Leaded Fuel vs. Unleaded
In his "Compleat Idiot" book John Muir admonishes us strongly to NEVER use unleaded fuel in a VW engine. With respects for his memory, Muir is completely wrong in this regard. Of course now we can't buy anything BUT unleaded fuel in the U.S. and the rest of the world has eliminated lead from fuels apart from one or two countries. Volkswagen has stated that all of their heads from about 1966 on have been made with hardened valves, seats, and valve guides. Even the pre-66 heads had hard enough seats to resist Valve Seat Recession for a long time. For this reason the aircooled VW engine does not need lead (or lead replacement) to protect the engine from Valve Seat Recession (VSR) and so the octane number is the only consideration. If your heads have been rebuilt, then the machinist probably used the good silicon-bronze or aluminum-bronze valve guides. Also, the valves need no rubber valves seals (as they point spring-end-slightly-down to drain excess oil off), and so get plenty of splash oil on the spring end, which is enough for the valve guides.
Unleaded fuel is just fine for the VW engine. The octane number of unleaded IS sufficient, provided you are using the "standard" VW compression. Rob usually uses mostly Shell unleaded, without any problems, but the formulation is different between different oil companies, and so it might be worth trying different brand of unleaded, just to test for any differences. Dave in the U.S. found that the cheap "supermarket" brand of fuel (like Woolworths sell here in Australia I guess) performed very badly in his VW, but Chevron of the same octane worked fine. The formulation is different between different oil companies, and so it might be worth trying different brand of unleaded of the same octane number, just to test for any differences.
There is a lot of confusion around the world about octane numbers. This has been caused mainly by the USA deciding to use a different measurement of octane numbers compared to the rest of the world, when the previous method worked just fine. There are two methods of measuring the octane number of any gasoline/petrol/l'essence/bensin/bensene (different names for the same product). The two methods of testing are called the Research Octane Number - RON, and the Motor Octane Number - MON. Both use the same test engine, but the MON method uses a harsher test regime which is closer to the on-road performance of the fuel, than the RON test. The "rest of the world" uses the original RON method. The MON is always lower than the RON, for the same fuel. The USA decided to use a combination of the two numbers and called it the ANTI Knock Index - AKI, sometimes called Pump Octane. So when talking about octane numbers, the SAME fuel will have a USA AKI about 4 digits lower than the rest of the world's RON. The USA has I believe 87, 89, 91 and 93 AKI Octane fuels. This equates to the rest of the world having 91, 93, 95 and 98 RON octane.
At a compression ratio of 7.5:1 you can use 91 Octane RON or 87 AKI. At 8:1 it needs 93 RON or 89 AKI, and at 8.5:1 you need 95 RON or 91 AKI.
Note: Compression ratios for some common engines are as follows: 6.0:1 for the 36hp 1200 engines, 6.6:1 for the 40hp 1200 engines, 7.3:1 for the 1300s, and 7.5:1 for most 1500 and 1600s outside the USA. In the USA, from 1971 onwards some engines were built with normal 7.5:1 compre4ssion ratios and some where built with lower comrpression ratios (7:1) using dished pistons - mainly AE series engines.
The Wisdom of Rob Boardman
Octane Numbers, Detonation and Lead in Gasoline
As the fuel is ignited, the flame front races across the cylinder, taking 2-3 thousandths of the second. The temperature and the pressure of the remaining as yet unburned mixture increases dramatically - the final combustion temperature reaches about 2000 degrees. So the as-yet unburned mixture is put under extreme heat and pressure, and WILL start to decompose into various sub molecular components as the flame front approaches. Some of these easily auto-ignite, and if this happens, you end up with the remaining mixture self-igniting before the flame front reaches it, resulting in a opposing super-rapid pressure wave in the cylinder. THIS is what produces the hammer blow to the piston instead of the nice steady push, and causes the distinctive harsh clicking/knocking sound of detonation.
Many folks mistakenly link detonation/pinging/knocking with PRE-IGNITION. The two are completely different. Pre-ignition occurs when the fuel lights BEFORE the spark, usually the result of hot spots in the cylinder like hot carbon deposits. Detonation occurs AFTER the spark, but before the flame front as finished racing across the piston top burning all the fuel/air mixture. Occasionally pre-ignition can also cause detonation, but once again, detonation occurs AFTER the fuel has been ignited, but before all the fuel/air mixture is burned by the flame front racing across the piston top.
Earlky in the 1900s, engine designers did knot know if detaionation was caused by the engine design, the igntion process or during the fuel burn. Combustion specialist Harry Ricardo (later Sir Harry) in the UK was fairly sure it occured DURING the combustion process using his test engine (the very first in the world) during tests in 1918.
This was confirmed by Thomas Midgley in the USA in 1921 when he used a test engine to look at the problem of engine-knock. The test engine had a window in the cylinder head through which they could take high speed film of the combustion process.
Early in the 1900s, manufacturers were limited in the compression ratio they could design in their engines by the occurance of KNOCK (detonation), and were desperate to find a way of increasing compression ratios to improve the power in their engines.
