Octane rating vs Performance. Discuss!

[ph001]

OK, so I'm just a little perplexed on the octane ratings on the fuel flap of the 3.0Si. The info seems to state that anything from 91RON to over 100RON is OK but must be a minimum of 91 AKI.

AFAIK, 91 AKI is considered Super Premium in the states and certainly doesn't equate to 91 RON (more like 95 RON).


The (American) manual states:

Required fuel:

Super Premium Gasoline/AKI 91 is highly recommended.

However, you may also use gasoline with less
AKI. The minimum AKI Rating is 87.
If you use gasoline with this minimum AKI Rating,
the engine may produce knocking sounds
when starting at high outside temperatures.
This has no effect on the engine life.


It doesn't state anywhere if there are any gains to be had from switching from 91RON to 100RON. What octane fuel is the specified output of 265bhp done at??

It's a little different to my E46 M3 which specifically states that 97RON fuel is required to meet it's specified output power. Any less than that and the knock sensor will retard the timing slightly so you get a few less ponies to the wheels. No issue in doing this in terms of reliability of course.

Most modern engines automatically retard timing if knock is detected but if the Z4 is saying it makes it's 265bhp on 91RON fuel, you have to wonder how far it advances ignition to take advantage of higher octane. And how often it checks!

This is of course all on the assumption that you simply cannot run the engine at it's most efficient PURELY as a consequence of low octane fuel. That seems a little disingenuous to me.

 

[Smartbear]

Hi, I don’t think there’s much to be gained on a normally aspirated engine by running super apart from the cleaner engine aspect, the 265ps would be a maximum output even for super I reckon.
I would have thought the engine management would be continually looking to advance the timing until it gets the nod from the knock sensor that something bad is starting to happen
Rob

 

[ph001]

Yeh, so the ECU always runs it right on the edge of knock for maximum efficiency - that would make sense to me. But then again a lot of people say it can take at least a couple of tanks of high octane juice before the ECU compensates. Not sure where the evidence of that comes from.

 

[Steve84N]

As noted above it's 265PS so about 261bhp.

Even my old M54 330i asked for super unleaded so I've assumed it's necessary for full quoted power on the N52 as well.

 

[Smartbear]

Yeh, so the ECU always runs it right on the edge of knock for maximum efficiency - that would make sense to me. But then again a lot of people say it can take at least a couple of tanks of high octane juice before the ECU compensates. Not sure where the evidence of that comes from.

I had my car Dyno tested & the 3 full power pulls all showed an increase in power as the ecu adjusted itself on each run-the operator said it would continue to improve slightly over the next couple of tanks until the settings reached there optimum levels.
It wasn’t a fuel test, but shows the ecu is constantly looking to fine tune for the best results

Rob

 

[markalp]

I always stick to Shell V-Power, clean engine, and the ECU doesnt need to keep learning what fuel I have put in.

 

[ph001]

I always stick to Shell V-Power, clean engine, and the ECU doesnt need to keep learning what fuel I have put in.

True, but it still has to keep learning where knock is due to altitude and ambient - both of which make a fairly big difference to how much ignition advance you can run.

 

[buzyg]

Just treated the Zed to a tank of Vpower for the run up North, this weekend. Last few tanks have been Tesco's finest Never gets 95 Ron any where near it. S54 Though. :wink: Surely, as above, the manufactures, put high Ron petrol in to get the brochure figers.

 

[Smartbear]

When you think how much we pay for these cars & how much we love them I can’t imagine why someone would worry & skimp over a couple of quid to fill up with the good stuff :roll:
Rob

 

[Ed Doe]

Just for clarity, I searched the differences between octane ratings between the states/Canada and the uk/Europe. The below was a post from pistonheads from someone who I'm assuming works in the petrochemical industry!

I will now attempt to give all you great unwashed a master class on fuel 

RON (Research Octane Number) is the octane number you will see at all European roadside fuel stations. In America they display the AKI (Anti Knock Index) which is the average of RON and MON (Motor Octane Number). The British Standards minimum requirement for Premium Unleaded is 95RON and 85MON and Super Unleaded is 97RON and 86MON. 

MON is determined in a single cylinder engine similar to the RON engine with a few changes that make operating conditions more severe and therefore the octane numbers are lower. The MON engine run at 900 RPM with a 300ºF (148.9ºC) mixture temperature. Spark advance varies with compression ratio. In a real world engine, MON is necessary to satisfy octane demands at wide open throttle. This is a very important number for racing engines since they spend a high percentage of their lives under high speed and high load conditions. Racing engines cannot afford to be short on octane quality, since detonation or preignition will quickly reduce a racing engine to junk.

The MON appetite of an engine with 13:1 compression ratio and a four inch bore varies with operating conditions but is normally around 101. To cover yourself for extreme conditions, it is wise to have an octane cushion, but there is no advantage to using a very high octane quality product if you do not need it - more octane does not give more power in that case which is a normal misunderstanding.

