Fluid dyamics discussion - someone educate me.....

Joined
Jul 7, 2009
Messages
32
Location
Virginia Beach
I'm trying to understand the effects of restrictions on oil pressure. Restrictions being things like an aftermarket oil cooler and/or a different oil filter. Some on here are saying that unless your oil pump is in relief you will never see a pressure change becuase the fluid flow is determined by the capacity of the system. But that doesn't make sense to me becuae when you change your filter or add an oil cooler you're effectively changing the fluid system. Can someone expain the basics? In my example... would removing an aftermarket oil cooler from a car make the oil pressure go up or down? A buddy is saying that since the cooler is a restriction it makes system pressure go up, so removing it makes the pressure go down. However, people say the opposite for oil filters. That a more restictive filter lowers oil pressure, while a more free flowing one increases it. I'm guessing the answers here are different depending on whether you're at idle, cruise, or WOT? Thanks
 

Brian_Bowers

Thread starter
Joined
Jul 7, 2009
Messages
32
Location
Virginia Beach
Here's something along the same lines from a Miata forum...
 Quote:
Having recently done my first oil and filter change on my stock '97 and installing an accurate oil press. gauge, I was a bit concerned that the pressure was only about 35psi at 3000 rpm when warm. Researching oil pressure on this forum to see what others were getting I came upon the Fram filter thread. Having just installed a Fram I read the posts with interest. I saw that a few posters mentioned higher oil pressures after changing out the Fram for a diff brand filter. My question is this. My understanding of engine lubrication systems is this. Pressure is determined by the pumps output (which is always designed to supply a surplus of oil to the main oil gallery under all running conditions), the clearances of the main bearings and mostly by the spring in the oil pump relief valve. If this is so, then how can an oil filter effect the pressure? Am I missing something or is the Miata's oil system diff. than what I described? BTW, my pressures DID indeed increase by 10-12 psi with a Purolator filter. Thanks for your input. Mike
 Quote:
Mike, Although I agree generally with the results posted above, theoretically you are right. There is supposed to be enough overpressure available from an oil pump to compensate for oil flow restriction of the filter. The oil pump is designed to discharge a fixed volume of oil on each revolution, regardless of how much pressure that requires, up to the point where the pressure relief valve in the pump begins to open and allow part of its output oil to fall back into the pan. Apparently the Miata oil pump/valve setup does not have the extra pressure capability to overcome the restriction in some commercially available oil filters, and therefore its bypass valve apparently opens, changing its output from constant volume to constant pressure. ---- Bill
 Quote:
Bill If it were not for the oil pump's relief valve (which on the 1.6L engines bypasses at 50-64 psi), meaning that the pressurised oiling system were a closed loop, then the issue of more/less restrictive filters would be nearly moot as the flow rate (gpm) of oil to the engine would not be significantly affected (until something blew). However the relief valve does exist (for very good reason) and by design reduce's the oil pump's delivery volume (by shunting flow to the pan) whenever the pump's output pressure exceeds 50-64 psi. While in theory this would mean that the output pressure could never exceed 50-64 psi, the relief valve's flow capacity relative to the pump's output volume comes into play here. When the pump's output exceeds the valve's flow capacity the pressure will rise beyond the valve's trigger point--this occurs whent he oil is cold (which is why we can observe pressures or 90+ psi at startup), and perhaps (I suspect) at higher engines speeds when the pump's output flow is very high. What this all means that more restrictive oil filters will cause the pressure at the pump's output (and the relief valve) to reach 50-64 psi sooner and more often than will a less restrictive filter. Therefore a more restrictive filter will cause more oil to be shunted back into the pan, and LESS oil to be pumped through to the engine. This is not just a pressure issue, the oil pump relief valve's behaviour makes it a flow rate issue.
Here's a LSX's oil flow diagram: So... if at idle the oil pump doesn't provide enought excess flow to reach relief a more restrictive oil filter would indeed reduce system pressure correct? FYI part #3 on the diagram is the pressure sending unit. But I'm still confused on the oil cooler... most everything I'm reading online says oil coolers create pressure drops and to be honest I didn't pay close attention to mine when it was in because I installed a high volume pupm and my cam at the same time. All I cared was that my pressure was good on track.
 
