Engine revs for saving petrol

Engine efficiency and best MPG aren't necessarily the same thing. An engine is at its most efficient in mathematical terms at around the peak torque figure, but there are other factors like wind resistance and transmission losses to be taken into account.

^ "Engine efficiency and best MPG aren't necessarily the same thing. An engine is at its most efficient in mathematical terms at around the peak torque figure"

Should you not add the clause that the engine must be at wide open throttle for that statement ???

My own tests have shown that at partial throttle openings, better mpg is obtained at lower rpms.

Also, when driving various late automatic gearbox cars, they have been programmed to change up very early. Petrol and diesel. For example, a Golf 1.4 TSI (122PS) DSG was in seventh gear (normal D mode) at an indicated 37 mph showing around 1200 rpm, and the car was struggling to maintain that speed when a slight incline appeared.

David

You can look generally look at the torque curve on the Superchips site, and see that it is fairly flat for a normally aspirated engine.

Keep the revs above 1500 when accelerating or climbing, and use the highest possible gear whilst on over-run and slowing down. If you have a TD and are not looking beyond the end of your bonnet, 3rd might be best.

Does this car have an economy meter? There was a time when a vacuum gauge did the trick but I think that went out with a carby engine.

My BMW Mini has a gear change indicator. If I used it I'd hardly ever be over 1800 revs. A smashing (petrol turbo) engine though - smooth to 6500 and pulls like a train. 35-40mpg.

Rob

The old fashioned advice was to fit a manifold pressure gauge (vacuum gauge) and drive keeping it reading the lowest reading (highest pressure). I haven't seen one that would work with a modern turbo or supercharged car engine - though supercharged aircraft do have them.

Slatts

You're mixing up economy advice with safety advice. Driving in third gear around town helps to remind you to keep your speed down, gives you more useful acceleration if you need it and is probably kinder to the engine as many modern cars aren't really that happy at 30mph in 4th gear.

It doesn't help your economy though.

Tim

So what you want is this sort of thing?

1: upper RH chart on lower half of P13. they have plotted l/100km on the chart "Run in all gears". Lower is better. It's a lot nicer to plot mpg/speed for each gear. Also you won't be running down to 1200rpm in top so there are cutoffs. It's not a real engine as nothing would have that huge drop off in top making it only any use below 2000rpm. 2: Smart51 on pistonheads has computed it for his Locost. Found each gear had a best range. On 2nd page Othlith has got a plot for his RX-8, below 50mph 4/5th makes no odds, above it needs 5th. But as the Pontiac Geo Metro shows it's common to only get a top gear fuel curve. YMMV. 3: Far better presentation of the data on page 847 in "The Motor Vehicle" by Garrett, Newton & Steeds. This is a "double" overdrive 6 speed gearbox (top speed in 4th). The rule of thumb for this vehicle would be as slow it will go in as high as gear possible. (be quick, now I've linked to it, it will be withdrawn from preview) YMMV. You don't have this vehicle, you don't have this engine with its unique BSFC map, you don't have this gearbox etc. You will have a hard job to find this quality of real data for YOUR specific car. It's almost always a computation for a "what if?" in some SAE paper.

None of that applies for acceleration, the mixture is set rich for acceleration and fuel consumption depends on rate demanded as well as speed and gear.

Get a data logger (adurino etc) and connect it to your car (then apply for your PHD). You need to acquire the injection duty cycle, known injector size, engine rpm and vehicle speed. Go drive it on a perfectly level road, with no wind at a lot of steady speeds for each gear. Then go crunch the data in Excel or Open Office Calc. (then apply for your

2nd PHD)

Or how about the moon on a stick?

Don't use too a high gear for slowing down with overrun fuel cut, may need a lower gear to keep rpm up (auto click the O/D off). It only shuts the fuel off when rpm is above the idle soft landing speed. At engine speeds lower than that fuel is supplied so it doesn't stall. Typically around 1500rpm for a 4 cyl with 850rpm idle, maybe higher if idle is higher, lower if idle lower.

Overrun fuel cut has been the norm on all fuel injected engines since the late 80's. Even the K-jet had it - page 9. Surprised me, but it was added much later than the 1973 introduction.

Some electro carbs also had overrun fuel cut. The idle position was set by a solenoid instead of a fixed stop, so fuel could be shut off above (Bosch Pierberg 1400rpm) when throttle was shut.

It's called a "boost gauge" [1] and they are very easy to obtain.

Except to use it as economy gauge you have to keep the reading on most vac and out of the boost, which makes having a turbo pointless. As its for BOOST the vac scale is somewhat depreciated.

[1] Mines a "bust" gauge, so called as it's BOC welding cylinder O2 gauge that had the flame trap soldered up. So I stuck a nail though it and moved the pointy thing to show 15. I've learnt to deduct 15 from the boost and 5-6ish (-9psi to -10psi = 20ish inHg) is idle vac.

That's what I meant my 'highest possible' - you wo'n't be slowing down once the fuel is being injected again.

IME the fuel stays cut off until about 1000 rpm. Yes, I do have a car with an instantaneous mpg readout and a 4 cyl engine with 850rpm idle.

One car I had was above 1800 rpm for over-run cut off. Below that, it started to supply fuel again.

David

Have you been on one of those speeding courses too? :)