NYCB
Speaking as a pump expert, which I was IRL before retiring, I can answer
pump questions with authority and where I don’t have specific
information my assumptions are typically accurate.
First off, I have to make an assumption that the factory pump which I
believe is a PD [positive displacement roller pump] would be rated
typically in the 75 psig with a maximum of 125 psig for that type of
pump due to construction. I.e. a roller pump is a section of flex hose
that is compressed by a roller[s] progressively. Picture a wagon wheel
with spokes but no rim. At the end of each spoke there is a roller. As
the wheel rotates the rollers roll over the flex tube compressing it and
pushing the gas ahead of the roller. Now there are not as many spokes
on the pump as there are on a wagon wheel but you should get the idea.
This means the pump is limited by the flex tube burst pressure and the
fact that it is being constantly stressed by being crushed between the
roller and a hard place. Since the PRV, pressure regulating valve is
set to maintain pressure between 35 and 45 psig, the pump has to put out
enough pressure to allow the valve to work. That is it needs a pressure
differential, which is usually no more than 10-15 psig over the
regulated pressure. Adding 15 to 45 gives 60 psig minimum. You would
also have to add in the friction loss in the filter and piping between
the pump and the PRV which would be between 5-10 psig [10+60=70]. That
is why I would suspect the factory pump is between 70 and 75 psig rated
pressure output with the potential to go over that to say about 100
psig. All PD pumps must have a pressure relieve valve and means of
controlling the relieved pressure flow [recirculation system would seem
to do that and I am not sure if there is an additional relief valve. So
I suspect you have a 100 psig rated PD pump with 12v motor rated for a
fraction of a gallon per minute.
Adding two PD pumps in series is like adding 1+1. The pressure produced
by one pump is added to that of the second. Its not a good idea to add
a second pump of higher pressure to the inlet of a lower pressure pump
because you may well exceed the burst rating of the lower pressure pump.
If your pump is in tank then you have no choice but to do the right
thing put the new pump in series after the old one. If external pump
you must put the new pump after the old one and in series.
Putting the pump in series will do nothing to improve suction abilities
of the original pump. Whatever the NPSHR of the original pump limits
what the pump can suck in [PD pumps pull a vacuum]. Though it must be
considered that the tank is supposed to be under about 2 psig positive
pressure which would force the fuel into any known PD pump quite nicely
[assuming you evaporative system is functioning and has 2 psig [it does
go whoosh when you take the cap of doesn’t it?].
Does the Kragans have an internal relieve valve? How is the relieved
gas returned to the tank?
Frankly, if the original pump is working and has only marginal
difficulty for some unexplained reason it should only require a second
pump of less than 30 psig. Adding a 125 psig rated pump to the existing
pump that is putting out 60 psig could burst the system unless you have
a means to control the over pressure. Now, what is the pressure rating
of the clear plastic PVC tubing you are planning on putting in the fuel
rail [before or after the PRV]. Remember the PRV puts out 45 psig and
its a good idea to have a rating of twice that on plastic so you would
need 100 psig tubing after the PRV and 200 psig rated before assuming
both pumps are pumping at the rated pressure.
Now, the ability of the original pump to do its job is dependent on the
ability of the fuel to get into the pump. It will cavitate if it has to
pull a vacuum on the fuel in the tank to get it into the pump because
gasoline easily goes to fumes [gas state] at atmospheric pressure. If
you reduce that even further you will cause the liquid fuel to evaporate
in the pump suction. If there is vapor in the pump suction the pump
performance is reduced greatly. Now, the system is designed to have a 2
psig over pressure in the tank [my wife’s Stratus has a pump that
pressurizes the tank to keep 2 psig in it for example]. 2 psig should
be enough to keep fuel from vaporizing in the pump suction in the
original pump. Actually, if the first pump can not suck any liquid fuel
into it, adding a second pump wont increase the pressure since it wont
have any suction either. The only time the second pump will add
pressure is if the first one is getting some or all its required
pressure on the inlet. What I am saying is that the temperature of the
fuel in the tank is the critical issue since it effects pump suction
ability. The pressure of the fuel in the fuel rail is supposed to be 35
psig [far above the 2 psig on the tank] so its unlikely [but always
possible with a leaky or bad PRV] to vaporize in the fuel rail. So the
two week points are the emission systems 2 psig fuel tank over pressure
or 35 psig in the fuel rail. Adding a booster will help if either is
marginal but so will fixing the system so it functions as designed.
Since the emissions control system keeps 2 psig on the inside of the
tank the original pump should work under all but the most extreme
temperature of the fuel in the gas tank. If there is a marginal problem
adding a second pump may cure the pressure.
Those are the facts of life for pumps with some educated guesses as to
what you are dealing with.
Good luck
Jay 90 VDP Majestic
NYCBullet wrote:
Dear People,
I have found that KRAGANS has an aftermarket fuel pump for
abotu $130.00. What is most interesting, is that this pump
specs are 12Volt (of course) but has 125PSI!
It is a very nice unit, designed as a cylinder inline
configuration (one end is intake pipe, the other is output),
with two simple connectors in the middle for the 12Volts.
Very nice.
Now, the 125PSI is very interesting…I am attempting to
find the original specifications of the Bosche unit that is
in there. What is the PSI on the original pump.
A bigger question would be, is in the Jaguar (and Haynes)
recommendation to place a SECOND PUMP IN SERIES WITH THE OLD
PUMP–what is the SIZE of the second pump? Do you put two
high pressure pumps in series? Or is the second pump just a
slave (assisting pump) of normal (12-30PSI) pressure?
Would be nice to know the second pump pressure they are
asking us to place in series with the older Bosche fuel pump.
Now, this is my plan of attack so far.
1.) I’m getting some 2’’ clear tubing, along with some 2’’
PVC pipe. I’m placing (and cutting) the clear tubing so
that it will fit over the complete Fuel Rail. I’m then
going to insulate the fuel regulator, as well as all gas
fuel lines in the engine bay.
The Clear 2’’ plastic piping, (very nice looking, $3 a foot)
will go straight on the fuel rail, to the front of the
engine, then, I will elbow it with the hard plastic PVC
(Black) pipe and go to the right. Then I will go down to
the bottom of the engine/radiator, and again, a 90 degree
turn to the front of the engine/radiator where I will cut a
small 2’’ hole in the air dam so the system has fresh air
from the outside.
This way, the heat from the engine should be insulated from
the fuel rail and fresh air should cool it.
I have found that the temperature of this rail on a hot day
is at 150 degrees F.
2.) If this does not cure the problem, I will buy a small
6’’ radiator, and put the RETURN fuel line to the front of
the engine where this small radiator will sit. I think the
problem is cumulative. That is, it appears that once the
gas comes into the engine compartment, it gets heated, then
ported to the tank again, where it loops right back into the
system, and the heat builds up. Cooling it at its output
and returning cool(er) gasoline to the tank–probably will
mitigate this situation completely.
3.) I will insulate all the lines of the gas system
including the fuel pump wish is naked to the outside temp
right now.
4.) Adding a secondary fueld pump.
5.) I fully intend to put a pressure gauge at the fuel
rail. This must be at least 50lbs at least (found a PERFECT
$11.00 one–but only went to 15lbs. 
6.) Machine aluminum and put PELLITIER DIODES on the
aluminum. These diodes get very cold on one side and hot on
the other when voltage applied. Should cool the aluminum
and then the whole rail quite magnificently.
I’ll soon do these one by one and then tell you the results,
part numbers, etc.
RLCII
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