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Leaded Gas, Additives, and Octane Numbers — What Does Your 356 Need?

September 27, 2010 | Troubleshooting & Repair

By Steve Douglas, David Jones, Alan Klingen, and Pat Tobin

Editor: "Back in the day", when our cars were new, all American gas was leaded, and the oil companies touted its benefits, such as reduced engine valve wear. Then we all woke up to the fact that lead was a toxic substance and one way to reduce human exposure was to get it out of gasoline! But don't our vintage engines still need leaded, high octane gasoline? In short: NO. Here's why.

Steve Douglas, A coupe original owner: Lead will keep the valve seats from wearing out, but only on engines that don't have hardened seats. 356s have aluminum heads and so have steel seats that are already hardened. Old American engines with cast iron heads that did not have pressed in seats need lead to cushion the valves, keeping wear down. I don't think any 356 needs leaded gas nor any additive, just run the best grade of gas you can, adjust the valves regularly, keep the mixture a little rich and timing advance conservative. Exceptions might be some of the real early pre-As with soft seats, but a modern valve job adds the hard seats.

David Jones, petroleum engineer: There is absolutely no point in running a lead substitute in the fuel for a 356.

Regarding octane: if you have a normal engine then 87 octane gas and a max advance of 35° BTDC as originally specified will probably return the best performance. 87 RON octane now is the same or better than 91 ROZ/RON then. (* see footnote below for more about octane)

Check your Driver's Manual and you will see a recommended octane value for your car. That number is expressed as a ROZ value which was the European octane rating system. Today in the USA gas at the pump is labeled in RDON values. The 356C Driver's Manual states a requirement of 96 ROZ for an SC, which means that 93 RDON ("premium" gas in most parts of the USA except for California where it is 91 RDON) is more than it needs. [Editor: the 356A Driver's Manual states the fuel requirement is a minimum of 86 ROZ. I run my 356A Super engine, which is completely stock, on 87 RDON without any problems. Modified high compression 356 engines over 9:1 CR will require premium gas.]

Therefore, for a 356 or any other normally aspirated engine with a carburetor there is no point in using an octane rating that is higher than the engine can use. The required octane is that which will not generate detonation at the furthest point of advance of the ignition timing under the greatest load.

This of course can depend on the actions of the driver to a great extent. If one were to try to accelerate at 1000 rpm up a steep hill in 4th gear in a 1600 normal then it is entirely possible that one would hear the engine knocking in protest. There is a difference in BTU content between regular and premium gasoline but really not enough to make a significant difference. Just remember that for an SC engine the recommended octane rating in 1964 was 96 RON which in today's terms would equate to about 92 octane. The "A" engine required 86 octane which nowadays would equate to 82 octane so regular gasoline is way to good for a 1600 normal.

If you have a big bore kit and a high compression ratio it is still unlikely that you need more than 93 octane unless you habitually try to accelerate from low rpm in a high gear. Keep the revs over 2500 when accelerating and you should not experience any knocking.

Remember also that in modern engines with computer controlled ignition and fuel injection system that knocking is eliminated by the engine management system no matter how stupid the driver. In a 356 you are the engine management system. High performance engines run ignition timing up to a point called "incipient knock" which means that the ignition is advanced to the point where the knock sensor recognizes the onset of knocking then retards the timing to prevent it. One can run over 40 degrees of advance and get more power at higher revs from a 356 engine but the consequences of getting it wrong at 7000 rpm would be disastrous. My advice is "don't try it".

Alan Klingen, The Stable, San Francisco: The problem with the lead substitutes is that it is not lead so its effects are minimal. If you are not having a problem with pinging or run on stay with un-leaded. The fuels today are extremely good and in fact produce less build up on the valves. They also have additives that keep the guides happy. I have seen more trouble with the additives from the point of plug fouling, most of these plugs come out looking like they are rusty, I think this called magnesium build-up. Maganese I think is the common substitute for lead. Don't worry about modern fuels, they do the job well, just stick to good brands. In some respects they are better than fuels in the past as they run cleaner.

Pat Tobin: Lead or lead substitute is helpful only in engines which have cast iron (or possibly soft steel) valve seats. Without lead or equivalent, the iron microscopically welds to the valve face under conditions of sufficient heat and pressure. Then when the valve opens it yanks away a few molecules of the seat. That results in valve recession; the valve sinks slowly into the disappearing seat. Cast iron valve seats are found mainly in older liquid-cooled engines which have cast iron heads, as many do. In many engines the valve seat was just machined into the base material of the head. Chrysler is an exception – in at least some of their engines they used insert seats of a different material in the cast iron heads. Our cars, and all with non-cast iron heads, must use insert-type valve seats. In the 356 they have been hard steel since '60; brass(or bronze) prior to that. When the heads are off the older engines, the valve seats should be replaced with hard steel. That is not difficult, nor particulary expensive, for a shop that specializes in cylinder heads.

* David Jones: The octane number you see on the pump is the average of the two measurements made at the refinery when the gas is manufactured. Those numbers are measured on two variable compression engines called Knock engines. The two engines are run at different speeds and the two octane numbers expressed are the Motor Octane Number (MON) and the Research Octane Number (RON) The derived number is known as the Road Octane Number (RDON) The recipe for gasoline changes constantly with the price of crude and the price of the various components used to come up with the highest octane for the lowest cost. Additives are mixed in to the gas at the fuel terminals by the end retailer and vary only minimally.