just for future reference, i'm wondering what causes the stock twins to crack? I have a turbo timer, and i always warm the car up before i take it out... I'm wondering if there's another factor to this?

 

see how long you last in a 1000*+ of heat for 15 years.
-J-

 

The "twins" don't crack. The exhaust manifold cracks internally over time from the heat cycles and huge temp variations. Warm ups and cool downs don't affect this. Turbo timers are pretty much a waste on our turbos b/c they are water cooled.

Don't worry about it. Just enjoy driving the car.

 

Curious, when looking for a good set of twins, most seem to be cracked some. How much or how big of a crack is considered acceptable, and when is a crack or cracks big enough that a guy should go on to the next?

 

poor material selection

 

The general consensus is when the cracks extend out enough to leak past the gasket.

Dave

 

blending of your apex seal, corner seals into seals ala mixed in the turbos :P

 

I've heard from a turbo shop that they've taken a brand new turbine housing, ran it around the block, pulled it, and it was already cracked.

There's parts of the turbine housing that have a flat face and directly have exhaust gas hitting it. There's no way it won't over time with THAT much heat.

But, the good thing is the majority of cracks don't affect anything. The exhaust gasses don't care if the turbo looks funny - it still works fine. The only time you have to worry about cracking is if you have one that extends under the gasket sealing surface - gasses can build up under the gasket and blow it out.

Dale

 

no...the most common place for cracks IS in the turbine section of the "twins".

 

the other day, i just found out my max boost under WOT is .62 (with the power fc) doing a good ol' conversion, .62 is around 8psi. Less than stock... I think my turbo's are leaking. In fact, i can HEAR air escaping. I'm just wondering what can i do to prevent this if i go for another set of stock twins...

 

I think the cracks and the only running 8psi are two different problems.
-J-

 

The cracks themselves, as best as anyone can guess, are caused by the residual stresses in the casting in combination with thermal stresses in the manifold. These thermal stresses come from the fact that everything expands when hot, and there is a variation of temperature through the turbocharger assemble as well as different materials applying forces too. I think temperature differences are the problem more then the temperature itself.

The best 'defense' against this is to make sure the car is warmed up well before boosting, cool the car down after boosting, and always run with the heat shields in place. Make sure the manifold studs/nuts are torqued to the proper spec. You don't want to heat or cool anything too rapidly - this is what results in the greatest thermal stresses.

Dave