by Brad Williams

Tech Info Home



1. Get the right octane fuel for your engine.
2. Protect or replace your valve seats.



The main consideration for which fuel to use in your car is "what octane rating does my engine require?" This relates to compression ratio and valve/ignition timing settings. That is, if the engine is designed to take advantage of higher octane fuels by having a higher compression ratio and more advanced ignition timing, then the higher octane is probably a requirement, not an option. The main thing that the user can do to reduce the "pinging" that results from insufficient octane rating of a fuel is to retard ignition timing. (Things like a head spacer for reduced compression ratio or "dished pistons" are possible but are retrograde mods - why would you want to?) Or you could add octane booster to your fuel tank, but generally speaking (and subject to marketing anomalies that cause higher prices in some locations), oil companies can produce octane cheaper than the aftermarket additives - otherwise they would use them. You ought to be able to buy a higher octane fuel cheaper than adding aftermarket booster to achieve the same measured octane rating fuel. The only reason for using an octane booster that I can see is that you may need higher than 98 octane for your "hot" red six.

Valve Seat Recession

Once you have the right octane rating for your fuel, the ignition timing set up correctly and the carburetion adjusted for needs/taste, the next consideration is valve seat wear. On vehicles designed to use leaded fuel, the valve seat metallurgy is insufficiently hard, or robust, to tolerate the use of an unleaded fuel for extended periods. With leaded fuel, the combusted lead oxide was laid down on the combustion chamber and valve seats as a protective layer (that lovely light grey coating that tells you the mixture is OK). Taking the lead out of the fuel means this protection is lost, or depleted over time (hence some of the comments about a Holden head lasting a long time without such protection). So the oil companies looked for other metal compounds that provide similar protection (without the toxicity of lead) and there were several: manganese, phosphorous and potassium are the main ones. I'm not positive, but I think manganese is also the key ingredient in some aftermarket octane boosters. It works, but is not as economical as lead, which is why the oil companies didn't use it for 50 years - the dosage level required is much higher to achieve the same octane boost as lead gives.

The time is coming (maybe 3 - 5 years from now) when the oil companies will no longer provide LRP with AVSR (anti valve seat recession) additives, as they will be phased out. So, depending on how long you think your engine head will last without valve seat protection, how often you want to replace/recondition your head and how much you want to spend, there are three approaches you can take:

  1. Do nothing to protect the head and wait for the tell tale signs of burned exhaust valves*.
  2. Replace the valve seats in the head with appropriate metallurgy so that valve-seat recession ceases to be an issue.
  3. Use a fuel with additive, or an aftermarket method of delivering additive to the fuel system, to provide valve seat protection.

Obviously the first option is the cheapest in the short-term, but it will eventually force you into option 2. The second option is a more expensive one-off cost, whereas the third alternative you'll feel through your wallet every time you fill up. Your decision will depend on the value of your vehicle, its usage, how long you intend to keep it, and your economics. For my part, I lean towards getting the head fitted with hardened valve seats, leaving me free of additive packages and free to choose the octane fuel my car requires without that added complication.

More about Octane

It should be noted that having petrol with a higher than required octane rating for your engine does not necessarily mean that you will "blow it up". Higher octane fuels burn more progressively, rather than explosively. A high octane fuel can be compressed more without detonating, ignited earlier to reach full burn intensity at "top-of-stroke", burn progressively and push the piston steadily throughout the whole power stroke. A low octane fuel goes "BANG" once the spark is introduced. So, to increase the power output of an engine by increasing compression ratio, advancing the ignition to allow longer, steadier burn of the fuel, you need an appropriately behaving fuel, i.e. one with a higher octane rating. If you run a higher octane fuel in an older, less "tuned" engine, you probably just end up losing some of the unburned fuel to the exhaust system. The higher octane fuel is more calorific (has a higher energy content) and if it is all burned then it could increase operating temperatures and place extra load on the engine in that way. But higher octane does not mean more explosive, in fact it means quite the opposite.

Grey motors (with their lower compression ratio) were designed to run on "Standard" fuel, which had an octane level of 92 Research Octane Number (RON). For the red motors, the requirement was for 98 RON "Super" grade. As motors age and tolerances increase it may be that the requirement for octane is less rigid and you could probably get away with the 96 octane of PULP/LRP. However, older engines can also suffer from carbon deposits that increase the compression ratio by taking up combustion chamber space and actually increase the octane requirement because of the higher compression ratio and/or pre-ignition from glowing deposits. For a "tight" rebuild that's "hot", you should consider the 98 octane fuels.

