Calling all tech heads.
01-20-2004, 04:16 PM
#16
Can't answer all of that but each cylinder fires every second revolution ( in between being the exhaust stroke for that cylinder).
When an injector runs 100% duty cycle I assume it's open all the time that it can be. Whether that is all the time I don't know, but that seems unlikely as you wouldn't want it open during the exhaust stroke (would you?)
The 75# injector has a much bigger 'hole' than the 47# injector. That's the difference. The measurement is the weight of fuel it can flow with 3bar pressure if it's open for 1 hour (I believe).
I agree with your description of the characteristics of a pump - to pump into a higher pressure using the same impeller would result in less volume being moved.
BUT
The 75# injector doesn't require more pressure than the 47# - they are calibrated at the same pressure. Sufficiently increasing the pressure behind a 47# injector would allow it to flow as much as a 55# injector but there is a limit to how much the pressure could (should, safely) be increased and the bigger injector can flow the same fuel at lower pressure and with a shorter duty cycle.
I agree the pump should be matched to a point, but so long as it can comfortably pump more fuel than the engine will require, at a pressure sufficient to get the fuel into the cylinder. If it exceeds that specification then no problem - that's what the regulator is for, to smooth the pressure out, but as I said earlier only by reducing it. Once you have a pump that can supply sufficient fuel at a high enough pressure (and a regulator to control the pressure) then you need only fit injectors that can flow more fuel than the engine requires. These are then controlled by the ECU to a duty cycle that gets the correct fuel in. In a nutshell the whole system should be designed to supply a bit (20% maybe) more than required which means that the injectors can get the mixture right at a sensible duty cycle. I'm guessing that getting all the fuel required in as quickly as possible at just the right time gives the best combustion - in fact i know it does, that's how the new VAG FSi engines work, isn't it.
Finally, I think I read somewhere that the 944 already runs quite a high pressure fuel system.
Really finally: 1,800cc/min at 2.5bar is quite a bit of fuel. That's roughly an imperial gallon every 2 1/2 minutes. Does that mean the pump can keep up if the injectors are bigger - after all we are only increasing the pressure to try to get more fuel through the standard ones than they are designed to flow?
When an injector runs 100% duty cycle I assume it's open all the time that it can be. Whether that is all the time I don't know, but that seems unlikely as you wouldn't want it open during the exhaust stroke (would you?)
The 75# injector has a much bigger 'hole' than the 47# injector. That's the difference. The measurement is the weight of fuel it can flow with 3bar pressure if it's open for 1 hour (I believe).
I agree with your description of the characteristics of a pump - to pump into a higher pressure using the same impeller would result in less volume being moved.
BUT
The 75# injector doesn't require more pressure than the 47# - they are calibrated at the same pressure. Sufficiently increasing the pressure behind a 47# injector would allow it to flow as much as a 55# injector but there is a limit to how much the pressure could (should, safely) be increased and the bigger injector can flow the same fuel at lower pressure and with a shorter duty cycle.
I agree the pump should be matched to a point, but so long as it can comfortably pump more fuel than the engine will require, at a pressure sufficient to get the fuel into the cylinder. If it exceeds that specification then no problem - that's what the regulator is for, to smooth the pressure out, but as I said earlier only by reducing it. Once you have a pump that can supply sufficient fuel at a high enough pressure (and a regulator to control the pressure) then you need only fit injectors that can flow more fuel than the engine requires. These are then controlled by the ECU to a duty cycle that gets the correct fuel in. In a nutshell the whole system should be designed to supply a bit (20% maybe) more than required which means that the injectors can get the mixture right at a sensible duty cycle. I'm guessing that getting all the fuel required in as quickly as possible at just the right time gives the best combustion - in fact i know it does, that's how the new VAG FSi engines work, isn't it.
Finally, I think I read somewhere that the 944 already runs quite a high pressure fuel system.
Really finally: 1,800cc/min at 2.5bar is quite a bit of fuel. That's roughly an imperial gallon every 2 1/2 minutes. Does that mean the pump can keep up if the injectors are bigger - after all we are only increasing the pressure to try to get more fuel through the standard ones than they are designed to flow?