Last edited:
Joined
Nov 23, 2004
Messages
1,166
Location
N. Texas
Simple answer is an oil cooler should not be a restriction, if it is, it is the wrong size internal plumbing and it would have lower pressure on the downstream side and higher pressure on the upstream side.
 
Joined
Feb 1, 2009
Messages
8,859
Location
Texas
IMO, in a healthy engine the restriction of the bearings, lifters, and other clearances should NORMALLY be much greater than the restriction of the filter and cooler. The biggest restriction dominates the pressure/rpm curve, so the added restriction of the cooler generally goes unnoticed. SOME engines have very high oil flow and less "built-in" restriction- engines with full-groove bearings, large bearing clearanaces for racing, piston oil-spray nozzles (usually found in turbo engines) etc. If the engine manufacturer skimped on the oil pump- say used the same pump for turbo and non-turbo applications, then the system might be on the edge and adding a cooler could cause a noticeable drop. Also- look at where the pressure *sensor* is in that diagram- its at the top of the feed gallery to the engine, so its measuring pressure right out of the filter. It doesn't reflect the lowest pressure in the system, which would in this case be at the FRONT of the main oil galleries. Engines that put the pressure gauge sensor at the very tail-end of the oiling system give a much more accurate indication to the driver about what the last bearing in the oiling system is actually seeing.
 
Joined
Sep 23, 2008
Messages
9,966
Location
Ontario, Canada
In theory, shouldn't the oil passages should be sized large enough that they should act like a pressure "tank" so all the bearings receive equal pressure? Also the 1.6L and 1.8L Miata's did have piston sprayers as that engine was designed to have a turbo. So maybe they did skimp on the pump for the NA motors and have it running near capacity?
 

Brian_Bowers

Thread starter
Joined
Jul 7, 2009
Messages
32
Location
Virginia Beach
Here's what's going on with my car... a 2001 Z06 Corvette with 84,000 miles on it. In early 2007 I installed a new cam, high volume LS4 oil pump, and a Dewitts radiator with left side EOC (enclosed oil cooler). Running Amsoil 5w40 and a K&N filter I would see 47-49 psi at hot idle (210ish oil temp) and around 60 hot cruise and 78-80 at WOT. On track where oil temps maxed out at 250ish I was never below 50psi at WOT which was perfect. Now that the car is no longer tracked (have a dedicated track car) I removed the oil cooler and radiator earlier this year. I was seeing low 50s at hot idle and mid 60s hot cruise and 84 max at WOT. This was in February and I never really paid attention to it after that. Last week I hit a chunk of concrete on the highway and as I pulled off I watched the oil pressure closely to make sure I didn't tear the filter or crack the pan. As I cam to an idle I saw the pressure drop to 44... then at the next light it dropped to 35psi. Thinking I'd damaged something I pulled over and found no damage... even jacking it up the only damage was a scraped cross-member and a small scratch on my oil pan. So... after that I was seeing 40psi at hot idle, 60 at cruise, and low 80s at WOT. I chalked it up to it being the summer heat etc. but oil temps are the same when hot. Last weekend I put in fresh M1 5w30 and a M1 filter becuase the Amsoil in the car was 11 months old. Now I'm seeing 35-36psi at hot idle, 57-60 hot cruise, and low 80s WOT. My main concern in all of this is trying to figure out why my oil pressure went from 50ish to 40ish at hot idle... I don't see how hitting the chunk of concrete could have changed anything. If it did, I'd think it would manifest itself all the time i.e hot cruise and WOT, but the only difference I'm seeing is at hot idle. I'm trying to determine if removing the oil cooler actually caused my oil pressure at idle to go down, but when I first checked it back in Feb/Marh I may have not been looking at it at full hot. I know it's a bit long, but I'm trying to understand the effects of removing my oil cooler and then subsequently moving to a more restrictive oil filter. The real thing to do would be to put Amsoil 5w40 back in it w/ a K&N filter and see what happens, but I don't care to spend $14 on the fitler and $10 a quart on oil anymore when I'm not trackign the car.
 
