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Discussion Starter #1
I recently rebuilt a 351w and began having misfire issues. Upon compression testing and leakdown, I discovered a burnt valve. I swapped out the late 70's heads for a set of used, working heads from a 1970 Cougar. Here are the before and after numbers:

Previous heads - 1978 casting (lower compression heads)
1 - 105
2 - 105
3 - 105
4 - 90
5 - 105
6 - 0 (burnt valve)
7 - 105
8 - 105
Oil pressure - 40 psi running 10w-40 stock Melling pump

Current heads - 1970 casting
1- 125
2- 125
3- 125
4- 115
5- 135
6- 135
7- 135
8- 125
Oil pressure - 15 psi running 10w-40

My concern is that now the oil pressure is MUCH lower than before the head swap. Would doing a head swap effect the oil pressure that much? Also, the oil change was fresh and now it has a very dark color like it has tons of miles on it. No signs of coolant btw. Any thoughts or ideas?
 

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A couple thoughts come to mind. First, cylinder #4 has issues that while not critical yet, I'd be keeping my eyes on it.

Second thought is that oil gets dark when it gets dirty. Did the oil pick up a lot of dirt from the new heads or from debris left in the lifter valley?

There is nothing, directly, that would cause a drop in oil pressure just from swapping heads. Indirectly, there could be a number of possible explanations ranging from oil contamination (cleaning solvents, dirt, fuel, etc.), debris plugging the screen in the oil pump pickup, debris plugging the drainback holes in the heads, debris that has damaged the oil pump, pressure relief valve or an engine bearing, or a defective oil filter.
 

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If you had the timing off and kept trying to start it, it may have excess fuel in the oil that drained down through the cylinders? Thinner oil, washed walls creating the color? All I can think of on the spot....
 

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Discussion Starter #4
Bartl, thanks for confirming my thoughts. I will do an oil change and see if that helps.

Yeah, #4 is iffy. I am hoping to get through the season with it the way it is and tear it down over the winter. When we rebuilt the engine that cylinder we found that bored to .020 over by the PO while the rest were .010.
 

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Discussion Starter #5
If you had the timing off and kept trying to start it, it may have excess fuel in the oil that drained down through the cylinders? Thinner oil, washed walls creating the color? All I can think of on the spot....
Interesting thought, time for an oil change.
 

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I recently rebuilt a 351w and began having misfire issues. Upon compression testing and leakdown, I discovered a burnt valve. I swapped out the late 70's heads for a set of used, working heads from a 1970 Cougar. Here are the before and after numbers:

Previous heads - 1978 casting (lower compression heads)
1 - 105
2 - 105
3 - 105
4 - 90
5 - 105
6 - 0 (burnt valve)
7 - 105
8 - 105
Oil pressure - 40 psi running 10w-40 stock Melling pump

Current heads - 1970 casting
1- 125
2- 125
3- 125
4- 115
5- 135
6- 135
7- 135
8- 125
Oil pressure - 15 psi running 10w-40

My concern is that now the oil pressure is MUCH lower than before the head swap. Would doing a head swap effect the oil pressure that much? Also, the oil change was fresh and now it has a very dark color like it has tons of miles on it. No signs of coolant btw. Any thoughts or ideas?
Just to be clear - are the before/after oil pressure readings before/after the engine rebuild or ? Were the 70 heads installed at the time of rebuild or after?

Paul
 

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Discussion Starter #7
Hey Paul, Both oil pressure readings were after the rebuild. The 77 heads were initially installed after the rebuild. The engine ran fine for awhile with those heads. Then the 70 heads were installed after the burnt valve appeared.
 

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Discussion Starter #8 (Edited)
I drained the oil and let the car sit for 2 days to let gravity drain what it wanted to drain, changed the oil filter and replaced with 15w-40 Shell Rotella. Now the gauge is reading 50psi at idle and goes up to 60psi when revved. She sounds much better now.
 
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