N74DV

I'll try this one more time

look at it this way.... if your foot is off the accel and you are engine braking and if what you say is true that more fuel is being introduced then answer this....

what is happening to that fuel? is it being burned? if it is then where's the extra power that you would expect? remember... more fuel and more air equals power. Last time I engine braked it slowed the car down not sped it up due to increased power.

So... then we have to assume if what you are saying is true, that more fuel is introduced during engine braking, and the engine is not producing power (of course it's not... that's why engine braking works) then that fuel is not being burned.... you did say earlier that the spark plugs don't fire during engine braking..... another ridiculous claim.

That's crazy because that would mean the engine would be running extremely rich with raw fuel passing though.... can you say fouled plugs??

The ECU WILL NOT LET THIS HAPPEN!

are you seriously saying that fuel delivery is proportional to engine RPM???

the whole point of engine braking is to let the compression and drag of the engine slow the car down..... if more fuel was being used during this process the engine would be producing power negating the engine braking effect.

do some research guys..... educate yourselves before acting like you know it all.

install an A/F meter and try some engine braking. you'll soon find out that your A/F ratio stays the same.... if anything it may get a little leaner.... this is why the O2 sensor exists..... if too much fuel is being put through and not being burned it is detected and the mixture adjusted accordingly.... if it didn't our cars would never pass our strict emissions laws.

joust75

iTrader: (1)

You are trying to argue based on knowledge you have without knowing the truth. This is a quote from GMs website under how to improve fuel economy:

Do a web search on downshifting and fuel economy N74DV. You are the one that needs to do some research. I think this quote adresses your above questions.

Just ask any mechanic! Downshifting uses more fuel than idling or coasting and overall hurts fuel economy. Not as much as operating at the same RPM under load but definitely notable.

Ouch okay! You should heed your own advice though because on this count....

You are wrong!!!!!

msims

Again ... simple physics. It takes WORK to slow a car down. The faster you slow it down, the more work it takes. Remember high school physics? F=MA (Force = Mass X Acceleration). Deceleration IS acceleration, only not in the direction you are use to thinking of it in ... its called a vector, and it can be easily seen with simple vector mathematics. Did you take physics in high school? SO, if it requires WORK to slow the car down quickly, then the fuel going into the engine during engine breaking actually provides a source of power for the engine to perform the WORK that it does during engine breaking. This work is also shared by the tyranny, the tires etc.

Actually, this is entirely incorrect. Engine breaking works because the engine is being forced to produce power (resistance in this case) for the tyranny. Anything that does work generates heat. It is a natural by-product of work. If you took a cold engine, put the car in neutral and coasted down a hill, then dropped the car into third gear and let up on the clutch (assuming the key was in the on position), the engine would start to work against the car. It would also start to run and consume fuel (some people do this when their starters are broken). If the key was not on, the engine would still produce resistance for the car and would in fact get hot over a short period of time.

You need to re-read what I said. I said that according to your theory, the spark plugs would not be firing during engine breaking. The entire foundation of my argument rests in the fact that the spark plugs DO IN-FACT ignite the fuel in the piston chamber while the car is engine breaking. And that there is more fuel being taken into those chambers during engine breaking than during rotor breaking while the engine is at lower RPMs.

Yes! 3000 RPMs consumes more fuel per second during engine breaking than at 1000 RPMs. However, in a carbureted engine, if you are at ... say 1500 RPM, and you are only going 20 MPH, then you drop the tyranny into 4th gear and punch it, you will be wasting more fuel than if you were at 2500 at 40 MPH and you did the same thing. This same phenomena does occur in fuel injected engines, but the computer controls it much better than a carbureted engine.

This is totally TRUE!

This is where you falter. The power consumed by the gas during engine breaking is used to keep the engine running during breaking. If you were to turn the engine off during engine breaking, the car would slow down much faster than while the engine is running. Fuel is keeping the engine from dying. My point is, there is not that much fuel used during engine breaking than during acceleration. But there is more fuel used during engine breaking than if you let the engine idle while you break the car.

The reason I like to engine break, is that it takes work away from the break pads. When you break, the work (friction, MxA) needed to slow the car down has to come from somewhere. I prefer the work load be balanced between my engine and my break pads.

"Hello ... pot? Yeah ... this is the kettle ... your black!"

