15" at idle is plenty. I am running Accel DFI on my 468 and mine idles with about 11", plus it idles very nicely (almost too nicely, lol). I mounted my map sensor on the firewall, pretty close to the intake manifold fitting, so I am using a short piece of tuning.

When I first installed my Accel system it ran rich as hell. I tinkered with it - no luck. Then I hired a guy to come to my house and we drove it around and tuned for about 5 hours (cost $300). It was better, but still not right. Then I took it to a "fuel injection specialist" and left the car for 2 weeks while he frigged with it (cost $800). This time it was a little better, but still not right. It was stalling and hesitating a little. I called Accel and they were great. I sent them a copy of my ECM file and they were just laughing at how poorly it was set up. Bottom line - I gave them my specs and they whipped out a beautiful ECM file for me. I uploaded it and it ran great. Since then, I have tweaked it a little at the dyno, just to dial it in.

The moral of the story is - don't get discouraged. It's a little trickier than it looks and don't be conned like I was. Perhaps just call the manufacturer and they may have a setup file they can email you.

Also, I added a wide-band O2 sensor and that made all the difference. You can get them fairly cheap now. Mine cost about $1,000 a few years back but I think they're only a few hundred bucks now. The wide-band O2 sensor will give you real-time air-fuel ratios, which is imperative to know if you're trying to tune......................

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Picking nits again, gentlemen?
Pardon me while I edit out the crap, lets keep this post ontrack, Paul's looking for help, not an esteric argument on how to measure vacuum, kicked off by an obvious minor brainfart.

Paul, Have you checked your return line fuel pressure?
There is a plugged gauge port available to do this. Return line pressure must be less than 1-2 psi, or you'll NEVER get it to idle right.

15" of vac is plenty. I have 10" and she idles just fine @ 14:1 or wherever I tell her to go.

I'll look at your other post, too, my memory if foggy right now...

In the meantime...

Supply side fuel pressure?
Return side fuel pressure?
Values in the cells she idles in?
Pulsewidth in those cells?
Idle timing as shown by the ECU?
Actual idle timing with a timing light?

Also, I've been playing with this: http://temp.corvetteforum.net/c3/zman//CoPilot.htm
It seems to be VERY helpful.

 

Paul, I read through but I didn't notice if you were running in open loop or closed loop? Also didn't notice if you were running a narrow or wide band O2 sensor. Either way do you know how to use the data logger? My engine idles around a MAP reading of 60, whatever vacuum number that equates to. I have run idle in open and closed loop.

I guess bottom line is can you provide more information, even screen shots of your setup parameters. You could save your C950 file and post it, then I, Jim or anyone else with the C950 software could have a peek.

 

I know someone in Jersey who is a great tuner if you are serious. PM or email me.

You have to tune it to whateve the engine is. Vacuum is somewhat irrelevent. I've tuned speed density systems with less than 6" of vacuum at idle.