Fuel Trims Comparison...

[Diamond Black Geezer]

Has anyone ever compared live data of their motor on supermarket fuel, then given a week or two on 'super fuel', and compared the results? Quick question from me, this time. 

[biggriffin]

Has anyone ever compared live data of their motor on supermarket fuel, then given a week or two on 'super fuel', and compared the results? Quick question from me, this time. 

Er no.

[TheBoy]

If the car is working properly, they will still be 0% (DBW) or 127 (non DBW).

[Diamond Black Geezer]

Ahh, Perhaps I've misunderstood the internal workings of the engine, then. The knock sensors are supposed to detect a higher/lower octane/superfuel, and advance/retard the ignition accordingly, and I thought this affected the trims? Read on another forum this was the case (different car/engine) 

[Kevin Wood]

Knock sensors affect the ignition timing, lambda sensors affect the fuel trims. They are two independent systems.

With super unleaded, all you are changing is a few trace additives in the fuel to control its' knock characteristics. The mass of fuel required for the correct mixture will be the same, so the fuel trims will remain the same.

Knock control is more a safety mechanism to save the engine if it's fed with dire quality fuel. The mapped ignition timing isn't extreme enough for any "learning" to take effect in normal circumstances. I've never seen an Omega data log with any knock retard active unless the knock sensor has been misbehaving, so I wouldn't expect that to make any difference either.

[Diamond Black Geezer]

Many thanks, full, and clear description  Sorry, I know I sometimes bucket questions on here, but as I've said before, big thirst for knowledge, don't learn if I don't ask. Thanks again all 


So are you suggesting that the knock control can be 'taken down a peg' for poor fuel, but not 'up one' to take advantage of higher quality/octane? If so, I assume an engine remap would be able to do this, though?

[Mr Gav]

I think the knock sensors only retard the ignition not advance it so there`s no point in putting expensive fuel in as it probably won`t make any difference, it would only benefit if the knock sensors were retarding the ignition on 95 ron, then you would  reap the benefits of 97 ron fuel.

A remap would help with this but benefits on the Omega won`t be great, maybe 10% at best.

[tigers_gonads]

I think the knock sensors only retard the ignition not advance it so there`s no point in putting expensive fuel in as it probably won`t make any difference, it would only benefit if the knock sensors were retarding the ignition on 95 ron, then you would  reap the benefits of 97 ron fuel.

A remap would help with this but benefits on the Omega won`t be great, maybe 10% at best.

Yup
Used to sort out pre ignition caused by low octane / dodgy foreign fuel 

[Nick W]

Another thing to consider: just how beneficial is the small increase in octane rating? Exactly how big is Ron anyway

[Diamond Black Geezer]

Oddf they didn't factor in at the design phase that a higher octane fuel would come into existence. I appreciate at the time of design, (late 80s) they were just changing over from 4-star, but a shame.

So perhaps that's my question then - is there any scientific way of knowing that the engine does, or does not take any advantage of a higher octane fuel?

I stick in the Tesco Ultimatum for its 'detergent' properties, and frankly minimal difference in price, certainly cheaper here at 112.9p than much standard unleaded I've seen elsewhere in the country/motorways. Not wanting to turn this into a standard vs. superunleaded thread, just seeking a definitive answer as to the V6s being able to make use of, or not, the higher octane fuels.


And the next question after that - who does remaps? 

[Kevin Wood]

The basic requirement of an engine for fuel of a certain octane rating comes from the compression ratio or, for forced induction, a combination of the compression ratio and boost pressure.

If knock occurs, retarding the ignition timing and/or reducing the boost pressure allows the engine to run without damage but sacrificing power and economy.

On older engines, this was more of an issue, but a reasonably modern design of combustion chamber doesn't need particularly aggressive ignition advance to produce its' best power output so, for sane levels of compression such as are used in the Omega, there is plenty of margin before knock starts even on standard 95 RON unleaded.

You can't change the compression ratio, so you need a knock retard strategy to cope with very poor quality fuel, the likes of which probably doesn't get sold in the UK.

Forced induction changes things and some turbo/supercharged cars probably do reduce boost on 95 RON fuel.

If you wire up the engine with instrumentation as is done during development then yes, you can get a picture of what's happening in the combustion chamber by recording when the peak pressure occurs during combustion and checking for signs of knock with more sophisticated equipment than a standard knock sensor.

Without increasing the compression ratio, I doubt a V6 would respond to higher octane fuel - even if you remapped it, because an engine with a pent roof chamber and 10-ish :1 compression probably still has a good margin against knock when the ignition has been advanced to the point where you get no improvement in power.

[omega2018]

Has anyone ever compared live data of their motor on supermarket fuel, then given a week or two on 'super fuel', and compared the results? Quick question from me, this time. 

not that but on my old 2.5V6 i did the same stretch of motorway every day on cruise and compared the fuel consumption by resetting the trip computer.  over a series of days, when the traffic was light enough that i could safely stick on cruise between a couple of junctions, i got about 10% more mpg on super.  i'd expect the 2.6 improvement to be less as the compression is lower. 

that's my experience anyway

[Diamond Black Geezer]

I would also say similar migmog, thank you. However, Ive not been able to scientifically do it as well as yourself, I've had to basically say 'well, my mpg isn't worse' which isnt much help! 

From week 1 of driving Pissy, where old stale fuel and supermarket whatever gave me a week's average of about 18mpg, now she's doing 21.8 on superunleaded, with 2 years of miles on her, plus various enginework (all as-factory, we're talking thermostat, oil change etc..) Before the service a few weeks ago she was strugglign to get above 20mpg in town traffic. Very much apples and oranges, of course. But as I say, it's not making the engine worse. 

Kevin, thank you for the detailed response, that's a bit more knowledge about engine design added to the grey matter. 

[Shackeng]

Whereabouts is the knock sensor on the the V6?

[Kevin Wood]

Has anyone ever compared live data of their motor on supermarket fuel, then given a week or two on 'super fuel', and compared the results? Quick question from me, this time. 

not that but on my old 2.5V6 i did the same stretch of motorway every day on cruise and compared the fuel consumption by resetting the trip computer.  over a series of days, when the traffic was light enough that i could safely stick on cruise between a couple of junctions, i got about 10% more mpg on super.  i'd expect the 2.6 improvement to be less as the compression is lower. 

that's my experience anyway

Fuel consumption is another issue. We're talking about differences in the live data.

It's possible that different fuels have different calorific values, meaning the energy density changes and so does the fuel consumption for a given work done. The most likely mechanism for this to change from one brand or type of fuel to another is by the level of bio ethanol in the fuel.

You need half as much ethanol again in terns of volume to get the same energy as in petrol, so added ethanol is effectively "watering-down" your petrol.