Total Performance Solutions

We’ve covered in the past how race gas affects not only your spark advance but also the readings on a wideband and ultimately the calibration in your vehicle. In this article I want to show you an example of this on the dyno and then show you how to calculate the change in fueling if you know the stoich of the fuel you are using.

I get quite a few questions about what each race fuel will do to your calibration and how it will affect things. Before I give you the stoich for various race gas mixtures, I can tell you that generally speaking leaded fuel is going to be closer to pump gas than unleaded race fuel will be. When you start mixing unleaded race gas in your car tuned for pump gas you want to be extremely careful. The amount that you get leaned out might make the difference in a burned piston or not.

The green line at the bottom of the graph is a car run on the dyno with 93 and then run with a mix of 104 and some spark changes (street tune vs race tune). Notice how given no fueling changes the AFR shifted leaner.

Here’s how to figure out what a fuel will do for you. You first must know which fuel you were tuned on. We’ll work these ratios in AFR instead of lambda since that is what most DIY guys are familiar with. For the following examples we’ll assume you were tuned on non-Ethanol fuel at a 14.64:1 stoich.

If you empty your tank of this pump gas and load up some Sunoco 104 (GT Plus Unleaded – I run into this one a lot), what happens to the 11.8:1 AFR you were seeing going down the track? It jumps well into the mid 12s! Our two fuels in question have a 14.64:1 stoich and a 13.7:1 stoich – you can see the 13.7:1 stoich needs much more fuel per air than 14.64. About 7% more (original fuel divided by new fuel OR in this example 14.64 divided by 13.7). You could raise your wide open fueling up about 7% and compensate.

Now what if you’re mixing fuel – this is what happens for most of my customers that make a track visit in their track vehicle. Say you drive in with a quarter tank of 14.64:1 and want to mix in another quarter tank of 13.7:1 fuel. How much fuel should you add to compensate?

This is simple math – you’ve got half of each fuel, so (13.7/2) + (14.64/2) = 14.17. We can then use the formula above of old divided by new to get 14.64/14.17 or a little over 3%.

This isn’t all to the story – different fuels will make optimum power at different mixes, but this should give you a good overall understanding of what is going on.

I generally recommend my customers mix C16 at the track for their race tunes – it raises octane and the brief time you run it at the track will not hose your O2 sensors. C16 is easy to get and because of its close stoich to pump gas you do not have to worry about mixing ratios.

Without further adu, here is the list of stoichometric ratios I have compiled from talking to vendors, various literature on the fuels and other resources.

Pump Gas Non-ethanol: ~14.64:1
Pump Gas E10: ~14.08:1
Turbo Blue Unleaded (100 octane): 13.9:1
Turbo Blue Unleaded Plus (104 octane): 13.7:1
Turbo Blue 110: 14.7:1
Turbo Blue Advantage: 14.9:1
Turbo Blue Extreme: 15.0:1
Sunoco MO2X UL: 14.5:1
Sunoco 260 GTX: 14.4:1
Sunoco 260 GT: 13.9:1
Sunoco 260 GT Plus: 13.7:1
Sunoco Standard: 14.8:1
Sunoco MO2X: 14.5:1
Sunoco HCR Plus: 14.8:1
Sunoco MaxNOS: 14.9:1
Sunoco Supreme Leaded: 14.95:1
Sunoco Maximal Leaded: 15.01:1
VP Street Blaze 100: 14.16:1
VP C10: 14.53:1
VP 110: 15.09:1
VP C16: 14.77:1
VP C12: 15.00:1
VP C23: 14.93:1
VP C44: 12.87:1
VP MS103: 14.26:1
VP MS109: 13.41:1
VP Import: 14.15:1

When we left off in Part 1 we were discussing where our best torque would be made. We determined there was a spark advance that could be found for a particular engine combination that would make the best torque. We’ll delve a bit further now into the factors affecting how closely we can run with this optimum advance. I could not possibly go into the full extent of this discussion (see my earlier posts on some great textbooks to read for more detail) but I will give you some things to think on and by the end you should have a great understanding of our results and be able to begin thinking creatively how this could apply in other situations and on your own vehicle.

Let’s take a look at what can increase our power production. We have all heard that an engine is a glorified air pump, you improve how well it can get air in and out and you make more torque. The proper setups can even use the inhale and exhale of the engine to cause a ram and sucking effect to move air in and out of the cylinder. In our datalogging we can measure the inflow of air and measure the “volumetric efficiency” of the motor to see how well (or not) we are doing at pumping (how close to 100% of a cylinder full of air are you?). This also comes in handy when you are modding a car, you can judge very quickly if the CAI you just added actually got more air into the engine or if your butt dyno just needs a recalibration. I do this with every airflow mod I dyno test, it is a great way to judge what you have done alongside dyno numbers.

