Hypermiling techniques that are invalid (potential mechanical wear/cost)

[cfg83]

Christ -

Originally Posted by Christ

Woot for better mileage! I'm sure there's lots of places that you drive that would allow you to do this as well!

I did this last week on a steep downhill on the freeway (405 North on the way to the 101 East). I had to put it in 4th gear. I felt like a "sinner" because I was over 3100 RPM, but lo and behold the AFR gauge went lean so the fuel savings is there to be had.

CarloSW2

[Christ]

Originally Posted by cfg83

Christ -



I did this last week on a steep downhill on the freeway (405 North on the way to the 101 East). I had to put it in 4th gear. I felt like a "sinner" because I was over 3100 RPM, but lo and behold the AFR gauge went lean so the fuel savings is there to be had.

CarloSW2

Carlos -

Next time, wait till the indicator goes lean, then quickly shift into a higher gear without letting the engine idle at all. You might stay in DFCO at a lower RPM that way. Worth a shot, I'd think?

[Christ]

Of course, redline at WOT and redline at NoT are two completely different things. The engine isn't under anything close to as much stress while you're decelerating, so it's not as big a deal to rev it up a bit higher. (I used to downshift at or near redline on purpose, but upshift at 2500-3000)

[cfg83]

Christ -

Originally Posted by Christ

Of course, redline at WOT and redline at NoT are two completely different things. The engine isn't under anything close to as much stress while you're decelerating, so it's not as big a deal to rev it up a bit higher. (I used to downshift at or near redline on purpose, but upshift at 2500-3000)

Ok, I'll try the quick upshift next time. It would be cool if I can fool the ECU/PCM in this manner. It's just funny that 3100 RPM "feels" like a high RPM to me.

CarloSW2

[Christ]

When considering average range of operation, 3100 actually is high RPM. Most engines that aren't designed for spirited driving redline between 5500 and 6500, and you're halfway there... of course, you idle 1/6 of the way there.

Imagine you're driving a large diesel - you're engine braking at or near redline. :P

So, just to clarify - make sure that you shift somewhat abruptly, so that the RPM don't have to go back up while you're letting the clutch out. Try shifting without the clutch, if you're comfortable doing so.

I'm thinking that the ECU will think that you're just slowing down, and keep it in DFCO mode, unless it's still taking info from the VSS - at which point it may think you're crazy and:

[2000mc]

Originally Posted by Christ

Imagine you're driving a large diesel - you're engine braking at or near redline. :P

i believe gm got rid of the redline on the tach for the duramax after a couple years because of customer complaints of engine overspeed when decelerating or braking, because it was being used for engine braking far beyond where the engine makes useful power.

from bulletin #02-07-30-004C

For the LB7 diesel engine, maximum engine speed immediately following a preselect or grade braking downshift can be as high as 4,000 RPM. After a grade braking downshift, if vehicle speed continues to increase, an upshift will occur at 4,800 RPM engine speed.

[Christ]

Originally Posted by 2000mc

i believe gm got rid of the redline on the tach for the duramax after a couple years because of customer complaints of engine overspeed when decelerating or braking, because it was being used for engine braking far beyond where the engine makes useful power.

from bulletin #02-07-30-004C

For the LB7 diesel engine, maximum engine speed immediately following a preselect or grade braking downshift can be as high as 4,000 RPM. After a grade braking downshift, if vehicle speed continues to increase, an upshift will occur at 4,800 RPM engine speed.

But this only happens at 0 throttle input, correct? If you touch the throttle, that signals the TCU to upshift the transmission immediately, which lowers the RPM - at least as far as I can remember from driving my boss's old diesel Chebby.

[2000mc]

yea, just chiming in that even manufacturers use engine braking on diesels at such high rpm

[cfg83]

Christ -

Yup, my ECU/PCM is always messing with me :



CarloSW2