Auto in N question
 

The family and I went to see my Brother in NC this past weekend
We took the PT Crusier - auto tranny.
I did hook up the SG so i could see how we did.
I gathered data on the car at several speeds to get a feel for the impact MPH on MPG

While doing this - i also wanted to see if coasting in gear was better or worse then costing in Neutral.

From a consumptiuon point of view - coasting in N seemed to use slightly LESS fuel than coasting in gear. I suspect the PT does not have a fuel shut off as the MPG did not skyrocket.
Both cases had the instant MPG jump to about 75 ( N hit 80 a few times) hence i called it SLIGHTLY less fuel.

I wanna play with it some more and get more data - a little more contolled than "30 minutes of data crusing across SC)

Here is my observation and question.

When i shift the tranny into N - I expected the RPM to drop to Idle - which is what the Manual in my Altima does... but it did not?

going 65 mph was about 2400 rpm or so. shift to N - engine stayed near 2400 RPM ! HUH? Both the TACH and the SG said this..

the MPG did go up so i know less fuel is going IN -
Is this normal for an AUTO? Am i thinking "Manual Tranny" here?

I do not drive the Crusier much ( wifes car) so i had not played with it before. but am a little confused at this.

Steve

Wow, that is interesting. I can only speak from my own experiences with the Scion XA (automatic) and when I go to N from 45 (since that's where I usually drive at) my RPMs take a dive from 1800-1900 down to 700-900. MPG gains are immediate (instant and current trip) but I'm trying to get into the habit of DFCO coast first then N-coast once I'm below the minimum 1200-1400 rpm to make the most out of fuel consumption.

it may be an idle controler problem

mine drops to idle also.
what happens if you rev the engine a little? it might not be totaly out of gear.

What is the idle after it has warmed up?

My wifes 02 Taurus does that as well....it seems as if speed and RPMs are in direct corelation with each other.
The more I slow down, the RPMS drop......

My Jeep Cherokee works the same way. Makes the N coasting basically worthless on the Jeep. especially since it has the fuel cut off. The only difference I see is I can coast just a little bit farther. The only time I use N is when I'm sitting at a light. Drops the fuel use from 0.74gph to 0.60gph.

Within a couple seconds mine drops to 800-1000 rpm when going to N at any speed. Dont have my mpguino done yet but Im sure Im using less fuel in N as well.

Next time i am in it i will check a few things.
warmed up idle ( Stoped in D)
warmed up idle ( Stoped in N)
Coasting in gear ( at some MPH)
Coasting in N ( at same MPH)

i will check the GPH and the RPM at those cases.
with that info I will hit the Crusier forums and see if this is common or an error

thanks for the inputs

steve

A lot of this depends on the gear and control layout of the transmission.
Because there potentiall could be a big difference between the engine speed (low)and the trans output speed (high), and what gear range is selected from neutral to re-engagement.
Some control strategies will hold the engine above idle in a neutral coast to reduce trans load on re-engagement.

Hey All.
Just a quick update. I have not had a lot of time to play with wife's car - was in a conferance all last week in Upstate NY.

But i have managed to do a little with it. No SC info - only car's Tach.
@ startup (cold) the RPM hover above 1000 for a while then settle under 1K
@ startup (hot) the RPM touch above 1000 then quickly drop under 1k

driving at speed - Shift to N - the RPM do drop very slowly but will fall back to under 1K if long enough coast ( still at speed)
Neutral coast to a light - the RPM seems to drop faster then just a long N coast.

Conclusions - the engine does seem to be watching the transmissions RPM and keeping things close. As i noted before the engine does not have a Fuel Cut off - so for me it seems that coasting in N is the best way to go ( about the same MPG from SC - but longer coasts)

Thanks for the input - If / when i get the SG in her car - i can update with GPH info as well.

Steve

My Neon idles slightly higher coasting in neutral but not very much, 900-1000rpm vs. 600-800 rpm idling. It does coast alot farther in neutral than drive and uses less fuel doing so. Even coasting to a stop it uses less fuel coasting in neutral than drive.
How does the PT cruiser like shifting back to drive? Hopefully slips into top gear without a fuss.
Ian

The times i have shifted back - never an issue - no noise - not "bumbs" no blips - "Smooth as Silk" - which also makes me believe the engine is "following" the transmission RPM to some digree.

I am going to post this on a PT Cruiser forum to make sure thers are doing this as well.

Steve

does lower RPM equate to higher MPG?

when i'm driving on the road and shift to N, my RPM drops
but if i'm idling at a stop and shift to N, my RPM goes up

Not always. Many cars cutoff the injectors when coasting and allow the wheels to turn the engine at say 2000 RPM while using no fuel, but if at the same speed you shift to neutral and disconnect the wheels from the motor you still have to use fuel to keep the engine running at about 750 RPM. In this case you are actually using more fuel at a lower RPM.

Thats a special case though, in most instances lower RPM=higher MPG