fuel flow at high rpms but no throttle in a car

Matt13C

Well-Known Member
I know this is not aviation related but thought it might get more attention here from people who might know.

I was driving home from work in stop and go. I would get to second gear then go to neutral and brake, thinking that if I engine brake I am wasting gas. But am I?

In a plane when you pull the throttle the engine goes to idle. But in a car, if still in gear, the engine will run down slower, but does fuel flow go to idle or will it decrease with RPMs?
 

Minuteman

“Dongola”
does fuel flow go to idle or will it decrease with RPMs?
On fuel injected cars, the computer knows the load on the engine and sets the injector duty cycle as needed.

At idle, and above a certain RPM, the injectors will shut off completely. So coasting in neutral may use more fuel than allowing the wheels to backdrive the engine at a higher RPM.

As always (almost), what's good for fuel economy is not-so-good for the mechanicals.
 

ppragman

FLIPY FLAPS!
On fuel injected cars, the computer knows the load on the engine and sets the injector duty cycle as needed.

At idle, and above a certain RPM, the injectors will shut off completely. So coasting in neutral may use more fuel than allowing the wheels to backdrive the engine at a higher RPM.

As always (almost), what's good for fuel economy is not-so-good for the mechanicals.

:yeahthat:

The only time I down shift is to help in the braking process when its slick out.
 

Barty

Well-Known Member
I just leave it in gear and take my foot off the gas when I'm coming up to stop signs or traffic lights. I will downshift when going downhill or just before taking a turn since I know I'll need the lower gear when accelerating back out of the turn.

If the computer didn't shut off the fuel when the throttle was closed and the car was still moving, the exhaust would get loaded up with raw fuel, which could cause a nasty backfire when you get back on the throttle, potentially damaging the catalytic converters and muffler. In addition, as manifold pressure decreases, fuel required decreases, sometimes lower than the minimum pulse width of the fuel injectors. Along with the over-rich condition I described above, the excess fuel can also end up running down the cylinder walls, diluting the oil and causing premature wear to the cylinders.

Now you know...
 

WalterSobchak

Well-Known Member
On fuel injected cars, the computer knows the load on the engine and sets the injector duty cycle as needed.

At idle, and above a certain RPM, the injectors will shut off completely. So coasting in neutral may use more fuel than allowing the wheels to backdrive the engine at a higher RPM.

As always (almost), what's good for fuel economy is not-so-good for the mechanicals.
Confirmed or you read this months Car and Driver.
 

Matt13C

Well-Known Member
On fuel injected cars, the computer knows the load on the engine and sets the injector duty cycle as needed.

At idle, and above a certain RPM, the injectors will shut off completely. So coasting in neutral may use more fuel than allowing the wheels to backdrive the engine at a higher RPM.

As always (almost), what's good for fuel economy is not-so-good for the mechanicals.
Great.

I tend to not down shift, unless I am going around a turn, in which case I rev match to minimize engine braking and wear/tear on the clutch. I think that is why I have kept the stock clutch on my car to 34k when most people with the car had it go at around 12k.

I was trying to figure out if it is better to allow the car to engine brake or actually use the brakes sooner. I am chewing through my brakes/rotors and do not feel like replacing them any time soon.

Thanks for all the input!! :nana2:
 

OldTownPilot

Well-Known Member
What sort of car goes through clutches every 12k miles?

I've driven my car 43k since i've got it and it now had 113k and still on its first clutch.

Tires, body panels and paint on the other hand........
 

SteveC

Really?
Staff member
I just sold my '96 Jetta with 187,000 miles on it. Still had the original clutch.
 

Matt13C

Well-Known Member
Mitsubishi Evo....they made the clutches soft to protect the trans.

They gave the car a two step launch control. Instead of replacing transmissions under warranty, they made a clutch that would burn up. Once you put in grippier aftermarket clutch to get the power to the wheels, the warranty is voided.

I didnt buy the car to drag race, it is just fun to drive on twisty roads. But it requires some babying of the clutch and perfecting the heel toe to keep it going strong this long.
 

PGT

Well-Known Member
No downshifting to slow down will not have a bad affect on your clutch, unless you're driving aggressively. I asked a few other car guys what they think and they agreed that it will not damage a clutch.

I did this EVERY time I came to a stop for 2 years (as well as tons of launches, including three 7k RPM clutch drops) and nothing wrong with the clutch.
 

wheelsup

Well-Known Member
If you downshift it's probably a good idea to double clutch it. The clutch on my Jetta is still good but the syncros are fubar'd. We've had it for 20k miles and got it like that, the previous owner must've done something wrong. My wife and I have owned manual cars and put tens of thousands of miles on them and never had anything fail or syncros go out. That or the Jetta is just a POS, but that's pretty well known.
 

Matt13C

Well-Known Member
No downshifting to slow down will not have a bad affect on your clutch, unless you're driving aggressively. I asked a few other car guys what they think and they agreed that it will not damage a clutch.

I did this EVERY time I came to a stop for 2 years (as well as tons of launches, including three 7k RPM clutch drops) and nothing wrong with the clutch.
No it wont damage the clutch, it just adds wear and tear to it. Even if you rev match it is still engaging so you are wearing a small amount off. I choose not to do it unless I have to knowing the clutch is the weak point in the drivetrain.

I was thinking along the lines of fuel savings vs the cost of new rotors pads. If letting the engine run down in gear does not use fuel and slows me down it is win win, save gas and brake pads.
 

