1,8 gear ratio in 6th (split)

[FWH]

Originally Posted by Pottsy

It's not the manufacturers fault, it's the fault of the basic properties of wind resistance.

At low speeds there is a lot of "internal" resistance to be overcome. This is mostly the energy required just to keep the engine turning. So at 1 mph you go nowhere significant but use lots of fuel just turning the engine. So you get a low mpg.

At high speeds the engine has to overcome wind resistance. The trouble is that if you double your speed (say 30 mph to 60 mph) you quadruple your fuel consumption.

At some intermediate speed the wind resistance is low, and the internal resistance is negligible, and you get the best mpg. It was often thought to be around 50 mph, but a recent test showed that for small cars it is nearer 30 mph, and for larger cars 40-45 mph.

Anything above those speeds, the wind resistance progressively takes hold and your mpg goes down. Furthermore, the mpg drop between 50 mph and 60 mph is small, between 60 and 70 mph it's large and between 70 mph and 80 it's huge.

There's nothing a manufacturer can do about it - you just need to drive slower. For the record I get 57 mpg at 70 mph, 54 mpg at 75 mph and about 47 mpg at 80 mph (all indicated on an uncalibrated speedo).

But surely if you have a gearing which gives you lower engine revs at 70 mph this will reduce fuel consumption.

[Pottsy]

Originally Posted by FWH

But surely if you have a gearing which gives you lower engine revs at 70 mph this will reduce fuel consumption.

Say at 70 mph you need to make 40 bhp to keep the car at a constant speed. What you need to do is have the engine revs at an optimum, to make your 40 bhp as efficiently as possible.

Most engines have a rev range where they are most efficient. As a general rule this is when the engine makes peak torque at full throttle. So initially you'd think that the best way to make your 40 bhp is at peak torque revs. But in fact it's normally a little bit lower (especially in high revving petrol engines), because the reduction in efficiency running at lower revs is small, but the gain created by reduced internal friction is larger.

As the revs reduce significantly below optimum though, the engine quickly becomes inefficient and you will need a higher fuel flow to make your 40 bhp.

Let's guess that the engine makes 40 bhp at full throttle at 1300 rpm. With the tall gearing suggested, then you'd be cruising at 70 mph, full throttle at 1300 rpm, with the engine at a rev range where it really can't make the power efficiently and you'd be gushing the fuel away.

Lower revs does not mean better efficiency when below optimum revs. Now of course this is slightly simplified (because efficiency can change at different throttle / fueling setings) and manufacturers do all sorts of tricks to lower the optimum efficiency revs when in the cruise using tricks like variable intake length and variable valve timing. But they are tweaks, and will not change the overall picture that much.

[FWH]

Originally Posted by Pottsy

Say at 70 mph you need to make 40 bhp to keep the car at a constant speed. What you need to do is have the engine revs at an optimum, to make your 40 bhp as efficiently as possible.

Most engines have a rev range where they are most efficient. As a general rule this is when the engine makes peak torque at full throttle. So initially you'd think that the best way to make your 40 bhp is at peak torque revs. But in fact it's normally a little bit lower (especially in high revving petrol engines), because the reduction in efficiency running at lower revs is small, but the gain created by reduced internal friction is larger.

As the revs reduce significantly below optimum though, the engine quickly becomes inefficient and you will need a higher fuel flow to make your 40 bhp.

Let's guess that the engine makes 40 bhp at full throttle at 1300 rpm. With the tall gearing suggested, then you'd be cruising at 70 mph, full throttle at 1300 rpm, with the engine at a rev range where it really can't make the power efficiently and you'd be gushing the fuel away.

Lower revs does not mean better effiency when below optimum revs. Now of course this is slightly simplified (because efficiency can change at different throttle / fueling setings) and manufacturers do all sorts of tricks to lower the optimum efficiency revs when in the cruise using tricks like variable intake length and variable valve timing. But they are tweaks, and will not change the overall picture that much.

Understood, and I am sure that Honda´s engineers have not just guessed at the gearing.

Critical is of course what is "optimum" revs.
I would say anything over 2400 in the 1,8 from a torque perspective.
http://www.civinfo.com/forum/22148-post8.html
When driving the 1,8 petrol Civic I find that keeping the revs well below 3000 rpm is needed to keep fuel consumption at a good level.

Preferably even below 2500 rpm.
This ties in well with the torque curve and your statment about "slightly below".

This is not just motorway cruising, but general driving.

6th is however geared so that 70 mph is just around 3000 rpm. I would be surprised if gearing it to do 70 mph at 2500 rpm would not improve.

Or am I missing something here?

[Pottsy]

Yes, the 1.8 torque curve is almost completely flat! And yes, anything below 2300 is where the power producing efficiency is falling away - so keeping it down to 2500 is probably a good idea.

So the only significant gain would be in the internal resistance, which at low revs would be very small. But the maximum power available would be much reduced! With a significantly taller gear than you have, you would have much less ability to accelerate in 6th on the motorway, so I'm going to guess that a compromise gearing was chosen where the MPG was as near as perfect as possible but where you still have some surplus power available to accelerate.

What you need is an engine that develops power at much lower revs (to reduce internal friction losses) and one with no throttle valve (to reduce pumping losses). Such an engine does exist, and comes with a capacity of 2.2 litres...

[FWH]

Originally Posted by Pottsy

With a significantly taller gear than you have, you would have much less ability to accelerate in 6th on the motorway, so I'm going to guess that a compromise gearing was chosen where the MPG was as near as perfect as possible but where you still have some surplus power available to accelerate.

Probably a good assumption, but I would happily down shift to 5th for acceleration (I often do anyway) and have a taller 6th with less "pull" and lower revs at cruising speed (70 mph).

Originally Posted by Pottsy

Such an engine does exist, and comes with a capacity of 2.2 litres...

Except they don´t sell it in Sweden (yet).

[Pottsy]

Trouble is - (looking at the graph) any fewer revs on the petrol will give you so little power that you'd have trouble going up a hill at 70 in 6th. This would give the car such bad press... Much like that disastrous thing of the past, the Polo Formel E. They did just what is being discussed and put a very tall top gear on it for economy. It made little difference, but nearly drove you to suicide on the motorway as you had to constantly change between 3rd and "E" (4th). I'd rather slice off my left nut than have to drive one of those again.

[FWH]

Originally Posted by Pottsy

Trouble is - (looking at the graph) any fewer revs on the petrol will give you so little power that you'd have trouble going up a hill at 70 in 6th.

Yes, the 1.8 has very little low-end torque.

[nilsj]

Well, I believe that today most cars have far too low gearing on the top gears. With a 6-speed box, there is absolutely no problem to change down to 5th for a hill.

Stupid Motor Journalists always tend to complain about lack of acceleration in 5th or 6th and have given bad influence to the manufacturers. Acceleration in top gear is not the point, 5th and 6th should be overdrive for relaxed & economical cruising at low revs! With a 4-speed box, it is a different matter, but those days are long gone.