Measuring Fuel Required by the CDFP

[superskaterxes]

so in my quest for even more epic fuelz, ive come to realize that for my power goals i am going to need a little bit more then what the DW65c can provide.

anyway, before i start upgrading my fuel system even more, i need to know what the maximum flow and pressure requirements are for the LOW side of the CDFP.

we all know that we only need enough pressure and flow to fill the cavity inside the pump before it is compressed into the rail. this minimum amount of flow and pressure will never change because the volume of the cavity never changes. even at high rpm, the pump is still just filling and emptying that cavity.


Here is what the inside of the pump looks like:






so assuming the spill valve is wide open and the cavity is empty and ready for fuel, how can we calculate exactly what pressure is required to keep that cavity full, and what the maximum amount of flow is into the cavity?


so i guessed the volume of the cavity to be a cylinder with a diameter of 10mm and a height of 15mm (bore and stroke of the piston basically)

after doing the math this comes out to be 0.0011781 Liters of fuel


for 1 RPM, the cavity gets filled 2 times, so at 7500 RPM the cavity needs to be filled 15,000 times in 1 minute.

15,000 x 60 mins = 90,000 fills per hour

90,000 fills/hour at 0.0011781 liters each = 106 LPH worth of fuel.



this may sound like a small number but remember our CDFP is like a mini surge tank/pump. all it needs to do is keep the rail filled with fuel and at a super high pressure to be able to feed the injectors. Now dont forget the RV also helps fill this cavity with the excess fuel coming off the rail.


so right now i have 2 1000cc/min injectors at ~43psi low side pressure (after i subtract boost) which are at almost 100% IDC. According to their flow sheet this is about 120 LPH worth of fuel total.


here is dustin's fuel pump flow sheet from this thread:

More data on the new DW65c drop in fuel pump





this would mean that my dw65c is flowing about 210 LPH total at ~75 psi. so 120 LPH from AUX injectors + 106 LPH from stock pump = 216 LPH total required fuel



so a sanity check:

i was flowing about 535 g/s of air at an AFR of ~12.5

535/12.5 = 42.8 g/s worth of fuel needed total

42.8 x 60 x 60 = 154,000 g/hour

154,000 / 754 g/liter of fuel (40% e85) = 204 LPH worth of fuel.


do these calculations make sense to anyone? lol do my estimates for the CDFP cavity seem off?


Tomas djuosnteisn cld12pk2go SilverDemon SPEED6 KILLAH Lex 06Speed6

[silvapain]

Maximum would be with the spill valve duty cycle at 100%. Determine the displacement of the HPFP piston, and multiply that by 3/2 and you'll get the volume per crank rotation.

3 = lobes on intake cam for the HPFP
1/2 = speed that the cams rotate in relation to the crank.

Multiply that by your redline RPM and you'll have the maximum fuel volume per minute.

The diameter of the HPFP piston is easy enough to measure. The hard part will determining the stroke length of the piston.

Edit #2 : liquids are essentially incompressible, so the pressure the pump runs at is inconsequential.

Tapadatass

[silvapain]

It's 1.5 HPFP cycles per crank rotation, not 2.

7500 (RPM) * 1.5 = 11,250 cycles per minute.

11,250 (cycles/min) * 60 (minutes/hour) = 675,000 cycles/hour

675,000 (cycles/hour) * 0.0011781 (liters/cycle) = 795.2175 LPH

EDIT: That value seems too high. I suspect the liters/cycle value is incorrect.

Tapadatass

[SilverDemon]

Thanks for the tag, but I do not think I have much info on this subject. I have not studied up on it too much.

[djuosnteisn]

Originally Posted by superskaterxes ...so i guessed the volume of the cavity to be a cylinder with a diameter of 10mm and a height of 15mm (bore and stroke of the piston basically) after doing the math this comes out to be 0.0011781 Liters of fuel ....

15mm stroke sounds a lil bit high.

Also, assuming the hpfp cavity will out flow the injectors... it's maximum flow doesn't really have any effect on your low side fuel demands. Just calculate what the max DI inj flow for ~2000 psi to find a worst case DI flow demand. Not like we'll have higher flowing DI injectors any time soon, and until (if) those ever become available, they'll be the bottle neck, not the hpfp capability.

[dpolseno41]

The stroke should be around 15mm should it not?

[Ziggo]

Based on the apparent wear, yes. But really need to measure the lift on the cam lobe for the right number. Plus we don't know that the spill valve always closes right at the bottom of the stroke, the dynamic stroke could be less.

