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Head Gasket Preload by smutts on 05 Aug, 2009 14:26
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Whilst doing the headgasket on the GTD the other day, I noticed that the head bolts are stamped 10.9, which is a standard metric stress grade. So just out of curiosity I worked out a rough clamping load on the gasket. 10.9 means 1000 n/mm2 when it snaps, and it yields at 90% of that, the steel work hardens (gets stronger) as it yields but the bolt gets thinner as it is stretched, so the two effects more or less offset each other. This is the idea of angle turns, the designer can ignore what the bolt has been lubed with. So 900n/mm2 x 3.14 x 6mm x 6mm = 101736n = 101736/9.81 kg = 10371 kg = 10.4 tonne = 10.4 tons per bolt
Ten bolts hold the head on so thats about 104 tons holding the head on. But I can't be arsed to measure the m12 bolts to see what the minimum diameter is, if 11mm then this drops to 87 tons. Food for thought.
Yes I know I don't get out enough. Sidmouth singing tomorrow.
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#1 by Smokey Eddy on 05 Aug, 2009 19:24
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That seems like an unusual amount of force considering heads get lifted regularily with high (not by 104 tons standards) boost pressures... don't they? that is an extreme amount of force. has anyone actually seen a head physically lifted from the surface of the block?
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#2 by Turbinepowered on 05 Aug, 2009 19:34
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That seems like an unusual amount of force considering heads get lifted regularily with high (not by 104 tons standards) boost pressures... don't they? that is an extreme amount of force. has anyone actually seen a head physically lifted from the surface of the block?
It doesn't have to lift far to lose integrity, fractional millimeters is all it takes. You aren't going to see something move .5mm, but it'd probably blow all that pressure out rather quickly with that clearance. -
#3 by theman53 on 05 Aug, 2009 19:49
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Think of the boost too as pounds per square INCH and then add 23:1 compression...big #s
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#4 by Turbinepowered on 05 Aug, 2009 20:17
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Think of the boost too as pounds per square INCH and then add 23:1 compression...big #s
Lessee... 76.5 mm bore, right?
Off the cuff, that's 3.825cm diameter, giving you 45.94 cm^2 per cylinder of head flatspace. You have an atmospheric pressure of 14.7 pounds per square inch. With a 23:1 pressure ratio, this gives you, what, 338.1 PSI in that cylinder (14.7*23), so that is 338.1 P/ in^2 * in^2/6.45cm^2, which gives you 52.406 pounds per square centimeter.
52.406 p/cm^2 * .4536 kg/p = 23.771 kg/cm^2
Multiply this... 23.771 kg/cm^2 * 45.94 cm^2 = 1092.02 kg per cylinder.
Since all your cylinders aren't in compression at once, that's as far as you go from compression pressure. Combustion pressure will add to the strain, obviously, but again you only have the one cylinder under combustion pressure at any given time. You also will have exhaust pressure from the exhaust stroke, and a slight negative pressure from the intake stroke.
Also obviously, this is for naturally aspirated only. -
#5 by truckoSaurus08 on 06 Aug, 2009 03:42
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wow your good at math. So in other words. Alot.
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#6 by Turbinepowered on 06 Aug, 2009 14:01
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Just some more numbers...
If you are running 11 pounds of boost (~1.75:1 pressure ratio) then your kilograms force during compression becomes 1909.701 kg, or almost two metric tons.
If you run 15 pounds of boost (2.02:1 pressure ratio) then you are exerting 2206kg during compression.
25 pounds of boost gives you 2950kg...
You AAZ folks with your 79.5mm bores are 1179.68 kg (NA), 2062.43kg (11 pounds), 2383.43kg (15 pounds) and 3185.93kg (25 pounds) respectively.
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#7 by jtanguay on 06 Aug, 2009 14:17
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one main reason that head gaskets blow, is from the actual alloy material the head is made from. under high boost & heat, the head actually deforms. this is why andy2's compound setup kept blowing hg's. if the head was made from a strong titanium alloy, i bet it could handle 100 psi. but then you run into other problems like bent rods, and the actual cylinder walls collapsing from the stresses.
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#8 by smutts on 07 Aug, 2009 15:11
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Just dropped this one in for fun, as I enjoy rule of thumb engineering, my guesses on headgasket death is usually lack of decent corrosion inhibitors, and unsympathetic warming up. Aluminium alloys usually expand more than cast iron, so that is a massive shearing force on the gasket as the head drags itself across the block as it warms up. Don't forget that those pressures that you worked out for compression cycles are for when that compressed air has cooled down again, it then acts like a spring. But it heats up when compressed so that raises the pressure even more. (I think
) I would have paid more attention in physics class.
Assume 4 head bolts about each pot, 11mm diameter of meat between threads, 8.7 tons each = 34.8 tons.
peak cylinder pressures during combustion haven't a scoobie. But google is your friend, 15MPa for an intercooled 14 litre cummins N14-410, wouldn't know what one looked like but it will give a number, 15MPa = 15,000,000newtons/meter2 = 1,529,000Kg/m2 = 1.53 Kg/mm2
76.5mm bore gives 4,595mm2 so thats 7,029 Kg or 7 tons above that burning pot. That 14 litre engine is probably an industrial beasty that will run till the crack of doom, so will be well derated. Perhaps 50% even? Also direct injection, so 16:1 ish compression ratio? So that 7 tons a pot could go to 10 or 12 or so?
Lets pick 10 tons, Those head bolts when the head was put on were angle turned till they squashed the head and gasket with 8.7 tons then kept stretching plastically at 8.7 tons. Drive the car, put foot down and you add 10/4 tons to each bolt. The bolt yields again till it is at 8.7 tons, again. The yield strength of the bolt is now at the sum of the combustion load and the new gasket preload. For each bolt 8.7tons yield = 2.5tons combustion + gasket preload tons.
Gasket preload is now 6.2 tons per bolt.
So I am either totally clueless, or putting massive boost and fuel does nasty things to your head gasket. ARP it is then
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#9 by smutts on 09 Aug, 2009 13:36
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I bored everyone out of here!
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#10 by vwjones on 09 Aug, 2009 13:46
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wow, will you do my taxes?
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#11 by truckoSaurus08 on 09 Aug, 2009 16:21
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I think you should change your name to professor smutts.
No, but really, its very interesting to know these sorts of things. -
#12 by smutts on 13 Aug, 2009 14:21
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Quote
wow, will you do my taxes?
I wish I could have a clue about mine,
Any leads for UK tax geniuses for those in the construction, site engineering game would be gratefully recieved.
Be warned, I just mess about with the numbers to try and get some idea what is going on, but it was also scientifically proven that Bumble Bees cannot fly!