Cyber Scriber's guide to getting good fuel economy

[bradc]

Trev - in an N/A car that is normally below the point of max torque (more or less, generalising here). On a VR-4 that point is going to be about 4000rpm or so.

Brad - yes DSG's are cool, but VR-4's don't have them. CVT's however are plain evil and not suited to anything other than lawn mowers and mobility scooters.

 

[godzilla]

Trev - in an N/A car that is normally below the point of max torque (more or less, generalising here). On a VR-4 that point is going to be about 4000rpm or so.

Oh righto, so the limit is 4k? I might try it the next time i have a decent trip on the HWY and just run the car in 4th @100 and keep an eye on the throttle % and the vacuum on the boost gauge.
Thanks.

 

[bradc]

4000rpm in 4th is 130kmh, so you'll be fine to stay below it. I'm sure that running in 5th at highway speeds will be better for fuel economy though.

 

[godzilla]

4000rpm in 4th is 130kmh, so you'll be fine to stay below it. I'm sure that running in 5th at highway speeds will be better for fuel economy though.

I have an auto so would it still be 130?

I thought the whole thing was that it is better to do 100-110k at a higher rpm as to save fuel consumption?
I am confused, i will read Kenneth's post again.

 

[king_panther]

With cruise control on, my car will now sit on about 2400-2500rpm at 110km/h.

 

[cyber_scriber]

All this talk of 'labouring' too, what exactly is that describing ?

Brad.

My understanding of "labouring" is when you are trying to accelerate in a gear that's too tall for the engine to have adequate pull. This puts the engine under extra load and it has to "labour" to pull away.

From a practical perspective, it won't be hard for the engine to pull if you accelerate from standing still in first gear. However, try and take off in 3rd gear and the engine will be under the pump to get away cleanly. This "labouring" usually results in extra fuel use.

Another example if that you're doing 40klm/hour. In 3rd gear, it's no problem for the engine. However, in 5th gear, the engine will be struggling and labouring to pull.

 

[cyber_scriber]

40-70 in 5th is really low, if driving economically I'd stay out of 5th until at least 70kmh in a manual.

Really Brad? Perhaps it's my driving style or the car's very short gearing but in my stock manual, I find that there's still acceptable response doing 60 in 5th. This of course is assuming a flat surface. Uphill would be a different story.

 

[bradc]

Trev - yes I did those calculations with an Auto in mind. Stay in 5th for motorway driving.

Brad - 2500rpm at 110kmh is about right for an Auto.

Bruce - yeah in a manual the ratios are much much shorter and you can let it drop quite a bit lower and still have adequate pull

 

[Kenneth]

Its better to do less speed, as that is the main factor in your fuel consumption when you are cruising at a steady speed.

As I said, the only real way to answer the question is to monitor your injector duty cycle and see which method results in the smaller figure.

The point in the post was to say that at speed, gear ratio and rpm don't really make as much difference as you would think. As such, there are more effective fuel saving strategies. (Such as travelling 10kph slower and driving with very gradual throttle changes)

I have an auto so would it still be 130?

I thought the whole thing was that it is better to do 100-110k at a higher rpm as to save fuel consumption?
I am confused, i will read Kenneth's post again.

 

[Kenneth]

You have to be aware that DSG (dual clutch, dual shaft etc) gearboxes are based on a manual gear arrangement, not a automatic one! (i.e. it doesn't use a planetary gear arrangement)

A DSG gearbox is basically a computer controlled manual gearbox. The real advance is in the fact it can pre-select the next gear due to having multiple input shafts.

CVT is the way to go in my opinion... I would love a decent (i.e. not too heavy and able to handle the power) CVT, acceleration would be just phenomenal.

Unfortunately, most human brains aren't :001_unsure:

DSG/CVT/Dual clutch jobbies are the way to go. Less fuel use than manuals. Even Porsche are using dual-clutch autos. VW have stopped all development of autos to focus on DSG boxes.
Traditional gearing should have been dead decades ago, but so should have lots of things. Progress & evolution don't always move as fast as it should.

Interestingly, would there be much of a change on drag or effeciency between Series 1 & 2's rear wings on the Legnums ?

 

[Kenneth]

Your point is a good one, and comes under the bit where I said the variables are far too numerous to make a meaningful comparison.

