New engine, popping under boost??

[Sleaper]

Hey guys,
Just got my customers engine installed in the ol truck and I'm having an issue with it.
I haven't done too much troubleshooting, so feel free to suggest anything.

The truck runs great under normal and non boost conditions. As soon as it hits a few psi 2-3, the truck will start to backfire or pop out the exhaust.

Now when I first got this engine together, for some reason the #5 piston blew a head gasket. I immediately replaced it and now it's back in action.

I gapped the plugs at .025, probably closer to .023 so the 9.5cr doesn't blow the spark out.

Like I said I haven't done anything else, but I figured I'd post up and go through the troubleshooting stages and share.

I guess tomorrow, the process is:

Check plugs for consitency
Compression check - engine can't be hurt, too soon and not any hard runs on it.
Make sure gasket didn't blow again
Change out coils
New plugs

The tune is good enough to not cause anything , so I'm counting that out.

any suggestions?

thanks,
allen

I would say it sounds like plugs, wires or coils. Is there any wires that have been nicked or boots that aren't seated all the way? Are you using the stock coils?

[farmtruc]

...stock coils?

using stock coils, but i'm going to try a set of msd's tomorrow.