zoom44

http://www.cloverleaf-auto.com/fuel.htm

http://66.102.7.104/search?q=cache:B...ient=firefox-a

http://www3.bc.sympatico.ca/Volvo_Books/maint8.html

http://www.sdsefi.com/techcomb.htm

have to say that i am havimng troble with my google serched this wek. those arent what i read before but seem to be saying, after a cursory reading, what the others did.

MazdaManiac

iTrader: (3)

None of those pages suggest that gasoline is left partially uncombusted should the octane be too low.
In fact, the first page suggests the opposite.

pcimino

Okay, the confusion here is yes, lower octane burns quicker, but not more completely. Remember this is a dynamic system. The octane rating for a particular compression allows the fule/air to mix and compress to the proper ratio for the flame front. So what happens is the lower octane would cause knocking, if the computer didn't delay ignition. Because ignition is delayed to avoid knocking, the gas actually has less time in the cycle to burn.

Here's a technical link:
http://www.faqs.org/faqs/autos/gasoline-faq/part3/

And the relevant section, the key to understanding this is "retarding the spark lowers the octane rating...." and "If you advance the spark, the flame front starts earlier.... High octane fuels produce end gases that take longer to autoignite, so the good flame front reaches and consumes them properly. "

Section follows:

7.4 What is the effect of changing the ignition timing?

The tendency to knock increases as spark advance is increased. For an engine
with recommended 6 degrees BTDC ( Before Top Dead Centre ) timing and 93
octane fuel, retarding the spark 4 degrees lowers the octane requirement to
91, whereas advancing it 8 degrees requires 96 octane fuel [27]. It should
be noted this requirement depends on engine design. If you advance the spark,
the flame front starts earlier, and the end gases start forming earlier in
the cycle, providing more time for the autoigniting species to form before
the piston reaches the optimum position for power delivery, as determined by
the normal flame front propagation. It becomes a race between the flame front
and decomposition of the increasingly-squashed end gases. High octane fuels
produce end gases that take longer to autoignite, so the good flame front
reaches and consumes them properly.

The ignition advance map is partly determined by the fuel the engine is
intended to use. The timing of the spark is advanced sufficiently to ensure
that the fuel-air mixture burns in such a way that maximum pressure of the
burning charge is about 15-20 degree after TDC. Knock will occur before
this point, usually in the late compression - early power stroke period.
The engine management system uses ignition timing as one of the major
variables that is adjusted if knock is detected. If very low octane fuels
are used ( several octane numbers below the vehicle's requirement at optimal
settings ), both performance and fuel economy will decrease.

GhostRidr

An interesting article:

http://www.usatoday.com/money/autos/...emiumgas_x.htm

TRZ750

Didn't read all the post that got on the side track of octane. The issue is 15mpg to 22mpg. This difference was probably really caused by the first fill up being to a higher point than the second and/or MANY GAS STATIONS CHEAT ON THEIR GALLONS READING. Some have been caught with a switch that makes the pump hot only when wanted (or more likely only when needed to be accurate). Especially with a late at night - early morning fill up.

I usually get ~22mpg at 80mph so the second tank sounds correct, but not the first.

zoom44

thank you pcmino thats much more like i read awhile ago.

amartin

I'll try to be as exact as I can in explaining the events, and how I obtain my MPG.

First, How I full my tank:
I put the fuel pump in, and click the metal lever down. When the fuel pump self-shuts off, I remove the nosel. I do not squeeze extra fuel in. And I rely on the fact that the pumps should (with in a fairly small % error) cut off near the same point each time.

How I measure:
Miles Driven / Number gallons pumped in to the 3rd decimal.

200.3 / 12.456 = 16.08 mpg

1) The trip FROM austin TO SA is DOWNHILL, so my milage should have been better.

2) There is no way I was running 84MPH in 4th gear-- I'de have been turning 6500-7000 RPMs. And I'm a VERY alert driver with respect to what gear I'm in, and certainly 6500rpms isn't exactly "non-obvious" when you're winding the engine out for an hour at that RPM'age.

3) It wasn't a windy day, however, I can understand 5-10mph wind, but how much drag versus fuel are we talking about here? There was a >50%< different, not 10% difference in fuel.

