Best habit to shift gears on MT and fuel economy

[chrisl]

Originally Posted by Pete156

And they start pumping again if I engage the clutch coasting in gear?

Yep.

[chrisl]

Originally Posted by Marchy

from the link i posted above



i've personally noted this first hand in my 4 last vehicles, Mazdaspeed 3 (the speed was so good in fact i could cruise without touching the gas in 1st and 2nd gear) the brz, the wifes focus and my old ranger. The brz will roll around in 2nd gear without touching the gas so long as you aren't on a massive incline.

On my Outback, I think it starts adding a bit of fuel below 2krpm, based on how it feels when I'm rolling around in gear, and once you drop below ~800rpm, it starts adding quite a bit more fuel to keep it from stalling. It's actually pretty noticeable, if you look for it.

[Pete156]

While we are talking about DFCO, each manufacturer is going to have their own method of cutoff or reduction; the thermodynamic properties of different engines require that some maintain a combustion cycle while others do not. Some modern engines turn their injector pulse width down down to like .0002 while coasting (Hyundai Genesis). This means there is SOME amount of fuel being supplied to the cylinders, though a microscopic amount. Lets not lump all modern fuel injection systems together, because some are in fact slightly different.
Thanks to everyone for their responses!
I am learning a lot.

[993Fan]

Originally Posted by Pete156

I am learning a lot.

Wait! aren't you the one that said, quite definitively, "I know this stuff"...

j/k -- we must all keep learning

[Pete156]

Alright, throw me under the bus, will ya!

[Pete156]

I average 30.5MPG Boosted with an AVO Stage 1.5 at 8psi. I've driven the car 5K N/A and 5K FI.
The car gets better mileage FI.
Using the car three days a week and autocross

Sorry for being such an ass

[IronSean]

Originally Posted by ntron1

If you go to neutral, the fuel economy is better because there is less drag from the transmission.

But your engine burns gas to keep it idling. When you coast in gear it's your wheels spinning, moving your gearbox that keep your engine running at speed so no gas is used. The drag from the moving parts (and it's actually mostly caused by the vacuum created in the engine sucking in air while it moves without fuel) is actually what gives you engine breaking, and can help save your brakes in the process.

[strat61caster]

Originally Posted by IronSean

The drag from the moving parts (and it's actually mostly caused by the vacuum created in the engine sucking in air while it moves without fuel) is actually what gives you engine breaking, and can help save your brakes in the process.

Actually the valves open and close as if the engine is still being operated, due to the camshafts still being directly attached to the crankshaft so it's the compression stroke of the motor slowing the vehicle down as there is very little resistance from the intake stroke (comparatively).

Engine braking to save brakes is pretty poor logic, brakes are cheap to replace whereas using the engine to slow down puts a not insignificant load on the engine and transmission which are much more expensive to replace as they wear out over time. Given the choice to slow down by downshifting vs. using the brake pedal it should be the brake pedal every time.

This argument comes down to how people define 'engine braking' differently. Some guys think it means racecar downshifting to stay in the powerband while others take it to mean (on the city streets) that you're just downshifting per your usual driving rpms (in this car usually between 2k-4k) and a lot of heated arguments result.

I think it's more correct to say "I leave the car in gear as much as possible and when slowing down this results in gas savings because the injectors shut down". At least for daily driving. You don't want to be the guy that's showing off by redlining down to the stop sign bragging about how you're 'saving your brakes' and then you need an engine rebuild well before what is considered normal.

[993Fan]

Originally Posted by strat61caster

Engine braking to save brakes is pretty poor logic, brakes are cheap to replace whereas using the engine to slow down puts a not insignificant load on the engine and transmission which are much more expensive to replace as they wear out over time. Given the choice to slow down by downshifting vs. using the brake pedal it should be the brake pedal every time.

I had to suffer through a few days with a loaner Boxster S with PDK. (I hate PDK, so I mean this literally...)

In so-called sport mode, the PDK downshifted through all the gears at every stop. Of course, they were perfectly executed shifts with perfect rev-matching thanks to the microcontroller(s) in charge.

The gratuitous downshifts at every single stop became very annoying and I was glad when I was able to return the POS.

[chrisl]

Originally Posted by strat61caster

Actually the valves open and close as if the engine is still being operated, due to the camshafts still being directly attached to the crankshaft so it's the compression stroke of the motor slowing the vehicle down as there is very little resistance from the intake stroke (comparatively).

Most of that energy is recovered on the expansion stroke though - the air in each cylinder is basically acting as an air spring during the compression and expansion strokes when the engine is running this way. I would guess you actually do get a significant portion from the drag from the vacuum generated during the intake stroke. As for the valves opening - yes, they do operate normally. However, the throttle itself is closed, so a pretty significant vacuum is generated in the intake manifold as the engine pumps air from the intake to the exhaust side of the engine.

[strat61caster]

Originally Posted by chrisl

Most of that energy is recovered on the expansion stroke though - the air in each cylinder is basically acting as an air spring during the compression and expansion strokes when the engine is running this way.

You're right, I neglected that. That would be an interesting exercise but I've done enough math on this forum

Generally higher compression engines mean more work is done on the fluid which is why I figured engine braking is largely a function of the compression stroke over other factors, it generally doesn't cost nearly as much energy on the intake and exhaust strokes. In any case, I still think it's erroneous to attribute engine braking mostly to the intake stroke.

[ntron1]

Originally Posted by IronSean

But your engine burns gas to keep it idling. When you coast in gear it's your wheels spinning, moving your gearbox that keep your engine running at speed so no gas is used. The drag from the moving parts (and it's actually mostly caused by the vacuum created in the engine sucking in air while it moves without fuel) is actually what gives you engine breaking, and can help save your brakes in the process.

We'll just have to agree to disagree