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Fact of Bull****?

 
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Old 05-12-2011, 12:15 AM
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Fact or Bull****?

okay so in either PZ or SoCalPrelude there was a thread I was subbed in, and one guy said that leaving a car in gear while decelerating to a stop is MORE FUEL EFFICIENT than coasting to a stop in Neutral.


True, or bull****?

I've frickin been driving like a granny to save gas. I'm going down to los angeles friday and would like to know the real deal...
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Old 05-12-2011, 12:20 AM
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Re: Fact of Bull****?

Like 12 Yrs ago i remember my dad always saying use the gears to slow down. I don't know why though. So it habit for me now to down shift....
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Old 05-12-2011, 12:29 AM
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Re: Fact of Bull****?

I would say that's bull****. You figure your talking about the engine idling versus dropping gears and raising the rpms. Wouldn't be logical that that would save gas.
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Old 05-12-2011, 12:36 AM
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Quote:
Originally Posted by 2kLude View Post
I would say that's bull****. You figure your talking about the engine idling versus dropping gears and raising the rpms. Wouldn't be logical that that would save gas.
ha thought so. i only doubted myself recently because i was cruising through a 25 mile canyon road - about a third of it in neutral the way there. and my fuel gauge ended up being a bit lower than I anticipated when i got back home...

but what about cruising in neutral VS decelerating to 0mph in 5th (kickin the clutch before stopping)

Quote:
Originally Posted by drewmarie View Post
Like 12 Yrs ago i remember my dad always saying use the gears to slow down. I don't know why though. So it habit for me now to down shift....
yeah, same with my dad. it's safer to be in gear with traffic just in case you have to maneuver around something quickly
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Last edited by hotrod; 05-12-2011 at 10:12 AM. Reason: merging back to back posts.
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Old 05-12-2011, 12:58 AM
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Re: Fact of Bull****?

Quote:
You figure your talking about the engine idling versus dropping gears and raising the rpms. Wouldn't be logical that that would save gas.
Yeeah but is the car under more load at idle or when rapidly decelerating in gear? You have to keep in mind that RPM isn't the only parameter that effects fuel efficiency. There's also load, coolant temp, etc. IMHO, doing it one way or the other isn't going to give you noticeable results whatsoever.

Personally, I just shift into neutral and brake. It's probably easier on the trans/clutch.
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Old 05-12-2011, 01:33 AM
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Re: Fact of Bull****?

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Originally Posted by xandereyes View Post
okay so in either PZ or SoCalPrelude there was a thread I was subbed in, and one guy said that leaving a car in gear while decelerating to a stop is MORE FUEL EFFICIENT than coasting to a stop in Neutral.


True, or bull****?

I've frickin been driving like a granny to save gas. I'm going down to los angeles friday and would like to know the real deal...
my GTI says that its using 0.0l/100km when im coasting in gear, and more when out of gear depending on speed. Im going to say it is :) If its coasting and you aren't pressing the gas, there shouldn't be any gas going into the chambers, no?
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Last edited by shatterbox; 05-12-2011 at 01:42 AM.
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Old 05-12-2011, 01:58 AM
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Re: Fact of Bull****?

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Originally Posted by shatterbox View Post
my GTI says that its using 0.0l/100km when im coasting in gear, and more when out of gear depending on speed. Im going to say it is :) If its coasting and you aren't pressing the gas, there shouldn't be any gas going into the chambers, no?
hmm..well i think there wouldn't be any extra fuel going into the chambers, only just enough for idling rpm
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Old 05-12-2011, 01:59 AM
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Re: Fact of Bull****?

A non-tuned ECU dis-activates the injectors when you are off throttle when in gear or when there is negative load, but when in neutral the engine has have fuel added to maintain idle as well as opening the IACV.

I know my CR-X with its 1986 fuel injection only adds fuel when it is below 1200RPM, any RPM over that the injectors do not fire when decelerating in gear.

Some manufactures do let the injectors continue to add fuel under deceleration right as you let off, that is what the burbling sound is. SRT-4 comes to mind. Notice that most factory Honda's do not do this.
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Last edited by Dwend; 05-12-2011 at 02:02 AM.
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Old 05-12-2011, 02:06 AM
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Re: Fact of Bull****?

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Originally Posted by Dwend View Post
A non-tuned ECU dis-activates the injectors when you are off throttle when in gear or when there is negative load, but when in neutral the engine has have fuel added to maintain idle as well as opening the IACV.
so being my prelude is stock, its better to decelerate IN gear? (in 5th gear all the way to stop) rather than in neutral?
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Old 05-12-2011, 02:19 AM
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Re: Fact of Bull****?

i was always told that downshifting helps re surface the clutch, is that true?
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Old 05-12-2011, 02:22 AM
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Re: Fact of Bull****?