01-20-2004, 04:17 PM
#17
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So, this begs the question - Anyone experimented with using a high volume aftermarket fuel pump?
BTW, Marren Fuel Injection, who serviced my stock injectors. told me they are designed for up to 5bar of pressure. If, as Danno says, one problem is overheating the injectors and DME when run over 80% duty cycle, then why not upgrade the pump and FPR and run higher pressures instead? There are rising rate FPR's available where the rate of increase is adjustable as well, though I don't know if anyone has used one on a 951.
Personally, I'm running the stock injectors with a 3bar FPR and 16psi with a T04E and have plenty of fuel, but this is at high altitude
with only 310rwhp. I also don't like the idea of using injectors with an impedance mismatch with the DME.
BTW, Marren Fuel Injection, who serviced my stock injectors. told me they are designed for up to 5bar of pressure. If, as Danno says, one problem is overheating the injectors and DME when run over 80% duty cycle, then why not upgrade the pump and FPR and run higher pressures instead? There are rising rate FPR's available where the rate of increase is adjustable as well, though I don't know if anyone has used one on a 951.
Personally, I'm running the stock injectors with a 3bar FPR and 16psi with a T04E and have plenty of fuel, but this is at high altitude
with only 310rwhp. I also don't like the idea of using injectors with an impedance mismatch with the DME.
01-20-2004, 07:29 PM
#18
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One thing you might be missing is the pump is running all of the time and pushes the fuel into the rail through the damper then out of the rail through the regulator and back to the tank. So the regulator actually keeps the desired pressure by letting fuel pass by back to the tank to keep the pressure down.
01-21-2004, 05:09 AM
#20
Race Director
"Thus, I believe the pressure is needed to deliver more fuel within a given window."
More fuel is delivered at the same pressure by using injectors with bigger holes at the ends.
"The fuel pump moves the fuel to the engine through the fuel pressure regulator."
"How is this increased pressure derived? It would seem that the fuel pump provides this pressure. The fuel pump is constantly pushing against this higher pressure, thus the impeller or whatever is spinning is encountering higher resistance and the volumetric flow is reduced."
Fuel-pressure is generated by the restriction of the FPR at the exit of the fuel-rail. By restricting the outlet, it generates pressure in the rail. The FPR also has a diaphragm that adjusts to manifold-pressure and reduces or increases pressure from the pre-set value. A pump that has no resistance, like just a hose aimed out to the ground will flow LOTS of fuel at no pressure onto the ground.
"If the fuel pump is rated like other pump I've ever seen then as the head pressure increases the volumetric flow will decrease. Thus if you required all the fuel that 75# injectors could provide, you'd have to move to a higher pressure pump AND higher pressure hoses."
"Once at maximum width, I would have to increase the fuel pressure.
Thus, I believe the pressure is needed to deliver more fuel within a given window. "
I think I see your confusion. You have to separate pressure from volume delivered, they are indepedent and interdependent of each other. Here's a graph that shows the relationship in regards to the pump:
As you increase pressure, the maximum amount of fuel-flowed by the pump decreases. This is not the same as what's coming out of the injectors. That's determined by the computer and the duty-cycle it's sending (for illustration purposes, numbers are close, but not actual):
With the stock injectors, you can flow 0-1.0 ltr/min based upon the duty-cycle. Swapping to injectors with bigger holes, allows more fuel to flow at the same pressure. So swapping in 75-lb/hr injectors will allow you to flow 0-2.1 ltr/min over the same duty-cycle ranges , or roughly double. Simple. This is like using a 1/2" garden hose with the valve wide open. Going up to a hose with twice the cross-sectional area will let you flow twice as much water in the same amount of time.
""one problem is overheating the injectors and DME when run over 80% duty cycle, then why not upgrade the pump and FPR and run higher pressures instead?""
One problem is how to reduce the duty-cycle with increased pressure. Also increasing the fuel-pressure looks like this:
The actual increase in fuel-flowed increases to the square-root power of the pressure increase. So given:
P1 = 2.5 bar (initial pressure)
V1 = 1.0 initial volume
P2 = 3.8 bar final pressure
V2 = ??? final volume delivered
V2 = V1 * SQRT (P2/P1) = 1.0 * SQRT(3.8/2.5) = 1.23x increase in fuel.