If you decide you are tired of trying to improve the pump and want to instead ram air into the motor you can step up to a compressor (pick your poison, blower, turbo, centri) or nitrous which really is just cold, dense burnable oxygen. This now gets us well past 100% volumetric efficiency. Our 281 cubic inch motor in this example is now taking in the air that a 400 cubic inch motor would normally ingest.

This newfound airflow comes at a cost, we have to expend some energy to spin the compressor. Some compressors are more efficient than others but they all waste some sort of energy and as it relates to our discussion, they all create heat.

So you hear supercharged guys rattling on and on about how high their air temps are, especially down here in Louisiana during the summer. But what is it about heat that makes it so bad? To understand how big of a part this plays, let’s look at some factory Ford programming. We’ll consider the 03/04 Cobra platform at wide open throttle since it is what we are getting to in this discussion (in a round about way I know, but you need to understand the principles in play to interpret the data and the reason why we get the results we do).

On these vehicles from the factory once you cross over the 100 *F mark on your air temps, timing is pulled. For example at 150 *F you are down about 4* of spark. Now on the dyno we can say that every degree of spark is somewhere around 8-10rwhp just to give it a round number. So this is big news. You can understand why those boosted guys now whine about hot air temps and heatsoak, it matters.

On a side note, this is also where those little eBay mileage saver deals come from – they fake out the ACT sensor and report lower air temps and the PCM commands more timing/less fuel or some combination thereof. Some actually “work”, I’ve tested it on the dyno. You can begin to appreciate how this hack could lead you to a situation where the motor will knock. We are much better off actually calibrating the vehicle than just lying to it. Like the old adage goes, “garbage in, garbage out”.

Stay with me, so we understand now that spark advance plays into power. We understand that there is a spark limit of each engine combination, at some point additional advance will not help you. At this unknown point we are firing too early to reach optimum mechanical advantage (torque).

We now know that heat plays into not allowing us to run at this optimal spark advance, but how? As heat is added into our combustion event, our fuel becomes increasingly likely to pre-ignite.  This is where your intercoolers can make more power than non-intercooled car. As the temperature of the incoming air rises, we have to pull spark to keep us out of knocking territory. Water-methanol injection (I’ll do an in-depth article on this if there is interest) has a two fold benefit of both taking the heat out of the air and also increasing the octane of the fuel. There are also downsides but that is another subject for another time.

Now the results.

93 Octane Dyno – 24* advance

Here’s the first pull – normally you will tune a combo like this to lower 20* spark on a safe street tune. 24* is as far as I got before I lost power on 93 octane.

116 Octane Dyno – 24* advance (overlaid on previous chart)

Drained the tank, loaded in C16, corrected for change in stoich and pulled again with similar engine/air temps. Roughly the same – slight gain at the tail end – possibly seeing the beginnings of detonation up top over the 93 but otherwise identical.

116 Octane Dyno – 26* (overlaid on previous chart)

Power drops off here. As you now know, we found that MBT at WOT at higher RPMs was roughly at 24* on this particular combo.

93 Octane Dyno 24* vs 116 Octane Dyno 26*

Here’s the “money shot” on 93 vs C16 in this instance at 24 degrees and 26 degrees respectively. We also left the car on the dyno and ran the engine cold the next morning with minimal warmup time and observed no gain over 93 at 24*.

Conclusion, in this case we are not limited in power by the fuel we use. This is the exception to the rule and also why I thought it a good subject of this article. On almost all engines we are limited by the octane of the fuel that we have available. On this Cobra we were able to reach the point in which we can make no more power even with fancy fuel, in fact adding spark advance even in the absence of knock lost us power.

Now is this all to say C16 is pointless? Absolutely not. In fact even without the gain here, we pick up a good deal of safety running 24* timing and C16 should we be hot lapping the car, having a heatsoaked intercooler etc.

So where do you go from here to take advantage of the C16? Given proper internals we increase cylinder pressure while keeping IATs down (MORE BOOST!!!) and attempt to maintain our peak pressure at the optimum degree of crankshaft rotation, thus keeping torque high, tires spinning and the competition in our taillights.

Thanks for reading – if you have any questions or comments I would love to hear them. Feel free to drop me a line – wes@tpsperformance.com.