compression
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Junior User
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compression
i have a 70 f250 camper special with a 390. i want to do a compression test on the engine. can anyone tell me the specs on this. i know its old and will prob be a little low but i want to know how far i shoud go in the engine. also im looking to replace the cam, im looking for a cam that will be good for daily driving and pulling a 6000lb camper. any help will be apreciated. thanks
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What you are looking for in an older engine is for the cylinder pressures to be relatively close. Within 20-25%.
I would not put a cam in an older engine. Add-ons like intake, carb and headers would be as far as I'd go, as whatever is nearly worn out will show itself in short order as the power goes up. And if you are in it deep enough to change the cam, go all the way.
Does your truck still have the stock exhaust manifolds? The easiest upgrade is headers and dual exhaust. Stuffing more in does little good if it can't get out, and most 390s have pretty bad exhaust manifolds.
First things first...
I would not put a cam in an older engine. Add-ons like intake, carb and headers would be as far as I'd go, as whatever is nearly worn out will show itself in short order as the power goes up. And if you are in it deep enough to change the cam, go all the way.
Does your truck still have the stock exhaust manifolds? The easiest upgrade is headers and dual exhaust. Stuffing more in does little good if it can't get out, and most 390s have pretty bad exhaust manifolds.
First things first...
Fleet Owner
Joined: Jun 1999
Posts: 24,552
Likes: 74
From: Creedmoor, North Carolina
Big, I would suggest doing the test first and go from there. Trying to set or find a bench mark has no real value. You got what ya got.
Your truck is setup to pull from the factory and I agree with 85. I wouldn't recommend going into the engine without a complete overhaul.
John
Your truck is setup to pull from the factory and I agree with 85. I wouldn't recommend going into the engine without a complete overhaul.
John
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Joined: Mar 2008
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Hey guys,
I might have a similar problem with my F250 1970 as "big 390".
390 w C6 tranny, engine runs really good, but there is no power left when driving.
I had it at a chop several months, they changed rocker arms, rebuilt Holley carburetor etc etc, everything thats in the top... pretty much. Still about 100psi...
Anyone having a clue what to do next?
Have done a cylinder test and they are all at the same level.
Exhaust manifolds nice and new aluminum, also changed headers to aftermarket parts, new custombuilt Magnaflow 2,5" exhaustsystem....
A lot of the parts are almost new, like distributor and fuel filter etc etc.
Me and the mechanics have runned out of ideas...
My first thought were the piston rings... but as a girl -Im never seriously taken
(it used to have lots of power, and its been slowly going away..)
I might have a similar problem with my F250 1970 as "big 390".
390 w C6 tranny, engine runs really good, but there is no power left when driving.
I had it at a chop several months, they changed rocker arms, rebuilt Holley carburetor etc etc, everything thats in the top... pretty much. Still about 100psi...
Anyone having a clue what to do next?
Have done a cylinder test and they are all at the same level.
Exhaust manifolds nice and new aluminum, also changed headers to aftermarket parts, new custombuilt Magnaflow 2,5" exhaustsystem....
A lot of the parts are almost new, like distributor and fuel filter etc etc.
Me and the mechanics have runned out of ideas...
My first thought were the piston rings... but as a girl -Im never seriously taken
(it used to have lots of power, and its been slowly going away..)
Fleet Owner
Joined: Jun 1999
Posts: 24,552
Likes: 74
From: Creedmoor, North Carolina
John
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what im trying to find out is what should the psi for a compression test should i consider acceptable. i have put on headers with dual exhaust. i have removed the points and replaced with crane cams system and a new coil. new plugs, wires and fuel pump.it runs good and all but it doesnt have the power for pulling my camper especially up any slight grade hills i get slowed down to 45 mph with my foot to the floor in drive. the kick down linkage isnt on it becouse it wont fit prob becouse its old and someone has changed something or left something off i guess. im wanting to replace intake and carb but a cam is cheaper.
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Fleet Owner
Joined: Jun 1999
Posts: 24,552
Likes: 74
From: Creedmoor, North Carolina
Last year, I had issues and had two cylinders that were dead. I had head work done and because of sloppy work at the machine shop the new guides were shot and the valves were not holding compression.
I suspect from what you have described you have issues other than worn rings. The truck FE 352, 360, 390 pull like caterpillars. Until recently the FE was one of the best pulling engines that Ford ever built.
The headers don't do much for pulling, they do more for high rpms. A factory 2 barrel setup should tow anything that you want.
Go to Autozone or another place and borrow/rent a compression gauge and do the test.
From what I read you may have two or more sparkplug wires crossed, a bad vacuum leak but I doubt your compression is shot.
You gotta do the test, worring about it aint gonna tell you anything.
John
I suspect from what you have described you have issues other than worn rings. The truck FE 352, 360, 390 pull like caterpillars. Until recently the FE was one of the best pulling engines that Ford ever built.
The headers don't do much for pulling, they do more for high rpms. A factory 2 barrel setup should tow anything that you want.
Go to Autozone or another place and borrow/rent a compression gauge and do the test.
From what I read you may have two or more sparkplug wires crossed, a bad vacuum leak but I doubt your compression is shot.
You gotta do the test, worring about it aint gonna tell you anything.
John
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From: Simi Valley, CA
Y'all believe you have 390 engines, but are they?
They could be 360's, 352's, who knows?
These trucks are over 36 yrs old, so there's no telling what might be there now.
Measure the stroke: 352/360 = 3.50" / 390 = 3.78" / 410/428: 3.98"
Was it a 390 originally? The 4th digit of the VIN is the engine code 1968/72: H = 390 2V / Y = 360 2V.
Because they look identical, on sale day, 352 & 360 engines magically become 390's.
They could be 360's, 352's, who knows?
These trucks are over 36 yrs old, so there's no telling what might be there now.
Measure the stroke: 352/360 = 3.50" / 390 = 3.78" / 410/428: 3.98"
Was it a 390 originally? The 4th digit of the VIN is the engine code 1968/72: H = 390 2V / Y = 360 2V.
Because they look identical, on sale day, 352 & 360 engines magically become 390's.
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Joined: Apr 2004
Posts: 34,499
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what im trying to find out is what should the psi for a compression test should i consider acceptable. i have put on headers with dual exhaust. i have removed the points and replaced with crane cams system and a new coil. new plugs, wires and fuel pump.it runs good and all but it doesnt have the power for pulling my camper especially up any slight grade hills i get slowed down to 45 mph with my foot to the floor in drive. the kick down linkage isnt on it becouse it wont fit prob becouse its old and someone has changed something or left something off i guess. im wanting to replace intake and carb but a cam is cheaper.