How to over-ride computer RPM while coasting?

[csnyder]

Originally Posted by Manualhybrid

And it may affect the lubrication of your automatic transmission. Check your model of car to see if it is OK for 'dinghy-towing' or 'four-wheel towing' as if behind a motorhome. If it is not OK for dinghy-towing it may mean that with the engine off, lubricating fluid does not cycle through the moving parts of your transmission--even though you are in neutral. I used to engine-off-coast (EOC) in my automatic transmission Matrix till I was advised of checking whether the model was approved for dinghy-towing and, sure enough, it is not approved! I don't EOC anymore, fearing damage to my car. I just coast, engine on, in neutral.

You are approved for towing up to something like 20km at 30kph on the Matrix - I don't have the numbers right at hand - but it allows for low speed flat towing in an emergency, or towing with the rear end lifted with a standard tow truck.

[mcrews]

I had the same experience with a Taurus.

[csnyder]

Originally Posted by Enki

Most vehicles kick on the AC compressor for defrost; not entirely sure why other than possibly engine load (more heat faster) and temp control of the defrosting air.

The AC is turned on for defrost to dehumidify the air for defrosting. This more than doubles the effectiveness of the defroster. Has no significant effect on engine heat production or engine warm-up.

[Enki]

Well, it takes power to run the compressor, which is made by fuel, which in internal combustion engines is not 100% efficient. Thus, it will produce extra heat in the engine when running.

[csnyder]

Originally Posted by Enki

Well, it takes power to run the compressor, which is made by fuel, which in internal combustion engines is not 100% efficient. Thus, it will produce extra heat in the engine when running.

Note I said any "significant" effect. Turning the AC compressor on on my Ranger doesn't change the warmup time enough to measure (and affects the fuel mileage by less than 1/10mpg on a cold engine) On a warm engine it might make a little bit more difference - and it does make more difference at speed.

I can control weather the AC is on or not with defrost on the Ranger because it is a dumb-as-a-rock manual system I installed myself.

[ProDigit]

On my car, it idles at ~1k rpm when coasting.
I think you'll put more wear on the starter and ignition trying to turn on/off your vehicle while coasting.
On my 2003 Hyundai optima, there was no rev matching, but the car would jolt when shifted back into gear, with no rev matching..
Dangerous!

Ac does very little to warm up the engine, and I only use it when decelerating or stopping. Then also shift one or two gears down, to convert kinetic energy to AC. I turn off ac at acceleration, and turn it back on when coasting or decelerating. Saves me between 1 and 2mpg..

[LocoJason]

In looking at this for a moment, I felt I had a couple things to add. Due to wanting to keep my job, I can't give any data. I can give you some information to fuel the discussion.
-There are many reasons for hanging the throttle open at speed, even when the pedal is at zero. The biggest one is emissions, as was already pointed out. Keeping the cat "lit" requires both AIR and a little fuel. But also, this function also enhances the drivability by decreasing (or making more linar) effect of the varying deceleration rates of engine braking at different engine/vehicle speeds. Both the inputs and controls for this sort of feature vary widely.
-I can't give a "1RPM" discussion any consideration because there is no gasoline engine I know of in existence that runs this slow. When people say the load is more important than the speed, that's very true but only part of the story. At low engine speeds, port velocity and mixture motion is very low, so charge stratification (fuel/air separation) is rather high. This leads to needing more fuel at lower engine speeds in order to maintain stable combustion (Low CoV). The gradient varies greatly form engine to engine, but suffice it to say that with all other values being equal, there is a non-linear correlation between engine speed and fueling.

[RobertISaar]

it should be noted with a gasoline engine, there is a fairly direct relationship between engine airflow and fuel consumption. 1500RPM with a manifold pressure of 20kPa is pulling a lot less air through it than 1500RPM at 40kPa. probably not as low as an 800RPM idle at 40kPa, but it isn't nearly as dire as it looks.

also(since I don't have a position in the industry to protect), GM's term for this situation is(or at least, used to be) a "throttle follower" function. back in the days of a throttle that was separate from the IAC, a few variables went into the amount of additional airflow that was allowed through the IAC while driving, so that if/when the throttle was snapped shut, the engine wouldn't instantly react in a manner that would upset the driver/passengers. with a manual transmission, it was even more critical to prevent the possibility of a stall/near-stall while shifting. sometimes, it was overly boosted in that if you were to press the clutch in, the engine would actually accelerate, which is a weird feeling. now, with the IAC functions being dealt with via the DBW throttle, the PCM just doesn't completely close the throttle plate.

[csnyder]

OK - I did the numbers for my 4.0 Ranger. Ran the same road twice - same numbers. At 100kmh in 5th I'm at 2200RPM doing44mpg on the down grade. In neutral at 2200 RPM I'm doing72MPG, with it at 1100 RPM idle I'm doing 144MPG
Then I decided to check the GPH at different speeds with the AC on or off. The AC has an automatic adjusting orifice installed - otherwise it's a 1993 Ranger system converted to R410 and bolted on with only a switch for control.
With the battery fully charged (no alternator load) at 900rpm I got .28gph. At 1500RPM it was .53gph and at 2000 RPM it was .73gph
I the turned on the AC and got .75gph@2000, .53@1500 and .30@900RPM

You can do with the numbers whatever you like. They are worth at least what you paid for them - - -

[Wolf_Tm]

Originally Posted by mmonce

My 2012 Toyota FJ Cruiser has a 6 speed manual. The vehicles computer adjusts the RPM's based upon the mph. For example, if I am going downhill (45 mph) and put the tranny in neutral and foot off the gas, the computer idles up to 1,500 rpm. The faster the mph, the faster the rpm. Typical manual trannys allow the vehicle to idle as normal at 6-700 rpm.

Has anybody on this forum been able to over-ride their car computer to adjust the idle as it should be?

Sorry for the stupid question:
why do you have to go downhill in Neutral and not gear in?