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Old 05-28-2010, 04:25 PM   #1
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Driving technique confusion

I bought a 92 VX about a month ago and I have been trying to improve my driving techniques to get the most out of it that I can.

After reading many of the posts here I am both intrigued and confused by a couple terms and ideas. I have searched the forums for more information but I haven't found anything that definitively answered my questions.

If you guys could help me to better understand these concepts I would appreciate it.

Coasting in gear as opposed to coasting in neutral
I have seen some posters state that the injectors shut off completely when coasting in gear and therefore use less gas despite the rpm's being much higher than when coasting in neutral.

My questions are:

Is this true for all fuel-injected vehicles or does it vary from model to model?

Does this occur instantly when the foot is taken off the gas pedal or does it only occur after a certain period of time without throttle?

For example: if there is a stop sign 200 feet ahead and I am in third gear, would it be better to coast to a stop in neutral or should I take my foot off the pedal and leave the car in gear for as long as possible?

Are there any circumstances where coasting in neutral is superior?


Wide-Open Throttle (WOT)
I have seen it mentioned that driving under wide-open throttle can be a more efficient driving style. This one is hard for my simple brain to understand because it wants to believe that more gas pedal=more gas used. Also I can't imagine just getting in my car and flooring it through first and second gear, but there are times when I am in fourth or fifth where slamming on the gas doesn't seem quite as dangerous.

My questions:

Could someone please explain the theory behind this?

How do you control your speed at low gears while driving in this manner?

When is this a practical method to use?



Thank you for your help.
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Old 05-28-2010, 04:37 PM   #2
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Coasting in gear, or as we more commonly call it, DFCO (Deceleration Fuel Cut Off) is available on any fuel injected vehicle. However, there is a list of parameters that must be met before the injectors are disabled. For example, my 1998 GMC 4x4 will shut off the injectors after the vehicle is coasting in gear AND the TPS (Throttle Position Sensor) is reading Zero, AND the RPM is greater than 1,500 for more than 3 seconds. The injectors will remain off until the accelerator is depressed, or the RPM falls below 1,000. On some vehicles having accessories like air conditioning turned on will raise this threshold.

The theory behind WOT is that the engine is very inefficient at low RPM's because the engine has to strain to pull air past the restricted throttle plate. (pumping losses) Most engines are more efficient operating closer to peak torque. Also, many automatics are horribly inefficient at lower speeds because of the fluid losses in an automatic transmission. Its more efficient to get up to cruising speed faster, where the torque converter can lock up, and fluid losses in the transmission can be minimized.
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Old 05-28-2010, 04:47 PM   #3
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Another thing to note, your car is a VX. Find out what parameters you need to get it to stay in lean burn mode, and you should see an improvement in your mileage.
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Old 05-28-2010, 04:52 PM   #4
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Jay covered the explanations pretty well and succinctly, but I'll address a couple items:

"more gas pedal=more gas used": Stop thinking of it as a gas pedal. It is a throttle opener. Throttling is another word for strangling. When the throttle is closed, the engine wastes energy dragging air through the closed throttle. When it's open the air comes in freely. People spend loads of money on free-flowing intake systems but keep their throttles closed, totally wasting the money they spent (and generally the stock intake flows a lot more than even the open throttle anyway).

Your computer won't inject more fuel than is appropriate for the amount of air that entered the engine.

Controlling your speed in low gears at WOT: Shift up. That's the whole point. You keep your RPM down and your engine un-strangled, making the same amount of power/work at lower RPM using a higher gear. If you're accelerating too hard, shift at lower RPM.

In my VW I often shift when I reach 1200-1500 RPM, and enter the next gear at idle or 1000 RPM.
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Old 05-28-2010, 05:00 PM   #5
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Ok that clears up a great deal. I at least have an idea now of when and how to use these techniques.

I have another question regarding WOT though.

Is there any benefit to driving at WOT after you have gotten up to cruising speed?

Example: There is an incline on a freeway that I drive often. It is probably about 4-5 miles from top to bottom. It seems that if I am in 5th gear I can maintain 65mph at about 40% throttle. If I floor it, there is almost no change in speed. In this case is 40% or 100% better? or is there no difference?

RE: Leanburn
My VX is a California model, so no lean burn. I plan on converting it since it seems I only need a federal ECU to make the change. I just can't find one for less than $140 right now and that seems a little much.
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Old 05-28-2010, 05:25 PM   #6
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In that case I'm not sure. I've never had that happen (my car has stupidly low gears). You might alternate trying one way and the other for a few tanks of gas to see if you can measure a difference.
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Old 05-28-2010, 05:33 PM   #7
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Agreed. In that particular case you just have to experiment and see what works best for your vehicle.
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Old 05-28-2010, 09:46 PM   #8
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You should get the helms service manual for this car. Then do a tuneup for this car like adjusting the valve lash, something that is too commonly neglected. How many miles does this car have? You should be getting a lot better mileage especially if you aren't driving it hard. I have a Civic LX and I'm getting like 35mpg mixed driving, and my LX is no VX (VX is significantly better for mileage). I bought my car from an auto mechanic so I'm pretty certain it has been tuned up every which way.


OP, don't you have a SIL? (Shift indicator light) If you do, use it!
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Old 05-29-2010, 03:01 AM   #9
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No shift light unfortunately as it is a California model. I do drive conservatively and yes I would like to up my mpg beyond the ~40ish I am getting, but mostly I just wanted to understand some of the concepts that I see used on this site so I can try them out. I think my next tank of gas I will try using WOT to get up to cruising speed quickly and see what happens.

I am doing a tuneup, albeit slowly because I am trying to find information on the best parts to use when I replace old ones. So far I have changed out the old spark plugs for new correct ones but this seemed to make almost no difference in mpg despite the old plugs being NGK 5's instead of 4's and gapped at .65 instead of .44. I really thought I would see a gain on that.

Tomorrow I am switching out the engine oil and transmission fluid for synthetic. I am hoping for a small gain in mpg, but if not I am perfectly happy with longer change intervals.

Valve lash is something I would like to look into eventually, but since it will strain my mechanical abilities I am going to try the easier things first.
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Old 05-29-2010, 05:04 AM   #10
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My best recommendation is to buy OEM replacement parts whenever possible. "performance" parts rarely ever do anything to increase mileage, and if they do, its usually not enough to warrant the extra cost. Put your plug gap back to the factory spec too. If the wider gap was that much better, the factory would have set that as the spec. With your widened gap your car is running like it has severely worn plugs, no matter how new they are.
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