Well not to a stop because you'll stall. But when the syncros have no load on them, you can just slide it out of gear with little amount of pressure. Hold the shifter when you decelerate you should feel the syncros become unloaded and then they will reload when it starts to bog the engine. This is the transition from negative load to positive load on the engine.
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Old 05-12-2011, 02:36 AM
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Re: Fact of Bull****?

from what i learned its a very minimal difference but if your coasting for a long time going downhill its actually best to be in neutral to the the slower rate of deceleration u have more kinetic energy to keep moving forward. but when your decelerating to come to a stop it doesn't really matter if your in gear or not (provided that u don't let it stall)
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Old 05-12-2011, 02:37 AM
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Re: Fact of Bull****?

Quote:
Originally Posted by Dwend View Post
Well not to a stop because you'll stall. But when the syncros have no load on them, you can just slide it out of gear with little amount of pressure. Hold the shifter when you decelerate you should feel the syncros become unloaded and then they will reload when it starts to bog the engine. This is the transition from negative load to positive load on the engine.
lol of course :P at about 900 rpm it slides out of gear smoothly.
so you'd decelerate in gear then? haha
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ok so im in the mall with my wonderful girlfriend... shes buying **** with her own money (shes a keeper)...
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Old 05-12-2011, 02:51 AM
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Re: Fact of Bull****?

I downshift when coming to stops, was just the way I was taught. I never have my car in neutral while slowing down or driving, only once I am stopped. I feel safer having it in gear and ready to gas if I need to move out of the way.
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Old 05-12-2011, 08:24 AM
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Re: Fact of Bull****?

Whenever Im coming to a stop I just keep the car in d4. Jeez you guys have it hard.
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Old 05-12-2011, 09:38 AM
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Re: Fact of Bull****?

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Originally Posted by dbd6604 View Post
Yeeah but is the car under more load at idle or when rapidly decelerating in gear? You have to keep in mind that RPM isn't the only parameter that effects fuel efficiency. There's also load, coolant temp, etc. IMHO, doing it one way or the other isn't going to give you noticeable results whatsoever.

Personally, I just shift into neutral and brake. It's probably easier on the trans/clutch.
The only reason I don't engine brake is because my poor H22's valve seals are worn, and when I engine brake it smells like hell inside the car LOL.
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Old 05-12-2011, 09:55 AM
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Re: Fact of Bull****?

Quote:
The only reason I don't engine brake is because my poor H22's valve seals are worn, and when I engine brake it smells like hell inside the car LOL.
Yeah well we were only talking about doing this for cars in GOOD shape, so this doesn't really apply to you.

jk bro
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Old 05-12-2011, 10:03 AM
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Re: Fact of Bull****?

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Originally Posted by dbd6604 View Post
Yeah well we were only talking about doing this for cars in GOOD shape, so this doesn't really apply to you.

jk bro
lol everyone refers to my car as the ugly duckling. It looks haggard as f*ck, yet the interior is nearly flawless, and she won't die. I do need to get a newer car though, so I can rebuild the whole Prelude. The motor is starting to act funny when accelerating at low RPMs. That, and it's just f*ckin worn out, lol.
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Old 05-12-2011, 10:44 AM
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Re: Fact of Bull****?

Keep the car in gear will help you save gas. When your in gear and decelerating your throttle isn't open at all, therefore no air is entering your cylinders and thus no fuel is either. SO when you are decelerating in gear you are using no gas, which is why if you have a fuel gauge and decelerate in gear it says your MPG at that moment is 0 because it cannot be calculated.

This is also why when i decelrate from a long period of time my CEL comes on cause no exhuast fumes are coming out so normal air eventually travels through my pipe( not cat) and to my 02 sensor and is like omg somethings not right hahaha
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Old 05-12-2011, 10:49 AM
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Re: Fact of Bull****?

you might save a small amount of gas by putting it in neutral while coming to a stop, but I don't think this is going to offset the cost of replacing your brakes much sooner than if you were to downshift.
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Old 05-12-2011, 11:35 AM
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Re: Fact of Bull****?

Listen to Dwend. He is right and covering the theory pretty well here.

You will use less gas while in gear.

At idle, the injectors send a small amount of gasoline in the engine to keep it rotating (idling).

