Fuel Economy Improvements.

[Dom B]

Well not sure it's in the right section but here goes.

We all talk about tuning for power and in the end that just comes down to how much fuel and air we can get into the cylinders. There is some power to be won back from the pumping losses of the engine by fitting hard pipe kits, porting, bigger throttle bodies, better exhaust systems etc.

So the tuning for the car can give both better power and economy. I like many of us here love my vr4 legnum but the fuel consumption is awful there is just no other way of putting it. If i don't move my foot and cruise on the motorway all day with the wind behind me then i can almost manage 24mpg. If i press the loud pedal i can virtually see the fuel gauge dropping. I can empty the tank in little over 60 miles when gunning it!!! that's scary. It also doesn't help that the fuel gauge is the most non linear gauge ever not even slightly corrected for the shape of the tank! It site above full for 50 miles the top quarter dissapears in 10 miles and the bottom past zero seems to last ages before the fuel light is on so until you actually fill up you have virtually no idea how much fuel you have or how much further you can go.

So we understand that power needs fuel, there is no argument about that but why does cruising need so much damn fuel when we are only using the same power as a 1.1 metro not even at full throttle (well almost!!). If we could save fuel when cruising then we could have more ready when we want to use that power.

The basic design of the engine is good. Cross flow, very efficient low loss top end due to the roller followers, modern combustion chamber design etc.

Ok we lose some efficiency due to the lower compression ratio. But somewhere i saw some testing that showed that the legnum runs a rich afr almost all the time something like 11:1 in real testing with any sort of boost. Is this true?

Surely cruising at 80mph the turbos are not boosting the engine simply using exhaust waste energy to overcome the pumping losses and give only slightly over atmospheric pressure, but not a vacuum. Therefore that should easily make up for the loss from the compression ratio and then some. So the engine should be more efficient than the normal v6 surely?

Yes when pressing on the boost will consume fuel but just cruising the engine shouldn't eat it.

It should be feasible to get 30-35 mpg out of a vr4 with simple engine management changes surely. My 2.5 v6 alfa will do that and it hasn't even got a roller follower top end. Granted it is more aerodynamic and the cam profiles wont be made for vr4 levels of power but it's no slouch and we have had it dynoed at just over 200bhp.

has anyone tried to increase the mpg as well as power by bringing the net efficiency up as a whole.

has anyone had success remapping the car for uk fuel where they can run leaner then countries with low grade fuel? I was under the impression that jap fuel was some of the best but is this true?

Any ideas for achieving the end result practically and reasonably. Ie no HHo generators, no complete race car aerodynamics.

Lets just see if we can match similarly powered Bavarian mpg.

Discuss for fun lol

[fluffnik]

Lets just see if we can match similarly powered Bavarian mpg.

My slightly heavier and substantially quicker BMW ALPINA B10 V8 uses noticeably less V-Power than my Legnum VR4: 19-28mpg vs. 16-23mpg

A 340bhp 4.6l n/a V8 should not be more economical than a 260bhp 2.5l lightly turbocharged V6.

Both cars have 5-speed tiptronic 'boxes and are subjected to similar driving styles...

The big, and presumably power and fuel sapping, difference must be the VR-4 4WD system.

...though the fuel cooled anti-det can't help.

[Dom B]

Well there is the point exactly the Germans seem to get the best of both worlds power and economy. Surely the Japanese are right up there too with the technology and for the time these cars were built surely they were ahead of everyone.

Off topic a bit as its a diesel, but the telegraph did a economy drive between a Prius and a new 320d to the south of france and the BMW was far better fuel economy!! Go figure! lol Why buy a Prius!!

I have seen it published many times that the greatest power loss of a transmission is the 90degrees turn of a differential, so a BMW suffers almost of the same power losses we do when compared to a FWD two wheel drive car, but they consistently manage better fuel economy. My friends 2002 M3 even gets better mixed driving economy than i do and thats got 340ish BHP. He actually sold me the VR4 as it was too thirsty for him lol.

[vr4-fan]

my vr4 give only 14 MPG , always run less 2400 rpm , and is stock , so... your car give you good mileage

[Turbo_Steve]

but something is definitely not right with yours, VR4-fan.
Time to start looking at the lambda sensor to make sure it's reading correctly, or get yourself a copy of EvoScan, which will tell you exactly why your car is drinking so much fuel.


Transmission loss on a Tip VR4 is huge...something like 25%-30% from memory (someone will find the dyno thread and correct me shortly ) and as such a degree of our fuel goes into keeping this lot turning, even at cruise. And with a transfer box, gearbox and rear diff, there is plenty of scope for losses.

That being said, however, I agree that the base engine setup is ridiculous. Constant throttle previously hasn't seemed to bad on ours, but minute throttle changes seem to induce boost instantly, which results in a massive squirt of fuel by the ECU in compensation...and this deosn't appear to be particularly load related: it's a TPS delta calculation from what we've seen.

The best people with input here are the Aftermarket ECU boys. Brad is public enemy number one, here, with a manual transmission and complete aftermarket maps he seems to return fairly normal economy...and a few extra bhp to boot!