During the 70’s - 90's, for various reasons, there was some rationalisation of octane grades in consultation with the car industry. In the 90’s this was partly to offset the cost of reducing lead content during the phase-out. As lead was reduced, more "raw" octane was required to meet the finished product octane rating (since the lead itself had an octane boosting effect). In the early 90's, the 98 octane rating of leaded super was 97. When PULP was introduced in 1986 the octane rating of that fuel was 96. When leaded super disappeared, the 97 octane rating of leaded petrol slipped to the 96 of LRP which replaced it. The old Standard leaded grade was 92 RON and this was replaced in Jan 1986 by the 91 RON Regular ULP grade.

In the next 3 to 5 years, the current regular grade (91 RON) is planned to disappear, the PULP grade will become the regular grade at 95 RON and the Ultimate/Optimax/Synergy 98 RON grades will become the only high octane option.

The quality of these fuels is superior to the old leaded fuels because the components used to increase the octane level (that the lead previously supplied) are inherently better. They do, however, have a heavier boiling range and can be trickier for cold starting. Also, they rely on the ignition and carburetion systems being in good condition. So when putting LRP into "ol' Bessie" with a clapped out (but forgiving) grey motor with a sloppy cam, stuffed distributor, burned out plugs and highly resistive ignition leads that used to run OK, she will probably run lousy and it will be "entirely the new fuel's fault"!

After the introduction of LRP in WA (and before it was introduced in NSW, Victoria and Queensland), there were problems with boiling range volatility with LRP containing the heavier components. This was addressed by introducing a new specification called drivability index…a measure of the "weighting" of the distillation curve of the tested fuel. The spec requires that the fuel have a certain weighting toward the “front-end” or lighter boiling components to ensure satisfactory performance in a well tuned and adjusted engine. This led to a marked improvement in the performance of the fuels in less well maintained engines.

Many people are unaware that PULP and LRP are in fact the same fuel. One has AVSR additive, and the other does not. It is supplied as 96 RON unleaded (PULP) to various ports around Australia, and the AVSR additive (and others) are added at the truck loading gantry at the terminals. An interesting exception to this is that two of the oil companies specify a minimum 95 octane rating for PULP as opposed to the other two which specify 96, even though the legislated minimum is 95 RON. All LRP is specified as 96 minimum. BP is a 96 octane supplier for PULP/LRP. In fact, in WA where Kwinana refinery supplies all the PULP/LRP into the WA market, even though two of the other companies only require 95 octane for their PULP, because they receive their LRP into the same tank or they are hosted at a terminal by a company that does require 96 octane PULP, all the PULP is supplied at 96 octane. This will not necessarily be the case for NSW, Victoria and Queensland because of the larger number of refineries and terminals and generally more complicated distribution arrangements. You can always ask your service station what the spec for their PULP/LRP is - they should know what they are selling!!


Further information and interesting reading on this subject can be found in the following BP Fuel News datasheet (MS Word format download), released in May 2002 by the BP Marketing Technical Services department.

This is the third in a series of articles covering technical info that is not found in the Workshop Manual, or covers common methods of modifying an FE or FC. Many thanks to Brad Williams (fcfromscratch) for giving us the benefit of his experience and knowledge.

Important Note: This document is intended as a guide for those persons interested in repairing or modifying their vehicle. The FE-FC Holden Car Clubs of Australia take no responsibility and accept no liability for the information contained herein. You must ensure that all work carried out and/or modifications made to your vehicle are legal in your state, and we recommend you contact an engineer or your local Traffic Authority for further information.


AVSR    Anti-Valve Seat Recession, an additive in LRP to compensate for the removal of lead (and its protective qualities).
LRP Lead Replacement Petrol, the lead-free alternative to the phased-out "Super", available at the bowser. Introduced in 2000.
MON Motor Octane Number, MON measures resistance of a fuel to knock at high speeds and high engine temperatures.
PULP Premium Unleaded Petrol, the higher octane unleaded fuel available at many petrol stations
RON Research Octane Number, RON measures resistance of a fuel to knock at lower engine speeds and under acceleration
ULP Unleaded Petrol, the lead-free petrol introduced in Australia in 1985 (and mandatory for all petrol driven vehicles produced since that time)

* Burned valves let hot exhaust gases from one cylinder in exhaust stroke leach back into the cylinders that are under negative pressure on the intake stroke, causing pre-ignition of the incoming fuel and flash-back into the inlet manifold, thereby upsetting the whole pressure balance of the carburetion system, and frequently flames out through the carb venturi/throat. All in all it doesn’t make for smooth running.

If you have a technical question about repairs or maintenance on your FE or FC, please post a question on our Discussion Forum.

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