When in Gear and decelerating, the injectors do not send any gasoline in the engine at all as stated by Dwend. That is totally correct and true.

bCoy may have answered correctly, but his explanation is totally incorrect. At idle, the throttle is closed and air still gets in the engine otherwise it wouldnt continue to run. if the engine is rotating(ie running), air is entering and exiting the combustion chamber. Valves are opening and closing, pistons are moving up and down, and oil is pumping..



On the other Hand... Snooppydaballa brings up a really good point. Coasting to a stop versus decelerating. Decelerating while in gear does have a much larger braking effect on the overall than just coasting. Depending upon the situation, the distance, and the end goal coasting could be more efficient. I've coasted for more than 5 miles in the mountains once where I was able to maintain or slightly accelerate while in nuetral the whole time going down a slight slope. Had I been in gear I would have needed to accelerate to maintain and put a positive load on the engine multiple times and probably used 5x the amount of gas.
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Old 05-12-2011, 11:58 AM
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Re: Fact of Bull****?

pretty sure the consensus of this thread is that the difference in gas usage is minimal.

the big difference is that one wears out your brakes faster, and the other wears your clutch out faster. which would you prefer?
having said that, i almost always downshift from 6th-2nd, then just coast.
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Old 05-12-2011, 12:32 PM
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Re: Fact of Bull****?

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Originally Posted by lilbluelude View Post
Listen to Dwend. He is right and covering the theory pretty well here.

You will use less gas while in gear.

At idle, the injectors send a small amount of gasoline in the engine to keep it rotating (idling).

When in Gear and decelerating, the injectors do not send any gasoline in the engine at all as stated by Dwend. That is totally correct and true.

bCoy may have answered correctly, but his explanation is totally incorrect. At idle, the throttle is closed and air still gets in the engine otherwise it wouldnt continue to run. if the engine is rotating(ie running), air is entering and exiting the combustion chamber. Valves are opening and closing, pistons are moving up and down, and oil is pumping..



On the other Hand... Snooppydaballa brings up a really good point. Coasting to a stop versus decelerating. Decelerating while in gear does have a much larger braking effect on the overall than just coasting. Depending upon the situation, the distance, and the end goal coasting could be more efficient. I've coasted for more than 5 miles in the mountains once where I was able to maintain or slightly accelerate while in nuetral the whole time going down a slight slope. Had I been in gear I would have needed to accelerate to maintain and put a positive load on the engine multiple times and probably used 5x the amount of gas.
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Originally Posted by twokexlv6coupe View Post
pretty sure the consensus of this thread is that the difference in gas usage is minimal.

the big difference is that one wears out your brakes faster, and the other wears your clutch out faster. which would you prefer?
having said that, i almost always downshift from 6th-2nd, then just coast.
lilblue, thank you for concluding the thread, and the answer to my question :D OMFG
rep'd*

you too twok
lol

well, i'll be leaving it in gear from now on. too bad i used more gas than usual all this time!!! even if i was using my brakes more, they needed to be resurfaced anyway :P
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ok so im in the mall with my wonderful girlfriend... shes buying **** with her own money (shes a keeper)...
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Old 05-12-2011, 12:42 PM
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Re: Fact of Bull****?

Quote:
Originally Posted by lilbluelude View Post
Listen to Dwend. He is right and covering the theory pretty well here.

You will use less gas while in gear.

At idle, the injectors send a small amount of gasoline in the engine to keep it rotating (idling).

When in Gear and decelerating, the injectors do not send any gasoline in the engine at all as stated by Dwend. That is totally correct and true.

bCoy may have answered correctly, but his explanation is totally incorrect. At idle, the throttle is closed and air still gets in the engine otherwise it wouldnt continue to run. if the engine is rotating(ie running), air is entering and exiting the combustion chamber. Valves are opening and closing, pistons are moving up and down, and oil is pumping..



On the other Hand... Snooppydaballa brings up a really good point. Coasting to a stop versus decelerating. Decelerating while in gear does have a much larger braking effect on the overall than just coasting. Depending upon the situation, the distance, and the end goal coasting could be more efficient. I've coasted for more than 5 miles in the mountains once where I was able to maintain or slightly accelerate while in nuetral the whole time going down a slight slope. Had I been in gear I would have needed to accelerate to maintain and put a positive load on the engine multiple times and probably used 5x the amount of gas.
oh so i am right but also wrong haha. i didnt really read anyone elses post, maybe i should have lol.

but yeah with all the different variables that you would have to take into consideration i guess either way it is pretty close to the same and doesnt matter which you do
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