LOL neff

 

DED,

Yes I was referring to the sedan only, not the coupe which requires 91. The ECU does in fc tdetect gasoline grade, but i do not know how. I would think it derives "likely" effective octane by how much it can advance timing before the knock sensor detects knock. This, I assume, is how the ecu assertains things.

However, it is a certainty that the sedan requires 87 but DOES IN FACT improve in performance with 91 octane. The explanation is simple, a more optimized map in the ecu (1 of 3 total) is used when 91 octane is detected; and the "safer" one when a lesser grade of gas is detected (again, probably by indirect means).

The improvement is in the use of more advance timing and/or leaner gasoline mixture. Of course, tehre are many more variables that go into this equation, like coolant, intake air, oil, and ATF temps, etc.

E.g. with my w-band 2 meter, i can see the a/f getting leaner by about .2-.4 units of air/fuel ratio in the evenings (cooler) then in the high afternoon temps.

Gurgen

 

Q45 Tech

Good posts, learned somethign new! One think though, knock is detonation, not pre-ignition. THe latter is when the mixture lights up BEFORE the spark is fired. Correct me if I'm wrong, but I have read this in numerous places.

Gurgen

<P ID="edit"><FONT class="small"><EM>Edited by gurgenpb on 06/30/04 09:17 PM.</EM></FONT></P>

 

Thanks for the replys.

We are saying the same thing, ecu does not detect octane, it detects knock. But, if you are not knocking (i thought) you should get full power (timing advance) (different A/F maps?) no matter what octane you use. And (I thought ) max timing (for the sedan) occures using 87. For example, using 87 where conditions for knock are very low (cool ambients and such) will not give you more power than using 94 if there is knock (hot ambients). If there is knock, you will get less power no matter what octane you use.

If there is a more advanced setpoint for the sedan to get more power if using 94, is there a more advanced setpoint for the coupe if we use 100?

Meaning, is there a max setpoint or not? Are these points different for sedan and coupe? Does it keep advancing and using different maps until it senses knock, or until it hits a max.

One thing I do notice is a change in power when it's higher than 90F ambient. Does the temp mitigation override the knock mitigation? Seems even if you don't knock power is reduced and more octane would not help.

And these changes in ecu are not quantified. How much am I losing at high ambients? 20Hp? Enough to wipe out any NA mods? My thoughts are yes.

2004 G35C 6MT Black. Killer.

 

DED

Is it only ambients or can higher engine temps increase potential for knock?

I've noticed that when my car ('03 sedan auto) sits at idle for a while and (presumable) heats up, there is a noticable loss in power immediately afterward. Conversely, when running at highway speeds, especially in a gear that keeps the rpm's up, it pulls stronger (in the same rpm ranges) than when the engine is hot. I'm guessing this is because the ECU is sensing knock and retarding the timing until driving a bit brings the engine temp down. True?

Further, I wonder of some folk decide to "try a tank of 87 octane and see what happens" and then because they don't hear any knock, conclude that the car "runs fine on 87," not realizing that their ECU is way ahead of them, has sensed knock and retarded the timing and they are living with a significant power loss, all the while unaware of what is going on!

(Diagram that sentence, Miss Francis!)

 

Higher engine temps will contribute to knock.

I mention ambient because Q45 said there is an input to the ecu that changes the fuel map and timing when ambients get above 90F. I assume he means it does this even if there is no knock.

2004 G35C 6MT Black. Killer.