Timing advance on a car with hotter cams

[Buddyworm]

So I've been diagnosing/tuning a friend's CA18 build with 272's and a biggish turbo (greddy TD05-18G). I've got it running butter smooth through most of the revband on vacuum and full boost up to 5500-6000rpm where the turbo starts to "bite" and ignition starts to break up; loss of power, backfiring, but no bucking. Initially I suspected detonation so I backed it off from ~21deg to 17deg in the affected region to no avail. His wastegate is set at 1bar and I just don't see how it can be pinging at 17deg advance so my question is this:

How should timing advance be changed in order to accomodate the longer duration cams? Could it be that the cams themselves need to be timed differently?

Thanks for any input you peeps can give,
Kenji

[float_6969]

It's not the cams. The ignition system can't handle the boost/rev combo. Basically you're blowing out the spark. If I'm right, you should be able to gap the plugs DOWN and gain some revs/boost before it starts breaking up again. If that's the case, you need to address the weak ignition system and do something to increase the strength of the spark.

[Buddyworm]

Very well then! We put in colder plugs which improved things a bit, but the miss is still there. I've got a set of FET coilpacks on my car that we'll try.

[float_6969]

Did you try reducing the gap? Stock gap is 1.1mm. I'd gap them down to .9mm and see what happens.

[Buddyworm]

We've already gapped them down to .86, installed aftermarket coilpacks and the stumble is still there. I'm beginning to suspect a boost leak somewhere after the MAF (blowthru on the coldside) but there's not a lot of piping... It's only when the turbo hits its efficiency range.