Newbie needs advice!

[Tahrey1043]

Oh hell no, only when I have to get somewhere quick (or am trying to see how fast the car will go after some kind of change to it). It makes an awful noise and of course isn't very economical...... hence the main reason i was after the 5-speed. 65 up to 90 the engine is just too loud (at any throttle opening), particularly towards full speed. Much rather do 70-75 in a slightly higher gear and only change down when i really have to be somewhere.

I'm making it run in the 4500-5500rpm range for an extended period after all, and it's hard to make an argument that such treatment will do the engine or the fuel consumption any good. (Hell anything past 70 puts me into the 4000rpm range - ouch. rather stay at 3500 or less.)

Plus it tends to get me stranded in the outside lane uphill with the speed rapidly dropping thru the lower 80s and down, with a beemer behind and nowhere to pull across to

In my own driving experience, the more i open the throttle, the faster the engine spins and the car goes.. i don't have to open it wider at lower speeds on the level, unless it's really labouring (like... 1200rpm or less). And that's not a terribly good idea anyway - the car starts to shake.. it's only getting 20 spins per second from the engine after all, and with the 4-speed, thats what... maybe 20mph in top?
Against a major load, ie up a relatively steep hill, a speed lower (and higher?) than best torque will require a greater throttle setting to maintain the same speed, i'll grant that. But I have no qualms against changing down to go easier up a gradient! S'not like i'm constantly going uphill, like i suppose the non-gearbox/roadwheel linked ECU might assume at 90mph.
(i suppose what that comes out to is attempting to produce the same acceleration in the higher gear as the lower, but thats not what im trying to do)

So what are the answers anyway?

[Gareth_GT_Hatch]

So what are the answers anyway?

What are the questions?

Ok heres a fuel economy tip. Optimum fuel economy is obtained by mantaining constant throttle as much as possibe at about 60mph on flat ground, let the car slow down up hills and speed up down hills but keep the throttle in the same place, really boring driving I know but youll get 50mpg easy on a long run. I got 48.5mpg on my blue GT on the way to and part of the way back from inters, involving half an hour with the car stood idling with the lights on full beam while i put my tent up. and a couple of short journeys to and from the campsite in between. I was doing between 70 and 80 all the way there as well so I wasnt dawdling. Just goes to show how efficient the GT is at high revs, hits 4000rpm at 73mph in 5th, which isnt alot less than a 1.0.

You probably didnt need to know all that but Im a tight git so I enjoy talking about fuel economy probably about as much as you enjoy talking about gearboxes!

[DanW]

yeah even when I do put my foot down it's easy to get 30mpg at least, I think it was nearning the 40mpg mark last fill up. If I can help it I take the long way home so the engine gets warm and doesn't rust the exhaust, because I know the previouse owners had to replace it, due to it being used as a school runner, so only about 8miles (at most) a day I'd say it was being used for

[Tahrey1043]

So what are the answers anyway?

What are the questions?

The ones I wrote before

give me a mo later on and i'll try to extract them from all the guff.

My main one right now is, why would a 50% throttle setting at 40% revs be neccessarily worse than a 40% setting at 50% revs?

[Gareth_GT_Hatch]

My main one right now is, why would a 50% throttle setting at 40% revs be neccessarily worse than a 40% setting at 50% revs?

Well...Because it would.

The more throttle you use, the more fuel you use its pretty much as simple as that. (I cant think of anything right now to make it more complicated) And taking something you said earlier about engine resistance; The engine resistance at 40% (~2000rpm) and 50% (~3000rpm) wouldnt be alot greater. Engines are designed to rotate as they do, so engine resistances are minimised (to a cost budget for making the car) So forget that when thinking about fuel efficiency. Rolling resistance and drivetrain resistance may also come into it. But not so far as 40/50% either way in ur example, there maybe a situation where 48 and 52 doesnt make a difference either way round but in general the less throttle the better

[Tahrey1043]

OK... it may be putting in more air and fuel per revolution, but you've got less revolutions going on... is there a reason that the wider throttle setting vs a lower speed means more fuel used?
Think we're still on different wavelengths as regards what i'm asking/suggesting here.

i know the engine's oiled etc and i'm not on about friction - i'm on about having to accelerate several lumps of metal one way then the other many times per second. Even if it's bolted to a rotating crankshaft, changing something's velocity and direction to the complete opposite requires extra energy after all..... and the energy required to accelerate to a certain speed is a square function. Even in a friction free environment, it requires 4x the energy to accelerate your spaceship to 4000mph than to 2000mph (or... pistons to rpm).

