better MPG with #91 gas ???

[hangout]

I have the 2002 I-35,, I always put #93 gas in the car and get around 27 MPG.last week I drove to Maine state, they only sell up to #91 gas, I have to fill it up anyway.

after all, I found I have better MPG than the #93,, almost 30 mpg.. WHY ??

should I keep using the #91 ? ( i drive 100 miles a day )will the #91 gas cause the check engine light on??

please give me some ideas

[Q45tech]

Since 1996 OBDII ecu has used knock sensors to advance ignition to the knock point then back off every second or so. This allows an ideal ignition curve to be calculated for current fuel.

Depending on stroke length the PERFECT IDEAL max pressure should be created at 16 degrees after top dead center for each piston to maximize downward thrust on rods.

What ever fuel allows the oeak closest [below] 16 degrees ATDC delivers the best MPG at a steady rpm.............assuming the ecu is still in closed loop and not accelerating.

One variable not show on pumps is the BTU per gallon of the fuel. This determines the absolute peak power whereas the timing moves the peak on the time scale.

Average [area under curve] torque vs peak torque must be considered in how little fuel is required.

E10 is the big variable as it has higher tested octane but lower BTU per gallon and requires a 3.3% richer mixture to keep O2 monitoring system happy.

So in theory NON E10 PREMIUM should offer the best MPG but their are variables depending on air temperature.

[hangout]

PREMIUM GAS means #91 or #93 ??

[CStalin]

In California we only get up to 91.

This is news to me.

[MinisterofDOOM]

PREMIUM GAS means #91 or #93 ??

Depends on where you are. Altitude affects A/F ratio which affects detonation. At higher elevations, 91 octane offers the same practical knock resistance as 93, so 93 is not necessary. Whatever the top grade available to you is is premium. Might be 93 down at lower elevations. It's 91 up here in SLC. Low-compression engines can get away with running octanes as low as 85 up here.

The most likely reason you get better milage with 93 instead of 89 or 91 is that the car is able to run at optimum parameters. For 91 or 89 it retards timing, which decreases power and efficiency. With 93, no timing retardation is needed, so the car can run at its most efficient specifications.

Assuming your knock sensors are working correctly (and you'd know if they weren't) you cannot harm the VQ35 by running low octane fuels--it is smart enough to adapt. But if you're getting better fuel economy with premium, and you also know you're getting more power with premium as I noted above, the appeal of saving money up front decreases. The difference in cash in the long run is probably negligible if anything at all.

[nyvenom30]

theres a gas staion by my place has 108... lol... my bro goes there to fill his gn...

[Markc]

I think you may have misread the OP. He claims Improved mileage with Lower octane fuel.

[hangout]

I think you may have misread the OP. He claims Improved mileage with Lower octane fuel.

THANKS for understanding

[Markc]

I'm frugal, (not cheap! ), so I run 87 octane up here in Toronto, Ca. According to the trip computer, I'm getting 27 miles per U.S gallon combined city/highway.I'm probably missing out on a little performance though.

[hangout]

didn't try the #87,, thinking about the check engine light..

but, while I put #91 in, the car feels better.. the RPM is lower than #93..2400 RPM at 70 MPH2200 RPM at 65 MPH..

around 100 to 150 lower than the #93..

I think I will change to #91 from now on.

[duckmano]

My trip comuter is off by 2.5 to 3 mpg. I would not trust it. Mine always says 27.5 mpg, but I actually average 24-25 mpgs.

[hangout]

all the MPG number I talked about are the real number I calculated by the mileage and gallons of gas..

so I wonder why it can jump from around 27 MPG to 30 MPG