Various additives interfere with the decomposition process in the still unburned mixture, and allows the flame front to race across the cylinder normally to produce a fast but smooth burning process. Thomas Midgely first found in 1921 that adding 15-20% ethanol was a very effective way of eliminating detonation - yes, ethanol fuels have been around for almost 100 years! Unfortunately ethanol-added gasoline could not be patented, so he looked for other compounds which COULD be patented (think PROFIT) and found that TEL (Tetra Ethyl Lead) was the most effective compound, and had the added advantage that in old cast-iron head engines, it helped protect the valves seats and valve guides from excessive wear called Valve Seat Recession (VSR) - The added TEL prolonged cylinder head life. But he had to add two bromide/clorine based scavengers to the mix too - to stop lead building up in the heads and exhaust ports - so ALL the lead (and bromine and clorine) used was pumped out into the atmosphere.
So TEL was not the ONLY nasty chemical used. It was a mixture of TEL 61.45%, 1,2 Dibromoethane 17,85%, and 1,2 Dicolroethane 18.8%. The amount of Bromine and Clorine compound needed was called 1 "theory" and that mix above actually has about 1.5 "theory" in it to ensure good scavenging of the lead after combustion. Aviation gasoline 100LL (low lead) is only permitted one "theory" of scavengers, but is still allowed to have 0.56 grams per litre of TEL to this day. As at 2016, approximately 100 tons of lead is released into the atmosphere world-wide annually through burning aviation gasoline.
From the point of discovering TEL, Midgley (sponsored in his research by GM) ignored his findings about ethanol and concentrated on TEL (remember that there is PROFIT in TEL), despite many scientists at the time questioning the use of lead compounds being burned and poured into the atmosphere from the increasing number of vehicles on the road. There was, in fact, a continuing cover-up and misdirection campaign about the effects of lead poisoning for the next 50 years or so. Midgely himself at one point poured TEL over his hands then sniffed a jar of it to demonstrate that it was "safe". Almost immediately after that, he had to take a long holiday in Miami to recover from a bout of lead poisoning! Even though there were at least 17 deaths directly associated with the first production of TEL by the "Ethyl" company (Midgley was a shareholder), the misinformation campaign continued and profits soared. Eventually, cities like Los Angeles HAD to do something about the smog being created, and eventually the USA Clean AIR Act sparked the decline of TEL laced gasoline, because it ruined the catalytic converters fitted to new cars. The rest of the world agreed and the use of lead steadily declined, with health studies showing that the redcution of lead in the blood of people was declining and health problems associated with lead poisoning slowly improved.
As an aside, Midgely and his boss Charles Kettering (who invented the electric starter motor and the points-and-coil ignition system), went on to develop Freon as a refridgerant gas. This too has now been outlawed, and both have been included on lists of "the 50 worst inventions ever made".
In the USA, as lead was reduced then elminiated from gasoline, MTBE (Methyl Tertiary-Butyl Ether) and other oxygenates TAME ad ETBE were tried . MTBE was used for a time, but it made the gasoline smell like stale turpentine and had a very deletorious effect on underground water tables when it leaked from storage tanks, so was eventually stopped. These oxygenates did not offer any valve seat protection for older cast iron head engines either.
And that's where the current use of Ethanol fuels comes into the picture. You dont HAVE to have ethanol in gasoline/petrol these days - good refining techniques can increase the proportion of hydrocarbons with high natural octane numbers, but it does encourage clean burning, and the USA had politican and Agricultural lobbying, which resulted in ethanol being added to almost all fuels, and most developed countries have at least one blend of gasoline/petrol at the pumps containing around 10% ethanol. As and example, Australia has 4 main greade of petrol (gasoline), only one of which contains 10% ethanol - the other grade are straight hydrocarbons. Most modern cars with engine computers can adjust for 10% ethanol or ethanol free fuels, but our old VWs can not - so we have to make any adjustements needed manually.
Ignition Advance and Distributors
And dont be caught by the argument that high octane fuels burn slower and low octane fuels burn faster. This is not true, and the actual flame speed has nothing to do with preventing detonation.
The only thing which changes the flame speed significantly after ignition is the presence of burnt exhaust products from the previous power stroke - a higher concentration of burned gases slows the flame speed, and a low concentration increases the flame speed.
Let's look at the engine design. I'll use a 1600cc engine for the numbers here. If the VW engine was 100% efficient in filling a cylinder with fresh mixture, each cylinder would have 396cc of mixture (1584cc in 4 cylinders). But the cylinder head volume is 53cc, so even with 100% intake efficiency, there will still be 53cc of burnt mixture from the previous cycle mixed into the fresh charge, since the piston can't sweep out the head volume of burned gases. This is as fresh as the mixture can get. But the stock VW engine is only about 85% efficient at best in filling the cylinders (no car engine is 100% unless it's super/turbo charged) so that means instead of 396cc of mixture, the best you can do is the equivalent of about 336cc, still with 53cc of burned mixture. Now throttle the engine.... this will further reduce the amount of fresh mixture, whilst still leaving at least 53cc of burned mixture in there.
See what is happening? At various rpm (different airflow efficiencies) and different throttle settings, the proportion of burned/fresh mixture is changing enormously.