So with stock injectors, increasing fuel-pressure by 52% results in 23% more fuel delivered across the board. Same thing with using the larger injectors... up to a point. That limit is the flow-limits of the fuel-pump. Using the 75-lb/hr injectors at 3.8-bar won't work because at 67% duty-cycle, it will reach the limit of the fuel-pump at 1.7 ltr/min. Which is LESS than the same injectors at 100% at a lower pressure. Using 2.5-bar pressure will allow the 75-lb/hr injectors and pump to flow 2.1 ltr/min. Makes sense? That's why the largest injectors you'd want to use with the stock pump is about 65-lb/hr. Going larger than that can allow you to flow more fuel at the same duty-cycle, but you'll read the pump's flow limit before you reach the 100% duty-cycle limit of the injectors.
This is similar to the situation if you got 2-foot diameter garden hose and hooked it up to your faucet. Just because you opened up the delivery outlet by a large amount, doesn't mean that the pump is going to be able to complete fill it.
"Danno's explanation reads a little like the pump will put out 1,800cc/min with a 2.5 bar regulator but only 1,200 with a 3.8, but in reality it would still supply 1,800cc/min with a 3.8 bar regulator if that much were required, just that it would never get more than 2.5bar pressure into the rail. "
No, it's the other way around. The pressure in the rail is independent of the pump. If the FPR sets 3.8-bar at the rail, that is what's going into the injectors. If you put buckets under each injector and catch the output, you'll see that at 2.5-bar, the pump will put out 1,800cc/min. Increase the pressure to 3.8-bar and you'll only be catching 1,200cc/min in the buckets (100% duty-cycle, wide-open for both cases). Also above 5-bar, the volume delivered is pitifully small and not useable. The pump will also overheat and burn out quicker as well.
"Finally, I think I read somewhere that the 944 already runs quite a high pressure fuel system."
Yes, and only Bosch injectors have the capability of operating at the highest pressures generated by the fuel-system. Even then, they don't like to be pushed beyond 4.5-5.0 bar of fuel-pressure. You have to account for the rising-pressure FPR that adds 1psi of fuel-pressure for each 1psi of boost as well.
"So does anyone know the maximum window of time allowed for the fuel injector pulse? "
"When an injector runs 100% duty cycle I assume it's open all the time that it can be. Whether that is all the time I don't know, but that seems unlikely as you wouldn't want it open during the exhaust stroke (would you?)"
This changes with RPM. At 3000rpm, there's 40ms to inject fuel. At 6000rpm, there's only 20ms. Interesting to note is that 100% duty-cycle is different duty-cycle times and fuel-delivered at each RPM. Also due to the batch fire configuration with two on/off cycles per 4-stroke cycle, you have to incorporate two injector latency periods into those figures. Typically low-imp. peak & hold injectors have latencies of 1-3ms, high-imp. saturated injectors are 3-5ms. Subtract those from the times above and you end up with effectively only about 36ms@3000rpm and 16ms@6000rpm for low-imp. injectors. Due to the higher-latency of high imp. injectors, they only have about 32ms@30000rpm and 12ms@6000rpm. So for the same static-flow ratings, you can deliver about 25% more fuel at 100% duty-cycle with low-imp. peak & hold injectors. See this thread for some graphs of injector driver signals and fuel-volumes injected.
The timing of the injector pulses doesn't really matter that much except for idle & low-RPM ranges. See the Sequential vs. Batch Fire paper on my 951 RacerX website for diagrams of injection pulses vs. valve-opening. Above a certain RPM, the injectors will be firing during the entire intake-valve opening. The extra fuel will then be injected at the back of the closed intake-valve, waiting to be ingested the next time around.
" Anyone experimented with using a high volume aftermarket fuel pump? "
Sure, why not? Here's a Paxton pump that can deliver 3.5x the flow of the stock pump (at the same pressure):
The main problem most people in the past have had with using larger injectors is how to scale back the duty-cycle such that you have the same fuel-volume delivered as before. Attempts at signal-intercepting and massaging the incoming air-flow signal to fool the computer into thinking less air is flowing than actual to reference a leaner spot on the fuel-maps is clumsy. It also only work for 55# injectors because there's only one more data-cell leaner than idle and it's only 33% leaner, which kinda matches 55# injectors. All larger injectors will always run too rich at idle because no matter how low you change the voltage, the data in that last fuel-cell is only 33% leaner than stock.
