Oldtee

Let me see if I understand this. Today I filled the truck with regular at $3.25 a gal. The 928 was topped off at $3.45 a gal. Mid grade here is $3.35. I would never use less than mid grade, even if I were to consider it. The 10 cents a gal difference can be cut too, say, .075 becasue of slightly better gas mileage. In 15 gallons that is $1.12 a tank. Over say 10,000 miles we are talking about 45 tanks of gas for a savings of $50 a year. So, I give up one meal out with a girl a year. Small price to pay to treat the 928 to the style she prefers.

ROG100

What a waste - I have been running my 82 Weissach on Premium.
So if I understand what Rich is saying I can run it on regular gas with no problems.
Have I understood this correctly??????

I am running my 93 GTS on mid range gas (not regular and not premium) without any problems.

DANdeMAN

This post was to answer my own question of "what is RON 91" in my previous post. I was not sure what grade RON 91 was. Also I wanted to clarify what grade is recommended in the workshop manual, wich is RON 91. Accordingg to this RON 91 here in Canda and US is classified as Premium. The reason for all this, is that it was mention that we could run our cars on regular gas for MY 77 to 84 here in north America.

Now another member says that our RON 87 is realy RON 91 in Europe but the manuel says euro car uses RON 97 an US/CDN car uses RON 91 unleaded. In light of all that info, I tend to believe that the Workshop manual is right on the button with the RON 91 for US, wich is Premium gas. So I will "feed" my shark Premium RON 91 gazoline until proof to the contrary..

EDIT : The data in my post #9 indicates RON 98 for euro car not RON 97.

Rich9928p

There are different fuel standards.

Europe uses RON

The US uses (RON + MON)/2

So, if you have specs in RON they don't directly translate to the US standard!

more info here:

http://www.faqs.org/faqs/autos/gasoline-faq/part3/

6.5 What does the Motor Octane rating measure?

The conditions of the Motor method represent severe, sustained high speed,
high load driving. For most hydrocarbon fuels, including those with either
lead or oxygenates, the motor octane number (MON) will be lower than the
research octane number (RON).

Test Engine conditions Motor Octane
Test Method ASTM D2700-92 [104]
Engine Cooperative Fuels Research ( CFR )
Engine RPM 900 RPM
Intake air temperature 38 C
Intake air humidity 3.56 - 7.12 g H2O / kg dry air
Intake mixture temperature 149 C
Coolant temperature 100 C
Oil Temperature 57 C
Ignition Advance - variable Varies with compression ratio
( eg 14 - 26 degrees BTDC )
Carburettor Venturi 14.3 mm

6.6 What does the Research Octane rating measure?

The Research method settings represent typical mild driving, without
consistent heavy loads on the engine.

Test Engine conditions Research Octane
Test Method ASTM D2699-92 [105]
Engine Cooperative Fuels Research ( CFR )
Engine RPM 600 RPM
Intake air temperature Varies with barometric pressure
( eg 88kPa = 19.4C, 101.6kPa = 52.2C )
Intake air humidity 3.56 - 7.12 g H2O / kg dry air
Intake mixture temperature Not specified
Coolant temperature 100 C
Oil Temperature 57 C
Ignition Advance - fixed 13 degrees BTDC
Carburettor Venturi Set according to engine altitude
( eg 0-500m=14.3mm, 500-1000m=15.1mm )

6.7 Why is the difference called "sensitivity"?

RON - MON = Sensitivity. Because the two test methods use different test
conditions, especially the intake mixture temperatures and engine speeds,
then a fuel that is sensitive to changes in operating conditions will have
a larger difference between the two rating methods. Modern fuels typically
have sensitivities around 10. The US 87 (RON+MON)/2 unleaded gasoline is
recommended to have a 82+ MON, thus preventing very high sensitivity fuels
[39]. Recent changes in European gasolines has caused concern, as high
sensitivity unleaded fuels have been found that fail to meet the 85 MON
requirement of the EN228 European gasoline specification [106].

