COASTING -- & -- A/C vs WINDOWS

[add|ct]

Hmm. I wonder...if I'm driving my VX at any speed above 40 and I'm in 4th/5th, is it safe to drop it into 2nd until coming to the light and falling below 15/shift to N until light change?

Is that DFCO?

[theholycow]

On the "is it safe" question:
Safe for the car: It's extra wear on your clutch and synchronizers. You can mitigate it by rev-matching and double clutching but that may use more gas than you end up saving, I'm not sure.

Road safety: When you're engine-braking your brake lights aren't on. Other drivers expect brake lights to notify them that you'll be slowing, and there's a slight possibility you could get blamed if you get rear-ended. You can easily mitigate that by using your brake pedal just enough to light the brake lights.

On the "Is that DFCO" question:
That's one of the conditions required to initiate DFCO. I imagine that the VX isn't shy about DFCO so it probably would be pretty dependable.

You can be sure by using some sort of fuel system monitor, or possibly an O2 sensor monitor. The fuel rate meter linked in my sig is 100% effective at indicating DFCO and is how I learned that it's not as dependable as people think.

There's plenty of VX folks here and maybe someone who has used decent equipment can describe its DFCO behavior in detail. I think VX folks have reported that the DFCO floor RPM is raised when A/C or electrical accessories (wipers, fan, etc) are in use.

[add|ct]

Originally Posted by theholycow

On the "is it safe" question:
Safe for the car: It's extra wear on your clutch and synchronizers. You can mitigate it by rev-matching and double clutching but that may use more gas than you end up saving, I'm not sure.

Road safety: When you're engine-braking your brake lights aren't on. Other drivers expect brake lights to notify them that you'll be slowing, and there's a slight possibility you could get blamed if you get rear-ended. You can easily mitigate that by using your brake pedal just enough to light the brake lights.

On the "Is that DFCO" question:
That's one of the conditions required to initiate DFCO. I imagine that the VX isn't shy about DFCO so it probably would be pretty dependable.

You can be sure by using some sort of fuel system monitor, or possibly an O2 sensor monitor. The fuel rate meter linked in my sig is 100% effective at indicating DFCO and is how I learned that it's not as dependable as people think.

There's plenty of VX folks here and maybe someone who has used decent equipment can describe its DFCO behavior in detail. I think VX folks have reported that the DFCO floor RPM is raised when A/C or electrical accessories (wipers, fan, etc) are in use.

Yes, it is indeed raised. I'll look into your DIY. It would help to get my O2 sensor replaced on my car first, I'd imagine.

[theholycow]

The DIY I posted (thanks to help from other members; all I did was put the information in one place) measures a fuel injector wire directly. It won't be affected by your O2 sensor, though your car's behavior certainly would be.

[DRW]

My car also has DFCO. I like to keep the car in the highest gear while still keeping DFCO active. The force of engine braking is lower in higher gears, so the car will coast farther at 1200rpm in 5th than it will at 1800rpm in 4th (for example). I'm lucky that the DFCO point is consistant on my car. The things that raise the cutoff threshold on my car are a cold engine and using A/C.

I've also adjusted the brake light switch so it comes on with a very light tap on the pedal. I've gotten into the habit of flashing my brake lights as a warning to drivers behind me, since I rarely need to use the brakes above 20~25mph.

[add|ct]

Originally Posted by DRW

My car also has DFCO. I like to keep the car in the highest gear while still keeping DFCO active. The force of engine braking is lower in higher gears, so the car will coast farther at 1200rpm in 5th than it will at 1800rpm in 4th (for example). I'm lucky that the DFCO point is consistant on my car. The things that raise the cutoff threshold on my car are a cold engine and using A/C.

I've also adjusted the brake light switch so it comes on with a very light tap on the pedal. I've gotten into the habit of flashing my brake lights as a warning to drivers behind me, since I rarely need to use the brakes above 20~25mph.

Interesting. I've yet to feel out the VX for the DFCO points for each gear, conditions as you mentioned not withstanding.

I think I'd rather do that than shift into 2nd from 5th, unless I need to brake very quickly.

[theholycow]

DRW's strategy is the way I do it too. My car seems to care which gear it's in and 4th is most likely to give me DFCO, but if I'm already cruising in 5th then I'll generally get DFCO. Either way, I don't try to DFCO in lower gears; I take 4th or 5th down to my DFCO floor RPM and then shift to neutral. I use my brake pedal to provide however much braking I need.

[VetteOwner]

i wonder....

on a non fuel injected vehicle like a chevette would it be better to neutral coast or in gear engine brake?

with the engine turning at higher rpm wouldn't that cause a vacuum and cause more fuel to be sucked into the carb than standard idling?

[theholycow]

Carburetor does not have DFCO (except for a couple computer-controlled models from the end of the carburetor era). At higher RPM more air is moved and more fuel is sucked in. With a carburetor, neutral coasting is always better.

How about heavy throttle with a carburetor and a manual transmission? I imagine that the accelerator pump, squirting like a hose, has got to be pretty wasteful...probably better to accept higher RPM and pumping loss than to squirt at every shift.

[JanGeo]

Actually when aproaching a red light try to prolong the DFCO as long as possible since the longer you stay in DFCO the more time your engine is not burning fuel as you wait for the light to turn green. This means prolonging deceleration in the highest gear to keep the engine turning high enough to be in DFCO while not reaching the light before it turns green. The real killer is if the AC is on, my engine tried to keep the rpms higher by burning more fuel or increasing the idle air bypass. Typical burn rates are 0.12-0.14 gph no AC and 0.26gph with AC at idle or trying to DFCO it will still burn at 0.26gph unless the RPM is about 2000 or higher.