Joined
Oct 18, 2007
Messages
3,756
Location
CA
Are you running higher average temps without the cooler? I think the biggest factor is where your pressure sensor is located. Before the oil cooler *could* make it read lower without and after the cooler *could* make it read higher without the cooler. That's assuming the cooler and lines were actually a measurable restiction. I know you want to understand why the pressure has fallen but I would kill for your oil pressure. I'm always looking down to make sure my guage isn't going below 10psi hot idle and my pressure sending unit is right after the pump so it's giving me a best case scenario.
 

Brian_Bowers

Thread starter
Joined
Jul 7, 2009
Messages
32
Location
Virginia Beach
No... temps are the same without the cooler. My cooler was an enclosed unit in the left end tank of the radiator so the coolant would bring the oil up to temp. After that the coolant would cool the oil. I'm always around 210 hot cruise when fully warmed up. Takes it a bit to get from 200 to 210. Yes... I know the oil pressure figures themselves are good, but the delta is what I'm trying to figure out.
 
Joined
Sep 28, 2002
Messages
39,802
Location
Pottstown, PA
It's pretty simple. Below the relief level (that is seen at the pump) ..there is no such thing as a restriction. Assuming that the pump is of reasonable efficiency, the fluid will merely accelerate and will evidence a "pressure elevation" upstream of the choke. They will all fall into a "relative hierarchy" between each other in that single circuit model. You can stack as many chokes and bends and whatnot that you please. Once the relief opens, then you revert to a "pressure over resistance" model... and chokes ..lengths ..etc. do produce "pressure drops" as viewed from the supply ..and they may not be in the same order they were in terms of magnitude. Your bends will have impact with the disruptions of laminar flow ..your lengths will now "consume" pressure instead of the fluid just going faster through them ..your filter will show it's relatively high resistance ..where it was but a minor one (up to the limits of its throughput)... etc. You appear to have pressures in excess of your relief limits, which means that you not at attenuated flow just because you're in relief. That indicates that your relief limits can't shunt enough flow to keep the pressure at that setting. You've got a mismatch between your hot visc/volume/relief setting. This isn't a bad thing, it just produces the results you're seeing. If we had infinite VI fluids, it would make it much easier to make a fluid management system for all seasons/reasons.
 

Brian_Bowers

Thread starter
Joined
Jul 7, 2009
Messages
32
Location
Virginia Beach
English? If I'm understanding you correctly, then why wouldn't the pump always pump at the relief spring pressure at idle? Meaning shouldn't I always see 52 psi?
 
Joined
Oct 18, 2007
Messages
3,756
Location
CA
 Originally Posted By: Brian_Bowers
English? If I'm understanding you correctly, then why wouldn't the pump always pump at the relief spring pressure at idle? Meaning shouldn't I always see 52 psi?
Simple answer: It's not spinning fast enough. Once the rpms come up the pump has more volume than the relief circuit can deal with which results in the pressure above the relief setting. I'm sure Gary will have a much more detailed answer. I always have to read those posts 2-3 times but usually come out with some good info.
 

Brian_Bowers

Thread starter
Joined
Jul 7, 2009
Messages
32
Location
Virginia Beach
Ok... I get that part... at idle it makes lower pressure, then as the RPMs climb and the oil pressure created by the pump climbs to 52psi the relief valve opens and begins to dump pressure. I see 84 psi max becuase the the relief valve allows the pump to build more pressure. I guess I still don't understand why restrictions etc. wouldn't make a difference at idle if at 800 rpms the pump isn't providing max flow. I understand that it's a fixed output of oil per RPM, but if I'm only making 40psi of pressure why wouldn't different oil filters or an oil cooler change idle pressure?
 