4) On the return trip from SA, I stopped at the gas-station, filled up, reset the ODO and made my return.

-- Aaron

MazdaManiac

iTrader: (3)

However, this assumes the PCM retards timing just because the octane is lower, which it does not. There is only a negative change to the timing profile if there is a knock event. The PCM has no way of knowing that the octane is lower until there is a knock event.
Even if there is a knock event, there will have to be several in a row in a particular load range before the profile is changed significantly.
Even so, it would take nearly 20° of retard before it would be possible to delay ignition sufficiently to have the combustion process extend to the end of the exhaust cycle.
Remember, just because we are no longer in the combustion phase doesn't mean combustion ceases.

Sigma

The problem is that they don't. Not even close. You can be virtually certain the difference is a good half-gallon, and it's not unusual at all for it to be as high as a gallon or even more difference. There is no requirement that a pump shut off at the same point in time, and there's a fine line between a pressure that makes too many cars shut off way too early and a pressure that doesn't stop at all. Every station is different.

I use the same 3 pumps in my area depending where I need to fill up at. And I track my mileage religiously. As long as I use the same pump consecutively everything's fine, but if I refuel at a different location, my mileage appears to change by a good 3mpg one way or the other. Of course the average is still the same, but taking that take as a whole and the next tank as a whole, it's going to be a substantial difference.

For example.

If I always use the station by my house for a few tanks in a row, I get ~19.4mpg average. But if after driving 200 miles I then fill up by my work, that station shuts off about a gallon early -- my mileage appears to go up to 22.3mpg.

If I drive 200 miles and then fill up by my friend's house, that station overfills compared to the one by my work so it looks like I burned more gas -- my mileage appears to be 17.1mpg.

Going from 19 to 22 isn't that big of a swing. Going from 19 to 17 isn't that big of a swing. But if I fill up at work then the next tank fill up by my friends house, my mileage looks like it went from 22mpg one tank to 17mpg the next. But, in reality, it didn't change at all.

The only way to get a good picture of gas mileage is to either average your mileage over a larger period of time or, if you really want to compare 2 tanks, use the same pump.

amartin

Well-- as an experement, using the same pump I always use, at the same gas-station I always use (its right by my house), I filled up... and THIS time, I'm keeping my RPMs when cruzing over 4, but under 4.7k when possible.

So far, I'm now getting ~20mpg in the city, instead of my usual 15.3-15.7mpg.

Try it for yourself.

amartin

2nd tank completed..kept the rpm's @ 4400-5000 as OFTEN as possible...

Gas Milage: 18mpg ALL city driving (My normal is 15.3-15.7)

SAME routes, and gas station used.

How's THAT for data!?

zoom44

your data seems to be showing that if you make a concentrated effort to stay below a certain rpm then you get better mileage compared to when not paying particular attention to rpm. so it seems that your mpg is very rpm dependant which suggests that the first trip of poor mileage to SA was probably run at a higher rpm than you remember.

Sigma

Or his mileage has just recently decided to spike up for some unknown reason. It can happen. When I passed 10,000 miles on my Mazda6 it was like a switch was thrown that gave me an extra 5mpg. Literally. You can see the spike on my charts. Somewhere deep in the cavities of the ECU was some magical switch that determined the motor was broken in or something and it just happened.

MazdaManiac

iTrader: (3)

As noted elsewhere, keeping your RPM below 4200 or so makes a BIG difference in gas mileage because of the A/F targets in the PCM.

Sigma

Yes, but this guy's experience is that it's better when trying to keep the car north of 4400. Which reflects what others have experienced as well.

Mileage is a crapshoot on any car, but significantly moreso on the RX-8. Some people get better mileage if they rev the hell out of their can than they do when they granny it. Some people get better doing the opposite.

Whether it's just the engine tolerances when manufacturers or a byproduct of the result of a break-in process method, who knows.

JonsToy

I find my mileage is not too dependent on rpm range.

