Fuel Economy, Waste Gates And Boost Controllers

[jazza08]

My car's untuned and gettin 350ish kms and im driving below 3000-3500rpm all the time however i have a high load as im puttering around off boost all the time, and when boost starts to come on the load drops accordingly. Going to have the car tuned once i have a new turbs etc on, I think makin the lower rpm's run lean until low boost starts would be a good idea for fuel economy. I don't even know what the word is lol

[mad082]

<rants like an old man> Fuel economy? You young whipper snappers in your fancy 'sports car' don't know what the words mean. Why back in my day i drove from London to Brisbane on 3 thimbles of cod liver oil i found in a puddle on the side of a road.. We couldn't afford any of this fancy 'petrol' you see everyone using nowadays, so we just made do with what we could.

now we also didn't have any of these round wheels you see on cars today, we just got a wooden crate and bolted it on

and seats, well what can i say about seats....<trails off as i get carried into the mental assylum>

[GTaaargh]

Well, I used more revs and shifted later, but I actually used a little more gas. But 15km isn't a lot of difference. It doesn't prove it one way or the other.

I set up my Consult cable again and ran up the latest version of Nissan Data Scan. Then I checked up PaulR33's excellent page on Power-FC ECUs. It sort of explained the fuel map table, but I'm still a little unclear.

As I understand it, the fuel map works on revs and load (as measured by the AFMs). Does the number in the table represent the amount of time the injectors are open? I'm sure the question has been answered somewhere here, but I tried searching the forums, and there were over 450 threads that mentioned "fuel map", and it took around 15 mins to scan 2 of them.

If it is the time the injectors are open, is that a good indicator of the fuel economy (ie twice the number = twice the usage)?

[paulr33]

the rule of thumb you can try is

14.7 / INJ value = AFR

[Cubes]

GTaaargh,

RE: Post 20.

Drop the rev's open up that throttle a little more to reduce pumping losses and it will return better fuel economy than the opposite.

Thats providing the fuel map isn't tuned as such that afr's go way rich doing so.

[govich]

and the torque question... an rb30det making around 200rwkw makes roughly the same 'peak torque' as an rb25det making 300rwkw.

If it makes the same peak torque and puts out 100 less max kw at the wheels then it either doesn't rev hard enough or torque drops off something savage up high.

The stock ECU gives me around 380km/tank with FMIC, exhaust and filter. Power FC coming soon enough to lean out mixtures during normal driving... I'm testing out how stock and aftermarket atmo BOVs compare for fuel consumption at the moment.

[paulr33]

why would a bov affect fuel economy? unless it was obviously overfueling and dumping fuel?

[govich]

why would a bov affect fuel economy? unless it was obviously overfueling and dumping fuel?

Oh..i thought it was more common. Well my car was popping (out the exhaust that is) on overrun occasionally with the aftermarket BOV and hasn't been with the stocker (even when i've been provoking it) so i assumed so.. I guess the numbers won't lie when i'm done.

Edited June 13, 2006 by govich

[Cubes]

If it makes the same peak torque and puts out 100 less max kw at the wheels then it either doesn't rev hard enough or torque 'drops off something savage up high.'

As I would expect a 3ltr running a small stock sized turbo.

Cubes = torque but lack of rev's if you don't get it breathing via a nice exh. manifold, cams and suitable turbo that can keep up.

[khunjeng]

maybe I missed something but gentle acceleration will give u good results...in stop start traffic i.e. the CBD your always gonna be behind. I get well over 500kms out of my line but thats on more highway driving....in comparision on the same trip I will get close to 650km out of my NA FTO (2l) and I do give it a bit generally...

I think 400+ in stop start is not too bad really...if anyone has had a trip computer in their car you will see Km/100 figure go through the roof on even slight acceleration as moving ~1.5 tonnes from a standstill requires more energy than just mainating speed/momentum...year 12 physics heheh.

Edited June 13, 2006 by khunjeng

[Cubes]

maybe I missed something but gentle acceleration will give u good results..

You are right as gentle acceleration will give you good results.

Say we have two identical cars accelerating at the same 'normal' rate, one uses more throttle less rpm to do so, the other uses less throttle more rpm.

The less throttle higher rpm WILL use a bee's dick more fuel, why?

1. Higher pumping losses

2. Higher internal engine friction hence losses

Direct injection.

[rb26s13]

my simple conclusion to all this is dont like the fuel economy of the car? hello you bought a car thats not exactly the most fuel effiecent car.. it all comes with owning one of these things.. its the right foot that controlls fuel consumption most.. heavy foot = heavy fuel use

if you dont like it.. sell the bloody thing and buy a small cheap car that runs of the smell of a oily rag..

[Cubes]

my simple conclusion to all this is dont like the fuel economy of the car? hello you bought a car thats not exactly the most fuel effiecent car.. it all comes with owning one of these things.. its the right foot that controlls fuel consumption most.. heavy foot = heavy fuel use

if you dont like it.. sell the bloody thing and buy a small cheap car that runs of the smell of a oily rag..

The point is to get the most out of what we have...

Mine returns a nice 10.6-11L/100km's.. 3ltr.. I'm happy.

Can't get much better than that unless you pick up a little buzz box.

[psybic]

I would have suggested Cubes was way off the mark.

I do like the idea of low revs with more load, gets nice spool noises as opposed to light load, lol, but always feel I'm wasting petrol that way...

HOWEVER....

10/11 per 100kms.... thats freakin' awesome!!! I don't think my 2JZGE could do that. My SR20 was struggling to do that on a long trip.

