Rescue's Official Build Thread - 88 XJS V12 Oct 12 2019
#121
To add to what Greg said, the regime of lubrication a crankshaft or connecting rod experiences is called hydrodynamic lubrication. When the engine is running, the crankshaft will not touch the bearing, instead it is lifted off the bearing by the oil and it floats on a cushion of oil. That's why wear occurs at startup, when the oil film is not yet present and the crankshaft does touch the bearing.
As the oil is pumped into the bearing, it experiences shear. There is a velocity profile that goes in an inverted U shape, where it is zero at both the contact point with the bearing and the crankshaft and at some maximum near the middle of the oil film thickness. This change in velocity is shear, and the friction created by the resistance to flow creates heat. Higher viscosity oils create more friction and heat; and heat is the enemy of oil, as it causes the oil to breakdown faster. I have seen tests with thick oil where there was enough local heat created to partially melt the bearing metal, which is obviously bad!
So part of the selection of an oil viscosity is you want it thick enough to provide an adequate film thickness to lift the crank off the bearing and ensure there is no metal to metal contact under full load, and you want it thick enough to stay in place long enough to do the job. An oil that is too thin will run out of the clearance between crank and bearing and will not develop enough film thickness. You don't want it so thick that it creates extra heat and friction, as that is wasted energy. In cold weather that can significantly slow the starter, sometimes to the point the engine won't start.
So some factors that go into the selection of the viscosity, are the oil pumps flow rate, the bearing clearances and leakage rates, loads on the components and ambient temperature. As with most things, it's a balancing act.
As the oil is pumped into the bearing, it experiences shear. There is a velocity profile that goes in an inverted U shape, where it is zero at both the contact point with the bearing and the crankshaft and at some maximum near the middle of the oil film thickness. This change in velocity is shear, and the friction created by the resistance to flow creates heat. Higher viscosity oils create more friction and heat; and heat is the enemy of oil, as it causes the oil to breakdown faster. I have seen tests with thick oil where there was enough local heat created to partially melt the bearing metal, which is obviously bad!
So part of the selection of an oil viscosity is you want it thick enough to provide an adequate film thickness to lift the crank off the bearing and ensure there is no metal to metal contact under full load, and you want it thick enough to stay in place long enough to do the job. An oil that is too thin will run out of the clearance between crank and bearing and will not develop enough film thickness. You don't want it so thick that it creates extra heat and friction, as that is wasted energy. In cold weather that can significantly slow the starter, sometimes to the point the engine won't start.
So some factors that go into the selection of the viscosity, are the oil pumps flow rate, the bearing clearances and leakage rates, loads on the components and ambient temperature. As with most things, it's a balancing act.
The following 3 users liked this post by Jagboi64:
#122
I saw a presentation by an Esso lubrication engineer years ago and his recommendation was to use the lowest first number possible and then match the second number to whatever the manufacturer calls for. Thus if the original spec called for 10W30, you should use 0W30 if available.
The 0 gives great pumpability at cold startup, while the 30 matches the spec for hot viscosity. 90% of all engine wear occurs at starting, so the idea is to get the oil flowing into the engine as soon as possible.
The 0 gives great pumpability at cold startup, while the 30 matches the spec for hot viscosity. 90% of all engine wear occurs at starting, so the idea is to get the oil flowing into the engine as soon as possible.
Paul
#123
I used to have a 3.8 E type and unknown to me when I bought it, the block had been bored 0.030" over and one piston and rings was standard, the other 5 were 30 over. I put 5W30 in it at first and I was going through a litre every 60 miles! I switched to 20W50 and it dropped to a litre every 300 miles. After I changed the piston to match the other it went to a litre every 400 miles.
The following users liked this post:
Lucielle (10-25-2021)
#124
#126
I've been reading posts about tire sizes but it's more about different size rims with off sets that will fit. My question is what is the highest number that will fit on the stock rims in front and back. I will probably stick with a 225/60/15 tire but I want to know if say a 275/60 would fit rear or 255/60?
#127
I've been reading posts about tire sizes but it's more about different size rims with off sets that will fit. My question is what is the highest number that will fit on the stock rims in front and back. I will probably stick with a 225/60/15 tire but I want to know if say a 275/60 would fit rear or 255/60?
I dunno the largest size that will fit the car.
However, the sizes you mention are too wide to properly fit on the stock wheels, which are only 6.5" wide. Jaguar itself pushed the limits a bit using a 235/60 on those 6.5" wide wheels.
When you squeeze a too-wide tire onto a too-narrow wheel the tire wont work right....and looks a little weird to boot !
Cheers
DD
The following users liked this post:
Lucielle (10-25-2021)
#128
I'd probably try something a bit lighter, like 0W40 and see what the oil consumption does. If it's excessive, then go heaver. What does the owners manual call for?
I used to have a 3.8 E type and unknown to me when I bought it, the block had been bored 0.030" over and one piston and rings was standard, the other 5 were 30 over. I put 5W30 in it at first and I was going through a litre every 60 miles! I switched to 20W50 and it dropped to a litre every 300 miles. After I changed the piston to match the other it went to a litre every 400 miles.
I used to have a 3.8 E type and unknown to me when I bought it, the block had been bored 0.030" over and one piston and rings was standard, the other 5 were 30 over. I put 5W30 in it at first and I was going through a litre every 60 miles! I switched to 20W50 and it dropped to a litre every 300 miles. After I changed the piston to match the other it went to a litre every 400 miles.
Unrelated. Do the plastic caps come off the lug nutz? Even then tire wrench provided doesnt fit over them?
#130
Rescue119,
It seems that you have the nuts with the stainless / chrome cover caps. The nuts have a habit of corrosion building underneath causing those caps to swell. Use a fractionally bigger socket and, once, off, change the wheel nuts or considering removing the caps, cleaning the nuts and leaving them bare if in good condition.
Paul
It seems that you have the nuts with the stainless / chrome cover caps. The nuts have a habit of corrosion building underneath causing those caps to swell. Use a fractionally bigger socket and, once, off, change the wheel nuts or considering removing the caps, cleaning the nuts and leaving them bare if in good condition.
Paul
#131
Rescue119,
It seems that you have the nuts with the stainless / chrome cover caps. The nuts have a habit of corrosion building underneath causing those caps to swell. Use a fractionally bigger socket and, once, off, change the wheel nuts or considering removing the caps, cleaning the nuts and leaving them bare if in good condition.
Paul
It seems that you have the nuts with the stainless / chrome cover caps. The nuts have a habit of corrosion building underneath causing those caps to swell. Use a fractionally bigger socket and, once, off, change the wheel nuts or considering removing the caps, cleaning the nuts and leaving them bare if in good condition.
Paul
#134
#135
You can grab the actually nut and move that cover piece back and forth about 1/8th inch.
#136
#138
Exactly but 7/8th still doesnt fit over them. Probably rusty underneath and swollen. Oh well I ended up not needing to take the wheel off for now.
#139
#140