Good Vacuum???
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Lead Driver
Joined: Dec 2012
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From: Wentzville, MO
Good Vacuum???
Posted this on the Y-Block site as well
I Have rebuilt my 292; bored .030, "G" heads with hardened seats, valve job, high ratio rockers that are pressurized, "B" 4 barrel intake with a 390 CFM Holley, PETRONIX III IGNITOR & Flamethrower and a set of Ram Horn exhaust manifolds.. Everything else is Std or stock. I am running a C4 auto transmission via a Flat-O adapter and power steering.
I haven't got to take my F100 out for a drive yet (still working the column & steering wheel out) but when running in neutral I'm only pulling 12" to 13" of vacuum. I am wanting to run power brakes but am told I need 16" to 18" of vacuum.
Any thoughts on why my vacuum is not closer to the magic 18" vacuum number?
I Have rebuilt my 292; bored .030, "G" heads with hardened seats, valve job, high ratio rockers that are pressurized, "B" 4 barrel intake with a 390 CFM Holley, PETRONIX III IGNITOR & Flamethrower and a set of Ram Horn exhaust manifolds.. Everything else is Std or stock. I am running a C4 auto transmission via a Flat-O adapter and power steering.
I haven't got to take my F100 out for a drive yet (still working the column & steering wheel out) but when running in neutral I'm only pulling 12" to 13" of vacuum. I am wanting to run power brakes but am told I need 16" to 18" of vacuum.
Any thoughts on why my vacuum is not closer to the magic 18" vacuum number?
Fleet Owner
Joined: Jul 2004
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From: NM
If you've got new rings, I would avoid running it until you can drive it with a load on the rings. That "may" be part of your low vac condition, rings not seating properly. But that low sounds as much like an air leak.
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Joined: Apr 2016
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Have you checked basic stuff like, base ignition timing is not advanced far enough and/or idle A/F mixture is much too rich? Are you certain the cam is stock? As far as ring seat, a friend of mine owns a machine shop and has told me if the cylinders are honed correctly, rings seat pretty fast. Usually by the time the cam has went through the initial break in. I think if you can get 14" to 15" vacuum at idle in gear your power brakes will work pretty good. I also have a 69 Mach 1 with power brakes, with 14" vacuum at idle the power brakes work good.
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Lead Driver
Joined: Dec 2012
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From: Wentzville, MO
I got a dual groove damper from somewhere (can't remember where) that I added to run the power steering. The timing marks aren't very readable. I marked TDC when I put the engine back together and since I can't read the marks I timed it by ear and best vacuum. Set the Holley to specs and the auto choke is working. But still only 12" to 13" of vacuum. Seems to run and start alright but like I said earlier I haven't been able to drive it yet. I also have a Holley 450 CFM new that I pulled out of the box & installed. Same low vacum issue.
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Lead Driver
Joined: Dec 2012
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From: Wentzville, MO
Forgot to add. I installed a PCV to the engine. Hooked it from the lifter valley pan to the Holley PCV port. If you pull the line from the back of the Holley the engine dies. I also sprayed WD40 around the intake runners & around the carb. No change in idle. Thinking as I type; I haven't tried capping the port on the Holley to see if that makes a difference.
Elder User
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You might be on to something there regarding the PCV valve. I have heard a PCV valve that is too large will create low engine vacuum problems. I don't have any personal experience with that. Aside from vacuum, will there be any issues with the PCV drawing in oil because of where it is located? When they are in valve covers there is typically some type of baffling below the PCV valve to stop oil from getting to it.
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Lead Driver
Joined: Dec 2012
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From: Wentzville, MO
You might be on to something there regarding the PCV valve. I have heard a PCV valve that is too large will create low engine vacuum problems. I don't have any personal experience with that. Aside from vacuum, will there be any issues with the PCV drawing in oil because of where it is located? When they are in valve covers there is typically some type of baffling below the PCV valve to stop oil from getting to it.
This is how it was originally.
Here's mine.
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Fleet Owner
Joined: Jul 2004
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From: NM
I had that problem with PCV on my flathead. I had a number for a PCV valve, supposed to be for a 3.8L Buick. Turns out it was for 5.0 and 351 Fords. I had a hard time getting idle mix and speed correct. Switched to a PCV used on many Toyotas, Fram FV-306. A much better built piece, too. It worked much better. I didn't check vacuum before/after tho.
Believe Nothing
Joined: Jun 2004
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From: W. Central FL.
That vacant bolt hole in the head, just to the rear of the brass fitting is sometimes overlooked during final assembly. Left open, they will throw oil out at speed . Possibly affect crankcase vacuum as well.
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Lead Driver
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From: Wentzville, MO
I got a PCV for a 302. I figured it was close enough for a 292 + .030 or 296.
Thanks for the observation Ray, that picture was taken during assembly. It is were I attach an engine to cab ground & a short bolt so it doesn't bottom out and hit the pushrod. Also the spark plug wire loom is turned upside down in that picture. AND The larger hole on the opposite side now has the distributor in it.
Thanks for the observation Ray, that picture was taken during assembly. It is were I attach an engine to cab ground & a short bolt so it doesn't bottom out and hit the pushrod. Also the spark plug wire loom is turned upside down in that picture. AND The larger hole on the opposite side now has the distributor in it.
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