When the flame speed is high, we need less advance since it's taking less time to burn, and when the flame speed is low we need more advance. A high proportion of burned gases means the flame speed reduces so we need extra advance when that occurs. The aim is always to have all the fuel burnt at the point where the piston starts it's descent, so a maximum push is applied to the piston.
And when rpm is high we need more advance because the higher revving engine will move through more degrees of rotation every second, and we still want the fuel burnt by the time the piston starts back down. When rpm is low we need less advance. so we always need SOME advance - sometimes a little, sometimes a lot.
You can see that there are two competing events here - the proportion of fresh and burnt gases; and the different rpms which the engine operates at.
Fortunately, the vacuum distributor (and the combined centrifugal/advance of 74+ distributors) does a reasonable job of following these two conflicting needs. The vacuum port on solex carburettors is placed just UNDER the main venturi, close to where the throttle plate passes as it opens. So imagine the engine idling. The throttle plate is nearly closed, so there is a low airspeed through the main venturi - not much vacuum there or just under it (above the throttle plate). The vacuum port sees very little vacuum, and the idle advance setting prevails (7.5-10 degrees BTDC on most models). There is a high proportion of burnt gases mixed in with the small amount of fresh mixture, so some advance is needed, but the revs are low so we dont need a lot of spark advance.
Now open the throttle a little, so the edge of the throttle plate passes by the vacuum port. This creates a mini-venturi with very high air speed, which creates a lot of vacuum, so you get a shot of vacuum advance to help speed up the engine (this effect is entirely missing with the Bosch 009 distributor, which is what causes the "009 flat spot"). Since in a part-throttle condition you still have a high proportion of burned gases for a low flame speed, this high advance also meets the advance condition needed to deal with that too.
Now open the throttle right up. The throttle plate moves away from the vacuum port (no mini venturi) and so the MAIN venturi is providing the vacuum effect, but since the airspeed hasn't yet increased much yet (engine hasn't yet increased rpm), the vacuum signal is lower than part throttle, so the advance is reduced a little - just relying on engine rpm for those distributors having both vacuum and machanical advance. Perfect for a fresher mixture (lower proportion of burned gases with an open throttle needs slightly less advance remember?). Now the engine rpm starts to catch up with the open throttle, so the airspeed through the main venturi increases, vacuum increases, and the advance increases progressively, which is just what you want for the increasing rpm, since the crankshaft is rotating in less time so you need more advance to get that fresh charge completely burned at the right moment.
So the vacuum distributors allow for high advance at high rpm/open throttle, where rpm is the dominant factor; and also allows high advance at part throttle/medium rpm, where the proportions of burnt/fresh mixture is the predominant consideration. The SVDA distributor, with both vacuum and rpm advance, combines the effects, with up to around 30 degrees rpm related advance, but with an additional 10 degrees vacuum advance added when the engine needs it.
Therefore, the advance curve has less to do with the leaded/unleaded issue (fuel type); but rather it accounts for induction efficiency and rpm range. If you alter the efficiency of the engine (larger valves, different cam) your ideal advance curve changes too - nothing to do with the fuel you are using, so long as the octane rating is high enough to stop detonation.
But use a completely different type of fuel, and different arguments holds true - for example, LPGas or CNG is used as an alternative fuel in some countries, and they do have a lower flame speed than petrol/gasoline. So, if running LPG only, the spark can be advanced 3-4 degrees (and ideally the advance curve should be steeper too) to account for this. If using dual fuel (two fuel tanks and able to switch from LPG to petrol and back) though, you can only advance about 2 degrees or so, since when on petrol the total advance could be too high. Cars with ECUs are great in this situation - they can have a separate advance curve for LPG/CNG mapped into the ECU, and so you get an advance curve matched to either fuel. (I had an old Aussie built Ford Falcon with dual fuel, and have been run LPG cars for some years in the past, but since the Falcon doesn't have an ECU, the best I can do is 2 degrees additional advance).
Getting back to the gasoline/petrol though - Avgas has a very low volatility compared to road fuels (since you don't want the lighter fractions evapourating at low pressure as the aircraft climbs higher and causing bubbles in the fuel lines!), yet similar advance conditions apply. But in the aviation engine, a centrifugal advance system is often used since they operate at relatively constant rpm, quite unlike the car, and it's normal in aviation to slowly increase and decrease power, you dont "floor" an aviation piston engine, so a centrifugal-only type of distributor works quite well. When the VW engine is used as a generator or compressor (thousands were used in the 1960s), a centrifugal distributor is used because it's running at near constant rpm. So the engine design and use has more to do with the advance, rather than any variation in the liquid fuel.
Now you know why the 009 centrifugal-only distributor creates so many flat spot problems and increases your fuel consumption! It can't respond to the throttle movement (can't respond to the mixture of burnt and unburnt gases), and since it can't back-off the advance when you floor the throttle, it must be limited to around 30 degrees maximum advance to prevent detonation, where the vacuum and vacuum/centrifugal distributors can provide up to 40 degrees advance for economical cruising at part throttle and can reduce the total advance when floor it, until the rpms catch up with the throttle position.
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