OMG

 

Conceptually this make sense to me. How, then, does the engine make work without fuel when decelerating in gear? Isn't the point of an internal combustion engine to ... combust?

I'm not ripping on your theory. But, please clarify so we can understand you better.

 

Do you mean to ask why the engine keeps on turning ? It is trying to stop. But there is 3,400lb of car pushing on the wheels that still wants to go forward.

The engine is just pumping air during that time, driven by the inertia of the car.

 

I got you; but, it's so counterintuitive! So, you mean to say that if I am coasting down from redline in gear, as long as I am not stepping on the throttle then I am using no gas!?

 

you can also look at it in a more abstract way, e.g. energy balance:

If you would burn fuel when engine braking, then where should the brake force come from? The brake force comes from the wheels having to turn the engine instead of the fuel.

But that also doesn't mean that engine braking gives you higher MPG. It just means it's better to leave the car in gear than in neutral or clutched out (due to the idle gas)

But you'll still roll further without gas in 6th gear than in 2nd. So you actually do not really want to brake (engine braking or not) if you are looking for long distance without much fuel (= high MPG).

 

Exactly.
When I'm off throttle, I want zero power. So no fuel is needed.

You can experiment with your car. Accelerate in first gear and then let go of the gas. The car will slow down under engine braking and when it's near 1,000rpm it will pick up and keep on rolling at crawl speed at idle.

By the way, it's not just a theory. It's what happens. Plug an OBD2 reader in the car and look at the injectors cycle time drop to zero off throttle.

Maybe Pedroosan's explaination makes more sense to you. Some times a different approach helps understanding.

 

It's never good to clutch out or go in neutral for *longer* times, unless stopping and having no choice to idling.

Neither on the highway at high speeds, nor when slowing down so slow that it takes very long to reach a complete stop (rolling out).

It's just dangerous to go in neutral at high speeds (I don't know who came up with this idea, every driving school should teach otherwise), because you lose your feeling of where you are in terms of speed of wheels vs. rpm.
Putting the car back in gear can cause severe braking because of the lost sync (e.g. too low gear too little gas). On a RWD you would only brake the rear and it may come lose.
And it does not save gas due to the idle gas as opposed to leaving 6th gear in and having 0 throttle.
That's what the 6th gear is for: as little engine braking as possible to easily roll without gas (at least in the range of legal highway speeds).

For short distance/heavy braking (city-like traffic stops):
The time till you stop is so short, that it really doesn't matter for MPG if you clutch out before braking or just when you reach the stop. If you clutch out and only apply disc brakes, it's like sitting at the traffic light a few seconds longer in idle. Trying to downshift with very quick deceleration will wear you clutch, and the short distance to the traffic light probably will not give you enough time to blip on your downshifts (exception of heel toe). Besides if you are interested in MPG, you probably do not want to blip.

For long distance stopping/very light braking (more of rolling to a stop):
leave in gear, choose the gear depending on the distance you want to roll. Downshift will not really require a blip, because you are rolling out anyway so the rpm will match. If you want to roll further, stay in high gear, if you want to have more engine braking go in lower gear.

From that you can see that if you are purely interested in MPG, it's better to try to roll as often as possible without big speed differences. Anticipate early when you need to stop and roll out to that point.

But honestly I personally care more about heel toe in the Z than how to come to a stop.

 

"Truly, you have a dizzying intellect."

Name that movie quote

 

Ah yes, very thorough explanations. Thanks Pedroosan and Kolia.

It's good to know - but it has no practical implications for me as (1) I rev-match, (2) go WOT and (3) take it to 6k too often to be concerned with fuel economy. Just got back from a lunch time jaunt through the country-side and averaged 13.5mpg.

 

I like this fuel topic a lot more than the people complaining about gas!

 

that's it right there. no further discussion needed

 

Ok guys, here's the deal. During engine braking, alot of people here stated that the injectors shut off and you dont use any fuel. However, this is not correct. Even if you are going down a really steep hill with the car in gear, the injectors will still inject a very small ammount of fuel. HOWEVER, this is not done to keep the engine running. It is done to keep the cats warm, so you dont violate emissions. If the injectors were to completely shut off, a ton of cold air would come pouring into your headers, and cool the cats down.

Here's a way to test it. Get in your car when the engine is completely cold. Drive down a long steep hill while the car is in gear, but dont touch the throttle. The engine will still warm up (slowly), even though you weren't touching the gas. Why? 2 reasons.... First, there's the internal friction that warms up the block slowly, and secondly there's the little bit of fuel thats being injected to keep the emissions system happy.

BTW, I got this info from Car and Driver. They talked about it in their tech section a few years ago.

 

Derratte, what you say is true only when the engine is completely cold, the injectors will put in more fuel to keep the engine running smoothly until it reaches normal operating temp, kinda like the chokes in the carbs in the old days. In such a case, even if you lift off, the injectors will put fuel into the cylinders. This does not apply once the engine is warmed up.

 

Sounds like there is some controversy. Every engine/car has it's own idiosyncracsy. It's just up to us to find out what the truth is on the Z.

 

ya are saying that in neutral under 1000 RPM, the injector opens to keep the car at idling...what about when your car is in gear, and the RPM dips below 1000 and your car is coasting down, is the injector still spraying?

 

Yep. It won't stall, it'll keep moving to a crawl.

If the ground is level of course. I can still stall if on an incline.

edit: Oh and it probably won't be very happy about idling in gear. But it should do it. Mine does.