BMfinney,
Your question; How can you tell if it is the head that is prone to cracking? This is a good question and I will offer my viewpoint based on my experience. Typically, cracks are caused by stresses. There are many types of stresses; tensile, compressive, shear and typically in the real world these stresses are relatively low in a head design. The "killer" is thermal stresses in cast iron heads. When exhaust flow is restricted/slowed, the area around the exhaust valves increases in temperature. This causes the local high temperature area to have very,very high thermal expansion. However, the rest of the head is bolted down on the block. So the local very high temperature area expands but this expansion is resisted by the firmly clamped-down head. Result? The thermally induced local stresses exceed the yield strength of cast iron and cracks develop.
What to look for on a head and what to do;
1. Small exhaust valves; increase valve diameter/curtain area and the diameter of head valve seats.
2. Small exhaust runners in the head; open up/port the exhaust runners to increase cross-sectionsl exhaust flow area.
3. Ways to get the hot air out of the Jeep engine bay, i.e. vented hoods, raise rear of hood w/ !" spacer blocks, vent the fenders, wrap headers w/ insulation, etc. Drop engine bay temperatures to reduce thermal head stresses and prevent head cracking.
4. The biggest radiator in the world WILL NOT work if the air flow through the radiator is reduced/slowed by stagnated air not getting out of the engine bay.
5. And spare me the BS that the Jeep XJ cooling systems are adequate. They are not. I've designed thousands of cooling systems for a living. The number of over heating stories on the this website probably ranks XJ over heating as No.1, with poor grounds a close second.
Any way that's my opinion based on my engineering experience. Your experience may vary.
Best regards,
CJR
Your question; How can you tell if it is the head that is prone to cracking? This is a good question and I will offer my viewpoint based on my experience. Typically, cracks are caused by stresses. There are many types of stresses; tensile, compressive, shear and typically in the real world these stresses are relatively low in a head design. The "killer" is thermal stresses in cast iron heads. When exhaust flow is restricted/slowed, the area around the exhaust valves increases in temperature. This causes the local high temperature area to have very,very high thermal expansion. However, the rest of the head is bolted down on the block. So the local very high temperature area expands but this expansion is resisted by the firmly clamped-down head. Result? The thermally induced local stresses exceed the yield strength of cast iron and cracks develop.
What to look for on a head and what to do;
1. Small exhaust valves; increase valve diameter/curtain area and the diameter of head valve seats.
2. Small exhaust runners in the head; open up/port the exhaust runners to increase cross-sectionsl exhaust flow area.
3. Ways to get the hot air out of the Jeep engine bay, i.e. vented hoods, raise rear of hood w/ !" spacer blocks, vent the fenders, wrap headers w/ insulation, etc. Drop engine bay temperatures to reduce thermal head stresses and prevent head cracking.
4. The biggest radiator in the world WILL NOT work if the air flow through the radiator is reduced/slowed by stagnated air not getting out of the engine bay.
5. And spare me the BS that the Jeep XJ cooling systems are adequate. They are not. I've designed thousands of cooling systems for a living. The number of over heating stories on the this website probably ranks XJ over heating as No.1, with poor grounds a close second.
Any way that's my opinion based on my engineering experience. Your experience may vary.
Best regards,
CJR
