Fuel flow vs. HP

LETHALRACER · 05-19-2002, 09:15 PM

How much HP will a Walbro 255lph intank support? I have heard 450 to 620, which one. I have an inline T-Rex and the stock pump in the tank right now. My O2mv drop from 870mv to 500 and down to 4mv at 4200rpm at WOT. The engine doesn't pop but it won't pull any more. Does an inline fight the intank or do they work together? It seems that the intank can push as much as the inline will pull and vice versa. I want to run a TNT 150 shot but am scared to run lean. I am guessing between 650 and 700 crank HP on the gas. Can I run the T-Rex and the Walbro, or should I just go with the Walbro?

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91 Z28 ALL FORGED 383,EAGLE 6" RODS,11.2:1 SRP PISTONS,TOTAL SEAL RINGS,MINI RAM,ACCEL 58mm T-BODY,AFR 210'S
int. exh.
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G-Tech time 12.20@124 w/200lb passanger.

rskrause · 05-20-2002, 08:25 AM

Get rid of the T-Rex, it flows less than 255lph (=70gph). If the pump is working properly and getting enough voltage, and there is no restriction in the fuel line, 70gph should support somewhere in the range of 850hp (~750rwhp). A good "rule of thumb" is 1gph/12hp. I have a 255lph intank and a Kenne Bell "Boost-a-Pump" that is supporting 733rwhp (stock lines and regulator).

Rich Krause

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'95 Z-28 383 with Vortech, nitrous, etc.
"1FASTZ28"

Injuneer · 05-20-2002, 11:07 AM

Quote:

Originally posted by LETHALRACER:
How much HP will a Walbro 255lph intank support? I have heard 450 to 620, which one. I have an inline T-Rex and the stock pump in the tank right now. My O2mv drop from 870mv to 500 and down to 4mv at 4200rpm at WOT. The engine doesn't pop but it won't pull any more. Does an inline fight the intank or do they work together? It seems that the intank can push as much as the inline will pull and vice versa. I want to run a TNT 150 shot but am scared to run lean. I am guessing between 650 and 700 crank HP on the gas. Can I run the T-Rex and the Walbro, or should I just go with the Walbro?

What fuel pressure are you running? With heads that big you could be approaching 100% duty cycle on the 30# Accell injectors, if you are running at stock fuel pressure. But that may just be compounding the problems with a weak fuel supply.

A (healthy) stock intank pump, plus an inline booster should provide a reasonable fuel supply for the setup you are running. Adding an inline pump in essence, adds the pressure capability of the inline pump to the pressure capability of the intank pump... they are additive and will help each other, not fight each other.

The result of the additive pressure capability is to provide more flow at any given system pressure. You could go higher on fuel pressure and that might increase the fuel delivered by the injectors, but it sounds like there is a restriction in the current setup... failing intank pump?... clogged filter?..... faulty AFPR?... Have you checked the actual fuel pressure at WOT? That would tell you where your problem is.

You can't get a lot of info out of the "lph" rating of a fuel pump, at least not without the corresponding pressure at which the flow is rated.... a pump may supply 255lph at carb type pressures... 7psi... but at fuel injection pressure levels will have a greatly reduced capacity. So, the flow ratings quoted for most pumps are meaningless.

I have some good data on flow vs. pressure for a few different pumps in a Kinsler fuel injection manual.

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Fred
94 Formula A3: 381/TH400/N2O
Detailed Mod's List
11.513@115.59 on motor; 11.162@127.67, 1.643 60' on a 125-shot. Going with a 275-shot this year

rskrause · 05-20-2002, 11:57 AM

Fred: I don't think your explanation of a "booster" pump is correct.

I believe this is correct: If you have two identical pumps in series the pressure volume curve will be the same up to a point. As the pressure increases, the two pump system will maintain flow longer than a single pump. IOW, the pressure/volume curves will diverge. If the pumps are not identical, the flow will never be greater than that delivered by the smaller of the two. When comparing two pumps of different capacity with the larger of the two as a single pump, the pressure/volume curve for two pumps will be the same as the single smaller pump until a certain pressure is reached at which point they will diverge with the two pump system able to maintain more flow as the pressure rises.

