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  Reply # 2090502 14-Sep-2018 10:33
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Benoire: I've never tried 91 in my BMW 550i but like Coil I would expect a loss of efficiency and power, it feels sluggish on acceleration.  95 is the minimum for me and that is a trade off between slighting increased power of 98 vs the cost, although never tried.  The car is suppsoed to accept 91 fine but meh it would feel dirty!

 

Higher octane petrol is harder to ignite and has less energy per litre.  Provided your car is designed for 91 octane and provided it runs fine on 91 octane without detonation, then a higher octane fuel will give you LESS power and HIGHER fuel consumption. This is counter-intuitive and not how fuel companies market petrol.





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  Reply # 2090505 14-Sep-2018 10:37
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Coil:Not too sure if you can retard injectors to stop knocking. I know you can adjust the duty cycle and that can help. But I think the ECU doesn't even have control over this in most vehicles.

 

No, ignition timing and octane rating affect knocking. Injector timing (in a petrol port injected cars) makes zero difference to knocking. In fact, so little that Toyota 4 cylinder motors only have 2 injector channels. Fuel is (pre)injected at TDC for the cylinder at the top of it's compression cycle and for the cylinder at the start of its intake cycle simultaneously. It may help to consider that that carburettors have no timing.

 

Duty cycle is modulated to control the quantity of fuel injected, (more/longer at wide open throttle, less/shorter at idle). In traditional petrol's the fuel pressure is constant regardless of engine demand so duty cycle is the only way to control fuel quantity. Depending on design, common rail dis-esels modulate both fuel pressure and duty cycle, but their injection timing is as critical as spark timing in a petrol.

 

 


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  Reply # 2090521 14-Sep-2018 10:48
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mclean:

 

Benoire: I've never tried 91 in my BMW 550i but like Coil I would expect a loss of efficiency and power, it feels sluggish on acceleration.  95 is the minimum for me and that is a trade off between slighting increased power of 98 vs the cost, although never tried.  The car is suppsoed to accept 91 fine but meh it would feel dirty!

 

Higher octane petrol is harder to ignite and has less energy per litre.  Provided your car is designed for 91 octane and provided it runs fine on 91 octane without detonation, then a higher octane fuel will give you LESS power and HIGHER fuel consumption. This is counter-intuitive and not how fuel companies market petrol.

 

 

 

 

Bollocks

 

https://autoexpert.com.au/posts/should-i-use-premium-petrol

 

 

 

 





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  Reply # 2090522 14-Sep-2018 10:48
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tripper1000:

 

Coil:Not too sure if you can retard injectors to stop knocking. I know you can adjust the duty cycle and that can help. But I think the ECU doesn't even have control over this in most vehicles.

 

No, ignition timing and octane rating affect knocking. Injector timing (in a petrol port injected cars) makes zero difference to knocking. In fact, so little that Toyota 4 cylinder motors only have 2 injector channels. Fuel is (pre)injected at TDC for the cylinder at the top of it's compression cycle and for the cylinder at the start of its intake cycle simultaneously. It may help to consider that that carburettors have no timing.

 

Duty cycle is modulated to control the quantity of fuel injected, (more/longer at wide open throttle, less/shorter at idle). In traditional petrol's the fuel pressure is constant regardless of engine demand so duty cycle is the only way to control fuel quantity. Depending on design, common rail dis-esels modulate both fuel pressure and duty cycle, but their injection timing is as critical as spark timing in a petrol.

 

 

 

 

Most petrol engine cars are now direct injected, and they can and do control injection timing very precisely to prevent preignition / maximise efficiency of the fuel burn.

 

At the extreme. I understand that Mazda's coming generation SkyActiv engines will be running compression ratios of 18:1.  I think that's more than my common-rail diesel (OTOH the diesel is also turbocharged).  Direct injected turbo petrol engines are also running compression ratios unthinkable before direct injection, typically 12:1.


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  Reply # 2090536 14-Sep-2018 11:07
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mclean:

 

Higher octane petrol is harder to ignite and has less energy per litre.  Provided your car is designed for 91 octane and provided it runs fine on 91 octane without detonation, then a higher octane fuel will give you LESS power and HIGHER fuel consumption. This is counter-intuitive and not how fuel companies market petrol.

 

IBH that is mostly not correct, and is based on the tiniest sliver of truth, that is only true under specific conditions.