[vr4-fan]

but something is definitely not right with yours, VR4-fan.
Time to start looking at the lambda sensor to make sure it's reading correctly, or get yourself a copy of EvoScan, which will tell you exactly why your car is drinking so much fuel.


Transmission loss on a Tip VR4 is huge...something like 25%-30% from memory (someone will find the dyno thread and correct me shortly ) and as such a degree of our fuel goes into keeping this lot turning, even at cruise. And with a transfer box, gearbox and rear diff, there is plenty of scope for losses.

That being said, however, I agree that the base engine setup is ridiculous. Constant throttle previously hasn't seemed to bad on ours, but minute throttle changes seem to induce boost instantly, which results in a massive squirt of fuel by the ECU in compensation...and this deosn't appear to be particularly load related: it's a TPS delta calculation from what we've seen.

The best people with input here are the Aftermarket ECU boys. Brad is public enemy number one, here, with a manual transmission and complete aftermarket maps he seems to return fairly normal economy...and a few extra bhp to boot!

what is the lambda sensor???


I dont undeerstand why give me that poor MPG , I do the all mantenimi , and only give me 14MPG

[Nick Mann]

The lambda sensor is the o2 sensor.

I suspect that at 80 mph a VR4 is borderline between closed and open loop. Fit a wideband with gauge, and then drive so the needle is always bouncing around at 14.7. (Okay, this is impossible, but at least aim for it!) That will significantly reduce your consumption and help you see when the car is using a lot of fuel.

£150 easily buys a wideband inc. sensor and gauge, so less than 3 tanks of fuel.

A piggy back ECU will then allow you to start tweaking the fuel when on the throttle too, helping even more.

[Dom B]

My vr4 does significantly better mpg at 80 when the boost is just positive than it does at 70 when there seems to be some vacuum still. The exhaust is noisy at 70 too and only becomes quiet at 80 when the engine seems to be in a happier place. Cruising more than that the air resistance becomes huge factor and ruins the economy again.

I can easily get single figures mpg when pressing on and empty a tank in 60 odd miles which is frightening but a normal cruise a vr4 should be returning mid 20s mpg if not something is wrong.

If you watch a fre of the youtube vr4 drag races you can see the vr4 has clouds of steam coming out of the back but the other car doesn't. The vr4 wins but you can see just how rich the car runs on boost. I'll try to dig out the video.

Someone told me that the jap fuel is great in the city and a bit ropey in the countryside so the jap turbo cars run safe rich maps but other people tell me that jap fuel is far better than ours. Can a european map be leaner for our fuel? I always try and run mine on tesco 99 octane fuel but it is only available in selected tescos. There is a website that list all the 99octane tescos (part of some road rally sponsorship). However high octane itself shouldn't affect the mpg on cruise.

[Wodjno]

Empty a Tank in 60 miles ??

And even at 80 mph you should be cruising in Vacuum.. Unless your going up a hill

Jap fuel is better than our fuel..

And Higher Octane fuel will give better MPG at Cruise.. The nearer the fuel is in quality to the Jap fuel, the better economy and performance will be..

Be interesting to see which VR4 your You Tube Vid shows

[AlanDITD]

Well i lowered my car the other week, and not sure if it was a fluke but i got another 20 miles to 20quid meaning 120 to 20quid roughly that was some cruising and some booting it.

My back end doesnt stick up so much and theres less under car clearance so im wondering if it somhow has better aero dynamics and has helped the MPG?

Going to keep an eye on it the next fill and see how it goes again. But 25mpg in mine has aways been easy enough to do.

[Dom B]

Seriously, if you really hammer it and i mean track day hammering, you will be lucky if you ever see 100 miles from a tank. Normal driving is different of course.

I don't see how a higher octane fuel will give better economy. There is little load at cruise and certainly no pre ignition (knock) events unless someone had put cat pee and sugar in your tank!! so you should have a fully advanced ignition map being as efficient on 95 octane as 99 octane. Pressing on is a diffenent matter as load and knock starts to retard the ignition map and bring the efficiency down but i think it will have very little effect at cruise.

Unless someone know a scientific reason why? I am open to all angles.

As for boost at 80mph, my car starts whistling at 75mph and at 80 becomes really smooth. Has anyone got any boost gauge figures at 80mph. Probably not as everybody is obeying the national speed limit lol.

We should do a poll, what boost/vacuum does your car run at 80mph in top gear on the flat.????????????????

[AlanDITD]

my vacum at 80 reads well into the vacum, cruising takes barely any force to keep you there so theres no need for any boost. iirc its between 15inhg and 10inhg if im being careful. Tiny bit of pressure on the throttle and im into positive pressure.

Scientifically i have no idea why, but 95 ron which i put in when i first bought the car gives me half the MPG V-power does.

But that isnt just cruising. I can see what you mean that if theres no boost the car doesnt have the compression to create knock with lower Ron levels. But anything into +ve pressure i would imagine would cause the ECU to pull timing.