A/F is a RATIO ... not a linear representation of how much fuel is going into the engine. If the A/F ratio is the same during engine breaking as it is during engine idle, then you have proven my point. At higher RPMs, the engine is sucking more air, and simultaneously, it is sucking more fuel ... when the engine is idle, it is sucking less air, and proportionately sucking less fuel. Here are some ratio examples for you to consider ... I am in no way an auto mechanic or a mechanical engineer, but I do understand physics and mathematics quite well, so these numbers are more than likely not representative of real air and fuel quantities, but it will clearly demonstrate why you see the ratio as being constant during deceleration:

Lets say that at 3000 RPMs, the car sucks down 1.5 cubic feet of air per second, and simultaneously it sucks down .1 gallons of fuel per second. Your A/F ratio would be 15.

NOW ... lets say the car is still engine breaking, but it is now at 1000 RPMs. Because it is at a lower RPM, it sucks in less air per second ... say .8 cubic feet per second. At this RPM, the engine needs less fuel, so it is only sucking in .053 gallons per second. The ratio is still 15, and yet the car is consuming less fuel at a lower RPM then it would at a higher RPM.

You get it?

N74DV

yada yada yada......... ask your nissan service tech the next time you see him. then come back and post how I was right.

or for a lesson on EFI read this

http://www.edmunds.com/ownership/tec...3/article.html

no $hit sherlock.... but it also would show a rich condition as if excessive fuel were entering the cylinders during engine braking. If the fuel were not burned with the proper amount of air the mixture would be rich..... a lower A/F reading like this would be sensed by the ECU and fuel delivery would be reduced.

maybe we are arguing at two ends of a stick.... I'm saying a noticeable reduction in fuel economy will not be realized..... I think you think I'm saying zero fuel is burned during engine braking..... which I am not.

msims

Listen asswipe - no one is saying that there is EXCESSIVE fuel being consumed during engine breaking. We are simply saying that there is MORE fuel consumed during engine breaking than there is during NON engine breaking.

If you could pull your head out of your a$$ for two seconds and realize that you are dead wrong, this argument would end. Swallow your pride for once, and admit that you either mis-read the argument, or that you are simply too damn stupid to understand english.

ZBaby

...and we are all wondering why the resale price on a used Z isn't so good ?

N74DV

if you would read what the dude first posted that staretd this..... he said engine braking wastes gas.. to me wasting gas means a lower fuel economy is realized.

I still say it's not.

I'm done this time...... I promise.

msims

Specifically, he is right, and broadly, you are right.

I too am done with this.

jedoublery

when you downshift, engine revs higer due to momentum and shorter gear.

Is this a Civic forum? Are we girls? I can't believe we are even discussing this.
Ask your local mechanic.

ShaftEd

I have an 04 Touring Roaster with the 5AT. I'm not worried that manual downshifting is really hurting anything, keeping in mind that I'm not banging downshifts to go from 2k to 6k rpms either. Most of my around town driving is in "D". I go to the manual mode whenever I'm on the twisties, so the tranny doesn't hunt through the gears right in the middle of a corner. If you have ever tried to run your Z in "D" on a twisty road, you'll know it's pretty awful. Gotta use the manual here. Also, sometimes I will switch to the manual mode in city driving for a burst of acceleration. I don't like the way the "D" mode does it's downshifts when you are hard on the gas, and it shifts into too high a gear to soon. I'd rather downshift manual then hit the gas and shift manually through the gears. It is smoother that way. Once I get up to speed and on my way, I go back to "D" mode. I really like the way it works, even though an auto on a sportscar is kinda "gey" (jk). I also, like the way when you flip to manual mode from "D", it automatically shifts down one gear(the engineers must have figured if you are going to manual, you are needing some accel), then you bang a 2nd quick downshift manually. You can get down two gears quicker than just letting the auto do it in "D". As for hard accel off the line, just leave it in "D" (TCS off) and hit it. Perfect shifts at redline everytime.

I have to agree that if you are using the auto in manual mode almost all the time, then yes, it probably wasn't designed to be used this way, and the 6MT would have been your best choice. I like to think of the manual mode in the 5AT, as something to keep in reserve. Use it when you need it, and then go back to the "normal" driving mode.

msims

I agree. I love the manual mode for quick accelleration when its needed ... especially for passing someone on the freeway. down shifting first, then hitting the gas is much smoother than letting the transmission decide when to shife based on how buried your foot is in the gas pedal.