 

Being harder to ignite is a desirable property and in simple terms is the measure that decides the octane rating. Being hard to ignite does not detract from efficiency, quite the opposite. Low octane is prone to knocking which is the fuel igniting spontaneously and uncontrolled and is extremely bad mechanically speaking and also very bad for efficiency. (getting side tracked - modern diesels that rely on "knock' or detonation have achieved big gains in efficiency by reducing the amount of knock to the bare minimum necessary to initiate ignition).

 

You are correct that if your engine is designed for 91 you may be wasting money putting in higher octane ratings. This doesn't necessarily mean that if you engine is designed for a higher octane but tolerates 91 that you should put 91 in it.

 

The presumption of higher octane meaning higher consumption depends on how the refinery has increased the octane ratings. High octane fuel can have less, equal or higher calorific value than 91 octane depending on formulation. For Gull 98, they spice up the octane rating with ethanol which has a lower calorific value than when fuel is spiced up with aromatics such as toluene.

 

Regardless, higher fuel octanes allow engines to be designed with higher compression ratios which increase the dismally low efficiency of internal combustion engines (~20%). So if your engine requires 98 octane, it will usually produce better mileage/lower consumption on 98 (even lower calorific value fuels such as Gull E10) than 91, simply because they reduce the 80% of your fuel that is usually wasted as heat.

 

https://www.engineeringtoolbox.com/fuels-higher-calorific-values-d_169.html


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  Reply # 2090538 14-Sep-2018 11:11
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scuwp:

 

mclean: Higher octane petrol is harder to ignite and has less energy per litre.  Provided your car is designed for 91 octane and provided it runs fine on 91 octane without detonation, then a higher octane fuel will give you LESS power and HIGHER fuel consumption. This is counter-intuitive and not how fuel companies market petrol.

 

Bollocks

 

https://autoexpert.com.au/posts/should-i-use-premium-petrol

 

 

How come bollocks?  He seems to be saying exactly what I did - there's no point in using fuel with a higher octane than your car needs.





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  Reply # 2090578 14-Sep-2018 11:21
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So in summary, higher octane enables a higher compression ratio which improves efficiency which naturally leads to lower fuel consumption.

 

https://en.wikipedia.org/wiki/Compression_ratio

 

 


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  Reply # 2090583 14-Sep-2018 11:25
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i wonder, if minimum octane is under rated for the car to be sellable in different markets.

 

eg, say a car really likes 95, but can do 91 for places with no 95, so to cut long story short they rate it at 91, but that if you put 91, it gives less power, giving the experience of higher fuel consumption?


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  Reply # 2090584 14-Sep-2018 11:25
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tripper1000:

 

So in summary, higher octane enables a higher compression ratio which improves efficiency which naturally leads to lower fuel consumption.

 

https://en.wikipedia.org/wiki/Compression_ratio

 

 

 

 

makes sense (no haven't read the link)


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  Reply # 2090587 14-Sep-2018 11:32
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Batman:

 

i wonder, if minimum octane is under rated for the car to be sellable in different markets.

 

eg, say a car really likes 95, but can do 91 for places with no 95, so to cut long story short they rate it at 91, but that if you put 91, it gives less power, giving the experience of higher fuel consumption?

 

 

There is an element of this. Additionally when crossing boarders and getting cars from different countries you have to be careful because octane is rated differently in different jurisdictions. Octane rating is usual a combination of RON and MON.

 

For instance in the USA the advertised octane rating is the average of RON and MON which seem fair, but in N.Z. the advertised octane rating is allowed to be the higher of the two with a tolerance of +/- 1.5%.

 

This means that 88 octane in the USA can actually be a higher octane than 91 in N.Z. when rated using the same method. Wo when you are driving an imported car you have to be sure of 'whose' octane rating they are specifying the fuel in.

 

edit: spelling.


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  Reply # 2090588 14-Sep-2018 11:37
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Fred99:

 

Most petrol engine cars are now direct injected, and they can and do control injection timing very precisely to prevent preignition / maximise efficiency of the fuel burn.

 

At the extreme. I understand that Mazda's coming generation SkyActiv engines will be running compression ratios of 18:1.  I think that's more than my common-rail diesel (OTOH the diesel is also turbocharged).  Direct injected turbo petrol engines are also running compression ratios unthinkable before direct injection, typically 12:1.