6.8 What sort of engine is used to rate fuels?

Automotive octane ratings are determined in a special single-cylinder engine
with a variable compression ratio ( CR 4:1 to 18:1 ) known as a Cooperative
Fuels Research ( CFR ) engine. The cylinder bore is 82.5mm, the stroke is
114.3mm, giving a displacement of 612 cm3. The piston has four compression
rings, and one oil control ring. The intake valve is shrouded. The head and
cylinder are one piece, and can be moved up and down to obtain the desired
compression ratio. The engines have a special four-bowl carburettor that
can adjust individual bowl air-fuel ratios. This facilitates rapid switching
between reference fuels and samples. A magnetorestrictive detonation sensor
in the combustion chamber measures the rapid changes in combustion chamber pressure caused by knock, and the amplified signal is measured on a
"knockmeter" with a 0-100 scale [104,105]. A complete Octane Rating engine
system costs about $200,000 with all the services installed. Only one
company manufactures these engines, the Waukesha Engine Division of Dresser Industries, Waukesha. WI 53186.

6.9 How is the Octane rating determined?

To rate a fuel, the engine is set to an appropriate compression ratio that
will produce a knock of about 50 on the knockmeter for the sample when the
air-fuel ratio is adjusted on the carburettor bowl to obtain maximum knock.
Normal heptane and iso-octane are known as primary reference fuels. Two
blends of these are made, one that is one octane number above the expected
rating, and another that is one octane number below the expected rating.
These are placed in different bowls, and are also rated with each air-fuel
ratio being adjusted for maximum knock. The higher octane reference fuel
should produce a reading around 30-40, and the lower reference fuel should
produce a reading of 60-70. The sample is again tested, and if it does not
fit between the reference fuels, further reference fuels are prepared, and
the engine readjusted to obtain the required knock. The actual fuel rating
is interpolated from the knockmeter readings [104,105].

6.10 What is the Octane Distribution of the fuel?

The combination of vehicle and engine can result in specific requirements
for octane that depend on the fuel. If the octane is distributed differently
throughout the boiling range of a fuel, then engines can knock on one brand
of 87 (RON+MON)/2, but not on another brand. This "octane distribution" is
especially important when sudden changes in load occur, such as high load,
full throttle, acceleration. The fuel can segregate in the manifold, with
the very volatile fraction reaching the combustion chamber first and, if
that fraction is deficient in octane, then knock will occur until the less
volatile, higher octane fractions arrive [27,28].

Some fuel specifications include delta RONs, to ensure octane distribution
throughout the fuel boiling range was consistent. Octane distribution was
seldom a problem with the alkyl lead compounds, as the tetra methyl lead
and tetra ethyl lead octane volatility profiles were well characterised, but
it can be a major problem for the new, reformulated, low aromatic gasolines,
as MTBE boils at 55C, whereas ethanol boils at 78C. Drivers have discovered
that an 87 (RON+MON)/2 from one brand has to be substituted with an 89
(RON+MON)/2 of another, and that is because of the combination of their
driving style, engine design, vehicle mass, fuel octane distribution, fuel
volatility, and the octane-enhancers used.

Rich9928p

I checked all the service manuals and dug up the compression ratios and the factory recommended fuels. In 1978 and 1993 both RON and MON standards were included, so I provided the calculated results. Also, in 1984 CLC was used, where CLC is the same as (RON + MON)/2. So, using official Porshce numbers I can safely state that US 87 octane fuel can be used in 1987 to 1984 US SPEC 928s!!

I can also safely infer that RON = 91 is comparible to CLC 87. The NON-US 1985 - 1986 M28.45/46 928s also are spec'd to run on Regular grade unleaded fuel. However the US spec 1985 - 1986 M28.43/33 928s are spec'd for RON 96/MON 86, or [(RON + MON)/2] = 91] which is what most US premium grade is today.

I also found that Australian spec S4s were rated to run on RON 91 that corresponds with US Regular grade fuel.* Sooooooo, if you change the coding plug as shown in the attached scan, you can safely run your S4 (at least up to 1988 models) on regular grade US fuel.

One thing that is clear, the GTS ONLY can run on Premium fuel. The highest CLC fuel that I can find in my area is 91 octane Premium fuel, below Porsche's Min spec of 93 octane! So I expect that my spark knock detectors are kept busy on hot AZ days.

1978 RON/MON 91/84 = (RON + MON)/2 = 87.5, 8.5 : 1 compression

1980 RON 91 Lead Free 9.0 : 1 compression

1984 CLC 87 Lead Free 9.3 : 1 compression

1985 RON 96 Lead Free 10.0 : 1 compression

1987 RON 95 Lead Free 10.0 : 1 compression >>> Australia RON 91

1988 RON 95/MON 85 = (RON + MON)/2 = 90, 10.0 : 1 compression

1993 RON/MON 98/88 (RON + MON)/2 = 93, 10.4 : 1 compression

* the 1988 Service manual states "The ignition curve family of the control unit in cars for Australia is changed with the help of a special coding element, so that the engines can be operated on regular grade fuel with 91 RON/82 MON. Engine power is then 221 kW/300 HP." (a loss of about 20 DIN HP).

