400 Engine Compression
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4wd Low
Joined: Nov 2003
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400 Engine Compression
This is my first time to the site so I hope that this is the right place to ask these questions.
I have come accross a complete 78 400 for around $600. The guy who is selling it said that the dry compression reading varies from 115 to 126 PSI. When I asked him to do a wet one he said that they went up to 125 - 135 PSI a slight gain of 9 to 11 psi per cylinder.
I guess my question is, is this good compression or is it on the low side. I have been told that it should be at least 145 - 150 PSI to be a good motor. It doesn't burn any oil and I was told that it only has under 20,000 on a rebuild. It was ran for 200 miles the truck was totaled and then sat for 4 years and then used for the last year or so.
What is good compression and is it possible that the compression was lowered with different pistons at rebuild. Any help would be appreciated.
I have come accross a complete 78 400 for around $600. The guy who is selling it said that the dry compression reading varies from 115 to 126 PSI. When I asked him to do a wet one he said that they went up to 125 - 135 PSI a slight gain of 9 to 11 psi per cylinder.
I guess my question is, is this good compression or is it on the low side. I have been told that it should be at least 145 - 150 PSI to be a good motor. It doesn't burn any oil and I was told that it only has under 20,000 on a rebuild. It was ran for 200 miles the truck was totaled and then sat for 4 years and then used for the last year or so.
What is good compression and is it possible that the compression was lowered with different pistons at rebuild. Any help would be appreciated.
Posting Guru
Joined: Aug 2002
Posts: 1,214
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From: Minneapolis, MN
The stock 400 isn't a high compression engine to start with so those readings don't sound too bad. Possibly on the low end of the "good" range but IMO nothing to worry about. Could be the manner in which the readings were taken. The key factor being that the cylinders are close to each other and it doesn't use oil. Another thing to look at would be the oil pressure after it has run for a while, preferably down the highway (warm). These things are notorious for having the oil pressure drop away when they get hot, almost as bad as the FE's. It's amazing how long they will go though with very little oil pressure at idle. A good rule of thumb is 10 pounds for every 1000 RPM. Of course, it all depends on how you are going to use it.
Price? I don't know. I have bought complete trucks for less but they are usually basket cases. I've got 3 trucks to get rid of now (2 with 400's). Just bought a super duty so they're going cheap. I just hate to part with them.
Welcome to FTE
Price? I don't know. I have bought complete trucks for less but they are usually basket cases. I've got 3 trucks to get rid of now (2 with 400's). Just bought a super duty so they're going cheap. I just hate to part with them.
Welcome to FTE
Thread Starter
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4wd Low
Joined: Nov 2003
Posts: 12
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400 engine compression
Thanks for the reply. I currently have a 351 m in the stock 77 F250 High Boy that is in need of a rebuild badly. In your opinion what is a better motor to have the 351m or the 400.
Lead Driver
Joined: Nov 2000
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From: Albuquerque
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Elder User
Joined: Nov 2003
Posts: 987
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From: Bass Lake, CA
of course the 400...
also, for the 351m/400, i think the oil pressure guuidelines should read, 1 PSI for every 1000 rpm :-) cuz mine runs fine, and NEVER has over about 10 psi, but normally just sits to the left of the guage...right on the little stop.
i put new main bearings, and a HV oil pump in it...made a slight difference.
also, for the 351m/400, i think the oil pressure guuidelines should read, 1 PSI for every 1000 rpm :-) cuz mine runs fine, and NEVER has over about 10 psi, but normally just sits to the left of the guage...right on the little stop.
i put new main bearings, and a HV oil pump in it...made a slight difference.
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Senior User
Joined: Oct 2003
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From: in a house
torque, what a feeling in my Ranchero w/ the 400. wieght is 4,260 lbs with me and a 1/4 tank of gas and it will snap your neck in 1st gear if you goose the gas at or over 1400 rpm.
danlee, 6 grand? with bottom end work right? I only push mine to about 4800, sometimes 5 when she gets squirrely on a turn.
danlee, 6 grand? with bottom end work right? I only push mine to about 4800, sometimes 5 when she gets squirrely on a turn.
Postmaster
Joined: Aug 2001
Posts: 4,270
Likes: 5
From: Delaware
What is the limitation to prevent a 400 from turning 6000 RPM?
To turn 6000 RPM a 400 piston must average 200 in/sec. In a 351C or 351W turning 8000 RPM, the piston travels 233 in/sec.
The bearings will follow a similar analysis.
OK, I wouldn't recomend it everyday.
To turn 6000 RPM a 400 piston must average 200 in/sec. In a 351C or 351W turning 8000 RPM, the piston travels 233 in/sec.
The bearings will follow a similar analysis.
OK, I wouldn't recomend it everyday.
