More questions about P&G w/ an auto tranny

[Zvolen]

I also have an AT and am currently testing P&G in neutral, I previously glided in gear before and didn't have much of a increase to even notice. However I DON'T have a scanguage which I am sure would help in this experiment. I will post back my results if anyone is interested.

[swng]

Originally Posted by Zvolen

I also have an AT and am currently testing P&G in neutral, I previously glided in gear before and didn't have much of a increase to even notice. However I DON'T have a scanguage which I am sure would help in this experiment. I will post back my results if anyone is interested.

Yes, please post your results when you have them. Have you noticed any really useful drop in rpm when you glide in gear? I have not. Does your car jerk when you shift from N to D as you finish gliding and wish to start pulsing again. Actually, I personally think I should rather shift to the "3" position for gliding in the hope to make the car's computer shut off fuel supply to the engine.
Also, I wish to ask if I should accelerate to the desired speed fast or slowly at the "P" stage.
Any advice will be appreciated.

[swng]

I have tried some P & G. I glided in gear and the lowest rpm I could go was about 1,100 rpm. The cars were slow at that rpm. I will try more.

[Jim Dunlop]

I disconnected the throttle controller. It's nice because it idles at 650 RPM. We'll see how this tank turns out.

[n0rt0npr0]

You bring up a good observation to comment on! There are AT's that WILL jerk when put back into gear (my Celebrity) and my opinion is that those that feel that with thier own cars will not see a noticible gain from p&g. I will say that there could very well have been other factors that killed the effort to get the gains of p&g, but still, it just didn't work with that car.

Now my 96' Monte is a different story, the tranny freewheel's really good while coasting in gear......while gliding I see 110mpg at 50mph(in gear) and 150mpg while gliding in neutral at 50mph. So I have picked up a conservative 2mpg using p&g with my auto tranny monte. Rpms are sometimes as low as 890.....plus its smooth as silk when going back into gear at any speed

I also have not figured out if its more efficient to pulse hard throttle to speed or to do it gently?

Originally Posted by swng

Yes, please post your results when you have them. Have you noticed any really useful drop in rpm when you glide in gear? I have not. Does your car jerk when you shift from N to D as you finish gliding and wish to start pulsing again. Actually, I personally think I should rather shift to the "3" position for gliding in the hope to make the car's computer shut off fuel supply to the engine.
Also, I wish to ask if I should accelerate to the desired speed fast or slowly at the "P" stage.
Any advice will be appreciated.

[GasSavers_StanleyD]

Originally Posted by swng

... Actually, I personally think I should rather shift to the "3" position for gliding in the hope to make the car's computer shut off fuel supply to the engine.....

Dont think downshifting is a good idea. Just shift in neutral. Personally I think one of the BIGGEST BIGGEST advantages in P&G is the fact that you have eliminated engine braking for the duration of the glide. Leaving car in gear increases engine drag. The lower gear makes that loss even worse

WHY P&G beats steady speeds even for AT: When you drive in gear you are being affected by engine braking. For those that have ever driven manual, this is best experienced by trying to drive 50mph in 2nd gear. For an auto try doing 40mph+ in the '2' even 'L' position. The car CAN ACTUALLY attain those speeds but you have to 'force' the car to reach said speeds by revving the engine. As soon as you let the pedal go the car will experience a negative forward force and slow down. The faster you go the stronger this force is. Once you reach a certain rpm, tranny will upgrade to nexct higher gear tha is more suited for your speed. Problem is that tranny only has 4 or 5 speeds. So once you pass highway speed you are experiencing high engine braking after 55 mph, less at slower speeds when your car is in correct gear. So by GLIDING, you are not experiencing this braking which is eating your FE. Engine braking really kills you on hills. When I am at top of a hill doing 50 I might hit 70 by the time I coast to bottom of hill. If I coast in gear I might not even hit 60 by the time I reach bottom of hill. P&G also works better at higher speeds. P&G from 80/60 will be better than straight 70. P&G from 60/40 will also be better than straight 50, but the % increase will not be as high in the percent increase you experience at the higher speeds. This is because with high speed P&G there is more engine drag eliminated.
Personally I only P&G at high speeds and on hills where you MUST coast in downhills as oppossed to losing FE from engine drag

[itjstagame]

Originally Posted by GeekGuyAndy

I think this spreadsheat is correct (tell me if I'm wrong) and it shows that RPM / distance is much lower in P&G than steady speed, but time is longer. I picked what I thought was reasonable figures, but it's probably not quite that perfect.