The latter is the case with a 255lph Walbro (rated at 50psi, I think) and the T-Rex. I believe the T-Rex flows less than the Walbro at 50psi. Since his setup is NA, I assume he is running stock FP. The T-Rex is therefore not doing him any good (he is not trying to run at high pressure like a SC system) and may, in fact be restricting fuel flow or sucking needed voltage from the in-tank, depending on how it is wired.

The way to determine if the pump(s) are inadequate is to look at a FP gauge when making a WOT run. If the FP drops, there is a fuel supply problem: the pump(s), voltage at the pump(s), fuel filter, or a blocked fuel line.

However, he may be lean because of the possibly inadequate injectors, as you suggest. And I also agree that his pumps as currently installed should support the hp suggested by a 124mph trap.

Rich Krause

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'95 Z-28 383 with Vortech, nitrous, etc.
"1FASTZ28"

Injuneer · 05-20-2002, 03:47 PM

Rich...

These are centrifugal pumps in series... they both will flow exaclty the same volume, and the flow will be determined by the combined pump curves, and the configuration of the recirculated system.

To the second (booster) pump, the only thing it sees different when running in series is that the inlet pressure (which of course is the discharge pressure of the intank pump) is higher. It "adds" its pressure gain capabilities to the inlet pressure and produces a new, higher outlet pressure. In effect, you are "stacking" the pump curves, with flow at the x-axis and pressure as the y-axis. The pressures are additive. For any given required flow volume, you can provide it at a higher pressure.

But the flow-pressure relationship in the bypass/circulating system will reach a new equilibrium. There will be a slightly higher pressure loss in the line from the pumps to the fuel rails, because of the increased flow, the pressure regulator will have to adjust and bypass more fuel to maintain the required rail pressure, so the fuel flow will have jumped significantly, with a higher bypass/return rate... this is where you need to be careful, because at low loads, you can overheat the fuel.

At no point will the two pumps produce a new pressure curve that is somehow the same as the capacity/pressure curve of the original pump by itself to some imaginary operating point. Obviously, at some point, the smaller pump will no longer be able to match the flow of the larger pump and it will become a bottleneck. But when you consider that a 255 lph pump can in theory (assuming it is rated at 45psi) pump enough fuel to support 800crankHP, running into this sort of problem is unlikely with the pumps we're talking about here. A 500HP engine only needs 158 lph of fuel and I would think that both the stock intank pump and the T-Rex would be good in this range, particularly when you consider that each pump is only developing a portion of the required pressure, and hence "backed up the curve" on the capacity. Where you get into trouble is the situation where there is an extreme mismatch on capacity, and this is where a failing stock intank pump can contribute to flow problems.

LETHALRACER · 05-20-2002, 05:22 PM

Wow that is excellent info. Thank both of you. I set my fuel pressure at 47psi at WOT. Lately the pump has been jumping back and forth from 35-55psi in a constant vibration, in normal drive with the vac hose connected. At WOT the psi goes from 20-65psi in a violent motion. I can bearly see the needle. I thought it was a bad filter so I changed it and cut the old one open. The old one was clean and the new one changed nothing. All I have heard is that the injectors are too large. I thought they might be too small. I have an Auto X-ray. Is there a way I can check duty cycle on that. Or can I just crank up the pressure and see what happens. I have turned it up to 55psi before and my O2mv are about 930-950mv then the same thing happens. From what both of you are saying it seems that the 255lph should be more than adequate for the measly ponys my little motor will be pushing, even on the juice. I will just install the Walbro and take out the T-Rex. The T-Rex draws a lot of juice any way.

