Premium fuel

The question of the day is why do people put premium fuel into there car. Unfortunately for 85% of the people out there that use premium, the reason is stupidity. Being a mechanic I know the reasons for using premium and I can also tell you that in a shop full of mechanics there is only one or two that know anything about octane ratings.

So here’s the octane lesson of the day.  Premium fuel is required for higher performance engines that require it. The reason that they require it is due to the higher compression ratio that was built into the engine.

The thing that happens in a higher performance engine is the air and fuel is compressed into a smaller area in the compression stroke of the four stroke cycle. With the same amount of heat inside a smaller area you get a resulting higher temperature in the compressed mixture. This happens around the time that ignition (spark occurs at the spark plug) happens; then the problems occur. What happens is the pressure increases due to the rapid expansion of the burning compressed air/fuel mixture cause the pressure and temperature to rise quickly. The temperature in the cylinder gets higher than the self ignition temperature of the fuel and the fuel self ignites. Here’s where a picture helps lots but I don’t really have one, imagine a flame front (a nice wall of blue flame) propagating away from the spark plug, now imagine a second flame that has started on the other side of the combustion chamber traveling towards the first flame. Well as the books explain the two flame fronts collide they make a knock (detonation) and this is bad for the engine, bad to the point of melting holes in pistons. Personally I don’t buy the flame front collision thing, I think it gets so crazy in the combustion chamber that everything starts to self ignite at the same time, it makes the same clack that a diesel makes when it’s timing is too far advanced.  Sorry if I’m straying from the topic.  The solution for this problem is a fuel with a higher octane rating, the deal is the octane raises the self ignition temperature of the fuel and it also slows the burn rate of the fuel; this causes the fuel to burn slower. It’s the same idea as the type of  powder that you put behind a magnum bullet should burn slower so that the barrel pressures don’t get too high.

So that being said, lets say you put premium fuel into your economy car. Without the added compression you won’t have any chance of detonation (well unless you have a dangerously lean mixture or excessively advanced spark timing). So things work the same way as they would normally except that after the ignition occurs the fuel burns slightly slower than the engine was designed for which results in lower power output and reduced fuel economy. Nobody seems to believe this but you will get reduced power if you burn premium fuel in a regular engine. And don’t bother asking your mechanic, unless he is into the tech of engines or builds race cars he won’t know how it all works anyway.

Ok so now you may think well I have a high performance engine I need premium fuel, well my advice is to find out what your compression ratio is and ask someone who knows. I rebuilt a 5.0 Liter mustang  engine that had a full giver race cam oversized valves, ARP everything, fully rollered rocker arms, ported heads, higher flowing intake, oversized throttle plate, shorty headers, aftermarket pistons.  Do you think it needs premium fuel? Well it had stock heads and the pistons were the same style as the old ones, so the compression ratio was stock which the book says is 9.0 to 1 I measures it and I thought this one was 8.5 to one but either way it made around 330 horse power and regular fuel is what it is recommended to burn. Now for my 2.3 liter turbocharged motor with 7.5 to one compression that from the factory is to have 6 PSI of boost but I have it turned up to 16 PSI, the owners manual says to use premium fuel I use regular, it has a bit of detonation at 3500 rpm for a quarter of a second when at full boost and wide open throttle but meh, I haven’t melted a spark plug yet and I’m going to continue to run regular fuel.

Now lets say you have a 4.6L mustang with 9.6 to one compression (sorry all I think about is Ford) you still don’t need anything more than regular.