The octane quality of a gasoline is its ability to resist detonation, a form of abnormal combustion. Detonation occurs when the air-fuel mixture reaches a temperature and/or pressure at which it can no longer keep from self igniting. Two types of abnormal combustion are common: the first is detonation as previously mentioned and the other is preignition.

RON is determined in a single cylinder variable compression ratio engine that operates at 600 RPM with a 125ºF (51.7ºC) inlet air temperature at standard barometric pressure. Spark advance is fixed at 13º BTDC. In a real world engine, RON is necessary to satisfy part throttle knock problems. High RON values are more important in carburetor engines than in fuel injector engines as the filling of the combustion chamber is not as efficient.

All of this means protection from detonation NOT power! 
Detonation occurs after the sparkplug has ignited the air-fuel mixture and the flame front is moving through the combustion chamber. If, during this burning process, the unburned air-fuel mixture reaches a temperature and/or pressure at which it is no longer stable, it will explode (burn up to 10 times faster) and as a result increase pressure on the piston before Top Dead Centre (TDC). Piston burning and rod bearing damage are the result as well of loss of power.

Most of this is vital for race engines. It is written with that in mind because a lot of you trck your cars and even for road use the stresses are greater than a "Honda Jizz" so it is still relevent.

Are there any qustions? No? 

Class dismissed. 


The ecu will not be on the limit of knock by any means. It's designed to operate in the middle of the middle east summer time on s**t fuel, just well as in alaska on amazing fuel. Damage is already being done if the knock sensor is triggered. The car would be designed to pull spark advance quite dramatically so you'd notice a bit reduction in power during driving. race cars tend to have a finer tolerance before knock as they are entirely optimised for speed Remapping the car removes a bit more of the safety net of the tolerance window, meaning you should use the higher octane fuels.

 

[ph001]

The ecu will not be on the limit of knock by any means. It's designed to operate in the middle of the middle east summer time on s**t fuel, just well as in alaska on amazing fuel. Damage is already being done if the knock sensor is triggered. The car would be designed to pull spark advance quite dramatically so you'd notice a bit reduction in power during driving. race cars tend to have a finer tolerance before knock as they are entirely optimised for speed Remapping the car removes a bit more of the safety net of the tolerance window, meaning you should use the higher octane fuels.

I'm not too sure about the first bit. Minor pinking is in fact very common at wide open throttle for maximum performance (designed in even) and is not considered detrimental to engine longevity. Knock sensors are extremely sensitive and activate long before full detonation occurs.

In terms of more power at higher octanes, I guess it all depends if the ECU is able to run optimal timing for maximum power on 95Ron fuel (which we are assuming is what the specified output is measured on).

I know from my old kart tuning days that (generally speaking, ignoring almost all environmental factors) around 22' advance before TDC gives MBT (maximum brake torque) at high rpm - hence maximum power. So, using this purely as an example the question probably is:

...can the ECU use 22' ignition advance to get maximum power on 95ron fuel or will the engine knock?

I would guess that on S54 engines it would indeed knock, hence the RECOMMENDATION to run 97RON. But there seems to be no such recommendation on N52 engines apart from 91 AKI.

If it does knock then the ecu will use a more retarded timing (with reduced power output) until such conditions exist that it can get back to 22'. Such conditions could be an increase in altitude, higher charge temps or higher octane fuel. Given such dynamic variations, I would expect the engine to control that feedback loop pretty quickly (certainly in terms of retardation), perhaps it is not so important to advance again at the same rate so you could understand why that may take considerably longer.

The big question then is....

is going to something even higher like 99Ron ever going to give you an increase in performance?

I think the answer is complex - at low altitudes and cold intake temps where knock is much more likely, I suppose there is a chance that the higher octane fuel will allow the timing to get to 22' BTDC (where previously it could not due to knocking). But car manufacturers being as they are, possibly made sure the car could produce it's full power in the middle of a Russian Winter at sea level. Or perhaps they just factored in a mild performance drop in such extreme conditions in order to get the full output 99% of the time in more moderate climates. I suspect they were more concerned with engine longevity rather than performance in such extremes so it's not too hard to imagine the case for this.

I guess we will never know unless the very complex 3D engine map is fully understood. I suspect even the most reputed tuners may struggle to answer that question.

 

[Marlon]

I've been going with the V-Power or Super Unleaded since I bought the si, but did have a spell using standard unleaded to save a bit of cash and I honestly didn't notice any difference in performance.
It's not a track car of course, but in spirited drives I couldn't tell any difference.
Its like the wheel spacer debate - can I tell a difference driving on public roads with 15mm spacers on the fronts and 20mm spacers on the rears - no of course not. Could I if I was a racing driver on a closed circuit - possibly. I would suggest the same goes for differences in Octane ratings.
I'm back on the good stuff now as believe its better for the engine