[dpolseno41]

In theory, if the piston could be enlarged entirely, not just stepped, would there be a significant difference over stock? This would require a new piston seal housing (my own term, don't get upset with me). I'm awaiting a used fuel pump so I can get cracking on measurements/ design(s) of new internals so whilst this thread doesn't pertain particularly to this case, it's certainly intertwined.

[superskaterxes]

Originally Posted by silvapain It's 1.5 HPFP cycles per crank rotation, not 2. 7500 (RPM) * 1.5 = 11,250 cycles per minute. 11,250 (cycles/min) * 60 (minutes/hour) = 675,000 cycles/hour 675,000 (cycles/hour) * 0.0011781 (liters/cycle) = 795.2175 LPH EDIT: That value seems too high. I suspect the liters/cycle value is incorrect. Tapadatass

i thought about this ALOT before i made the post haha

if you start with the piston compressed. the cavity will fill, empty, then fill in 1 crank rotation (half the cam rotation)


so this is where i got 2 "fills" in half a rotation.



EDIT: fml i guess its 3 total fills in 1 cam rotation so u need to count it as 1.5 right?

[silvapain]

Yes. 3 total in/out cycles in 1 cam rotation, and the cam rotates at 1/2 of crank speed, so it's 3 * (1/2) = 1.5 cycles per crank rotation.


Tapadatass

Originally Posted by dpolseno41 In theory, if the piston could be enlarged entirely, not just stepped, would there be a significant difference over stock? This would require a new piston seal housing (my own term, don't get upset with me). I'm awaiting a used fuel pump so I can get cracking on measurements/ design(s) of new internals so whilst this thread doesn't pertain particularly to this case, it's certainly intertwined.

If you look at the diagram you posted, the top side of the piston is where it pumps fuel to the rail. The stepped side is only for piston lubrication as stated in the diagram.

Making the whole piston that diameter won't increase displacement. It's stepped so that the original cap can be used; otherwise a new one would have to be made and O-rings found to seal between the piston and cap.


Tapadatass

[dpolseno41]

Originally Posted by silvapain Yes. 3 total in/out cycles in 1 cam rotation, and the cam rotates at 1/2 of crank speed, so it's 3 * (1/2) = 1.5 cycles per crank rotation. Tapadatass If you look at the diagram you posted, the top side of the piston is where it pumps fuel to the rail. The stepped side is only for piston lubrication as stated in the diagram. Making the whole piston that diameter won't increase displacement. It's stepped so that the original cap can be used; otherwise a new one would have to be made and O-rings found to seal between the piston and cap. Tapadatass

I know why it's stepped haha That's blatantly obvious. I have no problem machining a new cap if an overall larger piston could offer any advantage over a stepped piston. I'm thinking in terms of sustaining pressure over displacement.

Sorry for the thread jack!

[Tokay444]

What you should be thinking is in terms of what that diagram is explaining, as I see that as the reason for so much of our fuel dilution.
Not ring blow by.
But we have fuel dilution issues on the stock pump, and it's one size all the way down.
Different scenario for a different thread.

[djuosnteisn]

I'm still confuzzled on why anyone would want to increase the flow capability of the hpfp, when it already dwarfs the capabilities of the stock injectors.

If indeed 15mm stroke is even close to accurate, and the ~800 LPH number Dan computed is even close to accurate...... the hpfp volume seems like more than enough, even if magical upgraded injectors became an option.

[Lex]

How about using a fuel flow meter?

Or building your own on the low pressure side by using a piece of hose/pipe with a known ID and length and two pressure gauges at each end.

[06Speed6]

After thinking about this some, I actually think that the feed line into the cdfp is undersized for upgraded internals. If that line is 5/16" like I think it is, I think your numbers may be high. I know the pumps demand isnt constant but I dont think there is really any sort of self pressurizing reservoir that the piston can draw from so its limited to line diameter for filling.

[Ziggo]

I believe the pump volume is fully filled every stroke. If it wasnt, what is filling the remaining volume? You are in bad shape if its air.

Every stroke that cavity fills with fuel and you do not have control of the stroke length or frequency. When the fuel demand is low it waits to close the spill valve till the piston partially up so some portion of the fuel back flows towards the fuel pump instead of being pushed past the check valve into the fuel rail.

I think where the calculation goes wrong is assuming that when operating at 100%, that the spill valve closes at the exact instant the piston dwells at the bottom of its stroke. I dont think that is necessarily true.