On the GURU chips, we have a scale-back function that reduces the duty-cycle by -37%, -45%, -54% across the board on the entire fuel-map for 55/65/75-lb/hr injectors (no signal massaging necessary). The same flowXrpm look-point is used on the maps to maintain proper ignition values. That means you can just pop in larger injectors and GURU chips, select the injectors to use and that's it. The larger inejctors will deliver exactly the same amount of fuel as before, but at a lower duty-cycle. That means instead of maxing out your stock injectors at 100% and 250-260rwhp, you can now use only 46% with 75-lb/hr injectors for a lot more overhead.
"There are rising rate FPR's available where the rate of increase is adjustable as well, though I don't know if anyone has used one on a 951."
Yeah, there's this funky alien-looking device from Bell Engineering:
It can adjust for initial-pressure and rate-of-rise as well. However you have to custom-map a chip for this FPR due to the different flow characteristics. Too much work really. Just use larger injectors and a standard rising-pressure regulator.
"Danno, do you have a chart of where each size injector maxes out, power-wise?"
No, but I can run it off the top of my head. All at 3.0-bar initial fuel-pressure (3.8-bar pressure will be less for 65# injectors and above):
80%=230-240rwhp, 100%=260-270rwhp for stock 34.6-lb/hr injectors
80%=310-325rwhp, 100%=360-370rwhp for 55-lb/hr
80%=335-345rwhp, 100%=375-385rwhp for 65-lb/hr
80%=365-375rwhp, 100%=395-410rwhp for 75-lb/hr
80%=380-390rwhp, 100%=420-435rwhp for 83-lb/hr
Anyone want to buy a set of 160-lb/hr injectors???
More fuel is delivered at the same pressure by using injectors with bigger holes at the ends.
"The fuel pump moves the fuel to the engine through the fuel pressure regulator."
"How is this increased pressure derived? It would seem that the fuel pump provides this pressure. The fuel pump is constantly pushing against this higher pressure, thus the impeller or whatever is spinning is encountering higher resistance and the volumetric flow is reduced."
Fuel-pressure is generated by the restriction of the FPR at the exit of the fuel-rail. By restricting the outlet, it generates pressure in the rail. The FPR also has a diaphragm that adjusts to manifold-pressure and reduces or increases pressure from the pre-set value. A pump that has no resistance, like just a hose aimed out to the ground will flow LOTS of fuel at no pressure onto the ground.
"If the fuel pump is rated like other pump I've ever seen then as the head pressure increases the volumetric flow will decrease. Thus if you required all the fuel that 75# injectors could provide, you'd have to move to a higher pressure pump AND higher pressure hoses."
"Once at maximum width, I would have to increase the fuel pressure.
Thus, I believe the pressure is needed to deliver more fuel within a given window. "
I think I see your confusion. You have to separate pressure from volume delivered, they are indepedent and interdependent of each other. Here's a graph that shows the relationship in regards to the pump:
As you increase pressure, the maximum amount of fuel-flowed by the pump decreases. This is not the same as what's coming out of the injectors. That's determined by the computer and the duty-cycle it's sending (for illustration purposes, numbers are close, but not actual):
With the stock injectors, you can flow 0-1.0 ltr/min based upon the duty-cycle. Swapping to injectors with bigger holes, allows more fuel to flow at the same pressure. So swapping in 75-lb/hr injectors will allow you to flow 0-2.1 ltr/min over the same duty-cycle ranges , or roughly double. Simple. This is like using a 1/2" garden hose with the valve wide open. Going up to a hose with twice the cross-sectional area will let you flow twice as much water in the same amount of time.
""one problem is overheating the injectors and DME when run over 80% duty cycle, then why not upgrade the pump and FPR and run higher pressures instead?""
