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Discussion Starter #1
400 on a 78 Bronco. Not too bad. Do you guys think the low cylinder is too low or within the realm of good?

And that #8 plug hole is a b*%ch to get to....

1 - 160
2 - 154
3 - 161
4 - 145
5 - 150
6 - 135
7 - 160
8 - 155
 

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Numbers 4 and 6 are of concern. I would do a leak down test to determine if it's rings or valves. Could be head gasket too.

Use shop air to pressurize the cylinders individually. Listen for air escaping into the intake, exhaust or block. Then look for bubbles in the coolant.
 

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#4 is what I would consider acceptable, vs your top reading, #6, not so much.
 

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Just out of curiosity, what prompted you to do a compression test?
 

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Discussion Starter #5
There was nothing specific that prompted the compression test. I could tell when I bought it that the guy who owned it before me had cut corners (like the window cranks being glued on) which made me wonder what corners he had cut with the engine. It runs fairly well. I have also been considering rebuilding a 460 in the garage and swapping out the 400. If I would have had one really bad cylinder it would have been a no brainer and I would not drop another dime into the 400. As it sits now I'm not quite sure what I'm going to do.

To me, the 135psi cylinder is obviously low, but is it low enough to worry about? Is it low enough to prompt a dedicated 460 build in the garage? Is it too low to consider swapping the intake and carb on the 400 (it still has the 2bbl stock setup)?

All of these questions contributed to the compression test.
 

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Start building the 460 and have it ready to swap. The 400's days are numbered with those lower cylinders.

My two cents...it's worth what you paid for it.

Russ...
 

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There was nothing specific that prompted the compression test. I could tell when I bought it that the guy who owned it before me had cut corners (like the window cranks being glued on) which made me wonder what corners he had cut with the engine. It runs fairly well. I have also been considering rebuilding a 460 in the garage and swapping out the 400. If I would have had one really bad cylinder it would have been a no brainer and I would not drop another dime into the 400. As it sits now I'm not quite sure what I'm going to do.

To me, the 135psi cylinder is obviously low, but is it low enough to worry about? Is it low enough to prompt a dedicated 460 build in the garage? Is it too low to consider swapping the intake and carb on the 400 (it still has the 2bbl stock setup)?

All of these questions contributed to the compression test.
No, it's not THAT bad.

If it seems to be idling well, has decent power, is not burning a ton of oil, and has at least some oil pressure above idle, I'd run it until one of those items I just mentioned changes (or you just feel like swapping motors at some point).
 

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Discussion Starter #8
Thanks for the input OX. It does run pretty well. I am running 20w-50 and manage to keep it right in the middle of the oil pressure gauge (stock gauge so who really knows), but it will go down to the line at a stop light when it is really warmed up (but goes back to the middle when things start flowing again). I use about a half a quart every two tank fill-ups, but I think I leak most of that out of the rear main. It has decent power, nothing earth shattering, but probably normal considering how badly Ford de-tuned the 400 in 78.

One odd thing that I noticed when I did the compression test. I had all the plugs pulled, WOT, etc. I could not turn the engine over by hand when I pulled on the fan. I figured that it should really turn easy with no compression. Does that seem normal to you guys?

And you are right, I am really just looking forward to a nice slow 460 rebuild over the summer in the garage. Take my time, measure, calibrate, research, etc. I plan to rebuild a C6 and a NP205 as well and swap out the whole drive train. There is some cavitation that I can feel now and I have to believe that it is somewhere within the tranny and TC. Maybe I'm wrong.
 

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Don't forget the crankshaft is still connected to that C6. It'll have a bit of friction to it. I'd be surprised if it easily turned by pushing on the fan. It should turn fairly easily with a wrench on the harmonic balancer bolts, though, or a strap around the hb.
 

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Thanks for the input OX. It does run pretty well. I am running 20w-50 and manage to keep it right in the middle of the oil pressure gauge (stock gauge so who really knows), but it will go down to the line at a stop light when it is really warmed up (but goes back to the middle when things start flowing again). I use about a half a quart every two tank fill-ups, but I think I leak most of that out of the rear main. It has decent power, nothing earth shattering, but probably normal considering how badly Ford de-tuned the 400 in 78.

One odd thing that I noticed when I did the compression test. I had all the plugs pulled, WOT, etc. I could not turn the engine over by hand when I pulled on the fan. I figured that it should really turn easy with no compression. Does that seem normal to you guys?

And you are right, I am really just looking forward to a nice slow 460 rebuild over the summer in the garage. Take my time, measure, calibrate, research, etc. I plan to rebuild a C6 and a NP205 as well and swap out the whole drive train. There is some cavitation that I can feel now and I have to believe that it is somewhere within the tranny and TC. Maybe I'm wrong.
The low oil pressure and compression issue pretty much the M block modus operandi. There are those on here with M-blocks with those exact symptoms....yet they run for decades like this!

If it were my truck, I would run the crap out of it and take my time building a 460.

The "cavitation" is not normal. It could be vibration from a dying cylinder, but it is more likely a bad U-joint/CV. Check your driveshafts.
 

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How did the plugs look on #4 and #6 cylinder, that will tell you if your burning oil. If plugs were clean, then I would look at replacing the head gaskets. Replacing the rear main seal isn't that hard of a job, just requires you to drop the oil pan, remove the rear main bearing cap and slightly loosen the other main bearing caps so the crank will drop away from the upper rear main.
 

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Discussion Starter #12
#4 and #6 both looked good. All the plugs looked good, maybe just a hair lean which is odd since the mixture screws are turned out close to 4 turns.

The rear main sounds like a project, I have taken off and re-sealed leaky oil pans in the past (that the dealer had overtightened), but I have never taken off a bearing cap. I guess I'll research...
 

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Thanks for the input OX. It does run pretty well. I am running 20w-50 and manage to keep it right in the middle of the oil pressure gauge (stock gauge so who really knows), but it will go down to the line at a stop light when it is really warmed up (but goes back to the middle when things start flowing again). I use about a half a quart every two tank fill-ups, but I think I leak most of that out of the rear main. It has decent power, nothing earth shattering, but probably normal considering how badly Ford de-tuned the 400 in 78.

One odd thing that I noticed when I did the compression test. I had all the plugs pulled, WOT, etc. I could not turn the engine over by hand when I pulled on the fan. I figured that it should really turn easy with no compression. Does that seem normal to you guys?

And you are right, I am really just looking forward to a nice slow 460 rebuild over the summer in the garage. Take my time, measure, calibrate, research, etc. I plan to rebuild a C6 and a NP205 as well and swap out the whole drive train. There is some cavitation that I can feel now and I have to believe that it is somewhere within the tranny and TC. Maybe I'm wrong.
You still have all the valve springs you are pushing down randomly during the full 4 strokes. Some spots will feel like it sticks, as more of the valve springs are being compressed than at other times. Put a socket wrench on the crank pulley bolt (sometimes can squeeze it in from underneath, but might need to remove shroud) and see how it feels then. M-blocks will run for years with literally "0" oil pressure shown at idle. As long as it comes up off idle, you are OK.

Also, what order did you do the compr test in.
Factors like weakening battery, cylinder wall wash down can all affect later readings in the tests.
 

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Discussion Starter #14
I generally have good oil pressure when driving, it will sink down to the "N" or a hair below when hot at a stop light.

I did all the cylinders in order but had a battery charger hooked up the the battery the whole time so I should be good. What I should have done was to squirt some oil into the cylinder and see what happened.
 
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