GTD+

[monkeyhanger]

Based on an abnormally high commute figure achieved yesterday, the jury is out on mpg gains based on this morning’s commute, the first one going to work with the box on.

I’m only counting the way to work in my comparisons, because the journey is constant – we always go directly to work, dropping off the wife 9 miles into my 12 mile journey.

The way home usually involves some kind of detour because the missus wants to go to x/y/z to see if they’ve got any a/b/c, and no two journeys are the same.

For going to work, since returning from my hols (Weds), the conditions have been the same – damp, dark and 10C ambient temp.

Thurs commute = 42.9mpg
Fri commute = 42.8mpg
Mon commute = 47.2mpg*
Tues commute = 47.2mpg

*I attributed Monday’s gain to having done a decent leg of dual carriageway driving at speed over the weekend maybe temporarily altering the car’s fuelling characteristics following that journey, Monday night’s commute home was quite thirsty – back down to about 43mpg, so the morning value did seem a blip.

Based on Thurs/Fri numbers and ignoring the Monday blip, we seem to have a 10% mpg gain over either day for this morning’s figures. Taking Thurs/Fri/Mon figures and averaging them, we see a 6.5% mpg gain. I did have a few blasts this morning, at points in the journey where the road speed jumped considerably e.g. joining 50/70mph dual carriageways from a 30 road, zipping straight up to cruising speed. Even on the middle setting the performance gains are noticeable with improved pick-up in 3rd/4th/5th/6th.

Any concrete gains seen in the MFD are assuming that the car is metering fuel use accurately, independently of the altered signals that the tuning box processes. I won’t know for sure that is happening until I fill up and see if the same disparity between MFD calculated mpg and actual mpg via brim method remains.

Looking to have some proper fun next week when I will probably crack 1000 miles.

It seems ridiculous to me that VW put out a global “one size fits all” map for each engine rather than one of a number of  regionally specialised maps that should give everyone better mpg/performance. If you move out of your designated region, get the car’s ECU flashed officially to the appropriate map of the region you moved to (how often would that happen for most people?). They could easily do this. When my 170TDI MK5 was having DPF issues, the car was eventually given a service remap which improved mpg and reduced active regens from twice a week to almost zero.

[Bruce]

In brief if anyone is interested:

Fuel usage is calculated by the ecu using fuel pressure and injector opening time. These boxes work as a piggy back ecu altering boost pressure and fuel pressure to fool the car into providing more of each. Diesels have no throttle plate so power is regulated purely by the amount of juice that is squirted in. The box provides the car with a lowered fuel pressure value so the car increases the fuel pressure resulting in more fuel being squirted in. Similar situation with boost, a lowered boost pressure signal will delay the opening of the wastegate actuator resulting in raised boost pressure.
The indicated mpg will be higher than actual due to the increased fuel pressure.
Economy improvements will only come about through the ability to use lower revs due to increased torque. i.e each injector opening lets more fuel in but if there are less injector openings then there is the potential for fuel saving.

Hope that makes sense!

[monkeyhanger]

Bruce:

Kind of makes sense, but I think you’re generalising with an accurate description of the way the £30 ebay specials work (just a box of resistors changing current applied to receiving sensors in a very unsophisticated way with no real signal processing) which just force more fuel through almost all of the time for extra power. With that explanation, all of these diesel boxes should cost coppers if they all work in the way you have described.

Given a tighter likely set of operating range parameters (temperature/humidity) for a regionally relevant map provided by ECU signal modification, are the better boxes not capable of providing a more efficient fuelling via different algorythm which will supposedly make the car run better than as per the “one size fits all” global map applied universally by VW,  as a matter of course to push down fuel usage rather than just relying on higher gear usage with the higher torque?

