Bypass vs. full flow oil filters

Posted by su8overdrive On 2012/6/4 16:46:32

Tho' i posted this on this Forum under the oil filter canister thread begun by new member '46 Deluxe Clipper, it occurs that this might be of interest to those not clicking on that specific topic. Most Packards, if they have an oil filter, have a bypass, so i thought the below interesting:

Expanding a bit on the above discussion of bypass vs. full flow filtration, here's an interesting letter in the
2006, No.5 Auburn-Cord-Duesenberg Newsletter by Al Light,
Williamsport, PA, which is, coincidentally, where Lycoming was, perhaps still is, based:

"I really enjoyed the editor's comments on the club website about the real value of having an oil filter.

In the early 1950s while at Ford Engineering I was fortunate to hear an argument of "Full Flow" vs. "Partial Flow" oil filtration. The pro "Full Flow" people seemed to be more interested in the positive sales pitch that this system provided while the "Partial Flow" people seemed to
have a more solid engineering approach. The chief engineer from Fram Corporation stated the big difference was the allowable filter pressure drop. The larger the pressure drop, the finer and more material will be caught. With the full flow system you are restricted to approximately 10 pounds per inch drop across the element, and also require a bypass valve that will open when this pressure drop is exceeded. This of course is to protect the engine from oil starvation in the event of a plugged filter. With the partial flow, or sometimes called bypass system, the pressure drop is full engine oil pressure since
the oil discharges from the filter directly into the oil pan and not under any pressure. The system is automatically
bypassed if the filter plugs. It was also noted that a denser filter element is used on the partial flow system.
The Fram engineer summed up his thought by stating the answer to the argument was obtained by considering the following question: "Is it better to filter all of the dirt from part of the oil or to filter part of the dirt from all of the oil?"

He also stated that with the full flow system the bypass valve may open under cold starts and sometimes opens at high engine RPM when the allowable pressure drop may be exceeded. This results in no filtration under these conditions. His other comment was that the biggest advantage to full flow was to catch the machining chips, etc. in new engines. Fram's final approach was to propose the use of a combination system that did both full and partial filtering. I'm sure that this would have increased their filter business. I am not aware of any such system ever being used."


I can only add that an auld aircraft/auto mechanic/machinist/pilot and blown '37 Cord 812-owning friend said they were taught in tech school that a bypass filter will eventually filter all your engine's oil after a drive of 60-70 or so miles.

Another friend with '59 and '63 Lusso Ferraris showed me that one of these cars, and i forget which, came with both full and bypass filters from the Modena factory, as do some more recent trucks and heavy equipment,some of the latter coming only with bypass filtration. Such equipment is likely to run hour after hour, shift after shift, so all the oil would easily be filtered.

As we've observed, for all the technological breakthroughs since Packard folded, much engineering is cost engineering,
but i don't know if this is strictly the case regarding bypass vs. full flow oil filtration with internal combustion engines.

Perhaps the above will be of some solace to those of us with Packards with the usual bypass filters. I leave further discussion to others, but would especially like to hear from engineers like Al Light, above, and members of the SAE.

IMHO, '46 DeluxeClipper, if you clean out your oil filter canister as the gent above describes, perhaps drop your engine oil pan to get rid of the inch-deep sludge and lead coating its bottom, assuming your engine has never had serious work since leaving East Grand,

then use a major brand 10W/30 oil and change it and the filter periodically, you'll be in fine fettle. If your original engine's worn but still serviceable, and you don't want or need to rebuild it, you might use a heavier weight oil.

Finally, never, ever start the engine unless you're going to drive the car at least 17-20 miles on the highway to prevent the formation of sludge, varnish, carbonic acid, the latter strong enough to etch concrete so you can imagine what it does to your bearings and rings. If you want to wash, wax, or work on your car outside, push it out the garage.

We still shake our heads at the alleged car buffs who start their engine and run it for a few minutes to show it off in the garage, then switch off the ignition and let the car sit for a month or two or longer.