Twice ive typed out a long explanation, only to have it disappear. This time, im going to post, then edit, many times, so i dont lose everything again. Be patient, as i work through this.
Quote "There is this theory, which is the most often repeated/used (and entirely plausible as far as I know)"
The reason i have this theory isn't because im repeating what i read elsewhere. I have a degree in mechanical engineering. Im borrowing from my engineering mechanics, thermodynamics, and materials properties background for the following explanations.
So, you can see the flange on the manifold is about an inch thick. I didnt measure, but im going to guess, about an inch.
Now, some materials properties: the coefficient of cubic expansion for cast iron is .0000060 inches, per inch, per degree F.
On a normal 70° day, during normal driving, you exhaust manifold will be about 1200° . Higher if towing up a grade, lower if idling, etc.
So, if the manifold regularly changes from 70° to 1200°, thats a change of 1130°
So, a little math: .0000060 x 1130 = .00678
That means the manifold flange thickness changes from 1.000 inches thick to 1.00678 inches thick, every time you drive. It then shrinks back from 1.00678 to 1.000 every time it cools down.
So, lets work out the numbers for a guy who lives somewhere cold, and tows his trailer up a mountain. Lets Let its 0° outside, and his manifolds heat to 1800° while towing up Donner Pass:
.0000060 x 1800 = .0108 so, in this case the thickness changes from 1" to 1.0108", the back, every time the guy tows up Donner Pass.
So, if the thickness off the flange changes, that obviously means the bolt stretches by the same amount.
Now, we will do some stress analysis on the bolts/screws.
Im going to calculate the tensile stress on the screws based on their normal torque values, them im going to add the additional stress caused by the thermal expansion of the manifold flanges.
First, the bolt tensile stress from normal torque alone using the equation T=cDF
The screws are m8x1.25mm and have a nominal minor diameter of 6.68mm
So, using the above equation i find 3461 lbs of force spread over .0531 square inches of section area (R^2π of the bolts minor diameter). It tensile stress, this = 65,301 psi of tensile stress.
A286 steel is used to make exhaust manifold screws due to its ability to maintain its tensile strength at high temperatures. Other grades of steel soften when heated to 1300° and above. So, Ram uses A286, which has a tensile strength of 90,000 psi.
So, when torqued to 16ft lbs, the screws are under 65,301 psi of tensile stress. They will yeild and permanently deform at 90,000 psi of tensile stress.
Now, the question is, how much more stress does it add when stretched .00678" more?
And, when towing up Mt Motherhumper, and they are stretched .0108" more, how much additional stress is added?
Lets work that out...
For the tensile stress calculations, im going to use Hookes law and associated equations. dl =FL/EA
Forgive all the typos. The fuking autocorrect and autoformat on my phone must have been programmed by an idiot.
Actually, nevermind. I dont have a pen and paper handy, and trying to do these calculations on my phones word processor is maddening!!
Suffice it to say, the reason not everyone experiences the same breakage frequency is pretty obvious. We dont all drive the same way.
I live in WA state and i tow over mountain passes. In winter, my manifolds heat from 0° up to 1800°ish.
If i lived in Phoenix, and used my truck to take the kids to the mall, then my manifolds would regularly experience going from 115° up to 1200°, and might last a couple thousand years.
This could be fixed with thinner flanges, larger diameter screws or some combination of the two.
