Do not use the in-tank pump... EVER
05-02-2011, 06:21 PM
#46
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The issue that caused the factory (and other Bosch customers) to use a two-stage pump system was fuel boiling in the inlet of the pump in hot weather. As Fred R stated, the net pump suction head needs to be higher than the boiling pressure of the fuel at whatever the temperature might be. On a hot day as the temperature increases, so too does the suction pressure requirement. It's not enough that the fuel is liquid as it enters the pump. It must remain as liquid as it is drawn into the first stages of the pump. With just the external pump, that means that there is pressure lost in the screen, the inlet tubing, the connecting hose, the external pump inlet fittings, and finally the inlet of the first stage of the pump itself.
Still with me? The casing temperature of the external pump is another variable, where 'a bit hotter' is the difference between pumping liquid and trying and failing to pump vapor. So we add a priming pump inside the tank. It's a small pump made to offer a fairly high volume with a small pressure difference between inlet and outlet. It sits in the liquid fuel in the tank so it doesn't get a lot hotter than the fuel. It's at the bottom of the tank so it starts out with tank pressure plus the head from whatever fuel happens to be in there. The head pressure isn't a lot-- it takes almost 3 feet of fuel to generate 1 PSI at the pump suction at 70ºF, and the hotter it gets the more head it takes to have the same suction pressure.
So the low pressure differential in that in-tank pump means there is less chance of boiling in the inlet bell and the first stage. It does increase the pressure enough to eliminate boiling in the inlet and first stage of the main pump.
Back the the proposal that using a higher-pressure higher-flow 044 pump means you won't need the in-tank pump. I propose exactly the opposite. If you drive in conditions where the fuel temp gets high, like driving in the hot desert, extended hot idling in stop-and-go traffic with the AC adding under-hood heat, these are times when the in-tank pump is needed.
It should be noted that the AC system actually cools fuel returning from the engine. That system was added to help with fuel boiling problems starting with the later CIS cars, and it continued to the end of production. Bosch initially supplied higher-pressure pumps for many CIS cars, but eventually the core problem was identified as pump suction boiling as much as it was a problem with boiling in the fuel distributor.
My too sense.
Still with me? The casing temperature of the external pump is another variable, where 'a bit hotter' is the difference between pumping liquid and trying and failing to pump vapor. So we add a priming pump inside the tank. It's a small pump made to offer a fairly high volume with a small pressure difference between inlet and outlet. It sits in the liquid fuel in the tank so it doesn't get a lot hotter than the fuel. It's at the bottom of the tank so it starts out with tank pressure plus the head from whatever fuel happens to be in there. The head pressure isn't a lot-- it takes almost 3 feet of fuel to generate 1 PSI at the pump suction at 70ºF, and the hotter it gets the more head it takes to have the same suction pressure.
So the low pressure differential in that in-tank pump means there is less chance of boiling in the inlet bell and the first stage. It does increase the pressure enough to eliminate boiling in the inlet and first stage of the main pump.
Back the the proposal that using a higher-pressure higher-flow 044 pump means you won't need the in-tank pump. I propose exactly the opposite. If you drive in conditions where the fuel temp gets high, like driving in the hot desert, extended hot idling in stop-and-go traffic with the AC adding under-hood heat, these are times when the in-tank pump is needed.
It should be noted that the AC system actually cools fuel returning from the engine. That system was added to help with fuel boiling problems starting with the later CIS cars, and it continued to the end of production. Bosch initially supplied higher-pressure pumps for many CIS cars, but eventually the core problem was identified as pump suction boiling as much as it was a problem with boiling in the fuel distributor.
My too sense.
05-03-2011, 10:28 AM
#48
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OK, so I have a nice buzzing in tank fuel pump on Gio's former '89 Supermodel. Gio, what external pump did you install? Should I go ahead and replace the in tank pump with the strainer? If so, where to buy and how much $$$$.
05-03-2011, 11:46 AM
#49
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I suppose it's time to run the tank out and take a look inside at my in-tank pump to verify that it's still there and not dangling from a broken hose section.
Z's post on using the 044 high-flow pump without the internal pump is well taken.
05-03-2011, 01:13 PM
#50
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Hacker, count me in for one uberstrainer setup for my 044 pump when they become available.
Are you going to include the 5/8 hose and adapter fittings into the 044 pump?