(which is also why driving more slowly is more efficient, i suppose - less energy expenditure to reach cruising speed)


regarding the "low down torque" of the carb'd engine, i cant comment on the full torque without a rolling road day, but here's the figures from the manual...

1.0 carb: max torque 74Nm @ 3600, max power 33kW @ 5600
1.0 Spi: max torque 76Nm @ 2800, max power 33kW @ 5200
(injector wins! but carb may feel quicker as it's a bit lower geared.. and the car is lighter)

but, admittedly
1.3 carb: torque 96Nm @ 3400, power 40kW @ 5200
1.3 Spi: torque 95Nm @ 3200-3400, power 40kW @ 5000
(for the record..
1.3 Mpi: torque 100Nm @ 3200-4000, power 55kW @ 5900 - mmm such a shame it has that low 5th)

So there ain't more than a feather to put between them, though the injector may be a mere % or 2 stronger at 2000rpm... vs 100kg extra

(apologies for the euro figures, but I thought if I was to get facts straight, it was best to quote directly, and it doesn't really require me converting to lbft and BHP.. suppose its academic as we're talking 1.3 vs 1.0 anyway)

[Gareth_GT_Hatch]

OK... it may be putting in more air and fuel per revolution, but you've got less revolutions going on... is there a reason that the wider throttle setting vs a lower speed means more fuel used?

Right, for you to meet the conditions you have just described you would have to be driving slowy in a high gear, say up a hill at 30 in 4th. As you start to loose speed you need to open the throttle more and more untill eventually you reach the top of the hill or you get to full throttle and ur still loosing speed and you have to change down. Can you see where the fuel has gone? ur going very slowly using lots of fuel (cos the throttle is fully open) And since economy is based on miles per gallon, at a low speed ur not gonna cover alot of miles, but at full throttle you are going to use alot of petrol. If you were going up the same hill in 2nd or 3rd you would not need as much throttle, but more revs, to get up it. And since the throttle is less open (but ur still doing the same speed), then your miles per gallon will be higher. Obviously this is an extreme case but it illustrates what Im talking about.

Its a similar situation at high speed. Lets say that u have a 5 speed box and ur doin 60 in 4th. This is probably at about 4000rpm. Now to maintain this amount of revs you will need more throttle than you would need at 3500rpm in 5th. Why? This comes back to the point you made about engine resistance. Coupled with all the other drivetrain resistances, engine braking (ie when u take ur foot off the throttle) is greater, and because its in a lower gear, the car slows down quicker. Ok now uve put it into 5th and ur doing less revs; therefore there is less drivetrain resistance and you need less thottle to maintain speed. Ive had to think about this alot, and the primary factor in engine resistance is the effort required to compress the air in each cylinder before it is ignited. It is this that causes the engine braking for a very large part. So engine resistance is not negligible (which I said before) and it does increase with rpm. So the less revs you are doing at high speed the better. So this has been an educating experience for both of us.

As for them figures, the lower weight does help. But as you can see I wasne lying about the mk2 1.3s torque. the 1.0 mk2 and 3 share the same gearboxes. (but theyre not all fitted with the same one)

PM me with ur email if you want that excel file with all that stuff on it.

as for this question;

My main one right now is, why would a 50% throttle setting at 40% revs be neccessarily worse than a 40% setting at 50% revs?

Its actually quite difficult to simulate those conditions (in the same gear) you would have to be going up a hill for the first one and be on the flat (or going downhill) on the second one. Its like that hillclimb stuff i described earlier. At high speed, those two different options arent both possible unless we bring hills into it again. As for different gears, It would also depend on load. The short answer at the end to this question is I dont really know; If you were to take the graph as linear; the fuel used for both of these would be the same. But since the graph is not linear, ie you are burning considerably more fuel at 50% throttle than you are at 40% throttle. This is because the amount of air going in is greater at more throttle so the fuel used needs to also increase to maintain the correct ratio. Let me break it down:

For arguements sake, lets say the amount of fuel used at 40% throttle is equal to 1 unit per rpm and 50% revs is 3000 rpm. So you are using 3000units of fuel per minute.

Ok, now lets say you are using 1.25 units of fuel at 50% throttle (assuming fuel usage is linear with throttle percentage, which it isnt, if anything, it is exponential) and 40% revs is 2500rpm, then again you are using 3000units per minute. But as I said, it is not a linear scale.

