Oil filter bypass

[vette427-sbc]

My oil pressure has been dropping under load recently. I was told that the oil filter bypass might be stuck, and it would be better to just block it off anyway. Any tutorials on how to do this? A quick google search yielded nothing.
Some background on the oiling system...
I still have the stock corvette baffled oil pan, had the stock pump, recently replaced with a high volume, high pressure Melling pump. (same pressure drops with both pumps, so I dont think it is starving at the pickup). Pressure is 25lbs at idle and peaks to 70+lbs at 5000 RPM before dropping to 25lbs again. Valvoline 10/40 conventional with a WIX filter. Any other ideas as to what it might be besides the bypass? Pressure does not drop under hard braking/cornering either.

-Chris

[Yoda4561]

The filter bypass valve is inside the oil filter, not sure how you'd lock it closed without destroying the filter in the process. Also if your filter is going into bypass it's to ensure full oil flow during high RPM/plugged filter conditions, to prevent starving your engine of lubrication. Is the filter only doing this on a cold engine, or does it still do it even when it's fully warmed up?

One thing I'd try before dinkering around with the bypass valve is the amsoil EAO oil filters, they're a 100% synthetic media that from all I've read has much lower resistance to oil flow than normal cellulose oil filters, while filtering as good or better than anything short of a mobil 1 or bypass filter. They're not all that expensive given that the media also has a greater holding capacity, so you only need to change it once a year or so on a mechanically sound engine. Even if you have to change it at normal intervals it's not any more expensive than a Mobil 1 filter (even better filtration, but it's a celluose/syn blend that has just as much restriction as a normal filter). Worth a try anyways, I'll be ordering some up for a 75 TA on the next oil change to see how it goes.

edit: dug around a little, about halfway down the page seems to be a list of EAO specs, I think your engine is an EAO25, and the bypass valve pressure is listed as N, which I assume means it doesn't have one. http://www.bobistheoilguy.com/forums...&Number=355077

[vette427-sbc]

Is there another oil bypass then? The guy I spoke to showed me a piece that he had blocked off by using a tap and a pipe threaded bolt.
I tested it right after an oil and filter change and it still had the same symptoms- hot or cold

[Yoda4561]

I don't know much about the finer points of GM oiling systems, maybe there's an internal bypass in the engine, hopefully one of the chevy guys can chime in and get you fixed up.

[vette427-sbc]

Here is the bypass or "relief valve" I was talking about...




I found a very good explanation of how/why it works written by TwinTurbo:

You have to realize that it's not the system pressure of 15psi that opens it but a pressure DIFFERENTIAL of that over the filter. Basically a pressure drop due to cold/thick oil, a clogged up filter......

Everything is relative so even with a 15psi spring setting on the relieve valve, it only opens when one side has 15 psi less than the other side, whether this is 30 vs 15psi, 50 vs 35, 80 vs 65...blah blah blah...doesn't matter.

The oil filter is NOT on a side loop, it only is if you forget the hidden plug in the rear vertical passage. The oil filter adapter only bypasses to keep the filter from bursting. I have always replaced the spin on adapters with non bypassing ones and used either large case filters or a System 1 mesh filter. Never had a problem. Especially with the System 1 filter it takes a ton of pressure to burst that thing. So, if you have a pressure drop of more than 12-15psi over the filter, that's when the bypass opens.

The bypass valve in the PUMP is what bypasses the oil back to the sump to regulate max. system pressure. If that one opens the oil gets routed back into the sump via the relief port.

Going to try and block it off tomorrow and see what happens

[MrQuick]

by your description your oil pressure looks fine.
Increase of 10 psi per 1000 rpm.

However you should be running a higher capacity pan with that higher volume pump. It will suck that 5 quart pan dry at 5000rpm. Edit* Big block HV pump will suck it dry at 4500 rpm.



The only thing the oil bypass does is bypasses the filter if it becomes clogged, it does not create or increase pressure in any way. Your oil viscosity, oil pump and bearing clearances do.

Vince

[Vegas69]

I don't agree with the HV pump will pump the pan dry ALWAYS. It depends if the engine was built properly. Bearing clearances control what oil pump is used by a professional engine builder. If it has stock clearances and a HV oil pum then I agree that you could very well be over doing it.

[vette427-sbc]

I don't agree with the HV pump will pump the pan dry ALWAYS. It depends if the engine was built properly. Bearing clearances control what oil pump is used by a professional engine builder. If it has stock clearances and a HV oil pum then I agree that you could very well be over doing it.

The car would still lose oil pressure in the same scenarios even with the stock pump, thats why I dont think the new HV is sucking the pan dry.
I plan on getting a road racing Canton oil pan, and maybe an oil cooler later on to help keep the oiling system under control as well.

[vette427-sbc]

by your description your oil pressure looks fine.
Increase of 10 psi per 1000 rpm.

However you should be running a higher capacity pan with that higher volume pump. It will suck that 5 quart pan dry at 5000rpm. Edit* Big block HV pump will suck it dry at 4500 rpm.

The only thing the oil bypass does is bypasses the filter if it becomes clogged, it does not create or increase pressure in any way. Your oil viscosity, oil pump and bearing clearances do.