This is what I'm using now but it'd be nice to move up to 5/8 hose.
Are you going to include the 5/8 hose and adapter fittings into the 044 pump?
This is what I'm using now but it'd be nice to move up to 5/8 hose.
05-03-2011, 01:39 PM
#51
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1. New tank fitting with o-ring
2. Stainless strainer
3. M18 to 5/8" barb for the pump with a bonded seal
4. 5/8" fuel hose
5. Hose clamps
6. Strain relief for the hose - bending a hose this size at the angle needed was another challenge.
05-03-2011, 02:18 PM
#52
Bosch 044. I would inspect the internal pump first and see if it is intact.
05-03-2011, 02:37 PM
#53
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Of course, my car is the odd man out. At 150K miles I drained the tank and pulled out the in-tank pump. Fully intact. Put it back in. But every other car I've looked at has had a split hose. The split hose is bad for high-speed performance as the pump output partially recirculates. You would never notice it under "normal" driving conditions, but when you get try to go over 120 MPH the car struggles. Fix the in-tank pump hose and the high-speed power is back.
05-03-2011, 03:15 PM
#54
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Following up on Dr. Bob's comments, re: vapor pressure of fuel vs. pressure on the suction side of the pump. I found some data on fuel vapor pressure vs. temperature, it's a bit confusing because different fuel mixes are used winter and summer, with different Reid Vapor Pressure. But here is what the absolute vapor pressure looks like.
Summer fuel has an RVP rating of around 8.3, the range of possible fuels are 6.5 to 15-- but I think the normal range is around 7.1 in the summer, and 8.3 in the winter. I've extended the 7.0 and 8.3 lines out to 140F, I don't know how hot fuel can get in the desert heat with a broken aircon...
Now here's the other side of the equation: Pressure vs. altitude.
When the curves cross, the fuel turns to vapor and the pump quits pumping. Near sea level there is not much problem, even at 3000 ft the atmospheric pressure is still 13 psi. So for summer fuel, as long as the temp stays below 140F, the fuel won't vaporize. But any pressure drop due to suction of the pump will reduce the absolute pressure, and reduce the margin for vaporization. The lowest pressure would be inside the pump, pressure drop is cumulative.
At high altitude things change pretty quick. At 10,000 feet (Tioga Pass in Yosemite for example) the air pressure is only 10 psi (absolute), and it can be pretty warm in the summer. The curves cross at a fuel temperature of 120F, above that and the fuel wants to vaporize, and that is not considering any pressure drop on the suction side.
Cheers,
Summer fuel has an RVP rating of around 8.3, the range of possible fuels are 6.5 to 15-- but I think the normal range is around 7.1 in the summer, and 8.3 in the winter. I've extended the 7.0 and 8.3 lines out to 140F, I don't know how hot fuel can get in the desert heat with a broken aircon...
Now here's the other side of the equation: Pressure vs. altitude.
When the curves cross, the fuel turns to vapor and the pump quits pumping. Near sea level there is not much problem, even at 3000 ft the atmospheric pressure is still 13 psi. So for summer fuel, as long as the temp stays below 140F, the fuel won't vaporize. But any pressure drop due to suction of the pump will reduce the absolute pressure, and reduce the margin for vaporization. The lowest pressure would be inside the pump, pressure drop is cumulative.
At high altitude things change pretty quick. At 10,000 feet (Tioga Pass in Yosemite for example) the air pressure is only 10 psi (absolute), and it can be pretty warm in the summer. The curves cross at a fuel temperature of 120F, above that and the fuel wants to vaporize, and that is not considering any pressure drop on the suction side.
Cheers,
05-03-2011, 04:10 PM
#56
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I would rather use an external fuel cooler on either the feed or return line than screw around with the internal pump.
05-03-2011, 04:26 PM
#57
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Following up on Dr. Bob's comments, re: vapor pressure of fuel vs. pressure on the suction side of the pump. I found some data on fuel vapor pressure vs. temperature, it's a bit confusing because different fuel mixes are used winter and summer, with different Reid Vapor Pressure. But here is what the absolute vapor pressure looks like.
Summer fuel has an RVP rating of around 8.3, the range of possible fuels are 6.5 to 15-- but I think the normal range is around 7.1 in the summer, and 8.3 in the winter. I've extended the 7.0 and 8.3 lines out to 140F, I don't know how hot fuel can get in the desert heat with a broken aircon...
