Can a 2.5 inch exhaust with a 2.5 inch cat cause the head gasket to blow on a car boosint 18psi? I am still trying to figure out why the keep blowing. The block and head were resurfaced. I am using the MLS gasket with mopar spray with arp studs torqued to 80 ft/lb using a snap-on torque wrench. No detonation as far as I know. Any other Ideas?

best time so far 14.3 @ 101.1mph(no traction)
fastest 2GNT with a hitch! until proven wrong...
www.cnnmotorsports.com

Well most of the guy's using the ARP's use 90-100 ft/lbs on those studs. Secondly how good is your mach. shop? because if they miss a crack or a flat spot then it can cause more issues to crop up. Other than that they do not blow magicly, so I would then direct you twards the tuning of the car...

im running around 16psi resurfaced head, block. mls hg, arp head studs, torqued to around 80-90lb's. hg isnt a problem. and i have 3inch d/p to 2 1/2 catback,. dont ask why.

I would not reccomend running 18psi on a 2.5" exhaust but it should not be causind the HG to Blow...

If you are sure your HG is blown, then there really only are a few things to point a finger at. Being boosted, and running that much pressure, first thing I'd say is detonation. Its usually the first thing that goes with knock, because the HG is the weakest link when it comes to the extremly high cylinder pressure associated with knock. Most say that 2.5" is good for around 300 CHP, which at 18 psi, you are over, but not by a whole lot, either way, thats not your issue(although, if you have too much exhaust reversion back into the cylinder due to backpressure, it can contaminate your intake charge and possibly cause it to detonate also) As for the torque rating on the ARP's, they told me directly since we have a aluminum head, that the recomended rating is around 65 ft lbs. I personally put a bit more on mine, around 76 ft lbs, and run 15 psi so far without any failure. I know a lot of guys run much higher torque on the studs, and it seems to work, but i just took the route and suggested ratings. I even asked ARP about running lots of boost on it, and they said it will be just fine. The main reasoning is that the aluminum head needs room to expand with heat, and I would think that the more boost/power you run, the more the head will expand, and need a "looser" torque rating on the studs. Its up to you what you want to do, i have not had any issues, and others who run 100 ftlbs haven't had an issue, so take it for what its worth.

Next issue is making sure your head sealing surface is perfect, and the block sealing surface is perfect. Other than these few things, you should never blow the MLS headgasket.

I'd be more looking into detonation really. I can get my motor to ping for a second at a 17 psi spike on the S16G. I run tons of fuel and cold intake temps, and it still does it. If you are running all stock ignition timing, and at a minimum of 93 octane fuel, at 18 psi, i'd say you have a good chance that your motor is knocking. If you are not sure what it sounds like on our motors, take a listen to this sound clip. Listen carefully at around the 19 second mark or so. You will hear that rattle sound..

http://www.tridiamondpower.com/car_media/Jasondet.mp3

You can also hear this sound inside the car, with windows up(radio off of course), and it sounds like it coming from the passenger side area of the dash.

Now after listening to the sound clip I think I might have a little detionation. I always thought it was the external wastegate. I will get a little crackle sometimes at WOT. SO... Looks like I need to upgrade the exhaust to a 3in and maybe som ingition retard. Chances are I am going to need to resurface both the block and the head so I am going to check into having it O-ringed to prevent this from happening again If I get a little detionation. Thanks to everyone who has posted.. Also ano other suggestions for while it is apart would be great..
Thanks
Paul

best time so far 14.3 @ 101.1mph(no traction)
fastest 2GNT with a hitch! until proven wrong...
www.cnnmotorsports.com