I have been running my car on unleaded, however it can be run on
superunleaded, now I know it's more expensive, though i'm fantasy bargaining
that there may be an increase in performance dictating the need to use less
thorottle and hence more mpg.
Am i just dreaming?
ps my ca is not getting the mfrs claimed top speed, although I have covered
6000miles.

Avanti wrote:

> I have been running my car on unleaded, however it can be run on
> superunleaded, now I know it's more expensive, though i'm fantasy bargaining
> that there may be an increase in performance dictating the need to use less
> thorottle and hence more mpg.
> Am i just dreaming?


Yes. AFAIK super unleaded contains better antiknock ingredients not
better petrol. It won't burn better.


> ps my ca is not getting the mfrs claimed top speed, although I have covered
> 6000miles.


And this is surprising because?

-- 
Malc

Malc (malcolm.white@ubht.swest.nhs.uk) gurgled happily, sounding much
like they were saying : 


>> I have been running my car on unleaded, however it can be run on
>> superunleaded, now I know it's more expensive, though i'm fantasy
>> bargaining that there may be an increase in performance dictating the
>> need to use less thorottle and hence more mpg.
>> Am i just dreaming?

> Yes. AFAIK super unleaded contains better antiknock ingredients not
> better petrol. It won't burn better.


Not necessarily.

It may be that the car has a knock sensor and a base map set high enough to 
allow the ignition to be advanced on 98RON, giving more power.

Most knock sensor base maps are set at the 95RON values, though, so will 
only retard if pinging is detected, not advance in it's absence.

In message , Malc 
 writes

>
>Avanti wrote:
>> I have been running my car on unleaded, however it can be run on
>> superunleaded, now I know it's more expensive, though i'm fantasy bargaining
>> that there may be an increase in performance dictating the need to use less
>> thorottle and hence more mpg.
>> Am i just dreaming?
>
>Yes. AFAIK super unleaded contains better antiknock ingredients not
>better petrol. It won't burn better.


In engines designed to take advantage of higher octane fuel, you'll get 
better performance / economy. In engines not so designed, (i.e. most of 
them) you won't.

-- 
Steve Walker

Malc wrote:


> Yes. AFAIK super unleaded contains better 


Anyone else read that as 'butter'?

Willy Eckerslyke (oss108no_spam@bangor.ac.uk) gurgled happily, sounding 
much like they were saying :


>> Yes. AFAIK super unleaded contains better 

> Anyone else read that as 'butter'?


I can't believe I didn't.

> ps my ca is not getting the mfrs claimed top speed, although I have
covered
> 6000miles.


That is because your speed is determined by the position of the accelerator
pedal, not on the number indicated by the odometer.

Christian.

>> Yes. AFAIK super unleaded contains better antiknock ingredients not
>> better petrol. It won't burn better.
>


Excuse the ignorance, but what exactly is knock?

Tom

"Tom Burton"  wrote in message 
news:VSXVe.6358$st1.1015@newsfe3-gui.ntli.net...

>
>>> Yes. AFAIK super unleaded contains better antiknock ingredients not
>>> better petrol. It won't burn better.
>>
>
> Excuse the ignorance, but what exactly is knock?
>
> Tom
>


Spark plug ignites petrol while piston still on way up on compression 
stroke. Noise is your big ends having a hard time. Ignition too far 
advanced.
Cars with knock sensors adjust the timing so the engine is *just* on the 
point of knocking, so always giving max power for a given set of conditions.

Slurp wrote:

> 
> "Tom Burton"  wrote in message
> news:VSXVe.6358$st1.1015@newsfe3-gui.ntli.net...
> >
> >>> Yes. AFAIK super unleaded contains better antiknock ingredients not
> >>> better petrol. It won't burn better.
> >>
> >
> > Excuse the ignorance, but what exactly is knock?
> >
> > Tom
> >
> 
> Spark plug ignites petrol while piston still on way up on compression
> stroke. Noise is your big ends having a hard time. Ignition too far
> advanced.
> Cars with knock sensors adjust the timing so the engine is *just* on the
> point of knocking, so always giving max power for a given set of conditions.


I thought that 'knock' is when the mixture self ignites before the spark plug
gives off the spark.

> > Cars with knock sensors adjust the timing so the engine is *just* on the
> > point of knocking, so always giving max power for a given set of
conditions.
>
> I thought that 'knock' is when the mixture self ignites before the spark
plug
> gives off the spark.


I'd just call that a cause of knocking.

Knocking is when the mixture ignites so early that it causes significant
amounts of the energy to push down on the cylinder when it is still on the
compression stroke.

The cause could be:

(a) Lower octane fuel than required. Low octane fuel burns more rapidly, so
can push back on the piston when a high octane fuel hasn't really started
burning yet, giving the piston time to get to the top.

