[ByronRACE]

The objective is to light the mixture in a particular crank angle range to make it possible for the engine to operate efficiently and avoid problems. This range varies depending on many factors, and fuel octane is a large one. By efficiently, I mean more than just peak power, but that is what most people are primarily concerned about. The other factors are exhaust and engine heat, economy, emissions, and other lessor factors. By avoiding problems, I mean avoiding carbon fouling from having to run excessively rich mixtures to combat detonation, avoiding excessive engine heat, avoiding having to retard the timing a bunch...and still having an engine that is detonation prone.

Generally speaking, the earlier you light the mixture, the more complete the combustion process will be. This creates efficiency. The limiting factor for how early you can light the mixture is fuel type and burn rate (octane), compression ratio, engine geometry and rpm (how fast the piston is moving), valve timing (cam profile), etc. When you go too far, detonation and engine damage results.

So, in your case octane is fixed or decreasing; 91 pump gas probably to decrease in octane over time, as history has shown. The next largest factor is compression ratio. The goal is to pick a compression ratio that will allow you to light the mixture at a reasonable crank angle to make an efficient running engine. Generally speaking, the higher the compression ratio you choose, the later you have to light the spark.

11:1 CR on 91 Octane CA gas is not going to put you in the middle of the efficiency range. You'll be lighting the mixture late to avoid detonation. Translate that to "Deg BTDC" and I'd be willing to bet that you would be timing this engine in the 24-28deg BTDC range through peak torque and running at mixtures around 12:1 to make it durable. It will probably detonate at 30-32 total on mixtures around 12.5:1, and will likely give you fits if you pick up a tank of substandard gas.

Build the same engine 10.3:1 would probably put you in the 32-34deg total range and your optimal mixture will be in the 12.7-13.3 range. Detonation will set in at 36-38 total. Much more efficient, much more durable.

If this was my own build, I'd build it 9.8:1 CR and run 34-36deg total. Why? 10rwhp isn't worth the chance of detonation. Our fuel burns so fast, the performance penalty is almost nil and you end up with a more durable engine over-all.

I'll close by giving you an example of what a low compresson engine can do on our 91 fuel. I had a customer show up with a 8:1 CR 347" small block that was destined for a blower build. He decided to drive the car sans blower for a few months to get break-in miles, so he brought it to the dyno and I tuned it for him. It made 338rwhp in a 5spd mustang. The timing advance was around 34deg at peak torque and around 38deg advance at peak power. Normal for a 10.3:1 347 (I've tuned over 100 of them) would be about 320hp to the tires and about 28deg at peak torque and 30 at peak power. In this particular case, the low compression engine made MORE power than average once tuned. More interesting than this is what it did at higher advance values and leaner mixtures. It NEVER detonated, it just stopped making more power. I took it all the way to 44deg total advance and 13.5:1 and at that point, power started to drop off. Considering how easy it is to detonate a performance engine on CA 91 fuel, I found this remarkable considering the power number. Later, we copied this build for a road-race customer and the results repeated. That engine has been the most reliable road race engine the guy has ever run.

edit: comments on Mass-Flo EFI

Unless things have changed recently, the Mass-Flo branded system is OEM Ford EEC-IV EFI with a GM MAF sensor and a frequency-to-voltage converter box. The only thing I dislike about the system is the placement of the air meter; not the best location for a sample-tube based laminar-flow meter. Accuracy suffers. They've calibrated the processor to do a decent job of masking the shortcomings of having the meter in that location, but you may still experience some off-idle behavior that is less than desireable. Not a show-stopper, just something to be aware of. The Ford EEC-IV is quite capable...I'm running a 5.0 Mustang box, 160lb/hr injectors, and a blow-thru univer meter on the blower motor. The only limitation to be aware of is that the processor can't handle more than 7500rpm reliably without extensive modification. I doubt this is an issue for you. As far as drivability is concerned, the algorithms in the Ford EEC-IV are still superior to most aftermarket EFI systems...just harder to tune.