LG John

Now I know the words 'impreza' and 'economy' don't go together BUT:

My daily drive includes several runs from 0-70mph cruising speed and generally I indulge in some full-boost action

However, when I try to drive economically I use the boost gauge to keep the pressure at about 0bar, hence no spool up. I got to wondering though if this actually saves fuel. From my experiences with instant consumption computers on VAG's, BMW's, etc I noticed that as soon as the engine is doing anything other than decelerating or maintaining the current speed the mpg drops big time and obviously drops further if you nail it.

When I thrash it I get to 70mph and cruise in about 6-7 second (no launches, hence time). If I'm careful and don't boost up it takes way longer, maybe 2-3 times as long to get to 70.

Which will use more fuel? A short but brutal burst or a sustained slog with no boost?

Rottie1

Hi Saxo Boy,

I f i remember correctly u have a MY99UK like me with a few mods.

I can get 320miles untill the light goes on, and my driving style is always light throttle and minimal(spelling) turbo spool up. 3k revs max and cruise at 70. Also use sccobs cornering to greater effect and dont slow for the corners,maintaing speed.

Boost must mean more fuel. Tho its bloody hard to not do it.


Cheers
Rottie

LG John

320 miles shock-a-roony!!!!! I get about 240 if I'm lucky before the light comes on and thats with a fair bit of motorway driving and a fair bit of off-boost. You must drive economically all the time!!!

Scotsman

iTrader: (3)

320 is good going. I get around 290 - 300 on average.

Razor2001

I get 15/16 mpg on my JDM STi-7 and that is with 80% under 3 - 4,000 rpms and rest few blasts to 60 mph. One thing I noted that is my car runs crazy rich and the 02 sensor on my delta dash usually reads above 1.0 and under accelleration can read as high as 1.015 !

I am sure if I leaned it out it would give me closer to 20 / 21 mpg; however, I have no det and big advance even under WOT up to 7,000 rpms so I think I will keep it slower and less economical in the interest of trying not to blow my engine up !

I have a full SS decatted turbo back zorst with 1.25 mm restrictor

Cheers,
Ray

bluto22b

Mr Saxo Boy you must learn to control your right foot old boy. I "average" 28-29 in my MY02 Wagon Shooting Brake. I do drive her gently though as my boy racer days ended about 30 years ago. If you wanted an economy car you should have considered the new Smart diesel (out this week)......Blutes



[Edited by bluto22b - 9/20/2002 8:00:31 PM]

LG John

I can't as soon as the boost tries to come in it would be rude not to floor it! I think the problem is runs like today where I did a 19 mile round trip site visit and booted it the whole time. Was great fun but probably totally screwed up the mpg for this tank.

Anyone got an opinion on my theory though! I'd prefer if this doesn't become a 'I do Y to get X mpg' thread. I'm just being theoretical here

bluto22b

Seriously, I would expect economy to suffer as soon as boost is induced. The forced air from the turbo is usually matched with a proportionate amount of fuel. In summary, More air=more fuel, as needed for the combustion process, all dictated by the ECU of course. A light throttle opening may not neccesarily improve MPG as the turbo may spool up to compensate (travelling up hill in a high gear for example). Turbo engines are notorious for using more fuel than their normally aspirated brethren.......Blutes

Forgot to add, but flat four engines use more fuel anyway because of the layout (lack of gravitational precipitation normally experienced in a traditional internal combustion engine)

[Edited by bluto22b - 9/20/2002 8:28:59 PM]

LG John

Yeah but its only 7 seconds of silly fuelling and then nice 30-35mph crusing. The slow way is still going to use a fair bit of fuel to accelerate the car but it'll be exposed to that use for much longer.

TAZMAN

WHY????

kevin stanton

i find it hard to drive econimically, can't help pressing the fun pedal.

i usually get 230/240 miles from a tank.

it dosen't help having a va dump valve, higher revs to here the pssst.


kevin

bluto22b

See my edited notes above

LG John

I don't know why taz, it's just a theory, I'm expecting someone to come on with a technical reason as to why it's all bolloxs

Raks

wanna hear something weird and wonderful???

sometimes i get higher mpg when i boost it, and then lower when i try and run conservatively!

i think it might have something to do with me having an older version, not sure what to do. Might reset the ECU to try and get it a little more predictable!

dbailey

i'm in the same boat as Saxo and Kevin. I get about 240 a tank in my 98turbo. i have to agree with saxo though, the fun pedal does get a load of use. i just can't help it. after 6 months i still get to work with a big grin.

now to go find those smilies...