I bought a previously owned, 6/03 built MT 8 in December '04 with 9500 mi. I tried very hard to keep the rpms under 3750 for the first 10 fillups, averaging 16.5-17.0 mpg in all city driving. At that point, I decided I wanted to appreciate the performance characteristics of this vehicle, gas mileage be damned. During the next 13 tanks, I raised the upper rpm range during acceleration to 6000, and once I reached cruising speed would back off to under 3750 rpm. I also try to mix in at least one WOT acceleration to redline once per day (to "clean out the carbon!"). My mileage? 16.5-17.0.

The only way I can get a notable increase in mpg is to mix in some highway cruising, but I haven't been able to do enough of it to get a highway mpg estimate.

So in my experience, I get similar mpg no matter how I drive my 8. Any question as to which style I prefer?

zoom44

yes jeff i understand that of course. that post was my way of saying "maybe the people whosuggested before that he wasnt using 6th the first trip" might have ben correct

RedSheDevil

Soooo...after all this, what is the optimal rpm to run for the best mpg? Or am I just starting up the battle again?

MazdaManiac

iTrader: (3)

4k

RedSheDevil

Good to know, thanks Maniac.

myfuncar

I'll weigh in on the mystery...I think that factors such as temp. and humidity influence how much fues you get into your tank before the gas pump goes pop and stops fueling. And even if you attempt to top it off, the difference between tanks can be enough to account for at least 3-4 MPG.
Wind is a huge factor, even 10 MPS winds, and although the winds may have been mild that day, over that long distance, you can encounter quite a few headwinds and crosswinds that can make a big difference.
My RX8 gets pretty consistent 19-22 highway mileage

Hanzo

Did you have extra weight onboard on the way back? Example, extra person, luggage, or just ate too much and gain some weight (just joking)? But weight plays a big role in MPG, obviously the more weight you have onboard the car the less MPG you'll get.


Also I think someone mentioned that if you fill your gas up in the morning when it's cold you'll get more gas for the money because gas is more dense when cold.

Hanzo

Oh did you use AC on the way back since you said it was hot outside? It will also decrease MPG. Even if you open the window the wind resistant from you opening the window will also cause drag, which increase load on the engine and decrease MPG.

rdfield1

19 MPG. 2004 MT, April '04 build date.

I do what might be called "mixed" driving- mostly small two lane roads where you have the occasional stop light.

This past weekend I took a 930 mile trip- from Philadelphia through the Catskills and then over to the Finger Lakes region and back. Of the 930 miles only about 240 were on 4 lane highways (including an hour and a half delay on I 80). On the two lane roads I would do anywhere from 45 mph to 65 (and much higher when passing), and on the four lanes from 60- 75. My average mpg for the trip was 23.74 (24.26, 26.88 {hose clicked to off when tank was not full}, 19.53, 25.17, 22.88- this last included the hour and a half delay). I feel really lucky to have achieved this when I see so many of you getting far worse mileage. And- I wish I could pass on some tips, but I have a feeling I lucked out and got a really good motor.

amartin

I'm onto my 3rd tank... Conciously keeping my RPMs between 4400->5000 (I'm talking cruzing, not acceleration--- I still goto red line on accel), if its >20mph, 2nd, >44mph -> 3rd, >65mph ->45, >74mph -> 5th (no use for 6th).

I'm getting 19-20mph now in the city. I USED to try to cruze in high gear, or at least gearing with the lowest RPMs for the given reasonable pace to safe gas. I'm proving to myself thats WRONG.

You WANT to be above or at ~4400 rpms to obtain MAX gasmilage WHEN cruzing. Below 4400 I drop nearly 25-40% in gas milage in "cruze" mode (ie. cruze control).

...and for whoever said that explains why austin->sa was better..blah blah cuz he might have been in 4th.. come on people... first SA->Austin was BETTER, and if I was doing 84mph in 4th I'de have been turing 6500+ rpms... HELLO.

Regardless, after 15000 miles of 13.5->15.3mpg city, and now getting 18-20mpg city simply by keeping my RPMs at 4400ish when "cruzing" (cruze control on the HW), after 3 tanks... there's something to be said.

Furthermore, another member told me he's trying this and is ALSO getting 25% improvement to gas milage.

All I can say, is for those getting 13-15mpg in the city.try IT and post your results.