Fuel pressure spikes.

[RonA]

When i get back from my tune up at PDR I am going to change from FASS pump to 2nd gen piston pump. I was told that the piston and diaphram pumps fluctuate alot when you lift off the throttle. I was wondering if a hollow cylinder that holds maybe a 1/4 pint of fuel and has a bladder inside like a well pressure tank, mounted between the lift pump and the injection pump would help to smooth it out a little. The bladder would allow the volume to vary a little without a big pressure change.

 

[bgilbert]

Well since nobody's given this a crack, Ron, what kind of pressure is your FASS supplying your P-pump? I heard it needs at least 25psi at idle and more with throttle?? I would believe you'd be fine with a 2nd gen piston pump, since you have a p-pumped engine. Mechanical lift pumps are much more reliable than electric any day of the week. As far as a hollow cylinder between the LP and inj pump, you'd have to figure out how to get the supply lines fabbed. I don't know if this would help or not. All I can tell you from my 'low pressure' piston lift pump is at idle the FP is lower than FP with moderate throttle, the FP actually increases some with throttle. Which its supposed to do, its driven by the camshaft, and as rpm's increase, it drives the plunger in the lift pump faster, it pumps more fuel- that is if it can keep up with the demand for the fuel. A dead mechanical LP may give FP at idle but as soon as rpm's go up, FP drops quickly. I would think a p-pumped motor would require a piston lift pump since it requires much more fuel pressure. I would hate to see a guy swap a p-pump on a VE powered motor, and try to run with the diapraghm LP. I guess anymore info on 2nd gen LP's you probly get more exposure in that forum. Good luck with whatever route you chose. BTw you outta get some good $$ for that FASS pump.

 

[RonA]

Bill.

My fass is putting out 24 to 26 depending on throttle. I bought a piston pump from PDR about 2 months ago. I will put it in as soon as I get back from PDR next week. I'm not an electronic gadget kind of person. Thats one of the reasons I changed cams was to be able to install this little piston pump. I have been told that the pressure fluctuates alot with the piston pump when you back off the throttle. P-pump is fine with it. But I was wondering if it could be smoothed out enough to allow the piston pump to be used on a 24 valve. Small pusher in the tank, filter, piston pump, pressure tank with relief valve, filter, vp-44. A cam costs about the same as a FASS or Airdog. The piston pump is about 140. It might be a different way to handle the 24valve pump issue. Cam is a nice boost in performance also. Small cylinder with spring loaded piston seal and simple relief valve might allow it to buffer the spikes in pressure. the volume could change quite a bit without a big change in pressure. Would be cheap and easy to make.

 

[Greenleaf]

Ron,



I mayt have missed something here, what will power the piston pump on a 24 valve engine block??



The VP is similar to the VE in that too much pressure into the pump at the wrong time will effect it negatively, will it not? They need to see about 10-15 psi not 25 psi unless I'm off base here.



Bill and I had looked into an electrical lift pump however due to the nature of the electric pump and plumbing required we investigated the piston pump and found it to be quite satisfactory for use on a high power VE. If your using a P pump the higher flow piston pump may work well.



Installing an electric pusher to supply the piston pump would seem like an unessessary expense. The piston pump starts and stops. It's not a contiuous flow like an electric motor vane pump.



There is a lot to disscuss here... ... ... ...



Scott

 

[RonA]

My motor has a lobe to drive the lift pump. Experiment is to see if pressure from piston pump can be lowered and stabilized.

 

[RedRam401]

I was wondering if a hollow cylinder that holds maybe a 1/4 pint of fuel and has a bladder inside like a well pressure tank, mounted between the lift pump and the injection pump would help to smooth it out a little. The bladder would allow the volume to vary a little without a big pressure change.

I wonder if one of the Moroso Oil Accumulators would work for this fuel storage device? They are available in 1. 5 or 3 quart sizes.

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[cerberusiam]

the piston and diaphram pumps fluctuate alot when you lift off the throttle

To begin with, why is this an issue? Letting off the throttle will reduce the fuel needed for injection. The P pump doesn't use fuel for lube so it doesn't care and the VE will still be getting enough to lube and cool it. Isn't there a one way valve between the filter and injection pump to maintain pressure at the pump? Or is that only on the 24 valves? I think there has to be on the P an VE also or there would be drain back and hard starting.



Do you have the whole FASS system or just a FASS pump? I would think the benefits of the FASS would outweigh the need for a totally mechanical solution. Maybe the RASP system would give you what you want and still be able to utilize the FASS benefits?



Just wondering.

 

[Greenleaf]

I have not found a check valve. The LP is the check valve be it diahpragm or piston. In fact the piston pump does have a check valve. Some engines will crank awhile prior to starting with a bad LP



I'm not concerned with the spike. When you let up off the throttle the LP is momentarily pumping a good bit of fuel and the injection pump is saying no more. Then the spike... right?



It also spikes down... ... ... ... ... when you hammer the throttle hard. But then the LP psi picks right back up... . unless you have a bad LP.....



GL

 

[cerberusiam]

With a positive displacement pump you will always have a pressure high/low condition if the pump is providing enough flow but those should be within a range that is tolerable by the components in the system. Advancing or retarding the throttle should not cuase large spikes or valleys due to the fact the excess fuel is always being routed thru the return line back to the tank. Large spikes would indicate there is a restriction in the lines somewhere. Large valleys would indicate the pump flow is not keepng up with the injection pump demands. Right??



I was wondering why Ron was worried about fluctuations after lifting off the throttle. There should always be fluctations due to the nature of the fuel delivery system but as long as they are within norms it should not be an issue. If it is an issue adding rubber fuel lines in place of the metal ones will help flatten the pressure differences.



I guess the check valve must have been added to the 24 valve filter housings becuase of the demands of the VP and the way the LP works.

 

[RonA]

I am working on a backup plan in case the the bugs can't be worked out of this conversion.

 

[JLEONARD]

 Large spikes would indicate there is a restriction in the lines somewhere.

The restriction would be the return lines... . they really can't handle much flow.



I agree with GL's eplanation/theory.



Not convinced an accumulator will really do anything... not in the practical sense anyway. Keep it simple. Get a LP that can pump enough fuel at the pressure you need and be done with it. There are engines that pump more fuel than ours do... . so there are pumps available. Regulate the pressure if necessary.

Just make sure the feed line size to the LP is big enough to insure a good supply. Err on the large side to be sure.



For example... . my marine 6bta 270 hp (P pumped) REQUIRES 1/2 inch ID fuel lines going to the filter assembly.



Jay

 

[RonA]

I want to see if I can smooth out the pressure from a mechanical pump and drop the pressure down to about 15psi constant. I have been told that the spikes occur when you let off the throttle quickly. The electronic pumps are not fond of the sudden spike in fuel pressure. If I can maintain the constant 25psi for my p7100 without spikes, then I should be able to drop it down to 15psi and use it on a vp44.

 

[Cooker]

I think it is a good thing that fuel pressure as well as volume increase with engine RPMs. That is the way the factory piston pump is designed. I don't know that you would want to take that away? On an heavily fueled truck, even the factory piston pump is not stout enough to keep with the fueling demand.