gas milage

[The Central Scrutinizer]

Does this latest gimmick hold any promise?

http://www.wholly-water.com/magnetizer/super.magnetic.fuel.saver.htm

No.

(and I didn't even have to click the link. And yet, I know the answer is No. Amazing!!)

 

[jimlintott]

You can't drive in neutral you can only coast.

Automatic or standard transmission? With a standard, the engine will spin over according to wheel speed and gear selected. That means it is pumping air, if not enough fuel is supplied for the volume of air it will lean out. In neutral it will rev lower and probably use less fuel. (I'm not sure this is true but it sounds good.)

It is usually considered unhealthy to coast automatic transmissions and this is less of an issue as they don't effect the engine the same way a standard will.

 

[davefoc]

I agree with jimlintott for standard (manual) transmissions. More air pumped past the carburetor venturi more gas sucked.

I think this is also correct for non computer fuel injection.

Computer controlled fuel injection might do something to try to save a little gas in this situation, but I'm not sure that's correct or possible.

Do some or all automatic transmission freewheel when going down a hill where no power is required?

 

[WildCat]

I agree with jimlintott for standard (manual) transmissions. More air pumped past the carburetor venturi more gas sucked.

I think this is also correct for non computer fuel injection.

Computer controlled fuel injection might do something to try to save a little gas in this situation, but I'm not sure that's correct or possible.

Do some or all automatic transmission freewheel when going down a hill where no power is required?

I would think they don't. My Jeep has a second neutral setting in addition to the one in the main gear shift, it's located on the 4WD gearshift. This one completely disengages the transmission from the wheels, most cars will not do this. But IIRC all Jeeps do, which is why they're so popular to tow "4 down" behind large RV's. Tow most (automatic transmission) cars w/ all 4 wheels down for very long and you're asking for trouble. Just check it out next time you see a those monster RV's on the interstate, 90% chance the car they're towing is a Jeep.

 

[The Don]

Here's a good question for debate: does driving in neutral downhill use more or less gas than driving in gear?

I mean, in neutral, it needs to supply gas to the engine to keep it idling, right? I would guess that in gear, the engine needs zero gas.

For a manual gearbox. Nope, for a given throttle opening (i.e. very little), the out-of-gear example will use less fuel because both will be on the idling throttle setting but the ou-of-gear example will be pulling fewer revs. Also, the in gear version will be providing engine braking, further reducing efficiency

In order of reduced fuel consumption - and increased danger

- In gear
- In neutral, engine on
- In neutral, engine off

 

[Iconoclast]

Re: To Bikewer:

*My experience has mainly been with V-8's, and we say up to a 33% increase in horsepower by modifying intake and exhaust systems.

You are ***king dreaming mate! That's the problem when people sit around bench racing, they pull these incredible numbers out of nowhere.

 

[ceptimus]

Lots of fuel injection systems inject no fuel at all when the engine is being turned over by the transmission at above 1200RPM.

So when coasting down a long incline with such a vehicle, you'll use no fuel at all if you leave it in gear, but use some fuel to keep the engine idling if you select neutral.

 

[Iconoclast]

I agree with jimlintott for standard (manual) transmissions. More air pumped past the carburetor venturi more gas sucked.

I think this is also correct for non computer fuel injection.

Computer controlled fuel injection might do something to try to save a little gas in this situation, but I'm not sure that's correct or possible.

Modern EFI engines are able to sense a coast condition and will stop sending pulses to either half or all of the fuel injectors.

Do some or all automatic transmission freewheel when going down a hill where no power is required?

Old style 2- and 3-speed automatics were fitted with a one-way clutch known as a "sprag diode" on first gear which prevents the transmission overrunning the engine. Thus, an automatic equipped vehicle coasting downhill would engine brake identically to a manual equipped car. For other gears the transmission side can overrun the engine to a degree.

