Hey, for those of you guys tuning with wideband O2. What are you shooting for as a target between say 5000-5400 RPM. Naturally aspirated, about 500HP with MPI. I have been using 12.5 as a target for upper RPM. Down a couple MPH on speed this year, is any one else running slower? Maybe this gas sucks more this year and others are seeing it too.

Need a little more info on the engine. What is the compression? What cam? Those 2 items will have a huge effect on cylinder pressures, which will play a large role in how lean you can go. Are you running the engine cold or does it have a thermostat? This will also effect it.
As a VERY general rule of thumb, 12.5 on an NA engine at WOT is about what you want to shoot for. The more aggressive the engine, the higher the cylinder pressures, the higher the water temp, the lower the octane, then the lower the AFR needs to be.
I have some of my larger engine (1300+ hp) on pump gas in the low 11's at WOT to keep them safe. You should look for low 13's at idle and part throttle cruise. This will keep the transom clean and the economy up.
If possible, monitor knock as well. You can have it tuned perfectly and still have it beat the bearings out of the bottom end. Give me a shout if I can help.
Eddie

Young, what knock systems do you use? I am looking for something stand alone.

The vast majority of what I build are efi engines using a Mefi ecm, which has it's own knock protection. There isn't much out there for a stand alone. On carb stuff, I just use one from MSD. It's not something you want to absolutely rely on. It's about a half a step above nothing at all.
There is one out there by J and S(???) I think that's the name. Quite a few guys have talked about it here before. I've never used it so I really can't comment on it. Maybe someone else will chime in with some info and the correct name.
Eddie

Thanks Eddie, here is what I have, 489 stroker, 8.9 comp, RM roller cam, Holley MPI with wideband, 160 deg therm, 36 deg timing, GM large oval iron heads, 89 octain fuel runs the best. I tune mine with a laptop and I can program my own targets. I have had it set to 12.5 since I switched to the wideband O2. I have had no problems, boat runs great, not a mis in it and no lack of power at all. Just a little down on top end, I normally see 73-74 MPH GPS under any condition. Now I am seeing about 70-71, down about 200 RPM with no engine changes. I pulled the the plugs, cap and rotor over the winter and everything looked good. I have ran a few datalogs and I have since made a couple changes to the fuel map, but it looked pretty good. Maybe I should'nt worry about it gas is too expensive anyway.

http://www.jandssafeguard.com/

That's the one. Thanks GPM.

Jeff,
Sounds like you pretty much have it nailed. When is the last time it ran 73-74 mph? Was it this year or last? Check the usual items....prop for dings, bottom for a slight hook, etc. Obviously something has changed. It's just a matter of figuring it out. Let me know if I can help.
Eddie

Thanks Eddie, I will leave the AF at 12.5. I have not hit the higher speed since last year, prop is perfect, bottom looks the same, it sits on a trailor in my building. The cap & rotor look ok, plugs looked ok. Maybe I will try changing the plugs, cap & rotor first and see what that does. No misses, but when you are tweeking things it does not take much to knock of a couple MPH. I will let you know what I find out.

If the weather conditions, boat weight and bottom condition are the same, my thought is to do a leak down test. Somehow, you seem to be missing 30-40 HP.

Maybe, I have rebuilt in stages, the rings have about 200hrs, heads only have 100hrs on last valve job. If tune up parts don't get the speed back I will take a closer look in the winter.