FE Power Forums
FE Power Forums => FE Technical Forum => Topic started by: rockittsled on January 25, 2015, 10:53:10 PM
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I am curious about what the idle vacuum was on the different manifolds and did the vacuum change notably between the different engines with the same manifold type. I am most curious about the BT 4v, the Performer RPM, and the Ford "T" manifold because that is my targeted goal for my truck. Starting with a 390 stroked to 416ci or 433ci, it appears to match to the 425hp 428ci, but with the MR ports. If the info is easily accessible without plowing through
reams gigs of files. If not, that is fine.
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Unfortunately, I don't have any data on idle vacuum. The dyno starts recording data at the beginning of the pull, usually 2500 or 3000 RPM, and then finishes recording when you reach the peak RPM of the pull. So, I don't have any record of what the idle vacuum was for the various manifolds. Sorry - Jay
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FUDGSICLES!! Oh well, I had considered that might be the case about the data record, but thought I would ask. I am graphing out to compare those manifolds and I noticed how close all the CRs were and that came to mind. Glad I bought the book and thanks to you and everybody else that put time and effort and loaned parts into the book
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I don't have any hard data, but while breaking in a motor a couple years ago I tried the following intakes (just to make sure they all ran ok & didn't leak for future use), same engine, 440cid, early vanilla heads, late 2bbl 192-199 duration cam, flat log exhaust:
* 428pi intake
* 390hipo intake
* Edel RPM intake
* Ford iron "Z" intake (nearly same as "T" intake)
I didn't record data but didn't notice any change in idle or vacuum.
I've also run the streetmaster, it was down 1-2" vacuum, no surprise being a single plane.
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I am not sure why idle vacuum would change
Theoretically it could take it microseconds longer to come up to the full amount, but the engine is pulling the same air at idle against a closed carb, once it builds vacuum it doesn't know how big the plenum is.
Of course that is steady idle only, NOT with changes in throttle position
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Should not be any difference in vacuum readings between those manifolds if the timing, carburetor settings, etc., are all optimized. The camshaft design affects vacuum more than any other change for add on parts. Joe-JDC
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I was looking at the volume figures for the manifolds and the differences in volume and wondered if that would have any effect on the vacuum at idle. I assumed that the difference would have an effect on acceleration, but wasn't sure about steady state, at idle or cruise
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I ran a Offy Port-O-Sonic on my 390 and had a vacuum gauge plumbed in so I could get my power valves sorted out. Vacuum was 5"w.c. at idle. (cam was pretty frisky) A year later changed heads and went to a Performer RPM. Vacuum at idle was still 5"w.c.