IIRC someone posted up a knock count on 95 ron and it was in the hundreds.

Or im just talking utter ****e, as i dont really understand any of it tbh and i never bought a leggy for fuel economy, i dont actually thing 120 miles to 20 quid is that bad.

[Nick Mann]

higher octane fuel contains more energy. Even when cruising, you should need a fraction less right foot to maintain the same speed. My wifes 206 gets better mpg and is faster on V-power than 95 and there's no turbos there!

[Dom B]

Perhaps the lag in 'learning' knock and lambda values can mean that a car that is cruising, is still running on a retarded or even rich map correction from when it was last town/country road driving, for enough miles that it won't have caught back up to normal condition. That's just an idea i don't know enough about these ecu's to know how they behave. I know that the motronic 7.1 takes almost 150-200miles of engine time to settle properly on fuel/lambda changes but only takes around 10 good clear spark events after a knock retard to get back to actual timing position. No idea how the mitsi one works tho.

I am not convinced that the higher octane fuel contains more energy. The octane rating only refers to the fuels comparable and quantifiable tendency to explode instead of burn. The internal energy (enthalpy) of a fuel can vary from where in the world it comes from (which particular oil wells), what additives it has how much methanol or ethanol if any is added (and you would be surprised how many supermarkets add both of these). Remember a few years ago there was a great big uproar where loads of peoples lambda sensors were damaged after some supermarkets were supplied fuel with too much alcohol content.

Diesel does have higher internal energy and does have octane raising properties as white diesel is almost paraffin. I have seen someone regularly add a couple of litres of white diesel to a complete petrol tank fill as an octane booster and they never had a problem with mot's and they swore by it in older classic cars. Not sure i am brave enough to try it on mine. Would this work? would it poison the cat, i wouldn't have thought so in those dilutions as most fuel additives are paraffin based. the only thing is you would be charged the minimum pump delivert of £5 every time you needed just 1 litre!

Here is another thought maybe the higher octane fuel stops the knock events on modern engines and allows more power to be produced assuming there is enough compression ratio to do so on most modern cars, so you have a bit more power with the engine running at it's peak, therefore you accelerate that bit faster or take less throttle to accelerate at the same rate and so save fuel that way. Just guessing now, but that's possible.

[orionn2o]

Higher Octane fuel doesn't contain more energy, that factor is down to the type of fuel.

Higher Octane fuel is however more resistant to explosions and is harder to get to activate a chemical reaction. The allows the engine to operate without an explosion in the cylinder before the time is right. Therefore by doing this, you have prevented explosions (knocks) and the car can run at full capacity.

Compression is related directly to power so therefore turbo/super charged cars can allow a much higher compression to take place in the block. Fortunately the higher octane fuel doesn't activate until required.

Obviously if your car does start to knock then ignition is retarded and you will lose power. So this is directly related, but its all there to protect your engine

[Dom B]

But with reference to economy when cruising you are no where near that knock area so on any petrol the car should be as efficient and running at 100%, remember at cruise is low load.

Also you are right about the higher compression when boosted in turbo cars, however when off boost we actually have a lower compression ratio and BMEP cylinder pressure as we only have a 8.5:1 static compression ratio and most N/A petrol cars have between 10.5:1 and 12:1. Lower compression cylinder pressure means lower efficiency.

Alan above has said that he is still running vacuum at 80mph so the cylinder pressures are still governed by the low compression ratio of the vr4 at cruise.

I am now wondering what if we made the turbos spool up earlier on cruise so they run at atmospheric weather this would help? I guess the waste gate is still welded shut at 80 if there is a vacuum. My VR4 is off the road at the mo. can anyone let me know what rpm we do roughly in 5th at 80mph next time they are driving. The turbos must be beginning to overcome vacuum at 80mpg surely.

[Wodjno]

It is dependant on engine load also..
Rough surface, slight incline, wind resistance, oil viscosity, tyre pressures, hot or cold, weight in the car....

They all play a part in whether the car will run vaccum or positive boost at any given speed..

RPM at 80mph.. If memory serves me correct it's a smidgeon of a needle width under 3000 rpm .. Thats for an Auto...

[Dom B]

Thats got to be almost coming onto boost under cruise load at that rpm.

[orionn2o]

If you assume on a general run you want to maintain a 14.7 air/fuel ratio on cruise, then I fail to see how boosting will improve anything as more air = more fuel.

[Wodjno]

Thats got to be almost coming onto boost under cruise load at that rpm.

I have done alot of testing boost on and off the air pedal..

It is possible to increase decrease the vaccum in the engine at a given speed by pressing the air pedal.. But not to gain any speed

Fuel efficiency differs quiet a bit by a very small adjustment of the right foot.. so small that the speed of the vehicle does not alter, but the vacuum increases.. Thus using less fuel.. Obviously there comes a point where the car is less efficient at a given vaccum as the car starts to labour.. It is very tiring driving with constant adjustments of the right foot and watching the boost gauge.. But it can be effective in gaining fuel efficiency..