Carnut

I've had Subaru cars for 11 years and they have always been standard apart from my b4rsk.I would like to stay with Subaru but my only option is the hawk sti if I want to keep it fresh.I am happy to spend some money for peace of mind on the engine which I am still a bit unsure about.What I can take from the threads is forged pistons a new HG and head bolts should correct Subaru failings .However if I was to justify the money I would want to get a bit more bhp but am not sure what this would do to the MPG.I know you will all say well why don't you just get a diesel. But with so many new cars to choose from ie focus rs s3 golf r etc with good MPG I am feeling the pressure.I would like to stay with Subaru but am finding it harder each day.I know these aren't new questions but have looked about for the answers but with no luck.I can only say that if it is possible to fit new pistons etc and tune to maybe 320+and keep MPG then I promise I will come back with some tech questions that may interest you.
Hope you can help and thanks for reading if not I am sure someone has a good thread so don't give up.

Steve's Sti

iTrader: (2)

I'm getting roughly 24mpg steady sitting at 70mph in the back roads with 450lbft/460bhp, slightly less than standard ppp map but the FMIC I think has made a big difference to this and also the fact I'm not flat out I just boost in 4th, mines hawk sti, get a good rebuild and torque wise there's not a lot out there just now that can keep up.

just to add if I drive harder it does go down to the usual 16/18 lol

romford-boy

Do bare in mind that plenty of manufacturers quote mpg figures that can't be achieved in the real world.

You try owning a 2.5 litre, 5 cylinder turbo charged Focus RS and getting a significantly better mpg than an Impreza.

Another example is the A45 AMG - quoted in the region of 40 mpg, but the one AutoExpress ran averaged 25 mpg.

Carnut

[QUOTE=romford-boy;11404490]Do bare in mind that plenty of manufacturers quote mpg figures that can't be achieved in the real world.

You try owning a 2.5 litre, 5 cylinder turbo charged Focus RS and getting a significantly better mpg than an Impreza.

Standard to standard I am sure that's true but that is why I ask the question about tuning.
Just to add to manufacturers MPG figures I am told that the way they get there figures is on a rolling road with no driver and no interior so to save a lot of weight.I don't know if this is true but don't find it hard to believe that they can be misleading.

RS_Matt

I'm doing 30-38mpg rare I see less even around town.

I believe I can achieve even more if I get some hard tyres and go even lighter. Maybe 40-42mpg.

It can be done, especially when you get rid of the parasitics and lower the rotational masses.

Carnut

Sorry don't understand parasitic and rotational masses.I may have an idea what you mean but don't won't to look any more stupid than I mite already.plus what car and spec do you have.

blackvenom

My m235i is 326bhp but I can get 40mpg in either Eco pro mode, about 32mpg in comfort and 20-25 in sport (driven hard), Not many cars where you can have both, my classic runs on an average 28-30mpg

DmcL

MPG on a midified engine with a tune is nearly all down to the tune itself. i spent alot of time constnaly revising and tweaking the tune i now sell on chips for old bmw E30 and E34 M20 2.0/2.5 engines. i also purchased many chips from both reputable big names and smaller unknown names but i found that 99.9% didnt tune the low load/low RPM areas as well as they could be tuned. similar would apply to any ECU being tuned. in the end up and after all the tweaking, testing, etc customers now report better MPG when driven sensibly and also not at the expense of power. my logic was fairly simple, first tune the lower load and lower rev areas as much as possible which gave power gains in those areas over others and over standard. after that i set about leaning out the fuelling where possible to as lean as can be run safely without reducing power.

id imagine the same can be done by a good scooby tuner though id imagine the only areas this would be safely possible would be in the off boost areas. i think also that potentially a higher HP build can possibly even run better MPG when driven sensible compared to a mild build as a bigger HP build will have a bigger turbo which takes longer to spool and as a result would give a wider area that can be optimised off boost so whereas with a TD04 you may only have an area of lower load up to like 2000rpm, with a bigger turbo that area may be increased to say 3000rpm so you would have a larger area to drive in on the "eco" tuned off boost area if that makes sense.

injector sizing, fuel pumps, etc wont matter at all as the engine only needs X amount of fuel at a given point so whether the injectors are 50cc or 5000cc the fuelling requirement is the same, larger injectors will just be running a far smaller duty cycle to supply that X amount of fuel..

