I'm doing some spreadsheet work while deciding which way to go for my build-up. One of the things I'm doing is comparing a big NA combo to a FI setup. My goals are quite stringent: must run like stock, idle like stock, burn 93 octane, etc. Must not require octane boosters, and I strongly prefer stock or better fuel economy. As we're all aware, the things that make more power tend to have adverse effects on these requirements.

So, as part of doing this I needed a way of predicting (and I realize that's all this is, an estimate) the octane requirements of various combinations of compression ratio, boost, timing, and air-fuel ratio. I found "Jim Bell's Supercharged/Turbocharged Performance Guide" on Kenne-Bell's site. It contains empirical relationships between all these factors. I needed to get this into a formula so that I could play what-if games in a spreadsheet.

Here are the relationships of interest, per Jim Bell:

1 point CR = 3-5 octane (I used 5 in my formula, to be conservative)

1 psi boost = 1-1.5 octane (I used 1.5 in my formula, to be conservative)

1 AF ratio = 2 octane

1 degree advance = 0.5 -0.75 octane (I used 0.75 in my formula, to be conservative)

In formula form:

Needed octane = 93
+ ((Modified CR - Stock CR) * 5)
+ (Boost PSI * 1.5)
+ ((Modified AFR - Stock AFR) * 2)
+ ((Modified Timing - Stock Timing) * 0.75)

Ok, applying that to an LS-1, let's make these assumptions:

Stock LS-1:

10.13 SCR (per my calculation)

23 degrees advance (average, per an experienced tuner I know)

13.2 AFR (factory tunes all over the map, but this is a number most stockers can pull safely on, again per an experienced tuner I know)

For any of these assumed numbers, if you feel another number is more correct, use your version in the formula.

Putting all the above together, we get:

Needed octane = 93
+ ((Modified CR - 10.13) * 5)
+ (Boost PSI * 1.5)
+ ((Modified AFR - 13.2) * 2)
+ ((Modified Timing - 23) * 0.75)

For a typical example let's look at a 7PSI blower on a stock engine that needs to run on 93 octane. We will richen it and pull timing to compensate for the added octane requirements or the boost. If we run 11.5 AFR and 14 degrees timing, that gets us there:

93 + ((10.13 - 10.13) * 5) + (7 * 1.5) + ((11.5 - 13.2) * 2) + ((14 - 23) * 0.75) = 92.77

Again, this is a rough estimate for comparison purposes and it leaves out factors like relative humidity, air temp, coolant temp, intercooling, water/meth injection, etc., etc. Once a real engine is built it will want what it wants. Engines don't read formulas. Still, so far I am finding this useful for playing what-if games.

Now, the question for the much-more-experienced folks on this forum: Does the above look reasonable to you? Does it look useful, or am I kidding myself?

Also, if I have made some bonehead math mistake, please point it out!

 

Moved from advanced tech. More eyes in here.