Cyber Scriber's guide to getting good fuel economy

[king_panther]

How is this possible Brad? Are you running different OD tyres?

What's OD tyres ?
I don't know how it's possible. Different wheels & tyres between cars. Grandpa has had the transmission replaced & the gearing definately feels different between them (Super seems to hold gear far longer than it really should).

Super cruises at a higher rev mark than the Grandpa. About 2300rpm vs 2700rpm, & the Super is empty all the time too compared to the Grandpa so it's weird.

 

[ygoslo]

I'll put it a different way - has anyone noticed a measurable difference in consumption when carrying a passenger as against driving alone? I'd contend that the difference would be minor (although I've never measured it, so I'm guessing), and that'd be carrying an extra 50-80kg...

 

[montgomery]

Outside Diameter

 

[godzilla]

Thanks Chris.
Brad, i would put down the "holding gear longer" to the auto ECU.
ASAIK, there should be no reason the auto boxes would be different. If there is any differences it would be wheels/tyres or final gears, but i am sure someone will tell me otherwise.
EDIT, just a thought, maybe your TC is not locking in at 100 etc. Mine definitely drops revs after i reach 100 and cruise momentarily which is the TC locking in. Maybe your super doesn't lock?

 

[king_panther]

Outside Diameter

Hmmm... never heard of that term before.

 

[montgomery]

Hmmm... never heard of that term before.

Generally used in measuring pipe/tube etc.

ID = inside diameter.

If you measure your id and od you can figure out wall thickness etc.

 

[godzilla]

Generally used in measuring pipe/tube etc.

ID = inside diameter.

If you measure your id and od you can figure out wall thickness etc.

Yep.
I am in the Plumbing industry so that's how we talk.

Brad, what are the tyre sizes on both your cars?

 

[king_panther]

Brad, what are the tyre sizes on both your cars?

Raining outside so I won't be able to check tonight, but it seems to me there's also mechanical differences between the 2.

 

[cyber_scriber]

Super cruises at a higher rev mark than the Grandpa. About 2300rpm vs 2700rpm, & the Super is empty all the time too compared to the Grandpa so it's weird.

Brad.

Aside from the gearing issues, perhaps another reason why the Super is using more fuel is because of its extra weight e.g. via the bodykit etc.

 

[king_panther]

If it was weight Bruce, the thing is that the Grandpa is always full with heavy work stuff whereas the Super is always empty inside. I'm thinking the weight of the bodykit would be far outweighed by the gear in my other car, but it seems to drive with less freedom than the Grandpa.

 

[bradc]

Both cars should be at 2500rpm at 110kmh once they have warmed up. I have noticed on some Auto's they tend to drive at about 2700rpm at 110kmh for about 10 minutes until they are fully warmed up.

Tyre size won't affect it because the speedo only reads based on the amount of tyre revolutions per time cycle, so no matter what the size of the tyres are, it will still think you're doing the exact same speed. This is why I suggest everyone runs 235/45/17 tyres (2020mm circ) over the stock 205/55/16 (1985mm) because it makes the speedo bang on accurate


I've deleted about 35 posts out of the thread too by the way to get it on topic Sorry to all that had their posts deleted but if you want to discuss driving styles, make a new thread. Before anyone complains too much, yes I deleted at least one post of my own because it was off topic

 

[Kenneth]

I hope people are not using RPM at speed as a gauge of fuel consumption... fuel use is a combination of engine load and RPM, not RPM alone.

Example:
A specific car traveling at 100kph encounters a air resistance which is determined by the shape and surface of the car, and the environmental conditions.

If the shape and surface of the car cannot be changed, then your air resistance is therefore determined by the conditions (i.e. head wind, tail wind, rain, fine, road surface etc)


So, 2 cars which are identical (not practically possible, but hey) except for say 5th gear ratio are traveling down the same road side by side. Since the air resistance is the same for both vehicles, both vehicles have to apply the same torque to the wheels to overcome this resistance.