(b) Bad ignition timing. The spark occurs too early. For maximum efficiency,
the spark is timed to go off before the piston hits the top, as it takes
some time for good quality fuel to start expanding, and you want it to do
this before the piston has disappeared back down the cylinder. The aim is to
light it as early as possible, but not to actually knock.

(c) Pre-ignition. The mixture explodes without a spark, possibly due to hot
carbon deposits.

Christian.

The message <43284e6c$0$1306$ed2619ec@ptn-nntp-reader02.plus.net>
from "Slurp"  contains these words:


> Spark plug ignites petrol while piston still on way up on compression 
> stroke.


That always happens. Knock is when the fuel goes BANG instead of burning
smoothly.

-- 
Skipweasel.
In the beginning was the word.
And the word was Aardvark.

The message 
from Johannes  contains these words:


> I thought that 'knock' is when the mixture self ignites before the
> spark plug
> gives off the spark.


No, that's preignition, or Peter's Pinking as it's known.

-- 
Skipweasel.
In the beginning was the word.
And the word was Aardvark.

On Wed, 14 Sep 2005 16:05:09 GMT, Tom Burton wrote:


>>> Yes. AFAIK super unleaded contains better antiknock ingredients not
>>> better petrol. It won't burn better.
>>
> 
> Excuse the ignorance, but what exactly is knock?
> 
> Tom


<looks round for Peter....>

It's detonation of the mixture in the cylinder rather than a burn of the
mixture. Tends to knock holes in things. The American name for it is
pinging which is a far better description of what it sounds like IMHO.

Can be caused by an excessivly hot engine, an excessivly advanced spark,
too high a compression ratio or too low a grade of fuel (octane number)

"Knock" is something of a mixture of all the things people have said
:-)

In normal combustion, the spark plug lights the mixture and the flame
burns smoothly out from there to the edges of the cylinder. This gives
a nice smooth release of heat and increase of pressure.

In "knocking" combustion, the rising temperature and pressure due to
the flame causes some of the mixture to ignite spontaneously, BEFORE
the flame gets to it. As a result you have a sudden very rapid release
of energy, which in turn causes a very rapid increase in temperature
and pressure. It is this sudden pressure and temperature rise that can
cause damage to pistons, head gaskets, etc.

"Knock" is much more likely to occur when the ignition is more
advanced, since this causes higher pressure and temperatures. If you
light the mixture late enough then the descending piston causes the
pressure to fall at the same time as the flame would tend to cause it
to rise, thus lowering peak temperatures. But the efficiency is
generally less (less work output per unit of fuel burnt) which is why
you need to advance the spark as much as you can without causing knock.

I would have to disagree with Christian's Point A ("Low octane fuel
burns more rapidly") - my understanding is that the burn rate is
similar with all fuels (at least in the "normal" octane range), it's
just that high octane petrol is less likely to spontaneously ignite
before the flame arrives.


To get back to the OP's question: you might get better MPG with super
unleaded, but the benefit will be tiny. Most of the time you are
driving at part throttle, where knock is very unlikely and so the
ignition is set optimum with both 95 and 98 RON fuel. You are however
likely (though certainly not guaranteed) to get more power with 98 RON.

www.fuelsaving.info wrote:


> You are however
> likely (though certainly not guaranteed) to get more power with 98 RON.


If the timing isn't adjusted where does the extra power come from?

Because on most modern engines the ignition timing IS adjusted when
using different octane fuel, thanks to the knock sensor.

The ignition may be optimised for 98 RON, in which case it will always
be heavily retarded by the knock sensor when running 95 RON. Or it may
be optimised for 95 RON, in which case you will probably still see some
change as there will be a little bit of knock and subsequent retard
with 95 RON, which would be eliminated if using 98 RON

"www.fuelsaving.info"  wrote in message 
news:1126722086.983138.36040@g43g2000cwa.googlegroups.com...

> >
> I would have to disagree with Christian's Point A ("Low octane fuel
> burns more rapidly") - my understanding is that the burn rate is
> similar with all fuels (at least in the "normal" octane range), it's
> just that high octane petrol is less likely to spontaneously ignite
> before the flame arrives.
>
>

That was my understanding too, like in the old days with four star and two 
star. It wasn't the burny bits of petrol that differ, it's the additives 
that prevent pre-ignition that differ, shirley.

-- 
Malc

"Your mother can't climb stairs"
Dalek playground taunt

Guy King wrote:

> 
> The message 
> from Johannes  contains these words:
> 
> > I thought that 'knock' is when the mixture self ignites before the
> > spark plug
> > gives off the spark.
> 
> No, that's preignition, or Peter's Pinking as it's known.