There’s no way you’d see 10% fuel gains just relying on extra torque to stay in a higher gear, and my dad did see verified 10% fuel gains (brim method) on his old box fitted to the MK5. There aren’t that many situations where you’d rely on torque to stay artificially in a higher gear to maintain a speed. For a brief spurt of acceleration in 4th/5th/6th then yes, you might rely on the torque to avoid changing down briefly only to change up again later, but it’s not like you’d sit in 6th doing 1000rpm at 38mph because of the extra torque rather than change up to 6th at 40mph. Torque available outside the range that the turbo is doing it’s work is largely unusable in the higher gears that would get you your economy gains anyway.

These boxes are sold on noticeable mpg gains (if you’re not hooning around all the time) and have a 14 day trial.

I fully intend to test the box out to determine what gains are to be had. To be honest, I kind of hope that any mpg gains are invisible to the MFD due to mpg being calculated on the basis of the pre-alteration ECU signals, rather than physical metering of fuel volumes passing through a feed pipe in the fuel system. It would be a lot easier to see true gains. Say for example your MFD is known to be 5% optimistic and you are getting 50mpg indicated on the MFD, which should be a true 47.5mpg. Since using the box, you drive the same way and your 50mpg indicated is an actual 52.5mpg on the brim method – you have seen a 10.5% increase.

If any gains are invisible to the MFD then you have a rock-solid way of knowing if the box has given you any gains because a difference in your driving or conditions on any given trip is taken out of the equation. Your gains would always be a change in disparity between brim (actual) mpg and indicated mpg.

[monkeyhanger]

Buddy: Have you filled up yet to know if you’ve seen any mpg gains (I appreciate you’ve set yours to the max and had a bit of fun with it, so mpg gains will probably be a minimum)?

[Bruce]

MH:
I was being somewhat simplistic with my explaination. These more advanced boxes do more than simply remove a fixed or % of voltage from the sensor value. There will be simple look up tables. Some even alude to rpm detection.
I have a fully programmable piggyback ecu on my petrol v8. This uses the signal from the crank postion sensor to provide a rpm reference and maf voltage to provide load which are the xy for a fuelling map which is adjusted by molesting the inlet air temp signal. These boxes have access to neither of these unless some sort of rpm detection is used. I could understand this if the sensors were fed from a 'dirty' DC supply but the DC supplied by the ecu is usually pretty clean.
Some interesting stuff here:
http://www.crdperformance.com/performance/diesel-tuning-boxes/

If I'm feeling particularly enthusiastic (and if I ever get my car!) I might hook the inputs and outputs to the box up to a storage scope and see exactly what it's doing compared with rpm.

If you read the small print on all the sites selling the boxes the mpg figures are usually compiled from customer feedback - which will be based on indicated mpg. They neglect to mention that this will read higher with the box.

Scientific tests are not that easy really especially with mpg improving with mileage. My last car took 15000 to sort itself out.

Great write ups by the way. I will be buying one of these boxes for the power gains but I've yet to be convinced about economy improvements. I'll be interested to see how you get on.

When I've done 10000 or so maybe i'll do 2k with the box and 2k without to compare consumption.

Another thought.....There are NOx and egr valve characteristics that will also be altered by these boxes due to increased fuelling - maybe there are efficiencies to be found there at the expense of emissions?

[monkeyhanger]

If MFD indicated mpg bears no relation to actual fuel usage once one of these boxes is fitted, then brim vs indicated mpg should tell you gains (or losses) for having the box, independent of gains made purely by running in/miles on the car, as long as you knew what the disparity between reading and actual was before the box was fitted. This would mean that mpg is definitely worked out by fuel metering usage for the common rail system (but what about over-fuelling return from the injector system, pre-injection squirt? Is this taken into account when working out mpg).

If on the other hand, mpg is worked out by metering fuel volume permanently sent towards the common rail system, before it gets there, this would mean that the indicated mpg is accurately showing (taking usual indicated/actual disparity into account)  what is actually being used post-box fitment.

