I had the opportunity to do some playing around with one of our shop vehicles running on both 93 octane and then race gas, tuning as close to the edge as I wanted. It is your basic no-frills 04 Cobra setup, ported Eaton motor making about 400rwhp at 12psi on the MustangDyne. Runs a mid 11 second quartermile.
So with the stage set I first pushed the 93 octane tune as far as it would go. I drained the tank and filled up with race gas and pushed some more. Now when you hear “race gas” thrown around you immediately think huge power but is this always the case? What is it about race gas that makes power? Like pretty much everything in tuning there is more than meets the eye. Let me show you some of the high points and give you some tech and things to ponder that you might not find elsewhere.
There is a concept we deal with in Ford calibrations called MBT. This value represents the maximum spark advance that Ford engineers have determined through rigorous testing that will produce the best torque on the engine combination. There are then other spark tables that are derived at a certain temperature and octane. The spark advance is further modeled with adder and subtractor tables based off of engine temps, air temps and a good many more things. As the vehicle runs, the PCM constantly analyzes all these tables and does quite a lengthy math equation to arrive at which spark advance to run given the conditions. To further complicate things, the maximum spark that the engine can support changes as you modify the characteristics of the engine, fuels etc. It is a somewhat common tuning practice to make use of the data Ford acquired at set conditions to produce best torque and run the car at this spark advance rate. You can imagine that this might get you close to always making the best power all the time, but unless you are dealing in the controlled conditions Ford calibrators were, you are selling yourself short and taking a risky move.
So what are we doing when we add spark advance? In essence we are timing the moment we fire the spark plug to achieve a peak cylinder pressure at the optimum time that will give us the optimum mechanical advantage on the crank. This is physics 101, certain levers give you more “push”. We are doing just that. This mechanical advantage shows up as “torque” on our dyno graph. What we have working against us is a phenomenon called spark knock. The fuels we use have a tendency to ignite before the spark plug fires as the cylinder pressures and temperatures increase. When this happens the pressures in the cylinder double, triple or more – it is these rapid pressure waves that you can hear as pinging and this is how things break, shatter or otherwise not happen as we intended.
On the dyno we can steadily increase spark advance and maintain a consistent fueling and watch our torque increase. There comes a point where we continue to add spark but no more power can be found. This type of tuning at the bleeding edge is where our loaded MustangDyne dyno really shines, with accurate aerodynamic loading it makes this type of research and pushing (or simply finding) the limits a breeze.
So with these concepts in mind (and hopefully a LOT of unanswered questions if you are reading between the lines), what happens when we push the edge of 93 octane fuel and then load in race gas and push even harder? Did I blow it up? Did it live? Is it running 9’s? Read part 2 to find out!