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Leadfoot64
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Joined: April 12, 2006
Posts: 11
Location: Peoria, il
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 Posted: Wed Apr 12, 2006 12:54 pm Post subject: Oil Pump Flow Rate |
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I want to install a front mounted oil cooler on my car. I have the 911 fan conversion with an oil cooler & fan mounted by the left rear fender. In summer this system limits me to crusing at 65mph which just doesn't cut it. I need more air flow.
I don't want to put in a system with too high a pressure drop but don't want to completely oversize it either. I have all the data I need except the oil pump flow rate. I realize this will vary a lot depending on conditions but does anybody have any information that might help? The cooler would be by-passed until the oil got to 180F. |
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volksmann
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Joined: April 11, 2004
Posts: 65
Location: Green Bay, Wisconsin
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| Posted: Wed Apr 12, 2006 4:35 pm Post subject: |
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It can be figured out using this formula:
MA is the major dia. of the pump gear
MIN is the minor dia. of the pump gear (dia. between the stars)
GW is gear width.
GPM is gallons per minute
MA=__________ divided by 2 =________squared x 3.14 = (Y)
MIN=__________divided by 2 =________squared x 3.14 =(Z)
(Y)________ - (Z)_______= __________ (X)
(X)_____ x (GW)______=_______(A) Area cu. in.
(A)_____ x _______RPM = _______ divided by 231 =_______GPM
GPM_______ x .85 = _________ GPM Actual Flow.
This is what a Schadek 26mm pump should put out at 4000 rpm's. All measurement are in inches.
Remember the pump turns at half of the engine rpm's
MA 1.29
Min 0.793
GW 1.025
MA= 1.29 divided by 2 = 0.645 x 0.645 x 3.14 = 1.306 (Y)
MIN= 0.793 divided by 2 = 0.3965 x 0.3965 x 3.14 = 0.494 (Z)
(Y) 1.306 - (Z) 0.494= 0.812 (X)
(X) 0.812 x (GW) 1.025 = 0.832 (A) Area cu. in.
(A) 0.832 x 2000 RPM = 1664 divided by 231 = 7.203 GPM,
The pump would put out 7.203 gpm if it was 100 percent efficient but, most pump are only about 85 percent efficient.
GPM 7.203 x .85 = 6.122 GPM Actual Flow.
So a engine running at 4000 rpm's should put out about 6.1 gpm if it had a Schadek 26mm oil pump.
Ken |
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HBRag
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Joined: April 29, 2005
Posts: 735
Location: Huntington Beach, CA
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| Posted: Wed Apr 12, 2006 10:20 pm Post subject: |
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| Why not just put a 30mm pump in with a relief cover, and don't worry about it? |
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Leadfoot64
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Joined: April 12, 2006
Posts: 11
Location: Peoria, il
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| Posted: Thu Apr 13, 2006 7:31 am Post subject: |
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Volksmann, thanks for the info. I can now at least size the lines to avoid too high a pressure drop. The 911 uses AN-12 piping which seemed like overkill for a Beetle and it is. Here's what I got for additional pressure drop due to relocating the cooler to the front.
Line Size (dP psi)
AN-8 (20)
AN-10 (7)
AN-12 (3)
FYI HBRag, a bigger pump would make things worse not better. In most cases a bigger pump is better, but these are positive displacement pumps which means they put out a fixed amount per revolution for a given size. A bigger pump will pump more however this will increase the pressure drop through the lines, filter & cooler. Pressure drop is proportional to flowrate squared, you double the flowrate and the pressure drop quadrulpes. We need to maintain enough oil pressure back into the engine to insure proper lubrication. This means we must limit the pressure drop in the filter & in the external oil cooler, this is done be reducing the flowrate or increasing the line size. Remember the smaller pump can output the same pressure as the big pump just with a smaller volume. |
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RIS
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Joined: December 18, 2003
Posts: 3044
Location: Youngstown, Ohio
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HBRag
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Joined: April 29, 2005
Posts: 735
Location: Huntington Beach, CA
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| Posted: Thu Apr 13, 2006 11:44 pm Post subject: |
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Ah, fluid dynamics is interesting stuff. I had to think about this for about 30 minutes. But I'm still missing it I think...
Isn't the output flow rate dictate by the clearance and surface area of the bearing. I understand that long lines will cause some amount of pressure loss, but if you increase the volume on the supply side with a fixed flow rate on the output side, wouldn't that result in a higher pressure? Which would in turn compensate for the loss in the longer lines. Or is that not how the positive displacement pumps work?
Cool stuff, thanks for sharing the math and theory,
HB |
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Desertbusman
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Joined: June 03, 2005
Posts: 14655
Location: Arizona
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| Posted: Fri Apr 14, 2006 4:09 am Post subject: |
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Here's one of my old hydraulic system design charts. It gives somewhat of a perspective on hose (and passage diamerers) sizes in relation to pump flow and pressure drop. This is based on hydraulic fluid. Engine oil has a higher viscosity so pressure drop will be a tad higher. Pressure loss thru the flow paths not only wastes energy but adds heat. Ideally the pump would be sized to provide the suitable pressure slightly above the relief valve setting at whatever rpm you wish to maintain that pressure. Adding a pump producing more flow than is needed to maintain the relief valve pressure only dumps more oil over the relief valve. Which in turn is more heat and all that. It does not add more flow thru the engine lube system. So basicly if the engine pressure is sufficient, a larger pump is a disadvantage. Adding the external system adds to the pumps GPM requirement if the added pressure loss is sufficient to increase the pressure requirement at the pump and reducing it's efficiency. Higher pressure means greater internal leakage within the pump. Again, wasted energy and heat creation.
