P&G question!!!
Hey guys, excuse me for my noobness. But to be honest I got a question about P&G on an automatic transmission car. While getting an awesome kick out of it, like most of you do..I noticed whenever I throw it back in drive, it slows down. I know this because it's in gear and obviously drops my speed. But I noticed if I rev match it to the speed and the gear I assume I'm going into it doesn't "jerk" into gear. It keeps the speed.
But the main question is, what hurts my fuel economy more? Letting the car slow down, or bringing the engine about <1,000 RPMs up so I can keep the speed and continue on my way? Should I even be P&G on an auto? I do know how to rev match correctly (previous fun and very not so efficient GTI's I'vd had on the past) BTW, this site is freaking awesome. I've been driving more "eco-like" and so far I got 1/4 tank left to go when I would be empty by now. :D |
I think it's pretty much a wash. While the kinetic energy that spins up the engine (when you don't rev-match) was likely generated at higher mean engine efficiency than you get while rev-matching, that energy had to go back and forth over the drive line, causing significant losses.
So go ahead and rev-match if it makes the journey smoother. |
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Rev-matching smooths out things, so it's OK, even if it means blipping the throttle a bit and using fuel. Quote:
That's going to cost more fuel than briefly blipping its unloaded engine to do 1000 rpm more. Quote:
Shutting down the engine could harm an automatic due to lack of lubing. If it can be flat towed for extended distances, you're OK. If not, keep the engine running. |
Well I figured as much. I meant the jerkiness when it goes back into gear from neutral. Yeah I don't turn off the car when I'm gliding. I rather not, reading up on what I should be and shouldn't be doing pointed it out.
Thanks man! |
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I have been able to verify that it is possible to pulse and glide with an automatic transmission. There is no need to shift to neutral, or do any sort of hardware modifications to your car. I've done this myself with my truck.
The idea is, once you're at speed, try to briskly accelerate to a few miles above your desired speed, then let off the gas to allow your vehicle to slow down to a few miles below your speed. Then, accelerate to go to a few miles above your speed, and so on and so on... If you accelerate and you find that your vehicle downshifts on you, you're using too much gas. Back off a little on the gas pedal, and try again. Also, if you have access to something like a ScanGauge, and you find that your ScanGague is telling you that your vehicle goes into open loop when you accelerate, you're using too much gas. If you use your brakes, well... that should be obvious that you just converted some gasoline into brake dust. I've discovered this method on my daily commute to work, where I would oftentimes be caught in heavy (but still moving) traffic. I used to wonder why I'd get as much as 8% better fuel economy on days that I'd have to deal with heavy traffic, than on days where traffic was relatively light. In either case, my travelling speed was not a contributor to increased FE. Heavy traffic on my commute typically travels about 8 over the speed limit, and I normally aim for 7 over. I first noticed this by noting total trip fuel consumption as measured on my Scangauge II at mile 25 of my commute into work. On heavy traffic days, I have observed as much as 21.2 and usually around 20.5 MPG. On light traffic days, I have observed around 19.4 to 19.6 MPG this past month. I have subsequently tested my "P&G" theory this past Sunday (a guaranteed light traffic day on my commute into work, thus no possibility of "drafting" to explain my FE gain, and no possibility of annoying somebody behind me), and got 20.9 MPG. So, yes, it is possible to P&G with an automatic. It's just different than for a manual. |
On my 02 Insight I could coast down to 16 MPH and still get 150 MPG with the engine idling. It had an instant FE bar graph on the dash and it would read max at 150 MPG down to 16 MPH coasting in neutral with engine on.
It worked out to .11 GPH idling, while coasting, with no accessory loads including radio. You needed to rev match the re-engagement of the transmission in the Insight. On the 08 Altima, also with the CVT, you could coast down to 16 MPH while still maxing out the bar graph instant MPG reading at 60 MPG, which was as high as it could read. On some of my local back roads with 35 MPH speeds I could get very close to 60 MPG average in the Altima using pulse and coast (engine on). In the Insight it was close to 90 MPG on the same route. I never coasted engine off in either vehicle but when coasting engine on, I divided the coasting speed by 16 and the mulitplied it by 60 (Altima) and 150 Insight to get my higher speed coasting MPG. Used wisely it could make a significant difference in overall MPG. regards Mech |
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Just curious, how's the CVT working out as in MPG. My manager has the same car, with CVT. From what I know CVT keeps the engine at a "sweet spot" for efficiency I assume. Does it give good basic gas mileage? |
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Every time you shift from neutral to drive, you're inducing a tiny bit of wear on the friction component of your transmission's geartrain. This is by design and is unavoidable. Also consider that automatic transmissions were designed to be shifted from neutral into gear while the vehicle is not moving, so as to minimize that wear I just mentioned. Shifting the automatic transmission in and out of neutral, while the vehicle is moving forward at a decent speed, is a sure way to shorten the life of the transmission, because you'd be inducing a lot of unnecessary wear in the geartrain friction component of the transmission. Finally, there is a safety aspect that you might not be considering (or that you might be discounting). If you were to need to be able to accelerate out of a situation, it's far better to just have to step on the gas, than it would be to shift the car back into gear if it were in neutral, and then step on the gas. By comparison, the lockup torque converter (at least in my truck) is designed to unlock and lock up while the vehicle is in motion, and while a significant amount of power is being fed from the engine into the transmission. My torque converter will alternately lock-up and unlock whenever I am stepping on the gas, then letting off of the gas pedal. However, that isn't going to wear out my lockup torque converter very much, and it's certainly safer than shifting to neutral and back. Therefore, I recommend that your transmission be left in gear. The small amount of gasoline you'll save by switching in and out of neutral, above and beyond the pulse-n-glide technique I described, isn't worth shortening the life of your transmission, nor is it worth reducing your safety margin of error. |
OP- thanks for posting this I have the same concerns and I've been battling this for a bit.
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