I’m confused. I’ve seen magazine stories that say you should plug off that oil filter bypass in a small block Chevy but I’ve also heard that you shouldn’t do that. But with the bypass, doesn’t a bunch of oil go around the oil filter? That can’t be good, right?
Jeff Smith: Essentially you are correct on all those assumptions except the last one. Before we get into this, let’s first examine why that bypass is there in the first place for all engines, not just Chevrolets. Oil filters create a small amount of pressure loss caused when the pump has to push the oil through the filter. While all good car people religiously change their oil, engine designers have to take into account the fact that not everybody cares about their engines like hot rodders. So for that one percent who don’t change their oil, engineers must design parts around these engine abusers. If an oil filter becomes plugged, there needs to be a safety valve or bypass that will allow the engine to pump oil – as dirty as it may be – to the rest of the engine. Dirty oil is still preferable to no oil.
In the case of the small or big block Chevys, the designers built in a simple bypass valve with a small spring behind it that is rated to open at roughly 17-18 pounds of force. This valve is located on the filter adapter. It’s also important to note that all oil filters filter by pushing oil from the outside toward the inside. So, if you look at the small block Chevy adapter, the valve is designed to open anytime there is 17-18 psi more oil pressure on the outside of the filter element than on the inside. This greater pressure pushes open the little flapper valve, compressing the spring. It’s a simple design that works very well. This does not bypass all the oil, only a certain amount. The majority of the oil is filtered.
This bypass situation also occurs when the oil is cold and the thick, which increases the pressure drop across the oil filter. Once the oil reaches its normal operating temperature – roughly around 190 to 215 degrees F, this valve is probably closed or only partially open as the oil has thinned out. Of course, this will change with the type of filter you are using. A typical oil filter like the Fram Extra Guard is rated at 95 percent efficient at filtering particles down to 20 microns (0.000787-inch). Comparing that to a Fram Ultra Guard Synthetic filter with a 99-percent, 20-micron efficiency you can see that it will probably be easier to push oil through the Extra Guard filter than it will through the Ultra Guard. These percentage numbers are established with a single pass test, so the oil only went through the filter once yet it was able to remove 95 to 99 percent of all particles below 20 microns. That’s pretty good. When you have this kind of filter efficiency, you can afford to bypass a little oil to protect against damage to the filter. It’s also worth repeating a great axiom given to us by an oil filter engineer – “The best oil filter is a really good air filter.” If you think about it, it makes a lot of sense.
The reason you don’t want to plug this bypass is really about not creating a bigger problem. I’ve witnessed Fram’s testing of filters in their Ohio test lab and almost always the filter gasket blows out before the filter can splits. If you want a lesson in oil pressure, try this little test with an engine on an engine stand. With cold oil, kneel down until you are at eye level with the oil filter. Now, using a pressure luber like a ½-inch drill motor, spin just the oil pump to create 60-65 psi of oil pressure (engine not running). You will be shocked at how much that filter housing grows with oil pressure. That will also make you think about not revving a cold engine with cold oil like some guys like to do. That’s just not a good idea for a bunch of reasons.
Even if just the filter gasket blows out, this doesn’t make it any less of a mess. It takes mere seconds for an oil pump to push a couple of quarts of oil all over the engine compartment. Worse, you don’t want to push hot oil on a set of extremely hot headers – that’s a recipe for disaster. The flash point of oil refers to the temperature at which oil will ignite and burn. Motor oil will flash at roughly 450 degrees F. Headers routinely operate at this temperature under load, so you don’t want that to happen to you.
Generally, oil filters are so good now yet they don’t present backpressure problems once the oil is up to temperature. Of course, running a 10w30 or a 5w30 oil is also a good idea as that oil will be thinner when it’s cold, making it easier to pump through the filter. It’s all about making life a little easier for your engine.