Sounds like you're on top of the situation Leadfoot!
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volksmann
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Joined: April 11, 2004
Posts: 65
Location: Green Bay, Wisconsin
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| Posted: Fri Apr 14, 2006 6:02 pm Post subject: |
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There's alot of restrictions in the fittings that are used. If you use one size bigger fittings then hose size, that will help, like if your using a #8 (1/2 inch) hose, use #10 (5/8 inch) fittings.
Ken |
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Bruce
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Joined: May 16, 2003
Posts: 17644
Location: Left coast, Canada
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| Posted: Sat Apr 15, 2006 1:10 pm Post subject: |
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| volksmann wrote: |
It can be figured out using this formula:
MA is the major dia. of the pump gear
MIN is the minor dia. of the pump gear (dia. between the stars)
GW is gear width.
GPM is gallons per minute
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Ken, you are making a fundamental error. You forgot to deduct the volume of the gear teeth from the the total displacement. |
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Desertbusman
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Joined: June 03, 2005
Posts: 14655
Location: Arizona
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| Posted: Sun Apr 16, 2006 2:18 am Post subject: |
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| Bruce wrote: |
| volksmann wrote: |
It can be figured out using this formula:
MA is the major dia. of the pump gear
MIN is the minor dia. of the pump gear (dia. between the stars)
GW is gear width.
GPM is gallons per minute
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Ken, you are making a fundamental error. You forgot to deduct the volume of the gear teeth from the the total displacement. |
No, actually Ken's formula is correct. Bruce it sounds like you are considering only 1 gear. If that were the case you'd be right. But there are 2 gears rotating the fluid. The amount deducted for the teeth in one gear is matched by the volume added by the 2nd gear.
The other night I was real thankful that volksmann had posted the data. I've recently been curious about the pump volume but had been to lazy to measure and calculate. The reason for the curiousity was based not only on proper line sizes but also on the oil turnaround or cycle time. And looked at the 30mm Shadek pump sitting here and did notice a problem, not in the foumula, but in the measuring. I had never payed any attention before, but there is quite a chamber between the tooth nose and the mating gear depth. So a fair amount of oil is carried back from the pressure side to the suction side. I assume this is to pressure balance the gears for reduction in wear from side loading. So the effective min. dia. is around .890 and volume would be around 5.3 at 4,000 rpm for a 26mm pump. Although 85% efficient might be a bit high, at least for a used and worn one. What did stand out as real important was the need for the correct gasket between the pump and cover. If the "other" gasket with larger i.d. was used it would open up quite an internal leakage path.
In regards to oil cycle time: With the stock 5.3 pints, at the 4 grand highway speed, the oil completely circulates 8 times a minute or every 7-1/2 secconds. This has been my curosity, and in addition to the added cooling properties, this why I think an add-on sump is very beneficial. Both to assure plenty of oil at the pickup, and to "rest" the oil. A general rule of thumb in an industrial hydraulic system is the reservoir being sized for 3 times the pumps GPM. And yes, the pressures are considerably higher, but the temp would be lower, usually around 120 degree max. Regardless the oil gets one tremendous workout from the increase and decrease of pressure. Time to recover adds to its life and lessens breakdown. Also a good thing for an ACVW.
Back to the gears. VW has used 17, 19, 24, and 26mm widths. Then some aftermarket pumps are wider. But there is nothing ever said about the tooth size or form or diameters. I think the only design restriction would be the hole size in the case. Do different brand pumps use different diameter gears or differing teeth? Does one brand, or design, produce the same flow as the other brands? The real question is- do all 26mm pumps produce the same GPM?
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Bruce
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Joined: May 16, 2003
Posts: 17644
Location: Left coast, Canada
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| Posted: Sun Apr 16, 2006 3:28 am Post subject: |
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| Desertbusman wrote: |
| Bruce wrote: |
| volksmann wrote: |
It can be figured out using this formula:
MA is the major dia. of the pump gear
MIN is the minor dia. of the pump gear (dia. between the stars)
GW is gear width.
GPM is gallons per minute
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Ken, you are making a fundamental error. You forgot to deduct the volume of the gear teeth from the the total displacement. |
No, actually Ken's formula is correct. Bruce it sounds like you are considering only 1 gear. If that were the case you'd be right. But there are 2 gears rotating the fluid. The amount deducted for the teeth in one gear is matched by the volume added by the 2nd gear.
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Ken's error is twofold. First he didn't deduct the volume the tooth occupies, and second, he didn't double the output due to there being two paths for the oil. One error doesn't counter the other. |
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Desertbusman
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Posts: 14655
Location: Arizona
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| Posted: Sun Apr 16, 2006 4:49 am Post subject: |
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O.K. So isn't your method doing it like this? Deduct half the volume of the first gear. And then double the remaining amount to account for the second gear.
1 minus 1/2 times 2 equals 1
Kens shortcut way is 1 equals 1
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volksmann
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Joined: April 11, 2004
Posts: 65
Location: Green Bay, Wisconsin
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| Posted: Sun Apr 16, 2006 8:22 am Post subject: |
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I have always wondered if the tooth width made any differance. I'm no math wiz, far from it, but here is what I do know, I've been working in hydraulics for nine year. Rebuilding pumps is one of my dutys. There are times a pump comes in with no tag, no name and is 30 some years old, now that's a problem. The pump is junk and the piece of equipment won't run without it, so I have to find a replacement for it. Most of the large suppliers of pumps use this formula. It may or may not be 100 percent accurate, but it must be real close. I check it against some pump of known gpm's and was close. If it's wrong I am willing to change.
Ken |
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