Zigatapatalka

[djuosnteisn]

Are we certain the calculation is wrong? I mean, has anybody ever "out flowed" the CDFP?

It would be cool to rig a setup where the fuel rail is fed by a T in the low pressure line (cause we know the car will idle off ~50psi), and the CDFP output was directed into a bucket or something, and spill valve duty cycle forced for max flow... then someone could actually measure the amount of fuel the thing dumps over a give period at a given rpm.



Looks like 100% duty cycle = maximum fuel delivery (the inlet check valve will still be able to suck in fuel even with the pump is spill valve is energized). And while the ecu may never command higher than 50% in actual operation (cause there's no reason to energize the spill valve on the intake stroke, it would just unnecessarily heat the solenoid up), i'm sure it would be perfectly fine to just put 12 volts across the spill valve for a quick test.


Food for thought.

[Lex]

In terms of spill valve control and Ziggo's comment it would be very interesting to have a bit of insight here.

For example, is the spill valve controlled from a base calculation involving mass airflow, engine speed, fuel pump capacity and requested pressure? Then pressure feedback trims used to adjust pressure?

Because when we upgrade the pump we are changing the volume/stroke displaced and the ECU is either correcting based on fuel pressure feedback OR just riding the PRV which may be why we see the 2150psi PRVs cause cuts.

[phate]

Originally Posted by djuosnteisn Are we certain the calculation is wrong? I mean, has anybody ever "out flowed" the CDFP? Looks like 100% duty cycle = maximum fuel delivery (the inlet check valve will still be able to suck in fuel even with the pump is spill valve is energized). And while the ecu may never command higher than 50% in actual operation (cause there's no reason to energize the spill valve on the intake stroke, it would just unnecessarily heat the solenoid up), i'm sure it would be perfectly fine to just put 12 volts across the spill valve for a quick test. Food for thought.

When I was running full E85 and ramping in boost really early (like 25psi @ 3k rpm), you could watch pressure drop to ~1600, then creep back up to ~1800 by 3800rpm or so.

I also jumpered the spill valve back then, at idle, and it hammered the PRV open instantly.

I think it's pretty obvious we are relying on the PRV's to control pressure more so than we would like. Most cars with the higher pressure RV's get into a pressure overrun condition because they can't relieve the pressure any longer. Makes me think there is very limited spill valve duty cycle trims in the ecu, if at all...

Autotech's typically have pretty erratic pressure fluctuations - I've always considered this to be due to hitting the PRV limits. It seems to me ptp internals and cpe's don't do this...and it could be directly related to tolerances or piston diameter. I would measure some PTP's, but they're seized in the barrel piece right now

Anyway, I don't think I have anything valuable to add to this discussion, lol. Wanted to throw out some experience, though.

[djuosnteisn]

Originally Posted by phate When I was running full E85 and ramping in boost really early (like 25psi @ 3k rpm), you could watch pressure drop to ~1600, then creep back up to ~1800 by 3800rpm or so. I also jumpered the spill valve back then, at idle, and it hammered the PRV open instantly.

What do you think your torque was at 25psi / 3k rpm? If that torque was held constant til 7500 rpm... we could ricer guestimate the power you would have made, and thus the fuel required.

[Ziggo]

I dont think it times opening and closing with the cam. At least that is not how I would design it. Like you said, a soft enough spring will let the pump suck fuel past it so you dont have to time it. And who cares how the pulsewidths line up with the pump stroke as it will still be proportional and if you need more, 100% will give you the maximum flow the system is capable of, timed or not.

Lex, that has been my theory regarding the control loop for the spill valve breaking down with upgraded internals causing it to ride the PRV. I think it has minimum DC associated with load and rpm somehow so that even if its overshooting targets it wont allow the control loop authority to drop the duty cycle below the minimum, meaning you can have internals that are TOO good and blow PRVs. This should be unlockable by cobb though and its what I have been waiting on to upgrade my PRV. This is backed up by people getting the fuel pressure cut at the same RPM regardless of targets. That load/rpm table is driving the duty cycle too high

Zigatapatalka

[silvapain]

Assuming that the programming in the ECU for controlling rail pressure uses the rail pressure sensor for feedback, changing the displacement of the HPFP with upgraded internals will change the response characteristic of the loop, making the response (pressure) change more for a given input increase (spill valve duty cycle). My theory is that this is why the rail pressure is unstable for upgraded internals; to use controls terminology the loop is underdamped.