I'm going to try the low revs, higher load technique.

[jazza08]

I think you might find low throttle high rpms and high throttle and low rpms actually turns out a fairly similar figure...

Its true ive just given up on fuel economy, ill try to get some with the tune i guess

[Cubes]

psybic,

So basically.. shift as early as posible (2.5k) with a steady right foot, don't hold gears out with super light throttle as it increases pumping and friction losses.

Read the traffic - lots of deceleration in gear (fuel cut), no warm ups just get in and drive it normally.

Maintenance.

My injectors are spot on (when they weren't i was getting 350km's to a tank no matter what)

Nice tune that runs stoich when off boost

No vacuum leaks

Motors in good nick

*unsure how the rb26 cam specs affect fuel economy* I run rb25 cams with around 10degree's overlap, so similiar overlap to the gtr cams, similiar IVO. EVO is different. So it shouldn't be too far off.

As stated there is very little difference in economy when it comes to the two driving styles, But one does offer a little better than the other.

Say it is 15km's to a tank thats for example from 12L/100km's down to 11.5L/100kms.

Combine this with a good driving style, motor and sensors that are in 100% perfect working order with squirters that atomize fuel and there's no reason why a modified rb can't return factory like fuel figures.

I do admit I drive like a granny the majority of the time as I usually have my little 15month old rugrat in the car and boy is he starting to become a little rugrat allright.

Its possible but there does appear to be a LOT of rb25's that return terrible economy no matter what.

[Cubes]

I've decided to do a little googling to see what I can find on the topic.

First a wiki..

Most of what can be legitimately achieved is done by reducing parasitic losses of accessories e.g. fans, water pump, and pumping losses by taller vehicle gearing.

Taller gearing reduces pumping losses as it requires you to crack open that throttle a little more while using a lower rpm.

http://en.wikipedia.org/wiki/Fuel_saving_devices

Then a little from BMW

High Precision Injection couldn’t be more important. Like Valvetronic, which reduced pumping losses by eliminating the need for a throttle butterfly valve, High Precision Injection can improve fuel economy in every BMW gasoline engine.

http://www.greencar.com/index.cfm?content=topstory3

All though not really that clear..

.The higher the gear, the lower the frictional losses. By getting to 5th quickly, the total number of engine revolutions is reduced, with a corresponding reduction in frictional losses. The second reason is pumping losses. This is the work done to force air past a partially closed throttle plate. The more closed the throttle is, the higher the losses.

http://www.insightcentral.net/KB/faq-efficiency.html

and then there's this which pretty much sums it up.

The efficiency of a car engine also increases when a high engine load is used (giving more gas at low engine RPM (revolutions per minute)).

Which is pumping losses.

http://www.ecodrive.org/newdriving/easytolearn.html

Bed time.

[discopotato03]

Cubes my understanding of the losses due to throttling is more relating to effective or dynamic compression ratio . Cylinder charging tends to be pretty poor with not much area to breathe through so with less air ingested there's less to compress so the dynamic compression ratio will be way down .

Driving in a higher gear usually means the throttle will be opened further so cylinder charging will be better and dynamic CR higher .

The system that has no throttle is actually throttled by the inlet valves so in theory it has no upstream restrictions . That system can do some amazing things to valve timing and lift like having no overlap at engine cranking and low speeds . You would be surprised just how much altering the valve open period and ignition timing variation affects dynamic compression .

The next step is direct chamber injection because you can have more of the Diesel engines advantages like injecting very late in the compression stroke and cashing in on thermal efficiency .

Naturally with this system fuel pressure is very high compared to port injection because fuel rail pressure has to be higher than compression pressure . The higher pressure makes for better fuel atomisation so in theory a more even fuel/air mix .

I like to let an engine idle when started from stone cold (been standing overnight) for about a minute or two as I think combustion heat taking the chill off the piston crowns valves and chambers means less fuel hosed down its neck just to keep it firing - cold gun effect .

A good friend once pointed out to me that if you drive an engine for torque it generally gives good consumption - this assumes its tuned and functioning properly .

Cars these days have so many lounge room accessories its a wonder they don't have a rudder and screws . If people could live without the 6 stacker power windows A/C etc they would be much lighter and have better power to weight ratios . Depends on you priorities I suppose . I believe if you keep it light and the state of tune not too high its possible to have it both ways - the thing to keep in the normally used rev range is torque .

Cheers .

[dangerous_daveo]

My comments on this.

I tend to agree Cubes. Every car manual (as in the one with the car) I have read have said 'use moderate acceleration, for best fuel economy' Not light not heavy.

Being smooth also helps. Try and get good control. Being in cars with other people they are constantly on off on off, it adds load, uses more petrol. It also is really annoying.

I also (up to a point) drive by my boost gauge. Feel the car, you can feel when its straining, feel when its not doing anything and its time to change gears.

I feel I can say this as Im getting 550-600kms out of my RB20DET cefiro (so heavier than most of you skyline boys) Say 300kms of that is highway to be honest, but its still not bad.

In regards to the lounge room stuff... Personally I know 40* ambient (hotter inside of the car) in QLD summer is NOT safe. When you get that and rain, windows down dont work, you get wet. A/C is a MUST for Qld. You cant think straight otherwise, make mistakes, and crash. Ditto with stackers. The idea is so you dont need to change CDs while driving, being safer again. I do a lot of driving, and listen to music the whole time, so I can justify the extra weight and use of engery for that. As I say tho, thats just me...

[mad082]

stuff the stacker, just get a mp3/cd player. you can get about 10 hours onto a cd.