Pulse & Glide question

[Matt Herring]

pale...

i disagree that the higher the speed the less mpg gain u see from pulse and glide. my in gear sweet spot is 57 mpg according to my scanguage and at higher speeds it does go down from there.

however when i get it gradually up to 70 and neutral coast down to 55 the engine is working less in the 15 mph drop in coast than it would be starting the coast at 65. plus i coast farther going down from 70 which means im driving farther in neutral coast than starting glide at 65.

the golden rule is the longer the neutral coast the better the mpg gain. anytime the car is in neutral it is working less and using less fuel...as long as its above 20 mph for my vehicle...my scanguage confirms it.

[Matt Herring]

ill also add this disclaimer...at least for my vehicle...

the mpg loss that occurs when i go above 57 to get to 70 is far exceeded by the gains on the coast back to 50. ive tried both ways on the same stretch of unobstructed highway and over a 2 mile stretch i saw more than 1 mpg gain by going to 70 instead of 65.

my rpms are only gradually affected to get to the extra 5 mph but i can coast nearly 1 mile on the flat stretch by getting up to 70

[Piwoslaw]

Matt, I'll not question your numbers, but I can't agree with your theory.
As Pale stated before, drag increases with the square of speed. At 70mph you are going only 7.6% faster than at 65mph, but your drag is 16% higher. By increasing your speed from 65mph to 70mph, you have to overcome additional growing drag for the whole time interval that you are accelerating. Once you switch to neutral your deceleration rate is greater between 70 and 65mph than between 65 and 55mph. The extra coasting time you get isn't enough to compensate for the extra fuel that went into accelerating over 65mph to 70.
What I remember from school physics says that P&G between 55-65mph should be better than between 55-70mph, BUT every car is different and your's just might have the right acceleration at the right speed and rpm to give you the figure you got. You definately have a slightly higher average speed when doing 55-70.

[PaleMelanesian]

I totally agree about, "the longer the coast the better the mpg gain." It's all about maximizing the glide. I often pulse uphill so I can glide down the other side.

Higher speeds: Here's where we get into YMMV territory. My car, at 70 mph, is running 3200 rpm. There is NO WAY to make good mileage in that zone. Your automatic, I assume, runs a much lower rpm. That's half the battle right there. The other half is aerodynamics, which will still hurt you. The extra speed at the top end costs you more on the pulse, and gains you less on the glide. Whether the total is good enough, or bad, depends on the specific case - ymmv.

I find the best results in the 55-60 range in my Odyssey, dwl or nearly steady speed. I haven't been able to do better with any other techniques.

[Matt Herring]

id like to say im smart in regards to aero but ill defer to you on that as i trust your knowledge more than mine...as far as my vehicle goes ive driven the same 2 mile stretch on my way to the mall probably 10 times with my scanguage.

i reset the sg current mpg as i passed an exit sign 2 miles from my mall exit going 57 and then pulsed to 65 and on another trip up to 70...ive done it at least 4 times. a 2 mile stretch is a small sample size but i got more than 1 mpg on all 4 passes in neutral coast and i coasted further from 70 than from 65...it seemed to make sense for me to start from 70 than 65.

the whole reason i tried it was to prepare for my 400 mile trip home next week.

[MetroMPG]

Christ - you really need to add your vehicle(s) to the garage so people have context for your posts.

The technique I think you're describing in post #7 is what I once called "pulse and bleed", where the bleed is a very light load / high MPG gradual loss of speed (need instant mpg readout to maximize MPG while bleeding off speed). I've tried it on multiple occasions at different bleed rates in my car and was never able to make it work better than DWL at the same average speed.

[Christ]

Metro - Sorry about that, but I'd hate to add it, then get rid of it, and at this point, we're still unsure of what we're doing with either vehicle, hence the reason that not even basic mods have been made.

I suppose, at some point, it will become necessary to step back, take a look, and realize that I'm not benefitting anything by being undecided, and I either need to jump into something, or stick with what I have. There are large life-factors involved though.

The thing I'm asking about here, is essentially P&G - without using neutral. My wife's GP idles @ about 1200 - 1500 RPM while in drive - slightly higher than normal - when your foot is off the gas pedal, and you're coasting. The engine isn't dragging the speed down at this point, it's a GM thing, the engine idles higher while moving to compensate for potential load.. in case you hit the gas suddenly.

What I was wondering - regardless of speed - was if this is sufficient for a P&G situation where fuel mileage gains might be noted.

The problem with this, immediately noticeable, is that the TC locks to ~1500 RPM at 45-50 MPH, and doesn't go up much until around 70MPH, but under heavy accel, the TC won't lock, so I end up hitting engine speeds up to 3krpm during the acceleration, at only 30% throttle (to prevent down shifts).

Even if the TC does lock during acceleration (happens occaisionally), it unlocks as soon as there is no load - I let off the gas pedal to coast down - which means the next pulse speeds the engine to ~3krpm again, if it doesn't re-lock.

I'd love to control the locking phase of the TC manually, so I could accelerate at ~1500-2000 RPM up to 65-70MPH, then coast down at 1200-1500 RPM.

Make sense?

I'm going to try it out on the trip up to visit family, and if it doesn't work out, I'll just drive back with another method.

[mobilerik]

So Christ, it sounds like you don't have a ScanGauge, eh?

[Christ]

Not so much... one of my cars is non-OBD FI, the other is OBD-1 gen2 (not exactly OBD-1, not exactly OBD-2a either. It's a GM thing.)

Consequently, the SG won't do anything for me... MPGuino would, but monetarily, I can't swing it right now, especially w/ my wife having to drive Granny 1.5 hrs to school and back 4 days a week now... (carpool isn't available anymore.) More expenditures... I really hope this skool-ing thing works out for her.

[Christ]

Anyone here can feel free to send me one though! I wouldn't consider it out of line, or taking too much liberty AT ALL! LOL.

Was worth a shot.