fuel economy

hey all, i was wondering, would changing gears while the revs r lower decrease the amount of petrol your using? when im low on fuel im already in

5th gear when im going 60km/h.

any advice and help?

Sandro

Reply to
Sandro
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"Sandro" wrote

Usually lower revs are more efficient. If you take a look at BSFC-diagrams the line of constant engine power (needed for a given speed) is a hyperbolic one. The left top points give better fuel economy than the right lower ones.

Why not. Every modern engine can accept any revs / load combination from idle to red line (but idle speed may be too low for economy). Even if you hear a bad sound from the instrument panel, it does not harm the engine. It's only the panel beeing in resonance (not good for the panel and your ears, take a lower gear).

If you have a bordcomputer you can try what gear gives best mileage at a given speed.

Thomas

Reply to
Thomas Schäfer

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Reply to
William R. Watt

Nope, it doesn't 'really' work like that.

Engines have a 'sweet spot' where the power and torque is the most efficient.

If you rev lower than this it lugs the engine down causing it to burn way more gas or to need more gas pedal to do small speed changes.

For instance when I am on the highway at 105 kph (65 mph) in my Jeep I am turning at 2300 rpm which is the 'sweet spot' for power in my big 6 cylinder 4.2L engine and I get a nice 11L/ 100 km or 19 US mpg and I only need a twitch on the gas pedal to adjust for hills or wind. If I change up to 5th gear or overdrive, my rpm drops to 1700 or less and I drop to 15 or less mpg and I need to almost floor it to hold speed on the hills or in a headwind.

My Jeep has the aerodynamics of a brick, so things like that really are noticeable in it.

Every engine has it's own sweet spot for rpm.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

Sandro wrote:

Reply to
Mike Romain

::Rephrase: Would changing gears while the RPM are low decrease the amount of petrol your using?

Nope.

At low RPM the fuel injectors are in operation. Avoid changing gears below 1200-RPM.

-- Milo

Reply to
Indiån §ummer

Best fuel economy is achieved by keeping the engine in the fat part of the torque curve, which almost always means short-shifting. When you hit the next higher gear, you should be accelerating at a speed that is just above "lugging". You should not require a large throttle opening to increase speed in preparation for the next gearchange. On most cars, this means upshifting at around 2500 RPM, which should drop RPM to about 1500-1800RPM on the next higher gear. Rex in Fort Worth

Reply to
Rex B

so what rpm should i change at??

Reply to
Sandro

what would a civics sweet spot be? at what rpm should i change gears? at

3000 rpm or 4000rpm? what ideal?

Reply to
Sandro

2200 - 2500-rpm. Rpm shouldn't drop under the 1500-lugging rpm.
Reply to
Indiån §ummer

so changing gears at say 4000 rpm, is that bad for the motor? can u explain

thnx heaps!

Reply to
Sandro

"Sandro" wrote

When trying to save fuel you should try to apply a high load on the engine (open throttle, but not enriched mixture). That is more important than looking at revs (which can be calculated as a function from power and load).

It's not bad (*) to change at 4000 rpm, but not always fuel economical. Exact numbers depend on your engine. I.e. my small 75hp diesel is fine between 1200 and 3000 rpm, outside fuel efficiency rapidly drops.

While accelerating or climbing hills there is nothing bad with

3000 rpm (and still economical with high load), but I choose 4000 rpm only when I really need all the power (lane merging on ramp, >=100mph, ...)

(*) wear out will rise with approx. 3rd power of revs, but 4000 rpm is still moderate, so other factors are more important.

Thomas

Reply to
Thomas Schäfer

The last couple of 4 bangers I had ran with the best mileage around

3000- 3300 rpm if I remember right.

I really don't know on the Civic for sure.

On my Jeep engine I keep close track of the mileage on a highway run and judge it from there.

Mike

Sandro wrote:

Reply to
Mike Romain

Sweet shifting spot for most Civics are at 2100-rpm. This is mentioned by Honda. The HX also comes equip with a shift light that suggests shifting at 2100-rpm for economy. The same idea is program into automatics which also shift at 2100-rpm for economy or higher depending on the demand for power.

Sweet cruising spot for most Hondas are 2100-rpm @ 55-mph. The EPA suggest going higher will sacrifice fuel economy. Actually this is not a big problem as new engine technology such as the V-TEC-E which rearrange valve opening-patterns for efficiency at high rpm. The "E" stands for efficiency.

-- Milo

Reply to
Indiån §ummer

"Sandro" wrote in

Nope. Shifting at 4000-rpm is absolutely normal at operating temperature. Actually, healthier for the extra rugged-motor as it cleans out deposits once in a while at the cost of fuel consumption.

-- Milo

Reply to
Indiån §ummer

It depends entirely on the engine. If it can produce enough torque to move the vehicle at low RPM, then yes it generally saves fuel.But if its a little high-RPM mosquito of an engine, then its not only going to use more fuel, its harder on the engine's bearings, which are small and light for better high-RPM operation.

Reply to
Steve

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