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[Datsun 1200 encyclopedia]

Fuel Requirements

(Redirected from Octane)

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Categories: Fuel System | Maintenance And Daily Care

The Datsun 1200 was designed to run on unleaded "regular" grade of petrol (gasoline). Even the GX "low gas" engine runs on regular grade. Only the GX "high gas" version requires higher octane.


Octane Requirements

For a factory stock A12 engine, what does the Owners Manual say?

  • 1971 Export Coupe (9.0:1 compression): "gasoline above 90 octane"
  • 1972 Export (8.5:1 compression): "gasoline above 91 octane"
  • 1973 Export (8.5:1 compression): "minimum octane rating of 87 ... average of Research and Motor Octane Numbers"

Note that 1973's "87 Octane" is the same as 1972's "91 Octane". See below for more details on how the rating of the same fuel changed over the years.

Summary: the least expensive gasoline (87 R+M/2, or 90 RON equivalent) is just fine for a Datsun 1200.

1972 USA Owners manual (Page 22 ... ):

29357.jpg   29362.jpg
87 R+M/2, or 90 RON equivalent

Is Unleaded Fuel OK?

Sure is. A-seried engines have valve seat inserts and A12 engines starting from mid-1972 are produced specifically for unleaded fuel.

Japan-Market Lead-free Transition (~1972)
from engine number
A12 613858 Sunny (B110) High-octane
A12 638894 Sunny (B110) regular
A12 808954 Cherry (E10)
A10 061133 Cherry (E10)  
A10 828286 Cherry cab (C20)

What about 1970-1971 A12s and the earlier A10s? We have no reports of problems running unleaded fuel. Probably the early valve seats, although not designed for unleaded, were of a high quality.

Here's what the 1973 Export owners manual says:

Nissan Motors recommends the use of no-lead of low-lead (0 to 0.5 grams per gallon) to minimize emissions. 1st October 1972 Printed in Japan Publication No. OM3E-B110U1
The first USA model to require unleaded was the 1976 B210 california model, but before that unleaded was recommended -- three years earlier! This is due to the catalytic converter (lead will ruin the catalyst).

Tetraethyl lead was developed by General Motors, and added to gasoline starting in 1924 ago for two reasons:

  1. It improves the anti-knock quality of gasoline
  2. It was very inexpensive, compared to alternatives like alcohol blend

Also, as a side-effect it lubricates and protects the valve seats and valves

Lead is not needed for high-octane gasoline. Modern unleaded fuel uses other, potentially safer, additives to boost the octane rating.

To protect against valve wear, A12s and newer A-series engines use a special alloy valve seat inserts and special alloy valve guides. It is fine for unleaded gasoline. Most engines of that period used cast iron heads without separate seats or guides. Those other engines need lead, or lead-replacement additives, but not so with the Datsun 1200. Those Nissan engineers were clever fellows.

For more information on lead use in gasoline, see Overview: Leaded Gasoline History.

Which Fuel Should I run? Regular, Premium or E85?

  • For stock A12/A14/A15 engine, run Regular Unleaded gasoline
  • For A-engines up to 11:1 compression, run Premium Unleaded
  • For super-high compression (12:1 to 14:1) use either racing gasoline or E85

Racing gasoline is not legal for use on the highway, but E85 is. So E85 is the way to go for ultra-high-compression or high-boost turbo street engines.

For compression ratios up to 11:1 -- and naturally aspirated engines (non-turbo) -- Premium Unleaded is preferred over E85:

  • No special rejetting or hoses are required for Premium
  • Premium is available everywhere, E85 not available in some states

For turbo or supercharged engines, E85 is the way to go due to the cooling properties of alcohol. You can use more boost with E85.

Octane Ratings: RON and MON


RON vs MON ratings of BP Petrol (Australia)
Fuel typeRONMONR+M/2*
Regular Unleaded Petrol918286
Premium Unleaded Petrol958288
Leaded petrol (Super)968691
data source: BP Amoco chart from Chevrolet Australia
*estimated by ddgonzal (who created this table)

RON is Reseach Octane Number, the rating first used in 1929 when gasoline was first tested for anti-knock properties in a special one-cylinder test setup.

100 was assigned as the anti-knock property of a certain 100% octane-blend fuel. Gasoline, a blend of different fuels (benzene, octane, etc), commonly has a rating in the 80s and 90s using certain additives. However, some additives can increase anti-knock even more than the refernce fuel, hence racing and aviation gasoline with ratings over 100 exist. For more details, see Octane rating on Wikipedia.

Later as automotive engines become increasing more performance oriented, with compressions ratios no longer in ratios of about 5:1 common in 1930s vehicles, and commonly at 8.5:1 or higher in the 1960s, it was realized that the RON rating was not quite indicative of the anti-knock achieved in real cars. So the MON was developed. This is for engines under higher loads than the RON test.

In the USA, the average of RON + MON ratings was eventually decided upon for commercial retail automotive gasoline. So now you see the label on the gas pump: "87 octane (R+M/2 method)".

RON+MON average is sometimes called the Anti-Knock Index (AKI), Road Octane Number (RdON), Pump Octane Number (PON), or (R+M)/2.

Is High-Octane Fuel OK?

Sure, premium gasoline is OK, it won't destroy your engine. But you will be wasting your money, unless your engine is tuned to take advantage of high-octane gasoline.