Carnut

Dmcl that helps thanks for your time.

DmcL

no worries..

people often just think of full throttle or high load, etc. when talking about power and tuning.

more power at lighter loads and lower revs means you end up not having to use as much throttle to keep the car moving or to accelerate at a normal pace so couple less needed thorottle to move the car and maintain speed with running those low load/lower rev areas as lean as is safe and your onto an MPG winner in just about any application.

first time i put my wideband on my WRX i was shocked at the light throttle cruise AFR's. im seeing like 14.5 AFR or at extremely light throttle around 15 AFR taking measurements from the tailpipe with all standard cats in place. tailpipe readings on catted cars read leaner than if taking readings before the cats so its actually running more like 14 AFR at light cruise and 14.5 at extremely light throttle. for reference stoich is 14.7 so even a little less than that. my 325i ran similar AFR's at light throttle/low load and in the end up i found i was able to increase ignition timing in those areas anything up to like 20 degrees over standard as well as leaning out fuelling from the 14.7-ish to mid 15's and in some very light load areas as much as mid 16's before power would suffer at the expense of the lean AFR's. one thing to note though is that if combustion chamber temps play a part in ringland failure then if your wanting a tuner to ecomap your low load areas and run it as lean as possible with as much advance as possible it would be worth asking if they have done it before and maybe persuade them to use an EGT gauge/pyrometer to monitor temps so as to try and not increase combustion temps much because generally speaking leaner will run hotter, this is why people run rich on boost for safety and such, it helps cool things down which reduces det/chance of det and so on.

RS_Matt

Just engine assisted parts such as aircon, alternator, ABS and power steering, you can remove some or install lighter pulleys. Compromises though.

Rotationals: Carbon propshaft, light weight alloys, lightweight nuts, lighter pulleys and lightened flywheel can help the car use less power to potter around.

I think there's unsprung weight that should aid cornering and undoubtedly MPG a bit as a byproduct. Lighter brakes, suspension/coilovers etc etc.

I'm running a 2003 WRX. Conservative fuel conscious map with a laggy TD05. I've shed around 120-130kg with MPG boosting lightweight alloys and aircon removal. My tyre PSI is 35 at the front 34 at the back which is MPG orientated. They are fairly hard too which also helps. Jury is out over car stiffening products, I suspect a car is using a lot of energy trying to go straight when the suspension is bouncy and a car leans around corners.

All I've done has only given me a 15% increase over standard. (30 MPG)

I'm going to try Millers Nanodrive oil and DEI radiator relief to see if I can squeeze another 2-3%

The Trooper 1815

iTrader: (31)

I got about 8/9 better mpg after a remap. The torque and BHP went up but the map was not generic but mapped for the better air filter, better exhaust flow, less stressed fuel pump and better (bigger) TMIC.

Even PPP is generic and not mapped to the individual car/engine.

DeweyAXD

24.64mg since May 2012 on my Spec-D (forged pistons since August 2012). All v-max bar maybe 1 emergency Tesco 99. Remap at your suggested 320 power rating might improve MPG on motorway and off boost but likely make it worse when you push.

Oh and if/when you go the 2.5 route don't forget the secondary air pump removal while the engine is out. Dead weight unless you are in california and if it goes (which mine did @ 50k) it 'can' set the car alight. That is worst case but put it on your list (Roger Clark blanking plates are £50 and its not much labour if the engine is out.... it is a royal p.i.t.a if it is still in.

Tidgy

higher up the power band you go harder it is to safely use it, so cos you usethe right foot less mpg goes up. my hawk still got 34mpg on a long run, single digits when given large though hahahaha

The Rig

iTrader: (13)

Light throttle and cruising is governed by your 02 sensor so you will always have AFR`s at around 14.2 - 15, unless your 02 sensor is knackered that is.

i know what your saying about retarding timing to change AFR etc but at such lite loads, you can only retard timing so much before the car runs like a sack of ****e anyway.

DmcL

not retarding.. advancing. same principle as tuning full throttle, run as much advance as possible safely or as much as until you stop seeing gains then back off a little.

i forgot about the lambda sensor.. on the old BMW's there isnt one so what i tell it to run it runs, end of. on newer cars the lambda sensor related maps would need some work to allow the leaner AFR's depending how tight the lambda control is on the particular ECU.

Carnut

Thanks for the help guys this will help