Another note, lower octane fuel is OK for operation at higher altitudes. Fore example, when traveling you will notice that the fuels in Denver are several octane points lower than in the flatlands. That is OK, because there is less air densitiy, compression is lower, so lower octane fuel is used.

BrianG

I run regular (87 R+M/2) Canadian gasoline in my Japan-spec '87 S4 and have had no signs of pinging. It also runs just a slight bit quicker than my '86. US-spec car, which I have always run on regular fuel, without the timing jumper connected, and with no pinging.

Rich9928p

Note the addition of the resistor.

DANdeMAN

Thanks for the detailed info, I will try it on my next fill....

911Dave

Dandeman, the US uses RON+MON/2 instead of RON. To get the equivalent pump octane in the U.S., subtract 4 from the RON number. In other words, RON 91 means 87 octane in the U.S. Yep, unfortunately all this time you have been wasting your money unless you have modified your compression or timing from stock.

Also take note of the above post that talks about the lower octane requirements at higher altitudes. The owner's manual rating for our cars is based on low altitude. The octane requirement goes even lower as you go higher in altitude.

ROG100

Hi Rich,
Thanks for the detailed clarification.
I will be using regular gas for my 82 Weissach from now on.

As for the GTS - There are three of us in TX using 89 Octane gas without issues and two are getting very good mileage. I don't as I have a lead foot.

Roger

m21sniper

ROG100- there's some power to be had if you boost ignition timing and run on 93.

'Free power', as it were.

Just figured i'd point that out.

I run about 50 degrees of total advance...it's 93 or up for me.

928SS

guess there's no point in stealing gas from another 928 if I need premium.... hehehe....

Rich9928p

From the 1987 Service Tech manual: In case of knocking combustion the control unit will retard the ignition timing of the concerned cylinder by 3 degrees. If there is continued knocking combustion, the timing is retarded in steps of 3 degrees for each knock up to a maximum of 9 degrees when engine speed is above 3500 RPM. Timing is retarded with smaller values accordingly when speed is below 3500 RPM. If knocking combustion is no longer detected, the ignition timing sill be advanced in small stepts to its optimal value.

a. for Model years '87 - 89'

I think that there would be a benefit of using the "coding plug" to tell the EZK that lower octane fuel is being used. The EZK will not use as aggressive of spark advance so there will be less knocking in the first place.

b. for Model year '90 - '91
There is no longer any mention of the coding plug for Australia and a listing of RON 91, but the coding plug is still used. So perhaps by that time Australia no longer was limited to RON 91 unleaded fuel, but a coding plug change would still provide safe operation with RON 91 fuel.

The minimum octane is listed as RON 95/MON 85, so (RON+MON)/2 = 90 octane US fuel, which MAY be mid-grade fuel in your local area.

c. for GTS models

The 1992 GTS service manual states this in the Fuel Grade section:
The engine is designed to achieve optimum performance and fuel consumption when run on unleaded premium fuel with a minimum octane number of RON 98/MON 88.

If unleaded premium grade fuel with a minimum octane number of RON 95/MON 85 is used, the ignition is automatically adjudsted by the knock control of the engine.

>>> So, with the GTS you're OK with (RON+MON)/2 = 90 octane US fuel, which MAY be mid-grade fuel in your local area.

Mark

I just double checked the gas door on my 85 EURO.

It calls for 98 RON!

Rich9928p

I was very careful to specify US 928s in my threads. The earlier Euro S motors had high compression and without knock sensors it is much more dodgy running lower octane fuel.

The 1980 - 1983 Euro S used the transistorized ignition with vacuum and mechanical spark advance, so the only compensation would be to manually change the base spark timing which affects the whole advance scheme.

However, from Euro and ROW 1984 - 1986 (EZF spark control) there is the jumper for selecting lower octane fuel. If a Euro owner was to plug in the jumper and go for lower octane fuel I'd keep my ears open and listen for knocking.

I would hope that gas companies post the MINIMUM octane for their fuels and that in reality they actually come in a little higher. Perhaps a renlister who works in the petrolium industry could speak to the accuracy of their fuel specs.