Originally Posted by djuosnteisn What do you think your torque was at 25psi / 3k rpm? If that torque was held constant til 7500 rpm... we could ricer guestimate the power you would have made, and thus the fuel required.

It was a K04; there's no way it held the torque flat all the way to 7500 RPM.

[Ziggo]

Originally Posted by djuosnteisn What do you think your torque was at 25psi / 3k rpm? If that torque was held constant til 7500 rpm... we could ricer guestimate the power you would have made, and thus the fuel required.

Its not really going to work like that. The fuel injectors are power limited, the HPFP is Torque limited. Its an either/or situation. Above some rpm you are going to be injector limited. Below that rpm you will be torque limited by the hpfp

Zigatapatalka

[dadderllama]

Not to thread jack, but just a thought...
Are there any upgraded fuel rails? The larger the cavity before the injectors would mean less loss in pressure at wot... I know air is totally different than liquids when it comes to compressing/pressurizing, but think of air compressors. You have a 1gal tank, and a 5gal tank, both at 90psi let a very small amount of air out of each, you would see a drop on the 1gal, and a flicker on the 5 gal. Can we increase fuel rail overall volume and have similar results??
Maybe my thoughts are crazy, if so flame on...

tl:dr just flame on

[phate]

Originally Posted by djuosnteisn What do you think your torque was at 25psi / 3k rpm? If that torque was held constant til 7500 rpm... we could ricer guestimate the power you would have made, and thus the fuel required.

The car made 430lb-ft @ 3800 on the dyno. Virtual dyno (1.01cf) lined up with that perfectly, but peaked around 450lb-ft @ 3400.

Another good estimate for this is that I was spraying 35-40% more fuel than on 93 alone. Scaled MAF ftw

[djuosnteisn]

Originally Posted by silvapain It was a K04; there's no way it held the torque flat all the way to 7500 RPM.

Originally Posted by Ziggo ... the HPFP is Torque limited. Its an either/or situation...

Understood, but all of the equations in OP (and Dan's post) are for 7500 rpm... not 3k rpm. So if Clint did outflow the hpfp at 3k rpm, we can correlate his torque to the necessary flow at 7500 rpm, and see if the ~800 LPH is even in the same ball park.

[djuosnteisn]

Originally Posted by phate The car made 430lb-ft @ 3800 on the dyno. Virtual dyno (1.01cf) lined up with that perfectly, but peaked around 450lb-ft @ 3400. Another good estimate for this is that I was spraying 35-40% more fuel than on 93 alone. Scaled MAF ftw

450 ftlb at 7500 rpm would be ~640whp... multiply by 1.15 to get chp = ~740chp.

Using some cheese dick web calculator for injector flow per hp with 0.6 BSFC = 138.8 lb/hr injector... x 4 for total fuel injected = 555 lb/hr of fuel. Convert to liter per hour = ~350 lph.

Even with a 30% increase in fuel for the scaled maf, that only puts us around 455 lph of fuel. So if our HPFP's are only 56% efficient... then perhaps this all makes sense.

*** every ounce of the above was from the hip... i don't have enough time to invest any more than that right now ***

[Ziggo]

How confident are we in the 10mm bore number? That seems large to me but I have never measured or handled it. If you let the bore size be the variable, working backwards from phate's numbers gives me a bore diameter of ~6.6mm

Zigatapatalka

[Tokay444]

i can measure the stock bore tomorrow

[aggierandy]

Originally Posted by djuosnteisn Are we certain the calculation is wrong? I mean, has anybody ever "out flowed" the CDFP? It would be cool to rig a setup where the fuel rail is fed by a T in the low pressure line (cause we know the car will idle off ~50psi), and the CDFP output was directed into a bucket or something, and spill valve duty cycle forced for max flow... then someone could actually measure the amount of fuel the thing dumps over a give period at a given rpm.

I very much agree with djuosnteisn on this one. It would probably be more accurate (when we are talking about 1.1 mL increments) to determine the output experimentally.

Remember, any error in the cavity volume is being multiplied by a very large number (1.5 x RPM). This makes it hard to get an accurate number.

[Ziggo]

Honestly I don't think its worth the time till when (if ever) upgraded injectors are available. Given phates results the torque limit is likley at least 500wtq when not using pure E. At those torque levels you would hit the injector limit at 4200rpm. So for the hpfp to be an issue you need to be aiming for >500wtq before 4200rpm with no supplemental fueling to have an issue, not exactly a typical power curve or setup. Everyone I know of who can make 500wtq has both supplemental fueling and a torque peak after 4200rpm . With a built motor, a x71 turbo or one of them fancy efrs, a hatred for tires and a desire to break things I suppose you could pull it off, but I don't see why you would want to.

