O-470-R Fuel Pressure

NordicDave

NordicDave

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I'm having difficulty locating the fuel pressure operating range for a carbureted TCM O-470-R.
The TCM maintenance manuals appears silent on fuel pressure values.

Recollection is 4-5 psi.

Thanks.
 
Really appreciate the link!

The min/max parameters on page 3 Note 2: are listed as 15.5" Min and 6 psi Max.

question:
Why is the minimum pressure listed as 15.5 inches and not in PSI? How does inches relate to PSI in this context?
 
NordicDave said:
Why is the minimum pressure listed as 15.5 inches and not in PSI? How does inches relate to PSI in this context?
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There are various methods of measuring pressure. Usually when you see "15 inches" it refers to 15 inches of water column. And there are conversion formulas to move between the different values. If I had to guess 15" wc is probably less than 1 psi.
 
.56 psi to be exact, and when they say 15.5" in the TCDS? It is exact. It is interesting how the minimum FP pressure changed from psi in the early 470s to inches in the later ones while the max pressure limit stayed constant using psi. Maybe something to do with measuring equipment?
 
Stewartb said:
Maybe something to do with measuring equipment?
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Understanding this topic is a bit outside my pay grade, but from what I recall, yes it is related to the equipment but I believe it's more related to what reference point is needed. Pressure measurements are referenced either to a vacuum point or to a pressure standard point. One way will give you negative numbers and one way will give you only positive numbers. I think that is why some in the 470 TCDS the psi readings are a "minus." But I'm not 100% sure.
 
Bell206 said:
Understanding this topic is a bit outside my pay grade, but from what I recall, yes it is related to the equipment but I believe it's more related to what reference point is needed. Pressure measurements are referenced either to a vacuum point or to a pressure standard point. One way will give you negative numbers and one way will give you only positive numbers. I think that is why some in the 470 TCDS the psi readings are a "minus." But I'm not 100% sure.
Click to expand...

Ideally, all pressures should be measured in absolute because the “zero” point in gauge pressure is somewhat fluid due to environmental conditions. But gauge pressure is how most non-engineering minds think so that is what is commonly used. Another example of this is evident in this thread, questioning the unit of measure. People in the US often think of pressure in PSI but there are plenty of other units of measure that may (or may not be) more appropriate.

My guess is that inches of water was used in this measurement because it would produce a fairly small number when measuring in PSI, and perhaps because the PSI pressure gauges found in most maintenance shops won’t really go that low.

As a side note, anyone who has worked on engine calibrations, either as a hobby or as a job should be relatively familiar with absolute pressures measuring in some unit of measure other than PSI. I have not worked with an ECU from any manufacturer yet that does anything else.
 
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