There is potentially over-fuelling based not only on the DPF’s needs, but those for NOX and SOX. Maybe gains are made there somehow or the car normally overfuels to ensure similar performance the world over, despite climatic differences (which is what the box manufacturers would have you believe)?

Found a useful overview of the workings of the 2.0TDI CR unit (maybe a little out of date with current engine version improvements, this seems to be from 2008) which show metering of the fuel for the purposes of the injectors getting the right amount of fuel, but whether that metering is utilised to work out MFD indicated mpg, it doesn’t say. http://www.natef.org/NATEF/media/NATEFMedia/VW%20Files/2-0-TDI-SSP.pdf

Anyway, with a little bit of semi-scientific experimentation from me I think I’ll have a clear view of any gains if it is all in changes in the disparity between indicated MFD mpg and actual (brim) mpg in the next 5 to 7 days.

Would I put up with no gains at all in normal use? Probably not – it would be going back within 14 days. Could I put up with gains in normal use, but be prepared for modest extra thirst when I make use of that extra power? Probably.

[monkeyhanger]

Photos as promised.

Engine bay post fitting, with system fitted.



System fitted, cowling off. Red arrows are system plug-ins and green ones are original VW plugs, plugged into the wiring harness (left arrows from common rail sensor, right ones from the air intake sensor). Blue arrow is the box itself:-



Common rail sensors up close, same arrow colour scheme:-



Air intake sensors up close:-



The box up close (box is blue), with wiring harness (yellow arrow):-

[monkeyhanger]

This morning's commute was 47.3mpg, driving it with a heavier foot than pre-box addition. Not a noticeable change on yesterday, but as it was 5C today and 10C yesterday on my commute to work, i'd consider it a very good result. I am starting to think that the box is giving me some noticeable mpg gains. I will know for sure when I top the tank back up next week then have a delve into the high end box settings.

[Buddy]

Buddy: Have you filled up yet to know if you’ve seen any mpg gains (I appreciate you’ve set yours to the max and had a bit of fun with it, so mpg gains will probably be a minimum)?

Brim to brim. Range said 5 miles but when I refilled it was only 45 litres to the brim so I wonder if the extra fuel signals are bypassing the mfd or the range computer is just overly prudent. Anyway managed 450miles and that was with a largely heavy right foot for this tank. Around 45mpg I think?

[monkeyhanger]

Buddy: 45L = 9.825 Gallons, 450 miles done = an actual 45.8mpg. With a heavy right foot that is impressive. V-power or normal fuel?

I had 30 miles left on my first fill up (pre box addition) and got 43.69L in to fill it (414 miles/43.4mpg). If I’d taken it to “5 miles left” at the same mpg, I would’ve had to fill it with approx. 46.3L.

It would seem then that box on or box off, the car likes to still have at least 2/3 of a gallon left as a safeguard when it says it is empty (0 miles left). For that to be the same for you with box and for me without box, it suggests that either the box hasn’t changed your mpg appreciably either way because your heavier right foot has just wiped out gains seen the rest of the time, or that MFD mpg is worked out on actual fuel sent towards the common rail and not returned rather than being worked out on sensor signal on the common rail before the box alters it (this way would end up having the MFD understate actual, if gains were seen).

Did you take note of what mpg since fill-up was the MFD showing? If there’s about 5% disparity in the MFD display’s favour, I’d say actual fuel used is getting metered correctly to get the same disparity without the box attached. If the disparity is near zero or even if the MFD is under stating e.g. actual (brim) mpg is 45.8, but MFD says 45.2mpg, then it would suggest that MFD mpg is based on pre-box fuel usage signals and the box is making the car more efficient.

With V-power, pre-box addition I was getting 43.4mpg actual (brim method) on my first full tank. I suspect just using normal Shell it would’ve been 45/46mpg. On that basis and assuming you would have gotten the same pre box mpg as me, it seems that having a bit of a heavy right foot comes at little or no fuel penalty vs standard unboxed mpg.

Did you get sorted with the company for getting a dual channel box swap out?