Your chart makes sense, because you're staying below a certain speed on average (that's why your trip time is longer), but I wonder how your P&G 50-44 compares to a steady speed of 47 or 45. Comparing to a speed of 50 it will always be lower MPG.

[brucepick]

GeekGuyAndy, great chart! I'm jumping in late but want to add my .02 anyway.

swng, I'm glad you'll be testing P&G in your auto car. I've been doing P&G with my auto tranny, putting it in neutral for the glides. I can't use ScanGauge on my old beast but I'm pleased with the results from my version of P&G.

My 60 mile route is essentially a series of hills on a 65 mph limit road. Steeper than the usual interstate grades, so there is a definite climb and a possible good downhill coast.

True that an auto tranny will allow some slippage. What I do for that is, I don't accelerate hard at all. On the upgrades I peg my tach at about 2100, which is just enough in this car to climb the grade at 48-55 mph, depending on the particular grade. This is a "steady throttle" or "driving with load" technique but I only use it on the upgrades. Once I clear the peak I goose it up to 60-70 depending on conditions, but again I stay pretty light on the accelerator. Since it's a downhill it doesn't need much to get going. As soon as I hit the target speed I get off the gas and put it in N to coast. If my car had less aero drag, I could just stay off the gas and I'd by flying pretty soon just from gravity. But this boxy beast needs a boost to get going, even on a downhill.

This way I never (rarely, anyway!) step hard into the accelerator. I think wider throttle is where the auto tranny has most of its losses, so that's how I try to minimize those losses.

[Raccoonjoe]

Originally Posted by swng

Actually, I personally think I should rather shift to the "3" position for gliding in the hope to make the car's computer shut off fuel supply to the engine.

I agree....downshifting to the "3" position is a bad idea. Most auto transmissions are electronically controlled these days....WITH a manual override. That's usually pretty basic, especially if you have a floor mounted shifter, and RWD. (I believe this principle applies to all auto transmissions, however). The shift lever will usually allow you to over-ride which gear the computer thinks you should be in. However, this often disables/cripples the "freewheeling capability of the tranny. This leads to more forced engine braking, less gliding, and less FE.

Like I said above, I'm not sure about all transmissions, but I know this is how it works in my Jeep, as well as the wife's Blazer, and several other SUVs/PUs I've driven. I just don't get into cars that often =P.


*edit*
Also VERY interested to know y'alls thoughts.....Pulse at WOT, 80&#37;, 50%, 25%, or <25% throttle??

[brucepick]

GeekGuyAndy's chart and explanation look really good to me. I'll attempt another explanation of why P&G works.

The engine is more efficient at producing motion from fuel at wider throttle openings. Again, it's mostly the reasons given in the other explanations - the "overhead" costs in spinning the engine and all the other mechanicals like alternator and water pump that spin along with it. Another reason is that in closed throttle it's trying to suck in fuel-air mixture but it is fighting against the closed throttle. I suspect that if you eliminate fuel + spark and just spin the engine, it spins more easily with an open throttle than closed.

OK, so it's more efficient with throttle more open. But you can't drive wide open all the time for various reasons. Mainly becuase you want to keep speeds down because what you lose to air resistance (aero drag) goes up hugely with increased speed. Not to mention the state troopers.

So you alternate a more open throttle with coasting. P&G.

Another good reason to avoid WOT (wide open throttle) is that many engine computers will enrich the mixture at WOT. So 3/4 or 7/8 throttle is probably a good max, as long as it's a standard tranny without the losses of an auto tranny.