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The Datsun 1200 was designed to run on "regular" grade of petrol (gasoline). Only the GX "high gas" version requires higher octane. The GX "low gas" engine runs on regular grade.
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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)
Is Unleaded Fuel OK?
Sure is. A12 engines are designed for unleaded fuel. Here's what the 1973 Export owners manual says:The first USA model to require unleaded was the 1976 B210 california model, but before that unleaded was recommended -- three years earlier!
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
Tetraethyl lead was developed by General Motors, and added to gasoline starting in 1924 ago for two reasons:
- It improves the anti-knock quality of gasoline
- 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 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.
Octane Ratings: RON and MON
RON vs MON ratings of BP Petrol (Australia)
| Fuel type | RON | MON | R+M/2* |
| Leaded petrol (Super) | 96 | 86 | 91 |
| Regular Unleaded Petrol | 91 | 82 | 86 |
| Premium Unleaded Petrol | 95 | 82 | 88 |
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 wikipedia:Octane rating.
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 modified 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 (for example, shaved head)
- If the head is significantly shaved, or using a very thin head gasket, either of which wil raise compression
- Supercharged engines
- Turbocharged engine
- 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 normal. However, loud or extended 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 (160 or 180 degrees) rather than a 212 or 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. 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.
Pure ethanol or methanol is a form of alcohol. To run pure 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, except maybe the new-millennium B140 trucks.
However, at 10%, experience has shown no ill effects. It may even help lower emissions. Fuel economony changes are miniscule 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 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
- 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-performane 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
Do I need LRP Additives?
Lead replacment additives come built-in to LRP (Lead Replacement Petrol). However, if you are using ULP or PULP, with comes without lead or lead replacement and your engine requires it, you may with to use the additives.
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.
Ethanol
See E85