Ok, we have a discussion going on at my work on how a oil filter works. Mainly the internal bypass spring.

My dad's mechanic says that when the car is above idle, it bypasses the filter. I told him by doing that, all highway milage cars will not survive 30k miles and since most cars dont do alot of idling, it would be pointless to even put the filter on at all. But being that my dad owns the shop, that automatically means I dont know jack about crap. I am always open to new philosophys and theorys on cars, but from a design standpoint, that idea is dumb.

My dad and I say that years ago, when people actually changed their oil, the filters never had them. But with the introduction of the "Lets not do jack **** to our cars" fad, they installed them so when the oil filter plugs up, it bypasses the filter because a clogged filter will kill your oil pressure. Being that dirty oil is at least better than no oil, thats why they do it.

Do you guys have any proof one way or the other?

 

as far as i know, the by-pass valve opens only when pressure is real exessive, caused by...for example, a canadian winter cold start at minus 20 and down ( i live in Montreal and this winter was a killer), there is when is going to open, but on some magazine i've read, you can block it and have 100 filtered oil, BUT CONSIDERING, weather wise and FILTER wise.
Fernando.

 

The oil filter by-pass is in the oil filter adapter, not in the filter (at least on SBCs).
Not sure when they were first installed, but I believe they have been around a long time.

I've heard up to 10-15% of the oil by-passes the filter during normal operation (on a SBC); but I have no way of proving that.

LPE reports: "The disadvantage to this by-pass is that its lightweight spring allows un-filtered oil into the eng, especially in cold start-up conditions when press is high."

I plugged my L98 oil by-pass on the eng I'm currently building because I was concerned abt random debris being left inside my eng.
The eng was stroked, so there was a ton of filings all over everything.
I cleaned the eng good, but I'm still paranoid there's still a few filings inside there,, somewhere.

 

The bypass only opens when there is a pressure drop across the filter. In other words, if the filter was totally non-restrictive at all times, the bypass would never open; in reality, it opens for a short time at cold start-up, and whenever the filter gets stopped up with crud. As mentioned before, dirty oil is better than no oil at all.

There's no excuse for building a motor with filings in it. That's not a good reason for modifying the oil system. Next time you do that, take the engine to the quarter car wash after you get done with it, and spend a couple of dollars on plenty of hot, soapy water to clean it up.

I have never, ever, for any reason, in any motor I've ever built, modified the bypass; nor ever felt like there was any good reason to do so. The only times I've ever built a motor without one was back in the days of building blocks that came with canister filters, where I used the Fram adapter (with no bypass) specifically made for these blocks.

AFAIK the bypass does not bypass at all during normal operation, as long as there's a good clean oil filter and oil of the correct viscosity.

 

A better option to plugging it is to either use a stiffer spring, or shim existing spring. Oh, and don't leave the spring out either, did that once many moons ago, and engine lasted about 1000 miles before self destructing. OOPS
But to answer your question, the bypass is there basically to keep the filter from being hit by too much pressure and exploding or blowing off the block. I forget the exact amount of pressure the standard filter can take, but it's well within the capability of the stock pump if it hits a lot of restriction. If your rebuilding the block its a good idea to check it as a lot of times they get weaker with age.

 

The bypass won't protect the filter from excessive oil pump pressure. That isn't what it does or how it works.

It is "across" the filter; that is, it has the oil pump output / filter input on one side of it, and the filter output / engine on the other side. It has a fairly weak spring, such that it opens when there is a pressure difference of more than 5 psi or so across the filter. i.e. if the pump is putting out 60 psi, and the filter is restrictive enough that there's only 50 psi at its output, then the valve will open to allow oil through. If the pump's output is 200 psi (easily possible if the pump's regulator valve sticks), and the filter is of sufficiently low restriction that there is 197 psi on the engine side, then the bypass will not open; but the filter might burst.

If you leave the spring out, the engine will get mostly unfiltered oil, because the bypass will {drum roll please} bypass {cymbal crash please!!} the filter. So it can definitely cause excessive engine wear, but there would have to be some real issues for it to toast a motor in 1000 miles, namely lots of debris in the oil.

To give you some idea, old Chevrolet 6-cylinders in the 50s didn't even have an oil filter as standard equipment (it was available as an option, it hung from the intake manifold by 2 U-bolts), but often lasted hundreds of thousands of miles.

All that putting a stiffer spring in it will do, is make it not bypass until the filter restriction reaches some greater amount. Closing it off completely will simply force all the oil through the filter, all the time, with no path available around the filter to protect the engine in case the filter clogs.

 

Pre-1980 engines used a 10 PSI pressure relief spring/disc for the filter bypass. As long as there is a relatively clean filter on the engine, none of the oil will bypass under normal operating conditions. If you run a higher oil pressure relief spring, you may tend to get some bypass, but you're still getting filtration of the majority of the oil. If you use an incorrect filter (with too little media, incorrect flow holes, etc.), ignore the maintenance, use excessively viscous oil, or have excessive flow requirements from valve spring cooling manifolds or a worn engine, you'll likely open the bypass.

Incidentally, most ultra-clean hydraulic systems DON'T use full-flow filtration, but use a kidney-loop (bypass type) filtration that simply pumps a portion of the oil through filters to clean it. These systems commonly have contaminants no larger than 0.5 microns. The filters are so fine that even water molecules cannot pass through them. And they work just fine that way.

Anybody remember the old toilet paper oil filters?

 

It wasn't that I'm being sloppy. The block was returned to the machine shop after the latest round of grinding. Cam bearings (brand new ones) removed, and vatted again (for the third time). All the oil passages were scrubed, flushed and dried with air press.
Like I said before, I'm probably just being paranoid.



(I'll probably regret this, but since I don't have anything better to do at the moment, I'm going to argue this point. )

Filter protection from the bypass depends on how the oil press became excessive.

If the oil press regulator became stuck in the closed position and eng was revved-up, the filter by-pass would not prevent excessive press from reaching the oil filter. That's what you (RB) said and I agree.

However, since It's my understanding that the oil press bypass cannot flow the same capacity as the oil pump output at high RPM, excessive pressure could be placed upon the filter if the filter was plugged and the by-pass deactivated.



I'm running a Mr Gasket #26 spring, so I guess that qualifies.
Partial filtration wouldn't bother me so much if the eng had a few oil changes thru it already. Actually, I was planning to re-activate the bypass after a few oil changes.

Too bad the by-pass valve doesn't have some sort of external indication when they have by-passed like I've seen on other types of engs.