Attachment 533339
Now here's the other side of the equation: Pressure vs. altitude.
Attachment 533337
When the curves cross, the fuel turns to vapor and the pump quits pumping. Near sea level there is not much problem, even at 3000 ft the atmospheric pressure is still 13 psi. So for summer fuel, as long as the temp stays below 140F, the fuel won't vaporize. But any pressure drop due to suction of the pump will reduce the absolute pressure, and reduce the margin for vaporization. The lowest pressure would be inside the pump, pressure drop is cumulative.
At high altitude things change pretty quick. At 10,000 feet (Tioga Pass in Yosemite for example) the air pressure is only 10 psi (absolute), and it can be pretty warm in the summer. The curves cross at a fuel temperature of 120F, above that and the fuel wants to vaporize, and that is not considering any pressure drop on the suction side.
Cheers,
Summer fuel has an RVP rating of around 8.3, the range of possible fuels are 6.5 to 15-- but I think the normal range is around 7.1 in the summer, and 8.3 in the winter. I've extended the 7.0 and 8.3 lines out to 140F, I don't know how hot fuel can get in the desert heat with a broken aircon...
Attachment 533339
Now here's the other side of the equation: Pressure vs. altitude.
Attachment 533337
When the curves cross, the fuel turns to vapor and the pump quits pumping. Near sea level there is not much problem, even at 3000 ft the atmospheric pressure is still 13 psi. So for summer fuel, as long as the temp stays below 140F, the fuel won't vaporize. But any pressure drop due to suction of the pump will reduce the absolute pressure, and reduce the margin for vaporization. The lowest pressure would be inside the pump, pressure drop is cumulative.
At high altitude things change pretty quick. At 10,000 feet (Tioga Pass in Yosemite for example) the air pressure is only 10 psi (absolute), and it can be pretty warm in the summer. The curves cross at a fuel temperature of 120F, above that and the fuel wants to vaporize, and that is not considering any pressure drop on the suction side.
Cheers,
Can you rephrase this so my feeble mind can understand?
You talk about the curves crossing, crossing what? I don't get it, but I want to. Can you help a brother out?
05-03-2011, 04:31 PM
#58
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Non engineer WAG: different fuel mixes have different RVPs (propensity to vaporize) that increases with temp. And at high altitude, the counteracting atmospheric pressure goes away, predisposing to increased fuel evaporation. So if you're high enough and hot enough with a high RVP fuel, vapor lock (?)
05-03-2011, 05:56 PM
#59
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The way this is characterized is a curve of "vapor pressure" versus temperature, i.e. the pressure that is needed to "keep a lid on" the liquid and keep it from vaporizing. (First graph above). If the vapor pressure is greater than the pressure the atmosphere is exerting, then the liquid turns to vapor.
The only tricky bit is that gasoline comes in different mixes, winter vs. summer. Winter gas is more volatile, to help the car start on those sub-zero days. But it vaporizes too easily in the summer, contributing to pollution and vapor-lock. So it gets replaced in the summer with a mix that is less volatile, i.e. has a lower vapor pressure (and costs more).
The second curve is atmospheric pressure versus altitude-- as you go higher, there is less pressure to prevent vaporization. So fuel, which was happy to stay liquid at sea level, wants to turn to vapor at higher altitude.
To remain liquid, the atmospheric pressure (second curve) needs to stay above the vapor pressure (first curve, for whatever gas mixture is used, and whatever the temperature is). If the temperature is high enough, or pressure low enough, then the gas turns to vapor and the fuel pump quits.
So, a bigger hose into the fuel pump helps by reducing pressure drop, the in-tank pump helps by keeping the pressure higher going into the main pump, the fuel-cooler helps by reducing the temperature.
05-03-2011, 06:07 PM
#60
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It would be very interesting to see what my fuel tank temp is here in the summer. Car sits in a hot 90+ garage...driven in 100+ heat in traffic...parked at the airport on 110+ asphault..etc etc. and im at 2100-3000ft . When i do use the AC im sure it doesnt significanlty change the fuel temp in my tank at all.
I don't ever see any fluctuations in my Fuel pressure though as seen on my gauge?
Couldnt i just add a dash of ice to my tank to chill it