Stock engines

Stock A12 engines, built to the factory specifications do not need high octane fuel. They are designed for "regular" gasoline (petrol) and should not knock (ping/pink) if tuned correctly. In fact you may get worse fuel economy or higher emissions with premium high octane fuel.

You can use high-octane fuel:
- To mask tuneup problems

Generally speaking here are the reasons to use a higher octane fuel:

  • High compression engine. If you are using the factory A12 GX twin carb engine, it has a 10:1 compression ratio. It needs high octane fuel
  • If you are using high compression pistons
  • If you are using high compression heads (shaved head)
  • If the head is using a very thin Tomei or Nismo head gasket
  • Supercharged engines
  • Turbocharged engine
  • Racing engines subject to high heat and extended full-throttle running

My engine "pings"--should I buy higher octane fuel?

Whether you need high-octane fuel to stop an engine pinging (knocking, pinking) depends:

  • If your engine is stock, no. Instead you should tune it
  • If you engine is modified, Premium fuel may be needed

A very small amount of ping, usually under part throttle high load (e.g. going up hills) is fairly common, and setting the timing will fix it. Pinging can result in piston or valve damage, and so should not be allowed to continue. Do something about it!

If your stock engine pings, you can do this:

  • Adjust Ignition Timing to factory specification (7-10 degrees BTDC, usually)
  • Lower temperatures require less octane, so make sure you use a low temperature thermostat (180 degrees) rather than a 192 degree thermostat
  • Ensure the air/fuel (A/F) ratio is correct: Tune and inspect the Carburetor

The more expensive fuel has other benefits, right?

I've heard that premium fuel is better than regular, not just for anti-knock properties, but also it's just better, for example has better cleaners in it and won't clog my fuel system. So premium fuel is better even for my stock engine, right?

That's a common statement, but is not true generally speaking (at least not in the USA). Since 1994 the government has required all gasoline to include cleaners to protect fuel injectors. For example, Chevron, famous for its Techron cleaning additive, puts Techron in all grades of their fuel, even the least expensive regular gasoline.

It may be true that some dealers may have contaminated fuel, or so called "watered down gasoline" (cut, not with water, but presumably with some cheap hydrocarbon mix). So if you get "bad" gas, do business with another gas station rather than pay that station more of your dollars for thier "better" fuel.

But Premium Will Make more power, right?

Yes, high-octane fuel will make more power--if the stock engine is re-tuned to take advantage of it. Otherwise you may get slightly less power. High-octane fuel burns slower, so needs more ignition timing advance and will generate more heat. You may need to upgrade the stock Datsun 1200 radiator to cope with the increased heat. If your engine is tuned for regular gasoline, with Premium theoretically more heat goes out the exhaust. It won't hurt anything except your pocket book.

What about Alcohol in Gasoline, is that bad?

Is alcohol in gasoline bad for my Datsun 1200? Not really. Up to 10% alcohol is safe for Datsun 1200s with factory rubber parts. Cheap replacement hoses may have problems.

Pure ethanol (E100) or methanol is a form of alcohol. To run alcohol you'd need:

  • Carburetors re-jetted
  • fuel pump and lines uprated for about twice the fuel flow
  • Alcohol-resistant fuel lines and parts (seals, etc)

None of which is fitted to the Datsun 1200.

However, at 10%, experience has shown no ill effects. It helps lower emissions. Fuel economony changes are small at 10% mix. For a discussion, see: Question re: Australian petrol composition.

Glossary: What is LRP, PULP, etc?

  • REGULAR: leaded fuel, no longer available in US or Australia
  • ETHYL, the old nickname for premium lead fuel, for the ethyl lead additive. Trademark of the Ethyl Corporation, which now instead markets MMT
  • UNLEADED: Gasoline that used additives other than lead for anti-knock ability. Leaded gas for street use was outlawed after 1995 in USA and after 1999 in Australia.
  • ULP: Un-Leaded Petrol, introduced in 1985 and required for all new cars beginning at that time in Australia. For many years after this, both leaded and un-leaded fuel was available.
  • PULP: Premium Un-Leaded Petrol, higher octane grade of fuel
  • LRP: Lead Replacement Petrol, introduced in 2000 when leaded fuel was outlawed in Australia. Lead was replaced with different additives
  • AVSR: Anti-Valve-Seat Recession additive, the various ingredients added to Unleaded Petrol to make it compatible with engines designed for leaded fuels. Fuel with AVSR is called LRP
  • MON: Motor Octane Number, an octane rating for high-performance use
  • RON: Research Octane Number, the original laboratory rating for low-performance "theoretical" one-cylinder test setup
  • MMT: A popular lead-replacement additive. It is used in Canada and Australia, and also allowed in the US in small concentrations but under concerns about poison
  • BP Ultimate (99 RON octane with no-ethanol, equivalent to 93 USA octane)

Do I need LRP Additives?

No, Datsun 1200 engines do not need LRP.

Lead replacment additives are already in LRP (Lead Replacement Petrol). However, if using ULP or PULP, with comes without lead replacements -- and your engine requires it --- you may with to use the additives. However, this is not needed on the A12 or newer engines. They can run straight unleaded, and need no Lead Replacement additives.

With older cast iron heads, there were no valve seat inserts. The seats were cut directly into the "soft" cast iron, so needed tetraethyl lead lubricant to keep things right. With alloy heads which the A-series use, they already have all the hardened gear in them, so running unleaded fuel is no problem.


See E85


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