By-pass valves, blocked or not?
by David Chase
The stock, and when new, by-pass valve is rated at ~10PSI in pressure difference between oil entering the filter and leaving the filter. This information is from the factory service manual. None of us know how well the spring in the valve holds up after at least 13 years and like my car >192K. The actual spring pressure to open the valve is likely much less. We did was test the filters with the by-pass plugged to see how much oil pressure they would allow based on filter flow alone.
AC PF47 ...... 15 PSI
Remote large filter (no names) ........ 20 PSI
Biggie with PF24 ........ 22 PSI
This was tested on the same motor, oil, warm, one filter after the other, on the same day. Oil was 15W50 Mobil One. Here you see a possible pressure difference in the system of 7 PSI just at idle in this motor based just on filter flow rates. At higher RPMs it could be much more, but was not part of the test. Taken further, it could be reasoned that this motor would be by-passing oil with the car running at moderate RPMs even after it was warmed and cruising down the road ........ if the by-pass was not blocked. The 1980 Buick Free Spirit Power Manual says there is a pressure loss of over 35% in the 40 series oil filters. This means 100 PSI in and only ~60 PSI at the block. That will open any factory by-pass. And this is Buick factory information, in print. Now go back and review some old gnttype.org list traffic and look at the number of people who report seeing 60 PSI on their oil gauges and they are running the PF 47 filters. From what Buick says, there is a lot of by-passed oil in their motors. Many people are using the PF47 with the by-pass open and seeing very good pressures. But our tests isolating the filter for the flow tests, and the Buick information, show the filter just will not flow that much oil. That extra pressure has to come from somewhere. Please remember the pressure reading is taken at the block and after the filter/by-pass valve. Also you will be able to find people on the list who have tried blocking the by-pass and running the small filters. It can not be done. Even warm this will crush the filter internally, blow the sealing o-ring, and at times even expand the filter case. Again all that extra pressure is what normally goes through the by-pass. And this is with the motor warm, not just the cold start problems that are often worried about. By-passed oil is unfiltered oil. There is the problem, no matter what else you can say. We are just very lucky to have the quality of oils we currently can buy. A motor can run with no filter at all. The old VW Bug has just a screen in the sump which is cleaned out at the oil change. And they went miles and miles. Our problem is when we get some type of internal damage which puts trash in the oil. Like a head gasket blown or a wiped cam lobe. This trash can do serious damage to bearings in the motor and turbo. The only way to avoid this damage is to ensure all the oil in the system gets filtered, all the time. Right now the best solution is to do exactly what Buick says in the Power Manual. Plug the by-pass and use a larger filter. This has been almost a small novel, but I hope you can understand why Lonnie, and I, believe in blocking the by-pass in a performance motor. There is the old saying....."An ounce of prevention is worth a pound of cure."
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