increasing the fuel mileage on an n/a V6

[bradc]

My mum is complaining about the amount of fuel the Legnum ST-R is using, it tends to get about 9l/100km with her driving it, whereas when I drive it I normally get around 8l/100km...unless I'm listening to that lovely exhaust note at high revs....

Anyway, it's already got lightweight Enkei RPF1 17x7 wheels on it, they weigh 6.6kg each, the stock ones are more like 9kg each, and 215/45/17 tyres are lighter than the stock 205/55/16 tyres. Furthermore the 215/45/17 tyres are slightly undersize by 1%, which isn't much and would only equate to 5-6km difference travelled per tank, but the smaller radius compared to correctly sized tyres would affect the acceleration of the car too, so I'm willing to bet that the difference is a little more than 1%. I normally keep the tyres at 38psi front and 36psi rear, so it isn't due to them being flat!

It also had it's fuel filter, spark plugs, engine, trans and diff oils replaced at 100,000km, Amsoil was used throughout, it's done 9000km since then so it isn't as if old oil is causing any problems. I'm going to do an oil change on it in when it has done another 1000km though, so I need to decide what is best, and what my other goals are.

The things I can think of to change it are:


Install a new O2 sensor, the current one is most likely 11.5 years old and has done 109000km, it is possibly reading slightly incorrectly

Remove the cat, it will reduce back pressure a bit which might decrease fuel consumption

Install my S-AFC I have sitting in a box, it might make a small difference. I've only got one AFR graph of an ST-R, and it sits bang on 12:1 the whole way through the graph, increasing it to 12.5:1 would perhaps decrease fuel consumption, but would necessitate using 95 octane..which brings me onto the next thing

Use 95 octane? I've always put 91 into it, and my mum would take a lot of convincing to put in 95 due to the increase in price. Especially because it would take two tanks of 95 to get all of the 91 out so I could reset the ecu to get it to learn on the new petrol

Put a stock exhaust system back on it. I'm guessing the original one had rusted out, the exhaust after the rear axle has been replaced, perhaps the stock oue would be better, or perhaps use a VR-4 exhaust, or even a aftermarket VR-4 exhaust? My mum quite likes engine noise, she has the Monaro after all

Lower the front, it does lots of motorway driving and the front is very high on it, there is about an 8-10cm gap between the top of the tyre and the wheel arch. I'm thinking that some Legnum lowering springs in the front would perhaps help the aerodynamics of the car and this might make a small difference with the amount of 100kmh driving it does

When the tyres are worn down perhaps replace them with a type that mentions fuel saving in the specs? It has GSD3's on it at the moment which are performance orientated. Hopefully this would also help with the road noise, because it is quite loud with those tyres on it

Install a longer final drive - this one is really at the bottom of my list, but perhaps when the clutch gives up it won't be a bad thing to do? The ST-R final drive is 4.055:1, while the 2wd V6 is 3.722:1. I don't know if they would be compatible or not, but it would drop the revs down from 2640rpm to 2420rpm at 100kmh which might help, but obviously at the expense of acceleration and fuel economy around town.

Do the MAF mod, perhaps meaning I won't have to install the AFC at all, can't see it making much of a difference, but worth trying because it is effectively free

Install MIVEC heads and set the change over cam point at 4500rpm which my mum never revs to anyway. My dream But ultimately way too expensive. Good fun though


Any thoughts or any comments at all? Any new ideas on what to do?

[psbarham]

remind me about this tonight, ain't got time now

[mpau009]

Push it

You could always strip excess weight. I'll help rip out everything other than the drivers seat. And the aircon, thats good for more milage.

We'll see if she complains about an extra $10 a week then

[bradc]

lol nah, I wouldn't like that either!

[Kieran]

For UK peeps, 9l/100km works out at about 26mpg.

Brad... From your list and from my experience with the V6 Sport, I would say these are the 'hot' items from your list:

Use 95 octane or better - it'll be mapped for it.

Consider replacing the O2 sensor - Do you have an EVOScan cable? If you do you can take a snapshot and see if the 02 sensor is getting lazy.

De-Cat won't do much on the N/A engine. S-AFC won't either I think having seen a UK v6 on a dyno, the AFRs were only going to about 12:1 at full chat - not much more you can do with that unless you want a shedload of knock!

