Boosted engines create higher cylinder pressures which in turn require greater clamping force to hold the cylinder heads down. At some level of boost, the OE fasteners will fail and that will be a bad day. Studs versus bolts is a separate discussion for a later date. For now, we will stick to reviewing studs as they are far superior as a fastener type. We will use the LS engine as an example. 

Head stud upgrades to handle increasing boost can be summarized as “More”, “Better” & “Bigger” together with combinations thereof. "More" is going from an OE 4-bolt block to an aftermarket 6-bolt block. We will look at the Better and Bigger options in detail here.

Better Fasteners 

Head studs can be either Torque-to-Yield (TTY) or Torque-to-Spec. A stress-strain curve (Tensile Chart) shows that a given material first reversibly stretches in the elastic region. Once it reaches yield strength, stretching becomes irreversible. For that reason, TTY fasteners are not reusable. This allows the OEs to use less expensive materials as they literally push them past their limits. The chart below shows a generic Tensile Chart.

Aftermarket head studs use higher tensile strength steels and typically target torque at 80% of Yield. This is well within the Elastic region making these head studs reusable. Being well below Yield, they retain high tensile strength making them less prone to catastrophic failure compared to a TTY fastener.

Okay, so why do critical fasteners use the “80% of yield rule” instead of 90% or some other arbitrary amount? It’s because torque input is a notoriously unreliable method for controlling precise tightening as it measures friction rather than clamp load.  Tightening inconsistencies are a certainty. Decades of experience have shown that targeting 80% of yield builds in the appropriate safety margin to ensure the fastener will remain within its elastic limits. This avoids permanent deformation thereby preserving future predictability, performance and reusability.

P1 produces head studs from four types of steel alloys.

• E190, 190,000psi tensile strength, 8470 steel
• S220, 220,000psi, AMS 6304 steel
• H260, 260,000psi, H-11 steel
• P270, 270,000psi, Custom Age 625+ steel (Duramax & Chrysler hemi only)

Here’s a comparison of how these head studs line up in LS applications by alloy type and size: 

 
P1’s P220 series parts deliver a 14% increase in clamping force over their E190 equivalents. That comes with price tag about 20% higher. This is money well spent if you are bolting  light boost onto a GM LS block without pulling the engine. Dart's LS Next blocks were built to take a beating. With that in mind, P1's offer in limited to P220 and H260 head studs. The H260 studs are a lot more expensive but are still a bargain compared to pulling the engine and machining for ½” studs. One such scenario would be where you're swapping out a smaller turbo for a larger one. 

 
Important Note: Do not buy off-brand or imported fasteners. They are not the same and not worth the risk. Also, only buy from trusted retailers to avoid getting counterfeits. 

Bigger Fasteners

Apples to apples, a ½” nominal diameter fastener achieves roughly 40% more clamping force than a 7/16" fastener regardless of stud material. The increase is exponential rather than linear as clamping force is a function of cross-sectional area (radius squared). As shown in the previous table, the 1/2” E190 studs provide16,650 psi clamping force compared to 12,000 psi for their 7/16"equivalent.

As engine builders, machining for bigger head studs is our first upgrade choice. The engines is already apart so both block and heads will be on our CNCs for machining anyway. Our customers appreciate that the larger head studs are a clearly visible upgrade.

Bigger & Better
The advantages of size and tensile strength are multiplicative. For example, clamping force of a ½” S220 head stud is 58% higher than an E190 7/16” head stud. If you are going with heavy boost, "Bigger & Better" is the only way to go on any given block.

Conclusions
1. You should have a strategy for improving clamping force for anything other than Light boost. 
2. The best choice(s) for Medium boost depends on machining access, budget, and boost level.
3. Heavy Boost warrants More, Better and Bigger head studs.
4. Do not trust your engine to fasteners of questionable origin, quality or authenticity. It is a very bad bet. 

Note: Special thanks to P1’s Shannon Strothers for both the data and editorial input.