Zigatapatalka

[djuosnteisn]

Originally Posted by Ziggo Honestly I don't think its worth the time till when (if ever) upgraded injectors are available. Given phates results the torque limit is likley at least 500wtq when not using pure E. At those torque levels you would hit the injector limit at 4200rpm. So for the hpfp to be an issue you need to be aiming for >500wtq before 4200rpm with no supplemental fueling to have an issue, not exactly a typical power curve or setup. Everyone I know of who can make 500wtq has both supplemental fueling and a torque peak after 4200rpm . With a built motor, a x71 turbo or one of them fancy efrs, a hatred for tires and a desire to break things I suppose you could pull it off, but I don't see why you would want to. Zigatapatalka

I wish i could quote this a million times.... hahahahahahahahahahahahaha.

[Ziggo]

Addressing skaters original question, I dont think it matters how much the HPFP can draw. Using your power goal, just figure the fuel required to make it, regardless weather it comes thru the injectors or out the elephafifth port, it all has to come out of the tank. The conservative rule of thumb would be that for every 100hp, you need 60lb/hr of fuel.

Zigatapatalka

[djuosnteisn]

On that note, my surge tank came in a few days ago, and i'm picking up a slightly used 044 in a day or two. Just need to order some lines, and a AFPR, and i'll get starting on the next level of fifth port. I'm excited to see what a 70-75 psi aux base pressure will do.

[dpolseno41]

I presume you guys are talking about stock piston size? That would be .31465-3.47". Autotechs come in around .3858"

[superskaterxes]

Originally Posted by dpolseno41 I presume you guys are talking about stock piston size? That would be .31465-3.47". Autotechs come in around .3858"

woot!!! i was spot on with my 10mm guess lol

now can someone measure the displacement of the stock pump? even if you just measure the distance from max coil stretch to max coil bind we can get a general idea.


also as i was thinking more about this last night i remembered that CPE flow tests all their pumps at stock pressure and gives you a data sheet. Here is one i got from a friend that has some useful information:




it gives us flow (out of the pump) and efficiency.




btw the point of this thread isent about further upgrading the CDFP, its about figuring out what the max fuel requirement is of the pump and then subtracting that from your equation when designing a 5/6/7/8th port system. we cant just say 600whp requires 6 1500CC injectors because we have a completely different fuel system then that.

[Tokay444]

10mm is .3937

Stock piston is 8mm.

[Ziggo]

Originally Posted by superskaterxes woot!!! i was spot on with my 10mm guess lol now can someone measure the displacement of the stock pump? even if you just measure the distance from max coil stretch to max coil bind we can get a general idea. also as i was thinking more about this last night i remembered that CPE flow tests all their pumps at stock pressure and gives you a data sheet. Here is one i got from a friend that has some useful information: it gives us flow (out of the pump) and efficiency. btw the point of this thread isent about further upgrading the CDFP, its about figuring out what the max fuel requirement is of the pump and then subtracting that from your equation when designing a 5/6/7/8th port system. we cant just say 600whp requires 6 1500CC injectors because we have a completely different fuel system then that.

600whp on a speed 6 is ~ 720crank conservativly. So your ITFP needs to supply 432lb/hr. That works out to 265lph. So make sure you have a pump (or pumps) that can flow at least that much @60psi. Assume the stock system can handle 400chp of that which means you need 320hp worth of secondary injectors, so whatever cofiguration you have of them should add up to at least 240lb/hr. You dont need to know the HPFP flow capacity to figure any of it.

Zigatapatalka

[djuosnteisn]

Um, that Cpe "1500 cc/min" wouldn't even support 250 crank hp based off online calculators. Not on the specifics of their test tho.


Sent from my penis using magic

[Ziggo]

Originally Posted by djuosnteisn Um, that Cpe "1500 cc/min" wouldn't even support 250 crank hp based off online calculators. Not on the specifics of their test tho. Sent from my penis using magic

You need to know the cycle rate they tested it at for it to make sense. And then make (the rather dubious) assumption that the pumping efficiency doesnt vary with how fast the pump is being exercised and that the profile they are testing it with matches the profile of our cam.

Zigatapatalka