What is the head studs tightening torque... ?
04-26-2004, 01:42 AM
#31
Race Director
Hmmm, some basic metallurgy terms should be defined for these types of discussions:
Modulus of Elasticity (Young's modulus) - Typically measured in GPa, it's the elastic deformation of the material given a standardized sample size and stress level. Steel is typically 2x higher than titanium and 3x higher than aluminium. This measurement is fairly constant across all alloys of the base metal. This affects the stiffness/rigidity of a structure.
Yield Strength - in MPa and is the amount of stress needed to create a permanent deformity in the standard-size sample (a bend). Again, steel tends to be 2x stronger than titanium and 3-4x stronger than aluminium. But alloying content makes a big difference here, so the strongest titaniums can be stronger than the weakest steels.
Tensile (ultimate) Strength - in MPa and is the stress-level that causes a break in a part. Usually is 5-15% above the Yield Strength.
Fatigue limit - incorporates some combinations of all the above. This is the stress level where a material never fails. Steels have a limit where if all stresses are below this, the part never fails. Alumiunium has zero fatigue limit, so whatever stress level, no matter how small, microscropic surface fractures accumulate and the part will eventually fail. You can arm-wrestle your alloy A-arms and you'll eventually break it. Not possible with steel or titanium (which can be even higher than steel depending upon the alloys).
creep deformation - permanent deformation/elongation over time given a certain stress level. This is highly dependent upon the temperature endured by the material and typically is not an issue at 1/3Tm.
One distinction that must be made is that strain & yield levels occur immediately. If you place a 200-lb kid on the end of a steel high-dive platform, it will deform and bend a certain amount right away. A titanium platform would bend 2x as much as steel and an alloy one would bend 3x as much. If you have a heavy enough kid, the platform may be stressed beyond its yield strength and take a permanent bend. Again this is immediate.
Creep on the other hand, occurs over time. If you can heat up the steel platform enough, like to 1000K AND apply stress over the yield-limit at that temp, then yes, the platform would bend more and more over time.
With a Tm (melting-temperature) of over 1700K for the low/high-alloy steels, even a severely overheated engine won't place steel into primary creep region. That's really only possible if you've strained the material over it's yield strength at that temperature. I'm not sure you're heating up or stressing the head-studs by that much. If creep does occur, it would happen in the alloy block well before the steel head-studs. You can probably verify creep by measuring the length of the stud removed from the block versus a brand-new one (they most likely will be the same length).
What does happen over time is the headgasket fibre compresses with exposure to coolant and heat-cycles. This relaxes its compression-resistance to the tension applied by the head-studs. The other source of problems with head studs & nuts is vibration. The updated torque-specs appears to be a fix for both these issues. It's very similar to the problem the MKIII Supra Turbos had with finding head bolts finger-tight or even laying loosely on top of the head. Their solution was to use Loctite and update the torque-specs to 80 lb•ft.
I know many 944-spec and 951 racers who build their engines using 80-90 lb•ft on the head-nuts. The 944-GTR built by Fabcar used 14mm head-studs and torqued them down to over 120 lb•ft. Again, you're not even coming close to approaching the yield-limit of over 90,000psi on the material. Aftermarket studs have yield-strengths in the 110-120,000psi range. Here's a good site with info on threaded & bolted joints BoltScience.com.
Modulus of Elasticity (Young's modulus) - Typically measured in GPa, it's the elastic deformation of the material given a standardized sample size and stress level. Steel is typically 2x higher than titanium and 3x higher than aluminium. This measurement is fairly constant across all alloys of the base metal. This affects the stiffness/rigidity of a structure.
Yield Strength - in MPa and is the amount of stress needed to create a permanent deformity in the standard-size sample (a bend). Again, steel tends to be 2x stronger than titanium and 3-4x stronger than aluminium. But alloying content makes a big difference here, so the strongest titaniums can be stronger than the weakest steels.
Tensile (ultimate) Strength - in MPa and is the stress-level that causes a break in a part. Usually is 5-15% above the Yield Strength.
Fatigue limit - incorporates some combinations of all the above. This is the stress level where a material never fails. Steels have a limit where if all stresses are below this, the part never fails. Alumiunium has zero fatigue limit, so whatever stress level, no matter how small, microscropic surface fractures accumulate and the part will eventually fail. You can arm-wrestle your alloy A-arms and you'll eventually break it. Not possible with steel or titanium (which can be even higher than steel depending upon the alloys).
creep deformation - permanent deformation/elongation over time given a certain stress level. This is highly dependent upon the temperature endured by the material and typically is not an issue at 1/3Tm.
One distinction that must be made is that strain & yield levels occur immediately. If you place a 200-lb kid on the end of a steel high-dive platform, it will deform and bend a certain amount right away. A titanium platform would bend 2x as much as steel and an alloy one would bend 3x as much. If you have a heavy enough kid, the platform may be stressed beyond its yield strength and take a permanent bend. Again this is immediate.
Creep on the other hand, occurs over time. If you can heat up the steel platform enough, like to 1000K AND apply stress over the yield-limit at that temp, then yes, the platform would bend more and more over time.
With a Tm (melting-temperature) of over 1700K for the low/high-alloy steels, even a severely overheated engine won't place steel into primary creep region. That's really only possible if you've strained the material over it's yield strength at that temperature. I'm not sure you're heating up or stressing the head-studs by that much. If creep does occur, it would happen in the alloy block well before the steel head-studs. You can probably verify creep by measuring the length of the stud removed from the block versus a brand-new one (they most likely will be the same length).
What does happen over time is the headgasket fibre compresses with exposure to coolant and heat-cycles. This relaxes its compression-resistance to the tension applied by the head-studs. The other source of problems with head studs & nuts is vibration. The updated torque-specs appears to be a fix for both these issues. It's very similar to the problem the MKIII Supra Turbos had with finding head bolts finger-tight or even laying loosely on top of the head. Their solution was to use Loctite and update the torque-specs to 80 lb•ft.
I know many 944-spec and 951 racers who build their engines using 80-90 lb•ft on the head-nuts. The 944-GTR built by Fabcar used 14mm head-studs and torqued them down to over 120 lb•ft. Again, you're not even coming close to approaching the yield-limit of over 90,000psi on the material. Aftermarket studs have yield-strengths in the 110-120,000psi range. Here's a good site with info on threaded & bolted joints BoltScience.com.
04-26-2004, 02:43 AM
#32
Another reason to change your head stud specifically on our cars..
I found that liquid can accumulate in the gap between the stud and the block. When I took my head stud out I was shocked to see how much material rusted off the stud, not good.
I found that liquid can accumulate in the gap between the stud and the block. When I took my head stud out I was shocked to see how much material rusted off the stud, not good.