[Tahrey1043]

EDIT a quick thing to throw in... might have already said this i dunno
I may say stuff about the throttle opening restricting the engine and all, but i dont know how much real basis there is for it, only hearsay VW may not reccomend it, but i tried repeatedly going down a short but steepish hill in gear but with the ignition off (feels no different to the over-run cutoff...).. however much i had the throttle open seemed to have little difference on the engine braking. Fully open seemed similar to fully closed (and as ignition was off, no ISV to prop it). Just a seat-of-the-pants measurement in 4th and 3rd, but its a hmmmmm....
no matter how much air you let in, the compression effort is the same?

anyway
/EDIT

now *my* brain is hurting

think i'll have to retry the economy thing one day when i get a chance, but try to measure the fuel more reliably - coming to think that the tesco's pump cannot be trusted....
(how could i do that though)

first off i have to capitulate and apologise that i did indeed get the ratio wrong for my own 4-speed box.. it IS 4.267... and also i fluffed up the speed calculations in my own spreadsheet (got the tyre rolling circumference completely ballsed). so lots of room for improvement there. makes things look a little harder that i'd hoped, but still... there's goodness to be had

now back to the arguing!

course it's going to be uneconomical to go up a hill in 4th at a low enough speed that you have to change down, thats not what i'd be aiming for anyway. i want/ed to gear the car up higher, but it still had to be usable - not a case of reaching 70mph at 2800rpm on maximum throttle* and then having to change down any time there was a slight incline. It's for a cruising gear to be used on "wider", but not fully open throttle between say 50 and 70mph. For situations requiring full throttle for more than the few seconds a small rise in the road would produce, there's always 4th, or 3rd.

Somewhere between 20 (70@3500, 50@2500) and 23mph (70@3050, 50@2200) would be just right, with an increasing trade-off between maximum possible speed vs economy before the practical limit of sustainable speed is reached. Any ratio within this range would still allow for very comfortable cruising at 55... my foot is giving maybe a quarter throttle to the engine in 4th, which must be approaching the point of suffocating it and not so great for fuel mixing and swirl effects etc. (he said, relying on the flimsy logic of It Stands To Reason more than any solid engineering principles)

I still dont see how using full throttle at max torque can be any worse than part throttle at a higher rpm... i have a feeling graphs would have to be involved
I mean yes the throttles open wider because i'm having to make use of more of the motor's maximum potential at that speed, but that potential is only lower because it's spinning slower. It's still putting out as much grunt per spin, in fact more so, and quite likely being more economical with it. Again to take the argument to it's logical extreme, i'd be better off running it at 5200rpm up every hill, as thats where the maximum powers at? That even goes against the reccomendations for economy in the handbook. What's the difference btw of it going up against a hill, against wind resistance at high speed, or it's own internal friction?

Sure if i was going up that hill at 2000rpm, there'd be benefit in changing down, as by then i'd be dropping into the engine speed range more attuned for cruising around the city at part throttle with a general power demand of maybe 5hp, rather than attacking any kind of hill.

* oddly enough at that point the (theoretical) power output - 29hp - and maximum speed would be the same as that of a Citroen 2CV.. except my motor would probably be under less stress and running more frugally, as the gear ratio would be 25/1000 to the 2cv's 12...!
At the mo i think Metz's saloon 5-speed would be just about perfect, the one that's like 8P but with the same final drive as mine, particularly on my slightly smaller 155/70 tyres. In fact i only ran across this post again cuz i'm looking for the one where he metioned it - i want to find out how quick it'll go in 5th as opposed to 4th. 8P looks almost too high now (especially 4th being a touch too steep... and a 22.3/1000 top, making 75-80 about the max speed), but still a contender of sorts on the right tyres and with a small power boost.

Using the figures out of the back of the mk2 haynes as reccomended by yourself (and correcting the typo of "65/18 = 4.063"), heres some rough estimates... assuming 145/80 = 22.5 inch diameter and 155/70 = 21.9 inch

On standard 145/80s --
8P 4th = 18.5 (88@4750?), 5th = 22.3 (78@3500?) ...
metz's AYZ/etc 4th (and mine) = 17.6 (91@5150), 5th = 21.2 (83@3900?)

On 155/70s, 135/80s or similarly smaller (& wider? ) tyres --
8P 4th = 18.0 (89@4950?), 5th = 21.7 (81@3750?)...
metz/my 4th = 17.2 (91@5300, just right), 5th = 20.7 (85@4100?)... plus slightly lower ratios (that i'm already "enjoying") in 1st thru 3rd.

If that's accurate however, my dials need recalibrating something rotten!
Hmmm.........

nice knocking heads with ya anyway