Posted: Sep 22 2004 at 2:24pm | IP Logged

Cajun wrote: Unless you have found a way to advance the ignition timing and alter the fuel mapping, premium fuel is a waste of money and a reduction in horsepower. The timing curve in the ecu of these trucks is mapped for regular unleaded fuel. When using higher than the recommended octane, the timing is not advanced far enough to get a complete burn of the fuel charge. The lower the octane, the faster the fuel burns. That is the reasoning for advancing,(starting the burn sooner), the timing for premium fuel, it requires more time for a complete burn, so you start the ignition earlier. I have not had the truck long enough to experiment and map the learning ability of the ecu. It could be possible to run premium effeciently, if the ecu can change the ignition mapping and the fuel mapping for the higher octane. If this is possible, it is the first and only vehicle with an ecu capable of altering the parameters that far, no other in the US is capable of this feat, and we have tested quite a few. I guess the moral of all this ranting is to save the extra 4 or 5 bucks per tank and use the recommended fuel grade, until a programmer or any other method of re-mapping the timiing and fuel curves.   DT

Cajun,

That is one of the best explanations of how octane and ignition timing interact that I have ever read.  I mean that sincerely.  However, your statement that no vehicles in the US can alter the ignition advance that drastically is incorrect.  The Maximas from 95 up have that capability.  They have a knock sensor that listens for the pinging sound that accompanies pre-ignition.  The ECU gradually increases the ignition advance until it detects the slightest pinging, then it retards the timing accordingly.  I have proven that this system works.  I have 3 fourth generation Maximas, all 5 speeds.  One was running sluggish, very sluggish.  I read on Maxima.org that it was probably the knock sensor and that you can bypass the it with a 470K resistor, so I did.  It came back to life immediately.  I had always ran 93 octane because I knew about the knock sensor technology, but here was my chance to test it.  I put some 89 octane in after running the tank very low, and took it for a spin.  When I gave it substantial throttle under load I heard the pinging sound from the engine.  I repeated this only twice (to prevent engine damage), and heard it again both times.  I then stopped at a gas station and fueled up with 93.   I added a little octane booster since I had about 4 gallons of 89 already in there.  As soon as I got out and got on it again, it was back to normal, pulling like a freight train with that usual Nissan VQ torque.  I am under the impression that Honda is using a similar system in their high output VTEC engines now.  There are most likely others using it I am not aware of.  A buddy of mine at the local Nissan Dealer who is already modifying his Titan said that they have the same system and can full well take advantage of the better fuel with improved performance and economy.  I have not verified this though.  Anyway, not trying to be a mister know-it-all, just sharing my personal experience.  

Throttle

Posted: Sep 24 2004 at 9:22am | IP Logged

I think I agree with you.....it is over your head.   

Seriously, I was talking mainly about performance and with specific regard to Maximas.   In fact, your analysis about how much mpg improvement is needed for cost effectiveness is a good one.  However, folks like me would pay for the better gas just for slightly improved performance.  A fuel cost offset is just icing on the cake.

You mentioned why would not Nissan advertise of this benefit.  Good question.  Dunnno…..they haven’t with Maximas which have had this feature since 95.  I would guess the guys it matters to….someone like me…..are in the know anyway cause they read the C&D or MT article when it came out.  Not all advertising is on CBS. 

With regard to mfrs always recommending the best......well......have you ever heard of 7,500 and 10,000 mile oil change recommendations from mfrs.  I know of a situation where someone completely gummed up a BMW 540 engine at 100k by doing the 10k changes.  By contrast, I have 2 vehicles with nearly 200k miles that are clean as whistle inside.  I do 3k changes.  Why do you think some mfrs would recommend long oil change intervals???  Do you think auto mfrs want vehicles to last forever?? 

Posted: Sep 24 2004 at 10:32am | IP Logged

It is obvious that many buyers do give a "flip", hence the name of this thread.  That is also why Nissan is supplying such a wonderful powerplant for their vehicles.  They are "target marketing" this group.  In fact, that's also why a Maxima fan like myself just went out and bought a new Armada.  The reason for the Maxima post was not to advocate pepping up Maximas on the "trukin" site.  But to dispell the errors of a previous poster who said that engines could not advance timing enough to take advantage of 93, a topic germane to the thread.

With regard to your claims about mpg, there are others out there claiming their mpg improvement is offsetting the cost. I am currently testing this myself in a very controlled fashion.  It would be very easy to get the wrong results if you don't know what your doing.  Driving conditions and habits play a big role in mpg.  One should use the same fuel source (location even), same driver, and on a dozen tanks or more per grade.  One should refuel in same place and top off correctly (not relying on the auto shutoff).  Also, use the trip meter and a calculator, not the computer.  I have found that an accurate study of this issue beyond the interest level of most folks.

On top of that, an engine builder friend of mine says you can tell a big difference in an engine that has run 93 on tear down.  Less deposits at the base of the valve stem and less carbon buildup on the valve face and piston.  For folks like me that may keep it for 200k or 300k, this is a further consideration. 

As you can see, all this can be very complicated.  Someone like myself is interested in these issues and will pursue it a little further than most.  Everyone else will just subscribe to the "school of thought" that fits their biases best.