[u00mem9]

I know this question has been asked before my time, but I couldn't find it during a couple searches...

I have heard that the lean burn can only operate at low RPM. Can anyone tell me the maximum engine speed that can still utilize the lean burn cycle? I'm guessing it may be the VTEC crossover point which is ~2500rpm on the way up and ~2200 on the way back down...but can anyone confirm this from Honda literature?

Thanks

 

[Guillermo]

I've seen lean burn at about 2,500 RPM not sure how high it's possible.
VTEC starts at about 3,500 RPM

 

[kapps]

You can actually get lean burn upwards of 3k rpm. It just depends on engine load. Drafting at 75-80 mph, it's possible to still get it. Also, I've seen it down in 2nd gear at 2500 rpm. VTEC is around 3500 rpm.

 

[Insightful Trekker]

There's not a specific RPM value associated with lean burn "cutoff". Think of a downhill that only requires light throttle to maintain the chosen speed. You could downshift to the next lower gear and still maintain lean burn.

I think its generally agreed that 70 MPH is the top speed that any lean burn can be maintained. But the window is very narrow up there.

It is understood that lean burn is not so simple as to be only "related" to RPM. _Many_ factors are involved that enable or inhibit lean burn.

HTH!

 

[Foxpaw]

I think that lean burn is most closely linked to throttle, as opposed to a specific RPM. But that is pure speculation.

 

[Calpod]

Lean burn is related to engine load, so speed doesn't matter, I've had lean burn once following a truck and with a tail wind, Lean Burn just kept going and going and going, :lol: we were doing about 80 MPH, so it has to be related to engine load.

 

[Insightful Trekker]

<and Calipod essentially wrote:>

I think that lean burn is most closely linked to throttle, as opposed to a specific RPM. But that is pure speculation.

I think its a wee bit _more_ complicated.

In my commute home this evening in anticipation of answering this post I monitored the generic OBDII value of calculated % load.

It was basically impossible to maintain lean burn with this value 90% or higher. But once lean burn dropped out simply reducing throttle to a value less than 90% would not always readily re-establish lean burn. And as all you 5spd Insighter's know lean burn MPG does vary. I saw lower and higher lean burn MPG values for inverse % load values. E.g. sometimes with a lower % load mpg would be higher than a corresponding higher % load.

And this calculated % load value is somewhat in question in my mind as to "what" it really is telling us. "Normal" idle values are in the 25-30% range as reported in Honda Service manuals for various models. I can't imagine we're using 25% of the engine's available horsepower to merely idle.

Based on driving experience throttle angle isn't directly linked to lean burn either. You can "feel" this in the gas pedal when driving in the high MPG style.

I'll try to compare MAP (manifold absolute pressure) weather permitting. I'll follow up in this thread for those interested.

HTH!

 

[Foxpaw]

Hrmm. How does one calculate the % "load." When I think "load" I think resistance to some mechanical process that accomplishes a useful task. I think of it as a qualitative label - but you seem to be using it as a quantitative label.

So my question is, what exactly does "load" refer to?

 

[Tim Maddux]

IIRC someone on the old Yahoo! forums reported several years ago that the lean-burn on/off threshold was correlated with vacuum.

 

[Rick Reece]

I don't know exactly how they are related but when the vacuum falls the MPG bar will soon follow. As a result if you watch the vacuum you can ease up on the throttle to try and maintain lean burn. Have fun, Rick

 

[Insightful Trekker]

<snip>

So my question is, what exactly does "load" refer to?

Sorry about the long delay, Holidays and all.

The % load value I was referring to it a generic OBDII value and is available via a scan tool. I still haven't had time to dig up a reference book for its precise definition.

Tim Maddux and Rick Reece are basically correct. The best value that reflects engine load is manifold vacuum or the MAP sensor value. I tried monitoring MAP and throttle angle values and correlating them with lean burn, too many variables to watch and drive! And with the numerous hills in east Tennessee lean burn is constantly rising and falling except under ideal conditions, which are rare in the winter months.

Lean burn in a Honda system is available as long as the intake flow dynamics allow for maintaining the ideal mixture at the spark plug tip. And that emissions can be maintained, NOX purge being a known cause to drop out of lean burn. I suspect that several engine parameters indicating lean burn window are mapped out on a dyno then programmed into the ECM with additional sensors allowed to force an override e.g. knock sensor.

HTH!

 

[Willie Williford]

Def: Calculated Load Value

An indication of the current airflow divided by peak airflow, where peak airflow is corrected for elevation. If available.
This value is not engine specific.
It gives the service technician an indication of the percent of engine capacity being used. (with a full load as 100%)