seanW · 05-20-2002, 08:57 PM

So Fred,
Please tell me if this will help me or what else I should do. I have 30# injectors. The walbro 255 inline pump was hooked up, but I started to run rich. I left the pump inline and cut the power wire. The car still didn't feel the same. I changed the plugs. ended up they were fowled.
I dynoed the car with everything in the sig and only pulled 288hp/302tq. I think my opti is going south(changing it this weekend) I added a MSD6 2 days ago and gained 26rwhp and 15rwtq. My A/F was 13.2-13.7 from 2000-4500rpms and 13.7-14.5 from 4500-6500. is it because the pump is still inline the reason I'm lean on the top end? should I hook the pump back up? I have no knowledge on this subject. I would appreciate a Brain!

thanks sorry for being stupid.
seanW

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97z28, B&M ripper, Magnaflow SS, K&N FIPK, Lt4KM, Hooker LT's, Fluidampener, 30# injectors, 3.73 gears, ccex224/230, promag1.6rr, AFR LT4 heads, guideplates, Ed Wright tuning

Looking for 400rwhp NA!!
277 RWHP<---old
302 RWTQ<---old
??? RWHP<---new
??? RWTQ<---new

rskrause · 05-20-2002, 10:07 PM

Fred: here's what I been told (and it makes sense). Could you address each in turn and tell me which, if any, are wrong.

Two identical pumps in series:
1. Will flow the same as one pump into zero pressure.

2. As the outlet pressure rises, the volume will diverge slowly at first and then more and more rapidly as the pressure continues to rise.

Two identical pumps in parallel:
3. Double the flow at all pressures.

Two unequal pumps in series, the more powerful pump is first (upstream).
4. Flows like the smaller pump into sero pressure.

5. Hard to predict exactly what will happen as the pressure rises. I think that what usually happens is similar to what I first posted. At low pressures the volume will be pretty close to that of the smaller pump alone. As the pressure rises I am not eactly sure what happens, but here is what I observed. I have a Bosch 255lph in-tank and a T-Rex in-line. I dropping fuel pressure when activating the nitrous. When I removed the T-Rex the pressure drop was eliminated. My interpretation was that at this particular pressure (~55psi) the T-Rex was a restriction. I am not sure how to generalize from this.

Rich Krause

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'95 Z-28 383 with Vortech, nitrous, etc.
"1FASTZ28"

Injuneer · 05-20-2002, 11:23 PM

LETHALRACER:

You can't get duty cycle directly from Auto Xray, but you can get pulse width and rpm simultaneously. The rpm will define the amount of time available for each injector to operate - at 6,000prm, 2 revs (one 4-stroke sequence) takes 20mSec. The pulse width can be compared to that number... an 18mSec pulse width would represent a 90% duty cycle (18 / 20 = 0.90). At 5000rpm, the available time is 24mSec.

SeanW:

I think the non-operating inline pump is causing your problems.... I think a lot of fuel pumps are "gerotor" designs, or something like that.... I need to go back to my Kinsler manual and see what they say. But I think this design is what gives the pump the typically very linear (inverse) relationship between flow and pressure, unlike a pure centrigugal pump, which generally has a non-liner relationship. But I think you have a major obstruction in the fuel flow path.

Rich:

You fried my brain with that one... I need to think about it on a clear head, early in the morning before I burn up too many of the cells......

.

I see your point on the flow rates being equal at 0psi discharge pressure, and that also is supported by my explanation..... the curves are stacked... pressure of Pump1 added to pressure of Pump2 = combined pressure capability for a given flow rate. P1=0, P2=0, P1+P2=0. This becomes the starting point of the new "combined" pump curve, and the ONLY point where the pressure of two pumps = the pressure of the individual pumps.

This is the point where the curves start to "diverge" if I understand the way you are using the word.

As soon as you apply some obstruction to flow (the real world)... a piece of tubing, a fuel filter, a fuel rail, a pressure regulator valve.... the pumps will each contribute some portion of the total pressure in the system. And then the flow will change.... reaching a new equilibrium point... the point at which the frictional losses in the system for that particular flow causes the combined pumps to operate at a point on the new combined curve.