Vince

My oil pressure is fine when its not under load... 25lbs at 6000RPM is obviously far from ideal though.
Im thinking that this bypass valve is opening at a pressure way lower than it is supposed to due to a weak or broken spring. This is giving the oil the chance to flow through a perfectly good filter AND this bypass. Thats my my guess as to why it is dropping anyway

I appreciate the help guys

[Vegas69]

I don't see how an oil filter bypass could cause your problem. When it opens, the oil goes into the main galleys not the pan. I'm a big fan of blocking them off in the hot rod world though. I had an engine go south due to a distributor gear failure and it probably would've saved that engine. I'd try an extra quart to see if that helps. Was the oil pump pick up bolt on or welded?

[AMC Racer]

Just curious, often hear the comment that a HV pump will "suck the pan dry". Is the stock pump not able to maintain pressure? If the pressure is the same, flow is the same (assuming same oil temp). A HV pump will only flow more through the engine if the pressure@ a given rpm also goes up.

If oil pressure starts falling off only at high rpms, either the pump inlet isn't keeping up (insufficient oil in the sump / cavitating), the filter is too restrictive causing excessive pressure drop through the filter at high flows (assuming pressure is measured after the filter, e.g. 60 psi @ pump - 20 psi drop through filter = 40 psi engine), there is a problem with the pump relief (not the filter bypass), pump efficiency goes for crap at high rpms (flow drops off instead of increasing), or the oil is thinning out (doubtful, if good oil).

If the filter bypass is opening with warm oil, the filter is either plugged or too restrictive and you need a higher flow filter. Prefer to use a high flow filter with high internal bypass pressure if the stock filter bypass is plugged, e.g. Fram HP (22 psi bypass) ... rather have bypassed oil then none.

Does pressure drop at constant high rpms, or only when accelerating?

[MrQuick]

My oil pressure has been dropping under load recently. I was told that the oil filter bypass might be stuck, and it would be better to just block it off anyway. Any tutorials on how to do this? A quick google search yielded nothing.
Some background on the oiling system...
I still have the stock corvette baffled oil pan, had the stock pump, recently replaced with a high volume, high pressure Melling pump. (same pressure drops with both pumps, so I dont think it is starving at the pickup). Pressure is 25lbs at idle and peaks to 70+lbs at 5000 RPM before dropping to 25lbs again. Valvoline 10/40 conventional with a WIX filter. Any other ideas as to what it might be besides the bypass? Pressure does not drop under hard braking/cornering either.

-Chris

My oil pressure is fine when its not under load... 25lbs at 6000RPM is obviously far from ideal though.

Im thinking that this bypass valve is opening at a pressure way lower than it is supposed to due to a weak or broken spring. This is giving the oil the chance to flow through a perfectly good filter AND this bypass. Thats my my guess as to why it is dropping anyway

I appreciate the help guys

I guess I miss read your post. A peak to 70 psi @ 5000 rpm then drops to 25 psi with more rpm would to me indicate a loss of oil flow or cavitation.
I have seen the senario many times and its always after a high volume high pressure pump has been added with the stock pan. The pump flow all the oil to the top end, faster than it can return. A few where the pump shaft snaps from the pump galling. Its was just a suggestion.

Todds pickup suggestion lit a bulb, could it be sucking air when the flow becomes too great for the pick up? What pick up do you have? But then again you stated the issue was present before the pump change.

Could it be the gauge? Is it an electrical or mechanical type. That is another item that hasn't changed. I would put a hand held gauge on it and compare readings just to rule it out.

I seen to remember having a similar problems on a buick, ended up being a spun main bearing. Another in a Pontiac 400 with a faulty lifter.

"Im thinking that this bypass valve is opening at a pressure way lower than it is supposed to due to a weak or broken spring. This is giving the oil the chance to flow through a perfectly good filter AND this bypass. Thats my my guess as to why it is dropping anyway"

From what I understand of the system, a stuck open bypass would equal the oil filter flow so I doubt having both flow will decrease pressure. It doesn't act as a regulator. The ball and spring in the pump does that.

Maybe im just reading in too far but I am trying to help you and not being a Richard Head.

Vince

[vette427-sbc]

Well, heres the update, and its not a good one. I blocked off the bypass today. Didnt work. I did some tests though to possibly rule out some of the proposed problems:

Nothing changed (pressures, pressure per RPM...) But I did notice that if I slowly accelerate and bring it up to 4000RPM, pressure reads 70lbs. If I go WOT from there, pressure drops. If I engine brake at 4000RPM, pressure holds at 70lbs. All of this would point to the pickup starving under acceleration- Which is what I was telling myself it wasnt. Pressure wont drop under hard braking or cornering either. The pickup is welded on, and I closely inspected it before putting it on- Looked perfect.
Gauge is a new autometer sport comp (electronic sender). I have an old mechanical SW I can hook up to compare
Now, my car is no rocket, but its quick. But Ive been in cars that were just as fast with stock pans, and they never had any problems with oil starvation under acceleration. Plus, I think corvette specific pans have baffles in the pan made to help with starvation under acceleration. Somethings not right here...
Again, thank you all for the help.