The question is how negligible the difference is. If you consider that the main inefficiency in the engine is thermal, then a basic assumption is that the friction losses are minimal (the engine being healthy) and will provide only a negligible and possibly un-measurable difference. Especially if the engine is closer to its maximum efficient working RPM.

The thing is that you just cant know. We can make assumptions all day and I can show in theory that there should be no difference, but the only way to know for certain is to actually test it by monitoring what the ECU does.

I have a saying which I heard/read somewhere which seems to come to mind so often these days...

"In theory, there is no difference between theory and practice. In practice, there is."


My intention was to stop people comparing gear ratios and RPM and using RPM as a fuel economy measure.

Now i am not going to try and fool anyone into thinking i am across this info you have just put up for all of us but i have just had a thought in regards to your post, with the high rpm and low throttle v low rpm and heavier throttle and you coming to the conclusion that there is a negligible difference, where does the resistance from the engines own compression come into it? There has to be a point where to keep momentum and rpm the engine must need more fuel just to combat the forces behind this. :001_unsure:

 

[godzilla]

My intention was to stop people comparing gear ratios and RPM and using RPM as a fuel economy measure.

Thank you Kenneth, i do appreciate your response and what you have contributed in this thread, it does make for an interesting read (albeit a brain hurting one lol ).

 

[Gary31]

My intention was to stop people comparing gear ratios and RPM and using RPM as a fuel economy measure.

I agree with most of what you have said but RPM and gearing is the easiest and most quantifiable parameter to monitor, which is why I believe it is so commonly used for fuel consumption comparisons.

 

[smohekey]

I don't know if I'm off base here, but I've been monitoring boost to get an idea of load.

 

[harry]

Yeah I was just using that as an extreme example. I'm in 3rd @ 40, if I'm cruising @ 70+ I'll whack it in 5th. But thanks for the info, that was what I had assumed but nice to know I was on the right track with my thinking!

This is the whole reason manuals use less fuel. You can actually tell the car what you want it to do rather than it telling you. And no matter how many acronyms your transmission has, the human brain is smarter. Well, most human brains are...

that's not the reason.

firstly autos typically use more fuel around town than manuals and less on highway trips.

The reason they use more around town is because the auto transmission is not as efficient at a manual box (ie it costs more power to drive it) and as a result you use more fuel to make more power. Its not because you are smarter than an auto's ECU at choosing the correct gear for the speed (and load) you are doing.

autos (built in the last 20 years at least!) are typically better on fuel on long highway runs because they run at lower revs at highway cruise speeds than manuals and the torque converter is normally locked in this situation which all but removes the inefficieny problems involved around town.

lower rpm at highway cruise is generally better in the real world, if you have the torque to do it easily. Its the reason V8 Commodores have that ridiculously tall 6th gear that's unusable excpet for cruising along the highway, along with others like my previous TT Supra 6spd.

 

[bradc]

I drove 475km today and used 43.6 litres of fuel. This was with 95 octane in a stock 1996 manual Legnum with a set of wheels in the car as well as clothes and stuff for a weekend trip. I stayed below 3000rpm almost the entire time, and cruised at 105-110kmh the whole way, which is about 2700-2900rpm in a manual

About a month ago I did the same trip in a 96 auto Legnum, driving similarly, but with an intercooler and boost controller (although we hit boost so rarely it is irrelevant) On that trip the car used 48 litres. Most of the time we were sitting at 2300-2500rpm.

 

[harry]

hmy: from the fuel consumption thread it doesn't look like anyone is getting remotely close to that over here!

9.17l/100km!!!

 

[Kitty's VR4]

Harry,

I haven't got round to posting up my figures, but if you look here, you'd be surprised!!!!

 

[cyber_scriber]

hmy: from the fuel consumption thread it doesn't look like anyone is getting remotely close to that over here!

9.17l/100km!!!

Harry.

I just did a pretty good tank myself. As reported in the fuel consumption thread, I just recorded 9.96 litres / 100klms without really trying.

This was the first tank I did after a full service (oil, filter, plugs, ignition leads etc).

Before the service, I was recording around 11-12 litres / 100k so keeping the car in good nick does make a substantial difference.

 

[ygoslo]

Harry.

I've got nothing to add, just wanted to write your name at the start of a post to fit in.