Rpm Question
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New User
Joined: Aug 2006
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Rpm Question
hello all, i have a 74 f250 with a 390, 4 speed. i have been reading some of these threads and read that your guys rpm's are way down compared to mine. now i have no idea if my motor has been built or not but the previos owner said it was stock. the motor has factory intake on it with a big "S" on it and headers with a 600 cfm carb. i can run this thing up to 5500 rpm and still is pulling like a frieght train, i may be able to go higher but makes me nervous!!!! just wondering if i'm doing wrong but it has held together for over a year and still running strong. thanks for any input!!!
Post Fiend
Joined: Jan 2003
Posts: 8,600
Likes: 4
From: Iowa
Huh... If that's the factory motor in that truck, the intake should NOT have a big "S" on it. Are you sure the motor was not rebuilt before? Someone may have made some modifications, specifically in the camshaft area.
Post Fiend
Joined: Jan 2003
Posts: 8,600
Likes: 4
From: Iowa
That would explain a lot. They were built right from the factory. Someone also might have rebuilt it and put a good cam in it.
Most of these truck motors have small cams and low compression.
Sounds like you got a good one; dont break it!
Most of these truck motors have small cams and low compression.
Sounds like you got a good one; dont break it!
FTE Leadership Emeritus
Joined: Jul 2002
Posts: 42,561
Likes: 423
From: Long Island USA
If it's a highboy, and certainly 4x4, it's definitely NOT the original motor!
390's never came in the 70's 4x4's.
Don't go much past 5500RPM - it's not so much that the valve-train can't take it (rockers, pushrods), more that the valve springs won't keep the valve-float under control and you'll break something.
If it's a 4bbl, from a '68 fairlane, I would guess it's the 10.5:1 motor?
390's never came in the 70's 4x4's.
Don't go much past 5500RPM - it's not so much that the valve-train can't take it (rockers, pushrods), more that the valve springs won't keep the valve-float under control and you'll break something.
If it's a 4bbl, from a '68 fairlane, I would guess it's the 10.5:1 motor?
