine speed increases we need to advance the timing because the mechanical items are moving faster while the rate of fuel burn remains the same. The object live of all of this is to get maximum burn at or slightly past Top Dead Center. This give us maximum pressure down on the pistons. Advance it too far and we get fuel burn on the upward movement of the pistons. Fire it too late (retarded) and we get less power because the pistons as are already started down when the fuel is at maximum burn.

Thanks for doing this and supplying the results.

 

Mine is a 6mt mind you, and i like to drive fairly hard. That being said my average mpg on 205/50-15" high performance summer tires over 350 miles of mixed freeway/canyon driving was about 30 mpg on 87 chevron. I take the same exact route every day, highway 4 to vasco road (great canyon road) to i580, and with using chevron 91 octane for 5 consecutive tanks, my average mpg over the exact same route, same weather, same driving style (i heel toe and clip the same exact corners every day, full throttle up the same onramps every time, then cruise control at 70mph in 6th on the freeway) and my average mpg has settled to 35-36 mpg over the same 350 mile fill up interval. These tests were all performed within a 2-3 week span of very consistent weather in the 50-60° range and i make multiple trips a day for my line of work. I easily rack up 150 miles a day with my job, so ALOT of gas passes through my tank.

 

You're welcome, and for the most part you are totally correct, except in the case of our 11.5 to 1 compression direct injected motors. In these engine conditions, an ultra lean fuel mixture actually burns easier, and has a much higher chance of detonation, you can easily hear these engines knock on 87 octane under load in low rpms. The purpose of advancing timing in a high compression, direct injected engine is to maximize combustion efficiency with a higher octane, or slower burning fuel source. In our case, peak performance is realized at the very top of the piston stroke, where cylinder pressure is at its peak, and the combustion chamber mounted injector sprays a highly atomized pulse of extremely high pressure fuel a fraction of a milisecond before the spark plug fires. With lower octane, this injection/ignition happens far below the top of the piston stroke, resulting in a much lower explosion pressure, making less downward pressure on the opposing piston. These little high compression motors simply CAN make more power with a higher octane fuel because of the mechanical compression force its capable of creating.

 

 

 

OCD also explains why I have a spreadsheet of my MPG readings since I bought the car lol

 

sounds like u have a good neighbor 😂

 

 

I've driven 43k miles so far and I've filled the tank many times with either 87 or 91. I feel like the 91 gives me smoother acceleration and smoother sounds. With the 87 I need to apply more pressure on the throttle to accelerate at the same rate. It gives me better gas mileage though. Just my personal observation...

Oh I should add that I don't drive my Fit very hard most of the time.

 

Same EXACT EXPERIENCE you describe on my end over 35K miles.
I primarily (not always) use 91 in California.
Its getting expensive here though--yesterday 3.89 per gallon fillup!

No one is required to put 91 in the FIT's EarthDream Engine and Honda will avoid telling you to do so because it would cramp car sales. But I am a firm believer that at least for the FIT it makes a difference.

 

 

another added bonus of using 91 is the advanced timing allowing the injector to spray the backside of the intake valves as they are closing up during compression, this is a crucial benefit to direct injected engines, we need all the help we can get with keeping intake valves clean. if you look up the 3d animated renderings of the l15b engine u can clearly see the combustion chamber mounted injector sprays from BEHIND the intake valves as they are still slightly open just before top dead center.

 

 

Heres a beautiful 3d rendering of our engines in action, notice how the injector sprays the backside of the intake valves while they are open, the benefits of using premium gas go way beyond better fuel mileage and power, and i believe it will keep the intake valves quite clean despite being direct injected. Honda is brilliant, i guarantee the people with valve "coking" issues are not running premium gasoline.

 

So if all GK5 engines work as that video suggest, then running regular gas will still spray the back of the valves, yes? Would that not solve the valve coking issue you refer to? Also I have not seen any reports of GK5s having this issue. If you have links you can provide, I'd be interested in reading up on it.

I'm sure someone will mention top tier gas here in a few...

 

The short answer is yes they still do, but how effective the lower grade fuels are at removing carbon deposits is unknown. Just a few pages down in this very forum there are postings about "possible direct injection" issues, in where the OP had valve coking, and even injector failure from clogging. In most of these instances the OP was not using premium gasoline even though they did use "top tier". Top tier is just "brand name" add packs, it's premium gas you want for these engines, not just top tier. 91 octane or better Top tier is the good stuff for these motors. I myself stick to 91 chevron like glue when not doing my comparison tests.

 

#41
@jungle168
@jungle168 , 09-28-2017 10:06 AM
Carbon Deposits
Hi fellas,

I wanted to chime in about issues related to my 2015 Honda Fit LX. Bought the car brand new from a local dealership. Got the extended 6 year/160,000km Honda Warranty because my precious vehicle, a 2009 MINI Cooper S, bled me dry.

The first time I got CEL, VSA, EPS lights was at the 28,000km mark. They replaced the injectors, fuel feed pipe, and performed a valve adjustment. The second time was at the 82,000km. Same symptoms prior to my dash being lit up like a Christmas tree again. They replaced the injectors, spark plugs, coil packs, and performed another valve adjustment.

The car car threw the same codes again last week and this time I told the service advisor that they should check the intake valves for carbon deposits since the engine used direct injection. Sure enough this is what the technician found...

 

Im almost 100 percent certain this is a combination of low quality gasoline, overly long oil change intervals (yes 9k via the maintenance minder is TOO LONG), and mediocre oil with weak detergents to begin with. Most people who own these cars seem to treat them this way. Just because its an economic car doesnt mean the engine isn't sophisticated and requires premium fuel, honda just didnt want to tell people you need to run premium gas in their lowest cost car to achieve its maximum performance and efficiency.

 

Ok I think I remember seeing that before. Not sure if the problem is emblematic as you'd be seeing it reported much more broadly. Looks like that person had early problems with it showing up at 28k kilometers. So maybe there car had a manufacturing defect from the get go?

Like I said, I'm at 53k and have been using whatever cheapest gas I can find. Lately it's been Murphy's because it's the closest and cheapest station near me. I also change my oil at around 9k because that's around 10% oil life. Been using full synthetic 0w20 from Quaker state or Pennzoil. So far so good.

Other vehicle is an 2004 4 runner and still put 10w40 conventional as that's what it calls for. Also use 87 in it and that vehicle has 154k miles on it.

If people want to use higher octane gas in there car because that makes them feel better, more power to them.

 

I think this more of an issue than people are willing to believe. I think owner ignorance and sensitivity to the engine is a huge variable from person to person. While modern synthetic oils have shown to keep a strong tbn and retain alot their additives over 10k plus miles, they also gradually collect and suspend a HUGE Amount of carbon via detergents (calcium, phosphorus, etc..) And when tiny amounts of carbon rich oil are ingested through the pcv system, through the intake, the oil is nearly atomized and the carbon suspended within it gets transferred to metal surfaces like the intake ports, and backside of the intake valve and stem. Also the owners manual states use minimum 87 octane, wich essentially means you are giving the car its minimum octane requirement, not what it runs ideally on, so effectively 87 is on the razors edge of acceptible pre detonation threshold givin the 11.5 compression. I come from a background of building high compression, high revving naturally aspirated b series engines, my experience with octane/compression ratio, and experiments measuring timing changes based on octane leave me with absolutely zero doubts that these engines run better and will last longer with higher octane fuel.