Help me understand EOC (engine off coasting)
 

Hi!

Haven't been active here, been busy busy.

Did do some "mods" to my car though. Hit a deer early Thanksgiving morning, broke a headlight and crumpled my hood, lost my grill. Insurance considered it totaled so i had to fix it myself.

So I replaced my headlights with brand new ones, switched to all LED's inside and out, uncrumpled the hood to the best of my ability, did a full tune up, did some gap sealing around the outside with gask maker, cleaned my crank case and created a grill block out of LED light bars haha.

But I saw only negligible gains. I'm still in that 23-24 mpg area.

So I've started playing with EOC while I save up for some more aero mods.

I have a manual, but I'm not really sure how it works- when I leave it in gear and turn the key off, I have full control on brakes and steering (necessary, I live in a hilly, windy roads place) but I'm concerned it's just pulling in unused gas on revolutions. Is this the case?

It's also nice because I just turn the key and it jumps right back to life, smooth as peanut butter.

In neutral I lose brakes and steering, which can be kinda terrifying. But I've noticed I can just put it back in 5th gear after turning the car back on, even as low as 15 mph, and it'll turn on no problem.

Thing is, neutral EOC really only works for one stretch in my town.

The other issue is that when I turn my car key off, my headlights go with it, so I can only EOC during the day.

Would an ignition kill switch cure this issue?

Or should I look into fuel injector kill switches? They seem sorta complicated though...

The car automatically cuts fuel when you leave it in gear and coast, even with it "on". Your momentum turns the engine even though no fuel is being injected, and the accessories such as the power steering and brake booster keep power. This is called Deceleration Fuel Cut Off (DFCO). Some of your momentum is lost to spin the engine and accessories.

When you put the car in neutral (or better yet just put in the clutch) and turn the engine off, that is Engine Off Coast (EOC), and is the most efficient way to coast unless you actually need to slow down, in which case DFCO is the best method.

When you EOC, you don't need to have the key in the off position the whole time, and that can be a bit dangerous because if you turn it off to far, the steering wheel could lock when you go to turn. Simply turn it off long enough to kill the engine, then click it back to the on position, and when you're ready for the engine to come back on, let the clutch out while in 5th gear to bump-start the engine. No fuel will be injected until you bump the engine back to life.

The Outback is a bit of a pig and extremely difficult to mod for much better fuel economy.

There's a few older threads on here where members failed to achieve the MPG goals they were aiming for. Zerohour documents his efforts here:

https://ecomodder.com/forum/showthre...-5l-14034.html

Another member, Brucey, had an Outback of your vintage and has a few threads on various mods he tested. Do a search for threads by him if you're interested.

So if there's danger in putting it in neutral and coasting, why not leave it in gear and coast instead, beyond engine braking slowing you down a bit? I just turn it back to acc and the only things that I notice are the odometer quits and my headlights turn off.

What's a way around that happening?

And I'm really only aiming for about 28. I don't have any delusions about this car getting amazing mileage. I drive it because it's a tank. Deer heavy area. I have a close call probably 3x a week in my 50 hour work schedule.

I'd rather survive in this than be buried in a metro.

Post here before buying stuff.

I'm not sure you're understanding the difference between DFCO and EOC. There's no advantage to turning the engine off if you are DFCO (leaving it in gear and letting momentum turn the engine). The engine already cuts fuel entirely when doing this, even when it's "on".

As I said, the 2 strategies have different purposes. EOC is useful for maximizing fuel economy when you don't want to slow down. DFCO is useful for maximizing fuel economy when you do want to slow down. EOC doesn't power the accessories on your car such as power steering and brakes, which is why it's more efficient. It's not trading vehicle speed to power these things (and spin the motor needlessly). DFCO robs the vehicle speed in return for providing power to accessories.

The way I would use EOC is if there was a long downhill section that allows me to coast near to the speed I want to travel. I'd turn the key to ACC, which would kill the engine, then I would switch it back to run (not start). That way when I do finally want the engine to start, I just let out the clutch.

DFCO happens any time you're completely off the throttle and the engine is still turning, and you're slowing down. I use that all the time, anytime I need to slow down or stop.

I'm not saying you've got a bad vehicle, and my favorite car I've owned yet was a 1996 Legacy 5-speed manual. Best I ever averaged was 28 MPG, and that was with a 2.2L engine and lower chassis. I'm saying you'll have a very difficult time getting to that average considering the larger engine and greater frontal area.

Summary:

EOC (Engine Off Coast)- Used for minimizing momentum loss while coasting. Does not provide accessory power.

DFCO (Deceleration Fuel Cut Off)- Used when slowing is desirable, or losing accessory power is not acceptable.

Engine-off coasting is when you put it in neutral and then shut the engine off. This is the most efficient way to cover distance. If you leave it in gear instead, the friction and drag of spinning the engine slows you down - basically like riding the brakes.

As redpoint says, it's not necessary to key-off if you're just going to leave it in gear. By taking your foot off the pedal while in gear you're doing what's called "engine braking", which means you're intentionally slowing your car down by using the engine as a large friction brake. This also uses no fuel but robs you of momentum, so you only want to do it if you're actually trying to slow down.

The headlights are on a switched 12V source? Is this a Subaru thing?

The headlights turn off with the ignition, yes. Unfortunate, but how Subaru designed the lights. I'm in the same boat where I have to EOC cautiously or with just my parking lights on. The only lights that can stay on when the ignition is off are the parking lights. They have a dedicated hardware switch typically on the top of the steering column. I learned that soon after I bought my first Subie.

Shutting the ignition off while in gear is a good way to flood and damage it.

I never coast in gear unless I'm coming to a stop and I haven't timed my coasting properly.

Anytime I coast my engine is off if it is daytime and my 12V charge is at least 12V.

Nighttime driving drains the 12V too much.

I use the key because I'm too lazy to install an injector kill switch.

California98Civic installed an extra vacuum for engine off coasting.

LED lighting will reduce electrical load and allow you to engine off coast for longer periods without draining your 12V battery as much.