In fact, once you put the two pumps in a system, the flow pressure relationship is operating at a fairly constant point. If the engine is at idle, and the injectors are taking very little fuel out of the system, the pressure regulator will open to just the point where the recirculated flow fixes the system pressure to that point on the curve which corresponds to the rail pressure and the total recirculation. Start opening the injectors more, and the pressure regulator starts to close, to keep the pressure in the rails at a fixed level. In effect, the increased flow out of the injectors is offset by a reduced flow in the return line... the total flow through the pumps is probably changed very little, and the point on the curve where the combined pumps are operating is changed little if any. That is the beauty of the recirculated system... it is very stable.

I think this is why you are better off with parallel pumps when you have a step-change power adder like a nitrous system, as opposed to the (relatively) more gradual increase in flow through the range of engine RPM.

Another thing that I think needs to be reflected in selecting a pump for a particular HP level is to determine whether it is beneficial to always have sufficient excess capacity to insure that the pressure regulator never has to totally shut off return flow... is it necessary to use a 90% factor on sizing the pump, so that even at max fuel demand, there is always a 10% bypass..... would this create a more stable system? I don't know.

Let me think about it some more.

Fred

LETHALRACER · 05-21-2002, 06:28 PM

What do you guys think about the gauge bouncing around so much? Failing pump? Bad FPR? The FPR is less than a year old.

ZBass28 · 08-01-2002, 10:18 AM

Brought this one back up

I was re-reading this for some info (great tech info BTW) and also had a question regarding this matter.

Some of you may recall I had posted in another of the fuel flow threads about a high rpm stumble I had that I thought was fuel flow related. It turned out to be a faulty new plug wire (grrrr!) However, I still have some major fueling issues that I need to address.

Currently I am just running the Walbro/Holley 307 (255lph) in-tank. I am seeing some slight pressure drop under boost and can see the FAST system adjusting the IPW accordingly to compensate the lack of fuel. Even with 65# injectors I am seeing duty cycles approaching 90% at 6000rpms....there is no way my engine puts out that kinda power!

I fully anticipate approx. 600rwhp when I finally have this thing dialed in perfect, but still not anywhere near the capabilities of the 65#s.

At this point, since this is a street car, I would like to stay away from running a sumped tank and large external pump and changing all the lines around. Will there be any benefit to running a 255lph inline pump in tandem with the intank? Will I have to increase the overall system pressure for this to work well as opposed to staying at 44psi?

And a question directed at Rich....Where did you purchase your Kenne Bell boost-a-pump from? I tried the MSD version and didn't completely like it.

Thanks all

[This message has been edited by ZBass28 (edited August 01, 2002).]

rskrause · 08-01-2002, 11:19 AM

Quote:

Originally posted by ZBass28:
Brought this one back up

I was re-reading this for some info (great tech info BTW) and also had a question regarding this matter.

Some of you may recall I had posted in another of the fuel flow threads about a high rpm stumble I had that I thought was fuel flow related. It turned out to be a faulty new plug wire (grrrr!) However, I still have some major fueling issues that I need to address.

Currently I am just running the Walbro/Holley 307 (255lph) in-tank. I am seeing some slight pressure drop under boost and can see the FAST system adjusting the IPW accordingly to compensate the lack of fuel. Even with 65# injectors I am seeing duty cycles approaching 90% at 6000rpms....there is no way my engine puts out that kinda power!
I fully anticipate approx. 600rwhp when I finally have this thing dialed in perfect, but still not anywhere near the capabilities of the 65#s.

At this point, since this is a street car, I would like to stay away from running a sumped tank and large external pump and changing all the lines around. Will there be any benefit to running a 255lph inline pump in tandem with the intank? Will I have to increase the overall system pressure for this to work well as opposed to staying at 44psi?

And a question directed at Rich....Where did you purchase your Kenne Bell boost-a-pump from? I tried the MSD version and didn't completely like it.