I zeee. I thought that was the same thing. Some performance cars have
knock sensors for detecting pre-ignition.

The message 
from Johannes  contains these words:


> I zeee. I thought that was the same thing. Some performance cars have
> knock sensors for detecting pre-ignition.


No, they have knock sensors for detecting knock!

-- 
Skipweasel.
In the beginning was the word.
And the word was Aardvark.

www.fuelsaving.info wrote:

> Because on most modern engines the ignition timing IS adjusted when
> using different octane fuel, thanks to the knock sensor.


That's what I was asking.

IME no anti knock sensor, no performance gain.

So what you say confirms that.

"Malc"  wrote in message
news:5S%Ve.108781$G8.108401@text.news.blueyonder.co.uk...

>
> "www.fuelsaving.info"  wrote in message
> news:1126722086.983138.36040@g43g2000cwa.googlegroups.com...
> > >
> > I would have to disagree with Christian's Point A ("Low octane fuel
> > burns more rapidly") - my understanding is that the burn rate is
> > similar with all fuels (at least in the "normal" octane range), it's
> > just that high octane petrol is less likely to spontaneously ignite
> > before the flame arrives.
> >
> >
> That was my understanding too, like in the old days with four star and two
> star. It wasn't the burny bits of petrol that differ, it's the additives
> that prevent pre-ignition that differ, shirley.
>


No that's not real true. It's the combination of petrol compounds. More
simple chain hydrocarbons have lower octane values than branched chains and
aromatic rings. They're all "burny bits of petrol" but some burn better than
others. High octane fuel has more aromatics etc in it.

Guy King wrote:

> 
> The message 
> from Johannes  contains these words:
> 
> > I zeee. I thought that was the same thing. Some performance cars have
> > knock sensors for detecting pre-ignition.
> 
> No, they have knock sensors for detecting knock!
> 
> --
> Skipweasel.
> In the beginning was the word.
> And the word was Aardvark.


From a reliable book source: "If the compression ratio is raised too much,
the mixture, instead of burning smoothly, tends to go off with a violent
and damaging bang, emitting a noise known as pinking." 

Hence we're both partly right and partly wrong. Pinking and knock is indeed
the same phenomenon, but the ignition method is not the issue.

"Malc"  wrote in message
news:1126692665.934921.11820@z14g2000cwz.googlegroups.com...

>
> Avanti wrote:
> > I have been running my car on unleaded, however it can be run on
> > superunleaded, now I know it's more expensive, though i'm fantasy
bargaining
> > that there may be an increase in performance dictating the need to use
less
> > thorottle and hence more mpg.
> > Am i just dreaming?
>
> Yes. AFAIK super unleaded contains better antiknock ingredients not
> better petrol. It won't burn better.
>
> > ps my ca is not getting the mfrs claimed top speed, although I have
covered
> > 6000miles.
>
> And this is surprising because?


Not so much surprising, I thought the vehicle would have covered enough
miles to be considered run in.
NOt too much an issue as the top speed is more than twice the national
limit, so I doubt I would be doing that very often....

The message <LEfWe.69491$2n6.45109@fe3.news.blueyonder.co.uk>
from "Avanti"  contains these words:


> > > ps my ca is not getting the mfrs claimed top speed, although I have
> covered
> > > 6000miles.
> >
> > And this is surprising because?

> Not so much surprising, I thought the vehicle would have covered enough
> miles to be considered run in.


I think it was the failure to reach the makers claimed top speed which
was the issue, not the state of running in.

I don't know if cars are the same, but certainly bike magazines used to
test bikes and I don't think I ever saw one actually reach more than 95%
of the claimed max.

-- 
Skipweasel.
In the beginning was the word.
And the word was Aardvark.

On Thu, 15 Sep 2005 06:12:39 +0000 (UTC), neutron wrote:


> No that's not real true. It's the combination of petrol compounds. More
> simple chain hydrocarbons have lower octane values than branched chains and
> aromatic rings. They're all "burny bits of petrol" but some burn better than
> others. High octane fuel has more aromatics etc in it.


Yep - 5 star used to have quite significant amounts of benzene in for
example, 2 star had none (apart from the odd trace)

"Avanti"  wrote in message 
news:LEfWe.69491$2n6.45109@fe3.news.blueyonder.co.uk...

>
>>>
>> And this is surprising because?
>
> Not so much surprising, I thought the vehicle would have covered enough
> miles to be considered run in.
> NOt too much an issue as the top speed is more than twice the national
> limit, so I doubt I would be doing that very often....
>
>

Wot Guy said. I would be surprised if most cars actually achieved the 
makers' claimed top speed. Bearing in mind that they would test things like 
that in fairly specialised conditions.

-- 
Malc