(yes, yes, i have seen the other posts on smilies!)

hades

My guess is that when you're boosting on full whack in lower gears, you're doing somethng like 3 or 4 mpg instantaneous. When accelerating on light throttle openings, no boost, 10-15mpg.

You're therefore talking the difference between:
20 secs at 12 mpg =0.02 gallons; or
7 seconds at 3.5mpg = 0.03 plus 13 secs at 35mpg =0.005 Total 0.035 gallons
This all assumes an average speed over the 20 seconds of 60mph to make the maths easy, which won't actually be the case, but its probably close enough for here and now.
Therefore, the economy over that period is a hell of a lot better if you don't use boost. Is that a sufficiently scientific answer?

Of course, this is assuming that my numbers aren't absolute b*ll*cks, which could be the case.

In real world, my journey to work consists of 0-70, cruise for 200 yards, roundabout, do the same again lots of times. The guage certainly drops quicker if I use the boost than if I don't.

hades

P.S. I got 320 miles on my last tank too, although the first 60 miles was seriously using it cross country. It's just that the last 260 was spent at a steady 75mph on the A1.

Dbailey - you look to be local; you should come along to the maidstone & district meets which are quite convenient from here.

T5NYW

iTrader: (4)

I am certain that the short sharp blats give the worse Fuel economy as I normally obtain 120-170 miles off 50 Litres but have achieved 230 off 50 Litres due to rain/ nagging, trafic jams and Wife

Tony

AlexK

I remember reading somewhere (think it was AutoExpress, I'll check the website later) about a Swedish study into this. They found that driving using the throttle as little as possible *wasn't* the most economical way to drive. They found that accelerating moderately (i.e. not nailing it!) in each gear, changing around mid revs, until your chosen speed gave better results. It was concluded that this was because of where the engine's peak efficiency was located - driving at low revs isn't actually very efficient, but in the mid-range (typically close to where peak torque is developed) is far more efficient. I'll see if I can find the article.

AlexK

Yup, found it...


Foot Down To Cut Fuel Bills!

It sounds like pie in the sky, but a leading scientist has claimed accelerating quickly could save drivers hundreds of pounds in fuel bills.

Brisk power boosts and well timed gearshifts are the key to economical driving, according to Mark Dougherty of the School of Transportation in Borlange, Sweden. He's developed a dashboard device that advises on how to drive economically by showing messages, including 'Accelerate more rapidly'. Trials have resulted in fuel economy rising by 16 per cent.

"A car's engine operates at optimum efficiency whenever it's running at two-thirds of its power output," Dr Dougherty explained. "To achieve that means changing gear early, but don't be afraid to give it some gas."

dbailey

Thanks for the advice Hades. I am fairly local to Maidstone as o work down near Sevenoaks. How do I find out about the meetings?

LG John

Hades now you've applied some theoretical maths to it I see your point. I completely forget that after the 7 seconds you still have to cruise at 70mph for a further 13 to match the other scenario...doh!!

Looks like an interestion article. That was my other theory and is the way I'm beginning to drive, 'punching' up to 70mph on part trottle and part boost.

hades

dbailey - look in the "Southern" forum, down near the bottom of the board, under "Maidstone & District" meets, arranged by Hol. You've just missed this month's - it was last night - but the more the merrier for next month.

saxoboy - always glad to come up with theory, never sure if it works though! Be interested to hear what conclusions you draw from real world stuff.

LG John

It'll be difficult to test it. If I decide I'll behave myself for a tank to see what I can do I always end up wanting a blast at some point. For example, I have a local meet arranged on monday which will no doubt destroy this tanks average

Nigel H

Typical bloody scotsman

I was pondering the same myself - why not try some semi-scientific tests - a tankful on this style another on the other?

I can't see booting it being more efficent. Can't explain why, but it just feels wasteful.