 

Yes, hence I qualified with "port injection" however most of these cars are still spark initiated as I understand it, so ignition timing remains the single most critical non-fuel element in knock control.


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  Reply # 2090590 14-Sep-2018 11:41
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tripper1000: So in summary, higher octane enables a higher compression ratio which improves efficiency which naturally leads to lower fuel consumption.

 

True, but once you have the car you can't alter the compression ratio (very easily). The point I was trying to make was that if Benoire's BMW runs perfectly well on 91 octane fuel then he should not expect a performance or running cost improvement by putting in more expensive fuel.





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  Reply # 2090650 14-Sep-2018 13:33
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regarding additives that 'clean the system'
Occasionally some petrol station franchises advertise this. They do/did have different additives

 

Some history.
One petrol station company used an additive that burned out valves on some Euro cars , they admitted this when they found out.
It was only that ~brand~ of petrol station , unfortunately my local petrol station & yep, burnt out valve.

 

In real life use , would anyone notice the possible difference in power between different Petrols: 91, 95, 97 ....I very much doubt it .
Would additives used by some petrol companies significantly clean up your engine in real life use ... I doubt it .
Change your spark plugs, oil & air filter , that will make a bigger difference

 

:-)


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  Reply # 2090972 15-Sep-2018 09:37
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amiga500:

 

Thanks for all the comments guys.  My reference to supermarket fuels was about Britain & not NZ.

 

As for the injector cleaner I used on my previous car I think it was Wynns. The car was a Laser with the 1.6 DOHC engine.

 

After reading the comments I am now wondering if I had something else wrong such as fouled spark plugs caused by a lot of round town driving.  The car would start mis-firing when under load at low revs.  After a few days I would get the tank next to empty go and buy the injector cleaner and a full tank of petrol. 

 

After that I'd go for a nice long drive some of it in 3 rd gear & plenty of revs and it did seem to work.   Maybe it had nothing to do with fouled injectors though. A day or so later the problem had almost completely disappeared but would come back after a few months.

 

P.S. And yes, I did change the plugs from time to time!

 

 

 

 

Sounds like fuel filter, fuel pump and fuel tank. Either one or all could have been dirty.  (used to have a similar car in mazda brand). 

 

Edit: or just a cracked head....and with that, the compression probems.  Dodgy airflow sensors / air intakes also would have needed to be ruled out. 

 

 


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  Reply # 2090984 15-Sep-2018 10:14
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Goosey:

 

amiga500:

 

Thanks for all the comments guys.  My reference to supermarket fuels was about Britain & not NZ.

 

As for the injector cleaner I used on my previous car I think it was Wynns. The car was a Laser with the 1.6 DOHC engine.

 

After reading the comments I am now wondering if I had something else wrong such as fouled spark plugs caused by a lot of round town driving.  The car would start mis-firing when under load at low revs.  After a few days I would get the tank next to empty go and buy the injector cleaner and a full tank of petrol. 

 

After that I'd go for a nice long drive some of it in 3 rd gear & plenty of revs and it did seem to work.   Maybe it had nothing to do with fouled injectors though. A day or so later the problem had almost completely disappeared but would come back after a few months.

 

P.S. And yes, I did change the plugs from time to time!

 

 

 

 

Sounds like fuel filter, fuel pump and fuel tank. Either one or all could have been dirty.  (used to have a similar car in mazda brand). 

 

Edit: or just a cracked head....and with that, the compression probems.  Dodgy airflow sensors / air intakes also would have needed to be ruled out. 

 

 

 

 

Those laser/323 (Mazda B) engines have issues with the black plastic material they used in the long spark plug connectors breaking down with heat - and arcing through.  You couldn't see any sign of the problem by looking at them.  The OEM Mazda parts were expensive and rubbish quality.  The solution was to buy NGK branded lead sets - they were blue colour (leads and connectors), better quality, and much less expensive than the Mazda part.

 

The seals at the top of the connectors were also prone to perish, if driving them in real wet conditions all you need is a drop of water leaking in to the spark plug wells to cause erratic misfire issues.  Taking them for a long run, and eventually the water evaporates out - so that fixes the problem until next time the engine gets water splashed on it.

 

I'd almost guarantee that was the problem. 

 

 

 

 


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