Regarding newer vehicles fitted with lockup torque converters, I believe the engine management software will try to detect whether engine braking or freewheeling is preferred in any given situation, but I don't know jack about modern cars so I may be far off base here.

By the way, the habit of selecting neutral when going downhill used to be referred to as "Mexican Overdrive" in less politically correct times.

 

[scotth]

Here's a good question for debate: does driving in neutral downhill use more or less gas than driving in gear?

I mean, in neutral, it needs to supply gas to the engine to keep it idling, right? I would guess that in gear, the engine needs zero gas.

It depends on the car. Most fuel injected cars will use less when coasting in gear. The FI on my BMW detects that the idle is being supported by the motion of the car and stops fuel flow completely.

In a carburated car, you will just see a higher vacuum behind in the manifold and the result will be a slightly higher air (and fuel) flow through the carb.

 

[davefoc]

scotth said:

In a carburated car, you will just see a higher vacuum behind in the manifold and the result will be a slightly higher air (and fuel) flow through the carb.

My guess is that the additional fuel used in a car with a carburetor is roughly proportional to the increase in airflow which is proportional to the rotation rate of the engine. e.g. If the car idles at 800 RPM and the car is rolling down hill with the car in gear and the engine is revolving at 3200 RPM the car will use about four times as much gas per unit of time than at idle.

My guess is this is true of non-computer fuel injection also.

I don't drive automatic transmission cars very often but my recollection is that in drive the engine rotation rate would drop somewhat below the rate that it would when it is connected to the wheels. It can't totally disconnect though because there clearly is some engine braking which is especially noticeable in the lower gears.

I believe the people that suggest that at least some computer controlled fuel injection systems cut off fuel entirely when the throttle control is cut off and the engine rotation rate is above a certain level.

Even if this is true, though, it can still save gas to travel in neutral down hills if the throttle control needs to be depressed to maintain the desired speed and it doesn't have to be depressed with the car in neutral, I suspect.

 

[scotth]

scotth said:
My guess is that the additional fuel used in a car with a carburetor is roughly proportional to the increase in airflow which is proportional to the rotation rate of the engine. e.g. If the car idles at 800 RPM and the car is rolling down hill with the car in gear and the engine is revolving at 3200 RPM the car will use about four times as much gas per unit of time than at idle.

Not true.

Here's why:

Except for very high performance engine (with lots of cam lobe overlap specifically), manifold vacuum at idle is quite high. To increase the flow through the carb by 4 fold as in your example, you would actually need to increase the absolute pressure difference across the carb by 4 fold. Even maintaining a near perfect vacuum in the manifold only improves the vacuum by a few percent, not 400%.

Looking at by the numbers (probably clearer this way).

Lets say atmospheric preasure at your location is 14psi.
Also, we'll say that at idle, the absolute pressure is 1psi (or 13psi of vacuum as compared to outside).

At most, in this case, you could take the manifold pressure to 0.
Can't go lower than that.

So, at most, you would achieve a 5 or 10% flow increase through the carb.

Just ball park numbers, but it serves well enough to demonstrate why pulling the idle speed up through the transmission does not cause anywhere near a proportional increase in idle fuel consumption.

 

[davefoc]

Thanks for that response, I was hoping somebody would confirm my thought or alas as seems to be the case correct me if I was wrong.

I would like to put your answer in my words:

At idle the throttle is closed down so far that the engine is already sucking almost as much air as is possible past the throttle. This is evidenced by the fact that at idle the pressure inside the intake manifold is only about a pound thereby producing a pressure drop across the carburetor of about 13 pounds or so which is already pretty close to atmospheric pressure which is the maximum pressure drop possible for a normally aspirated carburetor.

Is this about right?

 

[Yahweh]

I recommend driving with under inflated tired, preferably uphill, in your lowest gear.

Open all of your windows, turn up the AC all the way.

Make sure as you begin your journey, you gun your engine as much as you can (and perhaps gun it for a few minutes in park before you start the race).

I recommend driving on grass, and ride the brakes a lot.