Thanks all

[This message has been edited by ZBass28 (edited August 01, 2002).]

Coupla things. First, the FAST system has no way of knowing the FP and at WOT it should be in open loop. thus ignoring the O2 sensors. So it is not "adjusting" the pulse widths but is just doing what it is programmed to do.

FP drop under boost (assuming you have a pressure referenced FP gauge) is a bad thing and most likely means that the pump is bad or inadequate. Of course there are other possible explanations: FP regulator not working correctly, restriction in the fuel supply line, etc. The FP should rise one pound per pound of boost.

On my setup I needed the Boost-a-Pump to support more than 500rwhp with a single 255lph in-tank. It seemed like a better (easier) solution than a second pump and it has proven to be so. In terms of a parallel (tandem) pump it will more or less double the fuel flow. But it is a major PITA to install. That's why I prefer the Boost-s-Pump. I got mine from Kennedy's Dynotune (716-693-5354) and on the Web at http://www.kennedysdynotune.com

This is a friend's shop and he usually stocks them (or will have one drop shipped). If you mention my name they will know who you are talking about, FWIW.

Hope this helps.

Rich Krause

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'95 Z-28 383 with Vortech, nitrous, etc.
"1FASTZ28"

89ProchargedROC · 08-01-2002, 12:12 PM

Quote:

Originally posted by rskrause:
Coupla things. First, the FAST system has no way of knowing the FP and at WOT it should be in open loop. thus ignoring the O2 sensors. So it is not "adjusting" the pulse widths but is just doing what it is programmed to do.

Actually, Zbass has the wibe band option on his box so the computer will adjust the a/f ratio for him by staying in closed loop.

Also, you say its a walbro/holley 307? You should be using the walbro 340m pump which is the highest flowing walbro pump and it supports high pressure (60+psi). I believe the 307 flows the same, but doesn't like higher pressures.

[This message has been edited by 89ProchargedROC (edited August 01, 2002).]

[This message has been edited by 89ProchargedROC (edited August 01, 2002).]

George Debski · 08-01-2002, 12:36 PM

I would NOT rely on published flow numbers. You need to flow the pump yourself at operating voltage, pressure and AT THE FUEL RAIL.

My buddy put a Walbro 255 in his Mustang. When we flowed the pump at the fuel rail, it generated 170 liters/hr at 13.8 volts and 40 psi.

If you add a fuel pump in series, instead of parallel, your inline will be the bottleneck. If the inline is much larger, it will cavitate, and if it's much smaller, it will restrict flow. Been there, seen it, done that.

As a result, I totally redid my fuel system which now flows 364 li/hr, measured at the fuel rail at 40spi, my normal WOT pressure.

Another problem which crops up is that the stock regulator has a 0.095" orifice, which in my case only went down to 42psi as the minimum pressure. I went to the Holley regulator with a 0.125 orifice, and was able to bring the pressure down to 20psi. That orifice size difference does not sound like much, but it flow 73% more.

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SSMOKEM

1995 Impala SS "Cherry Bomb"
Best ETs: faster than my '65 pickup
Mods: 93% stock

My Super Impala SS Website

rskrause · 08-01-2002, 12:56 PM

Quote:

Originally posted by 89ProchargedROC:

Actually, Zbass has the wibe band option on his box so the computer will adjust the a/f ratio for him by staying in closed loop.

Also, you say its a walbro/holley 307? You should be using the walbro 340m pump which is the highest flowing walbro pump and it supports high pressure (60+psi). I believe the 307 flows the same, but doesn't like higher pressures.

[This message has been edited by 89ProchargedROC (edited August 01, 2002).]

[This message has been edited by 89ProchargedROC (edited August 01, 2002).]

So the FAST system stays in closed loop at WOT if it has the wideband sensor option? I didn't know that.

Rich Krause

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'95 Z-28 383 with Vortech, nitrous, etc.
"1FASTZ28"