Toyota FJ Cruiser Forum banner
1 - 10 of 10 Posts

·
Registered
Joined
·
71 Posts
Discussion Starter · #1 ·
I just completed a trip from Oklahoma to California and back again, and noticed some quirks in performance and MPG.

For the majority of the trip I averaged about 19-21 MPG between fillups, except for one area between NM and AZ where I averaged 22-24 MPG and my engine felt like it had gained about 10 hp?!

The weird part is that I was at 4500 to almost 6000 ft elevation. I was expecting mileage and performance to drop due to thinner air. On the trip out I thought air temp may have played a part as it was night and 75-80 degrees. But, on the way back I noticed the same gain in this area during the day at 108 - 112 degrees outside.

What gives, do I have to move to a higher altitude to get optimal performance and mileage, or can the ECM be reprogrammed?

For what its worth, I use 87 octane, and have the auto trans and 7200 miles.
 

·
Registered
Joined
·
478 Posts
I just completed a trip from Oklahoma to California and back again, and noticed some quirks in performance and MPG.

For the majority of the trip I averaged about 19-21 MPG between fillups, except for one area between NM and AZ where I averaged 22-24 MPG and my engine felt like it had gained about 10 hp?!

The weird part is that I was at 4500 to almost 6000 ft elevation. I was expecting mileage and performance to drop due to thinner air. On the trip out I thought air temp may have played a part as it was night and 75-80 degrees. But, on the way back I noticed the same gain in this area during the day at 108 - 112 degrees outside.

What gives, do I have to move to a higher altitude to get optimal performance and mileage, or can the ECM be reprogrammed?



For what its worth, I use 87 octane, and have the auto trans and 7200 miles.
One possible explanation is that at 5000' unless you have forced air induction (super/turbo charger) the max output of the engine is greatly reduced. This reduction would likely allow the computer to advance the engine timing to its optimal setting. At lower altitudes with 87 octane gas it's possible the engine knock sensors are being activated and the computer is retarding the spark advance to protect the engine. All normal and the reason Toyota specs were rated with premium gas. Also because the air/fuel ratio stays the same less horsepower made will mean less fuel used.
 

·
Registered
Joined
·
71 Posts
Discussion Starter · #3 · (Edited)
Strange, I had beeen using premium up until about 2 months ago, and noticed absolutely no change in mileage or performance at my present altitude after I changed to regular. Maybe it is just at the tipping point of where the ECU changes the mapping?

I just wish I could figure out how to make it run like that all the time. By the seat of the pants dyno, it felt like at least a 10 HP increase and revved much quicker and was way more responsive than normal.
 

·
Premium Member
Joined
·
6,534 Posts
The ECM is designed to maintain a certain A/F ratio under certain loads at all times to provide engine protection, emissions and performance guarantees. The ECM was spec'd on PUL so the ignition timing map only goes so far as to handle the max recommended. (That's why putting in 100 octane fuel doesn't buy you anything). If you run RUL but don't load the engine to the point where it pings, then you will see little to no performance difference between RUL and PUL. If you use RUL and drive your vehicle in such a way to load the engine where it will ping, then the knock sensors will detect that and tell the ECM to back off the timing (only enough to eliminate the pinging), maybe even richen up the mixture (if the pinging is really bad) to protect the engine.

When you travel to a different altitude, the MAF sees either more or less air coming into the engine. The ECM makes sure the correct A/F ratio is maintained; so if your not loading the engine hard, it will provide less fuel to maintain the mixture. That is why folks will often get better mpg at higher altitudes when cruising flat highways. Also, since the air is thinner, you are creating less compression pressures in the cylinders so that you are producing less HP and TQ. But, the engine can also handle a little more ignition timing since combustion chamber pressures are lower. The ECM will want to give you as much ign. timing as the ECM was programmed for until the knock sensors have to intervene.

I hope that all makes sense.

DEWFPO
 

·
Registered
Joined
·
961 Posts
One other thing that has not been mentioned is wind resistance. Since the air is less dense at altitude, it causes less wind resistance. Since our trucks are wind tunnel tuned (NOT), they benefit greatly from less wind resistance.
Remember wind resistance increases with the square of the speed (eg. twice as fast = 4x the wind resistance).
Another variable just occurred to me - fuel formulations often vary from state to state. Perhaps you got a nice batch of gas too (in your truck).

HTH..
 

·
Registered
Joined
·
71 Posts
Discussion Starter · #7 · (Edited)
:bigthumb:Thank you for all the input, it makes a lot more sense now. I had also forgotten about the gas mix until Bartt and Redthunder mentioned it. Most of the filling stations out there advertised they did not add any alcohol. I did not think it would have that great of an effect, but that's what I get for thinking...:lol:
 

·
Registered
Joined
·
961 Posts
Hey Mising - Thanks for your service!

OK, now I'm going to throw a wrench in all this..
I have been tracking mileage, speed and some other stuff via a connection to the ECU on my laptop and have noticed that the stock tires yield invalid speed and mileage. I have verified this against both my GPS and a stationary radar source. It appears that at 65 mph, I am off by 3 mph (about 5%). When I run my other set of tires (285 70/17) I am right on the money.

So I started thinking.. (I know a bad idea but here we go..)
I would bet dollars to donuts that the original design was not racked, used 285's and got about 15 mpg. But Toyota had to meet some mileage standards, so they lowered the front and put smaller tires on it to comply. This gives a bit better wind resistance and fakes the mileage by inflating the distance traveled (and increasing the torque enough so the engine is working less).
 

·
Registered
Joined
·
734 Posts
maybe you just got a good tail wind?

other than that, having better mpg in the condition you drove is not possibe. your engine will not knock/detonate on 87 when you're cruising along like you did. If it's so, on a naturally aspirated car of 2007 made by toyota, we're in serious trouble. you were most likely on a down slope, when you were driving during the day in high temp.
 
1 - 10 of 10 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top