Lets make up a number and say the torque required to mantain 100kph is 100lb-ft for both vehicles.
Ill make up another couple of numbers for drive train reduction ratios
3.2 (higher rpm, 4.0 final reduction x 0.8 gearbox reduction)
2.8 (lower rpm, 4.0 final reduction x 0.7 gearbox reduction)

As the torque to the wheels is determined by the engine torque / the final drive reduction ratio, we can use the above known parameters to work out the torque the engine has to provide

So the required engine torques are as follows
31.25 lb-ft (higher RPM)
35.71 lb-ft(lower RPM)

Using the standard 225x50x16 tyre size we can calculate that we require 840.22 revolutions per minute to maintain 100kph

Using the drive train reduction ratios again, we work out that the engines are doing the following

2688.71 RPM
2352.62 RPM

We now have the following "known" data:
High RPM 31.25 lb-ft 2688.71 RPM
Low RPM 35.71 lb-ft 2352.62 RPM

Looking a the lb-ft data, we have the high RPM is 14.3% higher than the low RPM (that is High RPM = 114.3% Low RPM)

Looking at the RPM data, the high RPM is again 14.3% higher than the low RPM (that is High RPM = 114.3% Low RPM).

In an ideal world, these cancel each other out. The difference in RPM offsets the difference in torque, as a result you use the same amount of fuel, since torque is directly related to how much air/fuel you have in the cylinders.

Unfortuantely internal combustion engines are inefficient and therefore the RPM which is closer to the engines more efficient operating range will use less fuel. For the 6a13TT, this is apparently ~4000RPM.
The engine running higher RPM will result in more flywheel inertia making the engine more tolerant to minor changes in surface, which results in less work on the accelerator, which results in slower rates of load/TPS change which results in the EMS being less likely to use acceleration enrichment.
Not only that, if you are unfortunate to be close to a load boundry, running lower RPM could put the ECU into open loop and run a richer AFR. Ouch.
On top of all that, you have to take into account gearbox efficiency. i.e. Automatic transmissions are less efficient than manuals.

So, in conclusion, You cannot use speed / rpm as a measure of how much fuel you are going to use. There are far too many vairables which mean you can't even usually compare to cars which appear to be the same.


The only way to REALLY monitor which speed, gear and style combination uses less fuel for your car is to monitor the injector duty cycle.
Alternatively the MAF signal will probably give a good indication of how much fuel you use, though not as good as IDC.

 

[ygoslo]

Yeah I had been wondering about the whole low RPM = low fuel consumption thing too. Can someone answer a question for me? Would I use more fuel accelerating from 40-70 in 5th, where I need to use a higher throttle percentage and therefore more boost, or in 4th, where I can stay off boost, albeit with higher RPM?

 

[Kenneth]

Assuming you want to maintain a fairly reasonable acceleration rate regardless of gear, 5th will load up the engine much more and result in more fuel being used.

Personally, I would be in 3rd at 40 and probably change into 4th at 70. Each to their own though, and I have already said fuel economy isn't that high on my list of considerations. I usually do 8.5+ kilometres per litre around town and 9.5+ on open road.

My time on motorcycles ingrained in me the idea that the ability to accelerate is a important safety feature

 

[bradc]

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

As for ideal RPM, the stock (emphasis on stock) ECU map is to stay at 14.7 below 3000rpm. From 3000 to 4000 it interpolates to create a smooth line between the 14.7 figure at 3000rpm and 14.1 at 4000rpm. Thus you'd want to stay below 3000rpm in a stock setup

 

[king_panther]

Out of curiousity then, why does the auto box shift up to cruise at 60 in 5th ?

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

I put a new idle screw in the Super, & took the one that was in the Super into the Grandpa (which lost his). Both are changing gears far differently than before. Bizarre.

 

[ygoslo]

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...

 

[bradc]

Because the autobox is silly It should have been made to change at 80kmh minimum.

For labouring - floor a manual car at 800rpm and see what happens

 

[king_panther]

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...

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 ?

 

[godzilla]

In an ideal world, these cancel each other out. The difference in RPM offsets the difference in torque, as a result you use the same amount of fuel, since torque is directly related to how much air/fuel you have in the cylinders.

Unfortuantely internal combustion engines are inefficient and therefore the RPM which is closer to the engines more efficient operating range will use less fuel. For the 6a13TT, this is apparently ~4000RPM.
The engine running higher RPM will result in more flywheel inertia making the engine more tolerant to minor changes in surface, which results in less work on the accelerator, which results in slower rates of load/TPS change which results in the EMS being less likely to use acceleration enrichment.
Not only that, if you are unfortunate to be close to a load boundry, running lower RPM could put the ECU into open loop and run a richer AFR. Ouch.
On top of all that, you have to take into account gearbox efficiency. i.e. Automatic transmissions are less efficient than manuals.

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: