I think it's detonation. The way you describe it reminds me of mine. Did you retune for the smaller pulley? I would check fuel pressure first, and do a scan to check AFR and KR.

 

I did a re-tune for the pulley change and afr was good and made gobs of power with no tuning isues. I also read the plugs and they were clean, a little pinkish white but no glitter from detonation. every plug was identicle.

 

then its the boost reaking havic on the springs u need good springs for high rpm boost applications and go ahead and upgrade to the lifters

 

I wouldnt worry about the lifters, stock lifters will handle more then that. I would look at doing springs, pushrods and a cam of your choice while you are in there.

 

I agree^. How much boost are you making?

 

im selling some brand new comp 918 springs for 160 shipped. pm me if interested.

 

i don't know how many miles you have on the motor.
your stock springs are not strong enough for your upgrade on boost.
you need a set of stronger springs, LS6, 915, 918, or duel.

like it has already been said, now would be a good time for a cam
and spring swap that will work well with the radix.

 

There must be some weak springs floating around, mine stock springs held up to 30k miles boosted shifting at 5700 with no problem. (Stock Cam) Not saying its not a good idea to upgrade the springs, because it is.. Just throwing some info out there

 

my 918's sold...

 

With my stock springs I could hear a change in the engines tone above 5,500rpms. When I put it on the dyno you could see the valve float on the chart and it lost a lot of power quick up top. I installed a $50 set of ls6 springs and now it pulls great up to 6k with no valve float.