Go ahead and hang a decorative parachute from the back of your car.

I'll be surprised if you dont blow that 40 MPG barrier out of the water!

 

[Jon_in_london]

Tweaking the engine by way of porting/polishing, etc? You might gain a couple of HP....thus a tiny increase in efficiency.

Polishing the intake ports may give a tiny increase in efficiency. However, a radical redesign of the whole intake can lead to drastically improved power- although whether this will result in better economy is another debate.

I used to drive a bog standard Mini-1000. It was a decent car for driving in heavy traffic and quick trips to the supermarket but not much good at high speeds or flying away from the green light. I managed to get set of 4 carbs from a large (750 or 1000cc Kawasaki) bike and got a grease-mokey friend of mine to install and tune them up. The result was that instead of one carb and a single, very square induction manifold, I had one carb per cylinder with a short, straight manifold, The performance difference was amazing! I didnt have it for long enough afterwards to notice what effect this had on fuel efficiency tho....

 

[scotth]

Thanks for that response, I was hoping somebody would confirm my thought or alas as seems to be the case correct me if I was wrong.

I would like to put your answer in my words:

At idle the throttle is closed down so far that the engine is already sucking almost as much air as is possible past the throttle. This is evidenced by the fact that at idle the pressure inside the intake manifold is only about a pound thereby producing a pressure drop across the carburetor of about 13 pounds or so which is already pretty close to atmospheric pressure which is the maximum pressure drop possible for a normally aspirated carburetor.

Is this about right?

Thats it exactly.

 

[scotth]

Polishing the intake ports may give a tiny increase in efficiency. However, a radical redesign of the whole intake can lead to drastically improved power- although whether this will result in better economy is another debate.

If the car was driven in the "wide open or engine off and in nuetral mode", this would in theory provide some milage gain. I would be surprised if the difference was measurable/noticable though.

Driving at steady state throttle, I wouldn't expect any improvement.

By raising the maximum output of the engine, it would improve the ratio of output horsepower compared to horsepower used to turn the engine over at WOT.

 

[Bikewer]

Race cars are not known for particularly good mileage....

 

[scotth]

Race cars are not known for particularly good mileage....

And race cars generally drive at very high speeds, meaning that they must push alot of air. Remember that air resistance increases by the square of the velocity. Twice the speed requires 4 times the horsepower to do it.

And race cars generally use their brakes.... ALOT. Bad news for fuel economy as considerable amounts of horsepower is used to heat up the brake discs and pads.

And race cars are generally a disaster aerodynamically compared to street cars. All the wings and spoilers used to keep them glued to the track add significant drag.

And those wide tires are terrible for fuel economy.

Now, if you took a NASCAR for example, removed the spoilers, put narrow tires on it, and drove in the 30 - 50 mph range using the wide open or coasting in neutral driving style, it would get very impressive gas milage.

Your point?

 

[King of the Americas]

To Iconoclast :

Now wait a minute there, mate!

I believe I said I worked on 'used cars' as a youth. I never claimed to be able to make thoses increases in a new stock engine...

An old engine with fouled plugs, clogged intake, and crudded up inards, clean it out and replace all seals and port and polish everything as to see yourself in it, will infact make 'big differences' in horsepower. 'I' have personally achieved as much as a 33% increase in engine performance with application of 'better' exhaust systems.

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To the posters on this thread:

Thank you all, and please go about and be openly proud of your 'increased gas mailage'. We should ALL strive to go further on less, if only for the sake of efficiency.

I plan to 'finish' my car with a paint job that displays the number of MPG my car gets on the highway.

Wouldn't it be COOL of that is what we bragged about?

I smell another "Initiative" brewing...

 

[King of the Americas]

To Jon_in_london:

AWESOME.

So is it a pile of mangled metal now?

Depending on the transmittion you could have had built, you'd of had a highway monster on your hands...

You had you a nice little street rod there, what happened to it?