[bradc]

I don't have an evoscan cable, but some of the other guys in cvr4 in nz do, I'll have to harrass them Should be 14.7:1 at idle, and from Carstens graph above as well as your post, 12:1 under load seems to be what they aim for.


I'll take the 95 advice, I might put in a tank of 98 octane when it is near empty next time, then reset it and tell her to put 95 in it after that and see what happens.

Thanks!

[psbarham]

the V6's prefer the higher octane stuff, but i suppose the price difference between 95 and 99 over here doesn't acctually pay for its self in the extra milage, but i bet the difference between 91 and 95 will, the v6's also have a high dependence on back pressure, with the de-cat on mine had a lot more top end, but all the low down grunt had gone, as had the fuel consumption, leave the cat in.

the afr's on the v6 as standard are near as damn it perfect for economy, any adjustments you make to the fueling will cause it to guts more petrol. somewhere on here i have put up a screen print of a run with the lm 1 in.

and anyway 26mpg seems quite average really, especially considering yours is a 4wd, so give her a slap and tell her to stop moaning

[bradc]

over here at the moment 91 is about $1.75, and 95 is about $1.80, whereas I'd imagine 98 is going to be somewhere between $1.85 and $1.90

Imagine that 50 litres is used, and 550km is travelled with 91, which is about 9.1l/100km, it would cost $87.5 with 91 to travel that many km's. With 95 you'd need to use only 48.6 litres to cover the price difference, which isn't actually that much at all!

Maybe i could just get her to fill up with 95 and just put in 5 litres every tank when she isn't looking

[Subaru ETA]

first thing i would be doing is puting 95+ in it.

if you take it to mitsi they could do a data log on the o2 sensor, if you cant get hold of an evo scan cable

[bradc]

nahh there is enough people in the club with an evoscan cable, I'll borrow one

[psbarham]

nahh there is enough people in the club with an evoscan cable, I'll borrow one

tight tw@t

[Robotnik123]

What about converting it to FWD or RWD? The transmission loss thru the 4WD drivetrain could be considerable.

If I recall correctly, on the R32 GT-R, converting to RWD is as easy as pulling a fuse. It must be harder to do on a legnum though.

[bradc]

yeah it involves welding the transfer case and removing drive shafts, quite a bit of work really, and I do like the 4wd system a lot

[Kenneth]

I was going to suggest using a LC-1 to up the closed loop AFR to 15.2... But then it occurred to me that you really need to work out how much fuel you use, how much it costs and project any savings over the time you expect to have the car.

If the sole point is to increase the economy of the vehicle, you have to make sure you don't spend more on the hardware than what will be returned eventually.

For example, say it costs you $1500 for a map ecu2 fully installed and tuned for economy.

Thats around 750L of fuel (more at current prices, but lets be pessimistic)
If you get an average increase in fuel economy of 3KM/L (say from 10KM/L to 13KM/L) then your 33.3% increase in fuel economy will take around 25,000km to of driving to pay for itself.

Not 100% sure I got all the math correct, so feel free to correct me.

[Robotnik123]

Another solution would to convert the car to LPG. Expensive, but the Government might bring back subsidies for car LPG conversions - that would reduce the cost to you quite a bit.

[bradc]

Ken - yeah thats why some of the options on the list aren't really practical. A wideband kit would be kind of cool though and very useful

LPG isn't an option, we need the room for carrying stuff around (like goats)

[Robotnik123]

LPG isn't an option, we need the room for carrying stuff around (like goats)

I think there are small LPG tanks you can put into your spare wheel well or under the car. However, those may be too small to be convenient I guess.

[Subaru ETA]

lpg is cheap but the L/100km is quite high so id say it work out about the same?

[elnevio]

lpg is cheap but the L/100km is quite high so id say it work out about the same?

I think the mpg of lpg (sorry, couldn't resist ) is about 10% or so worse than petrol. But it costs half the price (well, in the UK, I don't know how great the diff is in NZ).

The main consideration is that you really need to do sufficient mileage to offset the original installation cost.

[Robotnik123]

Hmmm, I actually work for Rockgas (large NZ LPG supplier) I should really find out more about the options available for cars and LPG, me thinks...