Honda Civic VX?...better MPG by NOT clutchless coasting?

[jeffcat]

I know most people think the general rule of thumb is that when coming to a stop, pushing in the clutch and letting the car idle while it coasts down is a better approach to saving MPGs, than just letting it engine brake, but I just bought a Civic VX a few weeks ago, and while strolling across the manual, I saw this which I found rather interesting.

3. Fuel Cut-Off Control
During deceleration with the throttle valve closed, current to the injectors is cut off to improve fuel economy at speeds over the following rpm:
-D15B7 engine 870rpm
-D15B8 engine M/T: 920rpm A/T: 900rpm
-D15Z1 engine 850rpm
-D16Z6 engine M/T: 930rpm A/T: 920rpm

So, would it be more fuel efficient to just let the car engine brake to a stop AKA drive normally, as opposed to the classic way of saving MPGs by pushing in the clutch, where the VX only idles at 600rpms?

[NachtRitter]

This tip summarizes the recommendation: The most efficient way to slow down

The wording in the tip is key... the method you choose depends on how quickly you need to stop. If you know you need to stop, and you are approaching at a speed where you know you will need to use the brakes, then you may as well take advantage of DFCO so that you use zero fuel for a short period and you reduce your speed more quickly. But as your manual points out, under a certain RPM, you will begin to use fuel again. The delta between the idle speed and the DFCO rpm cutoff point is pretty small, so in the case of the VX it may not make much difference if you leave it in gear all the way to the stop or if you clutch in again when the RPMs drop below 850.

Of course, the best method from FE point of view would be to take the car out of gear (or push in the clutch) much earlier before the stop (if conditions allow it), so that the need to use the brakes (and therefore the need to use engine braking) is significantly reduced.

[jeffcat]

Originally Posted by NachtRitter

This tip summarizes the recommendation:

The wording in the tip is key... the method you choose depends on how quickly you need to stop. If you know you need to stop, and you are approaching at a speed where you know you will need to use the brakes, then you may as well take advantage of DFCO so that you use zero fuel for a short period and you reduce your speed more quickly. But as your manual points out, under a certain RPM, you will begin to use fuel again. The delta between the idle speed and the DFCO rpm cutoff point is pretty small, so in the case of the VX it may not make much difference if you leave it in gear all the way to the stop or if you clutch in again when the RPMs drop below 850.

Of course, the best method from FE point of view would be to take the car out of gear (or push in the clutch) much earlier before the stop (if conditions allow it), so that the need to use the brakes (and therefore the need to use engine braking) is significantly reduced.

But based on the nomenclature of the service manual, wouldn't the engine be using zero fuel when coasting in gear above 850rpm with no throttle input? If you coast and just push in the clutch...out of gear, the engine idles around 600rpms, therefore would be below the fuel cutoff of 850rpm and would be using fuel.

[NachtRitter]

That is correct. However, when you coast in gear, you will slow down more quickly due to engine braking. So it is a tradeoff... if you are simply coasting (with no intention of coming to a complete stop), you will coast x feet when you push in the clutch, but only y feet when coasting in gear, where the y is normally much less than x. Since you've slowed down so much when coasting in gear, you'll now have to accelerate again to get back up to speed, thereby nullifying the fuel savings you gained while coasting in gear and using no fuel. Typically, when covering the same distance, coasting out of gear (or clutch in) will give you better FE than coasting in gear and then accelerating.

The x and y is going to be unique to your vehicle and your route, so it might be something you'll want to try out (some fuel economy instrumentation such as the MPGuino would be really helpful for this)

[Superfuelgero]

You also have to consider that the idle speed (and hence DFCO cut off) is dependent upon load (electrical, a/c etc) and temp. If your idle speed with the clutch in is 1500, expect the DFCO to only last until 1750. Its rare that mine will run the minimum idle rpm (which is needed for the 850 cut off). Once past the cut off, it goes back to needing to add fuel not only for the engine, but also the wheels again.

[slownugly]

Yup when mines cold it only goes to 1500. Anything lower will use fuel until it gradually warms up. Anothe good thing to remember about the vx/cx specifically is the long legs in the transmission will allow that longer coast time without putting it in neutral. In fact I never put mine in neutral.

[jeffcat]

Ok guys...that's a great help. I'm looking at an Si chassis tonight to swap the VX engine and trans into, so hopefully, I will get more time to experiment, but that clears up a lot of what I was wondering.

[basjoos]

If you are concerned about fuel consumption while coasting in neutral, shut down the engine during the coast and clutch start the engine just before you come to a stop. While idling, the VX engine uses very little gas, my SuperMID shows over 200mpg (its maximum displayed mileage) while idle coasting in neutral until I get below 28 mph. Despite the tall gearing a VX/CX will coast a lot further in neutral than it will with the engine spinning in 5th.

[slownugly]

Basjoos- my afr gauge shows a richer ratio when I coast in neutral as compared to dfco in 5th. Maybe your aerodynamics play a role. Are you still runnin the p07-g01 ecu? If so do you run 87 octane with it? I picked one up on eBay from Europe but I haven't tried it yet.

[jeffcat]

Originally Posted by basjoos

If you are concerned about fuel consumption while coasting in neutral, shut down the engine during the coast and clutch start the engine just before you come to a stop. While idling, the VX engine uses very little gas, my SuperMID shows over 200mpg (its maximum displayed mileage) while idle coasting in neutral until I get below 28 mph. Despite the tall gearing a VX/CX will coast a lot further in neutral than it will with the engine spinning in 5th.

I don't like clutch induced starting an engine as it puts a tremendous amount of stress on the clutch, input shaft, and especially the engine itself, before the oil pump can keep back up with the pressure of immediately starting the engine. If it had a built in racing style oil accumulator, then perhaps I would consider it, but I just find it to be less practical to save $500 doing that then spending $800 on a new engine, but that's just me. To each their own.