Subject: Re: 2009 A4 TSI

Mark Rosenkrantz speedracer.mark at
Thu Jun 5 10:12:09 PDT 2014

Mike, often gas comes out of the same tank.  This is true.  The additive
package is added to each section of fuel, making up the different brands.
Sometimes this colors the fuel as well.  So although the same large tanks
are used, the gas at the pump is different because of the additives.

Mark Rosenkrantz
Typos sent from my droid
On Jun 5, 2014 1:01 PM, "Mike Arman" <Armanmik at> wrote:

>  Subject: Re: 2009 A4 TSI
>> As far as I know, the energy content (BTU) is the same regardless of the
>> grade of gasoline used.  The difference comes in the octane rating which
>> measures the gasoline's resistance to ignition.  The reason premium is
>> almost required for turbo engines is because the cylinder pressures
>> generated by the forced induction increase the likelihood of pre-ignition,
>> where the fuel ignites before the piston is in the power stroke.  This is
>> also known as knocking and is very bad for the engine.  When knocking is
>> detected, ignition timing is retarded (pulled) so the spark doesn't occur
>> until later in the combustion cycle. This leads to more wasted fuel,
>> poorer
>> fuel economy, and less power because the gasoline doesn't have a chance to
>> burn completely. When using good quality fuel, the ignition timing can be
>> advanced a little, where the spark occurs sooner in the combustion cycle.
>>  The fuel can then be burnt more completely, allowing for more power and
>> increased fuel economy. Most modern engines are sensitive enough that they
>> can detect a higher-octane fuel and adjust ignition timing accordingly.
>> I may have this completely backwards.... feel free to correct me if I do.
> Actually, you have it almost completely forwards ;-)
> The difference between regular and premium (high octane) gas is that high
> octane gas has additives which suppress pre-ignition. One of them is
> Tetra-Ethyl-Lead or TEL, a/k/a lead based chemical compounds.
> When the fuel-air mixture is compressed on the compression stroke, it gets
> hot. If it gets hot enough, it will fire off by itself without waiting for
> the spark. This is pre-ignition, and it breaks engines, sometimes
> spectacularly.
> The higher the compression ratio and/or the hotter the incoming fuel-air
> mixture the greater the tendency towards destructive pre-ignition.
> High compression ratios are used to get more horsepower out of a given
> displacement engine. High octane gasolines let us use high compression
> ratios (to get more horsepower) without risk of engine damage. There is NO
> more "power" in high octane gasoline than in regular - I can fill my riding
> mower with 100 octane avgas and it won't mow my lawn so much as one second
> faster.
> Turbocharged engines are worse, and need high octane gasoline even more.
> When the turbo compresses the fuel-air mixture (to jam more into the
> cylinder) it gets hot. This is a double whammy - the incoming fuel-air
> mixture is already hot, and now we compress it again and even more in the
> cylinder - instant pre-ignition. That's why we use an intercooler to lower
> the temperature of the fuel-air mixture as much as possible before we stuff
> it into the cylinders, and that's also why turbo engines usually run LOWER
> compression than the equivalent non-turbo engines (as well as using
> different cam profiles, usually with less overlap to prevent the compressed
> fuel-air mixture from blowing right out through the still-open exhaust
> valve). They have to, just to survive.
> Engines are equipped with knock sensors to to tell the CPU to retard the
> ignition timing slightly. They run right on the ragged edge of
> pre-ignition, and retarding the timing even a few degrees helps a lot. You
> will notice that knock tends to occur when you are going fast or open the
> throttle under load, such as going up a hill. The engine is now working
> harder (to make more HP) and making more HEAT, which is what causes the
> pre-ignition (knocking). Remember that a significant portion of the energy
> in the fuel is wasted as heat, and the more fuel you use to make
> horsepower, the more heat you'll also make.
> There are a number of ways electronic trickery can be used to run regular
> gasoline with very high compressions. One of them was the "stratified
> charge" system, which could run regular gas at compressions as high as 11
> to 1 (and I think I heard it worked up to 14 to 1). This was done for fuel
> economy, not horsepower, and allowed the engine to use very little fuel at
> small throttle openings. The newer Direct Fuel Injection systems can also
> do this.
> Does an engine run better on high test than regular? Maybe. Maybe not.
> Cheap regular is sometimes claimed not to have various engine cleaning
> additives, but this may be marketing hype. I do know that at Port
> Everglades, the fuel for everyone's brand (including the no-names) comes
> right out of the same giant tank . . .
> On engines that require high test, sometimes the timing gets retarded to
> stop the pre-ignition, and the CPU isn't smart enough to try again later
> and re-advance the timing to see if the knocking has stopped. As a result,
> the engine is running with the timing retarded somewhat, and won't make
> optimum power.
> There's also the proven "it cost more so it must be better" syndrome at
> work here plus a good dose of FUD factor. Nobody likes to admit they're
> wasting money, and it is not likely that many people have done deliberate
> A/B tests under properly controlled conditions.
> Basically, if you have a high compression engine which requires high
> octane (92 octane or more) fuel, that's what you should be running. If your
> engine is rated to run on 87 octane regular, running high test is a waste
> of money.
> Best Regards,
> Mike Arman
> 90V8Q, and almost 50 years of fussing with gasoline engines and collecting
> and reading engineering literature on them as well.
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