I think something needs to be said here.... otherwise people are going to miss a VERY important point.
Some tuners are only one, or maybe two dimensional. They only tune one boost point. When you hear people saying things like "I'm running 18 degrees under boost"... they're probably a one dimensional tuner. Running 18 degrees timing at all intermediate boost and rpm points... is stupid. Sorry, it's just stupid, there's no other way to say it.
That is possibly why you hear people suggest that you can just turn down the boost when using low octane. 18 degrees might work for 20psi on 110 octane. If your tuner is saying that at you can run 93 octane at 10psi, because 18psi is safe for that, too,... then your tuner is NOT tuning your car properly.
Timing tables are 3 dimensional. They typically look like saddles. You have two inputs, RPM, and Boost, and you get a timing value from that.
With boost, you can run more timing at MID-boost points. For example, if you're just trying to accelerate "quickly", at part boost throttle.... if your timing is the same as your full boost timing, you're leaving a lot of power on the table. Your car can take more timing between 0 and 10psi, than it can at 15 or 20. Your timing table should account for this.
Similar with RPM. Generally, for a given boost level, your car can take more timing at low, and high RPM levels, than it can at MID rpm levels. Your peak torque point (typically 4500rpm on a Zetec) is the point of MAXIMUM cylinder fill. More air actually gets into the engine for a given boost at 4500rpm than at any other time. That's why you need reduced timing there. You can have more timing at 3000rpm, and drop it off as you approach 4500 rpm. Then have it come back on approaching 6000 RPM.
Put these two things together, and you can see why a timing curve should look like a saddle.
Now, one thing I have to point out, MAF systems automatically account for the load variations with RPM, so your timing curve will probably look more like a straight ramp with RPM. You can usually use more timing as the RPM increases.
But with Speed Density, it HAS to look like a saddle, or you're leaving a LOT of power, and efficiency on the table.
My car is "tuned" for 10psi. I can turn it down to 5psi if I want, and not have to touch a thing. This HAS to be done, because I ask this:
If your tuner is only tuning your car for full boost, what happens when you're driving at half throttle?
You see part throttle problems mostly with old-school drag race tuners. The reason is because for drag racing, you typically trailer the car to the track, sit at idle at the lights, then go to full throttle when the light turns green. There really is no need, or understanding, of part throttle tuning.
Now... some may think this is directed at Tom. It's not. I have never watched Tom work, and I've never seen one of his tunes. I'm just using some example I've heard talked about here.
I've been to a number of tuners in Detroit, and Toronto, and have yet to find one who knows what they're doing. Even one who claimed to be an ex-Ford powertrain engineer.
The only one I know of who for sure is doing it right is the guy Chris F uses in Ottawa. You'll notice he has pictures of endurance race cars on his wall, not just drag or "street and strip" cars. Real race cars use part throttle, as do street cars.
Back to the tables...
So, if your tuner is doing his job right, you should be able to turn down your boost all you want, and still have the car run properly, and efficiently. Same goes for fuel. If you turn your boost down from 10psi to 5psi... but it's still shovelling in 10psi worth of fuel, and it's super rich, sooty, stumbling and getting horrible milage... your tuner is stupid.
Again, a MAF system makes this all much easier than speed density.
Generally speaking, I run 17:1 from 20" vacuum until about 10" vac. Then transision up to 14.7:1 at about 5" vacuum, and 13.5 by 0 vacuum. I think I get to 12:1 by 5psi, and 11:1 by 10psi. Again, it's 3 dimensional, and looks more like an elephant back. You need to be a little richer at 4500rpm than you need to be at 3000, and 6000. And of course it slopes sharply with increasing boost.
Some people mentioned this idea of tuning your engine for 93 octane up to 10psi, and then tuning for 110octane between 10 and 15. Then using a boost controller to keep it at 10psi on pump gas, and 15 psi on race gas.
It's true, as Rod points out, that you're obviously leaving power on the table at part throttle on race gas. But really, I don't think it's that bad of an idea. It's a pretty simple way running, instead of switching programs. Realistically, the only time you'd be running 110, is when you're racing. And at that point, do you really car about part throttle efficiency? There's no question it's safe. Basically the only thing you're losing is fuel economy. Important for an endurance racer, not so important for a guy running a track day.
If ever your tuner says something like "The tuning is different between the dyno and the track, because the dyno doens't 'load' the engine the same was as the track" RUN AWAY. RUN FAR AWAY. That's the first sign the guy doesn't know what he's doing.
Why not?
If the tune was made for 93 octane between 100 and 120 percent load. Then you put 110 in, you could ADVANCE the timing even more on that fuel. That would given you more power and more efficiency at those load points.
So yes, you are leaving power on the table.
The only time this wouldn't be true, is if your tune on 93 was limited by "Borderline Spark". That means, as you increase the timing (assuming there is no risk of detonation), there is a point where if you increase the timing more, you get less power. If that's the case, then you can't increase the timing on 110.
But on a 9.6:1 engine at 120% load, I guarantee that your timing value at 120 was set because of detonation, NOT because you reached borderline spark.
Therefore, Rod's statement holds true.
Tuning Question