Barty

Well-Known Member
If you downshift it's probably a good idea to double clutch it. The clutch on my Jetta is still good but the syncros are fubar'd. We've had it for 20k miles and got it like that, the previous owner must've done something wrong. My wife and I have owned manual cars and put tens of thousands of miles on them and never had anything fail or syncros go out. That or the Jetta is just a POS, but that's pretty well known.
There is no reason to double-clutch a synchronized transmission. The whole purpose is to allow the counter-shaft speed to match that of the input shaft, but the synchronizers take care of that for you.

If the synchros are bad, it is probably because someone had a habit of forcing the car into gear without the clutch being fully disengaged or most likely, they had the transmission "serviced" at some point and someone put the wrong grade of oil in there.
 

roundout

Bus Driver
I know this is not aviation related but thought it might get more attention here from people who might know.

I was driving home from work in stop and go. I would get to second gear then go to neutral and brake, thinking that if I engine brake I am wasting gas. But am I?

In a plane when you pull the throttle the engine goes to idle. But in a car, if still in gear, the engine will run down slower, but does fuel flow go to idle or will it decrease with RPMs?
In my TDI Jetta, if you coast in gear, the fuel injected mg/stroke goes to 0 and the MPG goes to 999.9.

Not sure about gassers.

EDIT - the fuel injected quantity only goes to 0 above 1200 rpms. From 1200 down to 903 (idle rpms) it starts injecting fuel again to keep the motor running.
 

Matt777

Well-Known Member
In my TDI Jetta, if you coast in gear, the fuel injected mg/stroke goes to 0 and the MPG goes to 999.9.

Not sure about gassers.

EDIT - the fuel injected quantity only goes to 0 above 1200 rpms. From 1200 down to 903 (idle rpms) it starts injecting fuel again to keep the motor running.
Is this standard for all cars?
I have a '93 Nissan 240SX and I used to get 22mpg, but when fuel became so expensive I let up a bit on my lead foot.
I take it out of gear anytime I am approaching a stop and let it coast to the line.
On the freeway, when there is a long downhill I accelerate to 75 and take it out of gear. When I take the freeway off ramp to go home I know right where to take it out of gear at 70mph so that it requires minimal braking approaching the stop sign.
My mileage has gone from 22 to 26.5 mpg.

So, let me get this straight, I am wasting fuel when the eninge is idling?
Leaving it in gear is better?
Then I'd have to keep applying power (fuel) longer to make up for the engine's drag.

What is most efficent when it comes to decelerating without trying to get braking from the engine?
 

Matt13C

Well-Known Member
Is this standard for all cars?
I have a '93 Nissan 240SX and I used to get 22mpg, but when fuel became so expensive I let up a bit on my lead foot.
I take it out of gear anytime I am approaching a stop and let it coast to the line.
On the freeway, when there is a long downhill I accelerate to 75 and take it out of gear. When I take the freeway off ramp to go home I know right where to take it out of gear at 70mph so that it requires minimal braking approaching the stop sign.
My mileage has gone from 22 to 26.5 mpg.

So, let me get this straight, I am wasting fuel when the eninge is idling?
Leaving it in gear is better?
Then I'd have to keep applying power (fuel) longer to make up for the engine's drag.

What is most efficent when it comes to decelerating without trying to get braking from the engine?
If your goal is to conserve fuel while losing the least amount of speed it appears the best bet is to be light on the throttle when on level surfaces and go to neutral when down hill or stopping.

If your goal is to save the brakes using engine braking has the added bonus of also keeping fuel flow below idle flow, or so I understand it.

I don't do the whole put it in neutral when driving, I just keep it below where boost starts to build, sub 3000 rpms when cruising on the highway. That and some easier acceleration has gotten me from 19mpg up to around 22mpg.
 

roundout

Bus Driver
Is this standard for all cars?
I have a '93 Nissan 240SX and I used to get 22mpg, but when fuel became so expensive I let up a bit on my lead foot.
I take it out of gear anytime I am approaching a stop and let it coast to the line.
On the freeway, when there is a long downhill I accelerate to 75 and take it out of gear. When I take the freeway off ramp to go home I know right where to take it out of gear at 70mph so that it requires minimal braking approaching the stop sign.
My mileage has gone from 22 to 26.5 mpg.

So, let me get this straight, I am wasting fuel when the eninge is idling?
Leaving it in gear is better?
Then I'd have to keep applying power (fuel) longer to make up for the engine's drag.

What is most efficent when it comes to decelerating without trying to get braking from the engine?
Depends on the vehicle. It just happens to be this way in the VW TDIs. I have a diagnostic tool/software and can data-log all sorts of fun stuff while driving. It only works in VWs and I've only got one (a diesel). Not sure if gassers are the same or not.

An OBD-II ScanGauge may help but they're kinda expensive.
 

roundout

Bus Driver
If your goal is to conserve fuel while losing the least amount of speed it appears the best bet is to be light on the throttle when on level surfaces and go to neutral when down hill or stopping.

If your goal is to save the brakes using engine braking has the added bonus of also keeping fuel flow below idle flow, or so I understand it.

I don't do the whole put it in neutral when driving, I just keep it below where boost starts to build, sub 3000 rpms when cruising on the highway. That and some easier acceleration has gotten me from 19mpg up to around 22mpg.
There are good reasons to leave the vehicle in gear rather than neutral coasting. Your economy may drop .5mpg but your safety will go way, way up.
 
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