Thread: Neutral Coast Not So Great for Fuel Economy

MPG for engine braking vs. coasting in neutral depends on how far you can coast. Engine braking will slow the car down, reducing how far you can coast and maintain speed. For a big long hill, you're better off in neutral and letting the injectors fire a bit to keep it idling. If you're coming up to a stop sign/light, then engine brake for a bit.

On my ECUtalk scanner, it pegs @ 99.9mpg coasting down in neutral until somewhere around 35mph, then it starts dropping. At that point it's advantageous to engine brake so the injector timing drops to 0ms.
At least this is my experience with my bolt on S4 cam B13, 14.6 1/4 on street tires, 162whp, up to 37mpg daily driver (in the summer, less mpg in WI winter) - daily commute 14.x miles, avg. 33mph for the trip, mostly through town.
"You're mileage may vary."


How will you take advantage of engine braking with the realtime ECU?
The stock ECU drops injector timing to 0ms under engine braking already - with my B13 it does anyway, not sure about B14/B15/etc.

The hills around here you can coast all the way down the hill in gear while staying a tad bit over the limit. But yes if i was in neutral then i would go over the speed limit (not a concern of mine) so you will not have to get on the gas until later to maintain the speed limit. So again it might depend on the hill or what is at the bottom. The longer the hill the more you might benefit from staying in gear.

Also remember that the engine pulling a vacuum is in effect creating a pumping loss. It takes energy to create that vacuum. A diesel engine has no throttle plate and therefore no vacuum. It is very efficient. Long story short, zero vacuum is best for fuel economy, but the fuel we use won't support the lean mixture it would take to keep the engine from revving itself to death.

Think of it this way, engine braking is essentially pulling a large vacuum to slow the car down. There is very little friction that takes place, the resistance of the pistons making a vacuum is what generates the braking.

If two cars ran down hill, one in neutral and the other in gear, the one in neutral is still pulsing the injectors to maintain an idle. The other one is not using any fuel at all.

I was just reading some of Mike Kojima's posts on Sports Compact Magazine, and came across this random article.



Now it is starting to make perfect sense why a turbo engine can get really good MPG's if you stay out of the boost.


Here is another interesting quote:


So on our engines the sweet spot would be between 4800 rpm and 6400 rpm

I guess it makes sense why I would get poor gas mileage when shifting at 4k RPM. I need to start shifting more at 5k rpm and down shifting to that point.

I know I've said this before, but in all honesty, I don't think it's worth the effort, all of this fuel saving driving and things. You're barley going to burn any different amount of fuel either way. After long periods of driving, it makes a little bit of noticeable difference. Maybe 10$ or 20$ for hundreds of miles. In my opinion, you might as well just enjoy driving instead of getting a headache over all kinds of small fuel saving techniques.

Coheed is 100% right in his post. There's no free energy. Either you're going to lose energy by engine braking but save fuel, or you're going to burn fuel but keep the kinetic energy. If you go down a lot of big hills or stopping frequently, you might get slightly better mileage engine braking. I think that engine braking is much less damaging to your engine than firing, even if you are just idling. There is no piston slap or knock involved with engine braking, and the friction will be no greater than if you are firing (albeit 3x or 4x more frequent due to higher RPMs). I don't think you'll cause any more wear and tear by engine braking than by idling. In fact, there will be reduced friction in the cylinder because of the lack of combustion pressure to push the rings out.

Haha true to that, but I think it all adds up in the long run

Engine vacuum takes energy, just remember this. Only a portion of a diesel engine's efficiency is derived from fuel and high compression. A diesel engine also has no throttle plate and therefore, no vacuum pumping loss.

There is always a trade off. You can use tuning to make a vehicle cruise at little to no vacuum, while still using less fuel. Getting this to work right takes all the right engineering, and will only work on some engines. Like I said before, less vacuum will increase fuel economy by making the engine work more efficiently. Getting it to work on an SR is another story alltogether.

On our cars, you will get the best fuel efficiency by not driving hard. Tuning can make a huge increase in mileage. I know some tuners will tune a vehicle for 70mph or so and will configure the timing and fuel maps accordingly.

Thats what I'm looking at doing too. That plus just regular day to day city driving...

I'm wanting to tune according to engine load, say if engine load is under 50 % then give good gas mileage, if load is after 50% then make it fast

Thanks to BenFenner for describing this, because before I wanted to tune according to RPM's, but that would mean the car would be dead slow until you hit the RPM range.

I wish my jwt tune was load based. It seems like they tune based on rpm...

Going with a Calum Realtime for that reason haha.