Joined
Oct 18, 2007
Messages
3,756
Location
CA
 Originally Posted By: Brian_Bowers
Ok... I get that part... at idle it makes lower pressure, then as the RPMs climb and the oil pressure created by the pump climbs to 52psi the relief valve opens and begins to dump pressure. I see 84 psi max becuase the the relief valve allows the pump to build more pressure. I guess I still don't understand why restrictions etc. wouldn't make a difference at idle if at 800 rpms the pump isn't providing max flow. I understand that it's a fixed output of oil per RPM, but if I'm only making 40psi of pressure why wouldn't different oil filters or an oil cooler change idle pressure?
They can. I think the point was the volume is fixed when not in bypass so a restriction here and the that raises pressure isn't important because the engine bearings are still recieving the same pressure and flow regardless of the change in guage readings after installing/removing a cooler or a less restrictive filter. If the sending unit is after the cooler and filter and right before entering the oil galleys, the pressure should go unchanged if the pump is not bypassing... Or Gary will correct me if I'm wrong.
 

Brian_Bowers

Thread starter
Joined
Jul 7, 2009
Messages
32
Location
Virginia Beach
Yeah I'm still confused. In my case the pressure sending unit is after the filter and the cooler (when the cooler was installed). I guess what it comes down to is in my case... would removing the oil cooler cause the pressure read at the sending unit to go up or down when idling? Would switching from a K&N filter to a M1 filter make the presssure go down becauase the M1 is more restrictive? Again... all at idle.
 
Last edited:
Joined
Sep 28, 2002
Messages
39,802
Location
Pottstown, PA
 Quote:
at idle it makes lower pressure, then as the RPMs climb and the oil pressure created by the pump climbs to 52psi the relief valve opens and begins to dump pressure. I see 84 psi max becuase the the relief valve allows the pump to build more pressure.
You have to look at it in terms of volume/flow below the relief limit. Your volume at idle produces xxx amount of pressure. Above the relief limit, your pump is dumping output. You're building 84psi because your relief port isn't big enough to dump the volume necessary to maintain the relief setting. If you sender is where it is in that diagram, you're propbably in excess of 100psi at the pump. Like I said, it's not a bad thing. If you want higher flow to the engine (however more that may be) ..then switch to a lower viscosity oil. You will no longer produce 84psi at that point in the system ..and the relief will be shunting a bit less flow.
 

Brian_Bowers

Thread starter
Joined
Jul 7, 2009
Messages
32
Location
Virginia Beach
So if the volume at idle is a fixed rate wouldn't changing the flow path characteristics via a more restrictive filter or an oil cooler cause the resultant pressure to change? What about the questions in my above post... I appreaciat the help... trying to understand what's going on.
 
Joined
Jun 21, 2009
Messages
2,115
Location
new york
i would like to understand too.why has gas prices stayed the same and oil dropped ten dollars so far this summer/ who should we contact about this ////does anyone seem to care?
 
Joined
Jun 21, 2009
Messages
2,115
Location
new york
got a question...oil has dropped 10 dollars so far this summer anyone seen a drop in gas prices????i am in nyc price can be as high as 3.00 gas places on highway charge that much.other place on li can be 2.77 who do we call ///anyone??
 
Joined
Jul 31, 2005
Messages
651
Location
Iowa
Assuming the bypass is closed, overall system pressure should be the addition of each individual pressure drop. If at idle it takes 40psi of pressure drop to force the oil pump output through the engine and 10 psi through the cooler the overall system pressure will be 50 psi. Remove the cooler the system pressure will be 40 psi. Remove the engine, the system pressure will be 10 psi. Another way to look at it is if you have two hydraulic motors. If they are connected in series and each requires 1000 psi to turn their load, the over all system pressure would be 2000 psi. Motor one would have 2000 psi inlet/1000 psi outlet. Motor 2 1000psi inlet, 0 psi out let. Remove one motor and the system pressure would be 1000 psi. I claim to know nothing about engine systems.
 
Top