One problem is how to reduce the duty-cycle with increased pressure. Also increasing the fuel-pressure looks like this:
The actual increase in fuel-flowed increases to the square-root power of the pressure increase. So given:
P1 = 2.5 bar (initial pressure)
V1 = 1.0 initial volume
P2 = 3.8 bar final pressure
V2 = ??? final volume delivered
V2 = V1 * SQRT (P2/P1) = 1.0 * SQRT(3.8/2.5) = 1.23x increase in fuel.
So with stock injectors, increasing fuel-pressure by 52% results in 23% more fuel delivered across the board. Same thing with using the larger injectors... up to a point. That limit is the flow-limits of the fuel-pump. Using the 75-lb/hr injectors at 3.8-bar won't work because at 67% duty-cycle, it will reach the limit of the fuel-pump at 1.7 ltr/min. Which is LESS than the same injectors at 100% at a lower pressure. Using 2.5-bar pressure will allow the 75-lb/hr injectors and pump to flow 2.1 ltr/min. Makes sense? That's why the largest injectors you'd want to use with the stock pump is about 65-lb/hr. Going larger than that can allow you to flow more fuel at the same duty-cycle, but you'll read the pump's flow limit before you reach the 100% duty-cycle limit of the injectors.
This is similar to the situation if you got 2-foot diameter garden hose and hooked it up to your faucet. Just because you opened up the delivery outlet by a large amount, doesn't mean that the pump is going to be able to complete fill it.
"Danno's explanation reads a little like the pump will put out 1,800cc/min with a 2.5 bar regulator but only 1,200 with a 3.8, but in reality it would still supply 1,800cc/min with a 3.8 bar regulator if that much were required, just that it would never get more than 2.5bar pressure into the rail. "
No, it's the other way around. The pressure in the rail is independent of the pump. If the FPR sets 3.8-bar at the rail, that is what's going into the injectors. If you put buckets under each injector and catch the output, you'll see that at 2.5-bar, the pump will put out 1,800cc/min. Increase the pressure to 3.8-bar and you'll only be catching 1,200cc/min in the buckets (100% duty-cycle, wide-open for both cases). Also above 5-bar, the volume delivered is pitifully small and not useable. The pump will also overheat and burn out quicker as well.
"Finally, I think I read somewhere that the 944 already runs quite a high pressure fuel system."
Yes, and only Bosch injectors have the capability of operating at the highest pressures generated by the fuel-system. Even then, they don't like to be pushed beyond 4.5-5.0 bar of fuel-pressure. You have to account for the rising-pressure FPR that adds 1psi of fuel-pressure for each 1psi of boost as well.
"So does anyone know the maximum window of time allowed for the fuel injector pulse? "
"When an injector runs 100% duty cycle I assume it's open all the time that it can be. Whether that is all the time I don't know, but that seems unlikely as you wouldn't want it open during the exhaust stroke (would you?)"
This changes with RPM. At 3000rpm, there's 40ms to inject fuel. At 6000rpm, there's only 20ms. Interesting to note is that 100% duty-cycle is different duty-cycle times and fuel-delivered at each RPM. Also due to the batch fire configuration with two on/off cycles per 4-stroke cycle, you have to incorporate two injector latency periods into those figures. Typically low-imp. peak & hold injectors have latencies of 1-3ms, high-imp. saturated injectors are 3-5ms. Subtract those from the times above and you end up with effectively only about 36ms@3000rpm and 16ms@6000rpm for low-imp. injectors. Due to the higher-latency of high imp. injectors, they only have about 32ms@30000rpm and 12ms@6000rpm. So for the same static-flow ratings, you can deliver about 25% more fuel at 100% duty-cycle with low-imp. peak & hold injectors. See this thread for some graphs of injector driver signals and fuel-volumes injected.
The timing of the injector pulses doesn't really matter that much except for idle & low-RPM ranges. See the Sequential vs. Batch Fire paper on my 951 RacerX website for diagrams of injection pulses vs. valve-opening. Above a certain RPM, the injectors will be firing during the entire intake-valve opening. The extra fuel will then be injected at the back of the closed intake-valve, waiting to be ingested the next time around.
" Anyone experimented with using a high volume aftermarket fuel pump? "
Sure, why not? Here's a Paxton pump that can deliver 3.5x the flow of the stock pump (at the same pressure):
The main problem most people in the past have had with using larger injectors is how to scale back the duty-cycle such that you have the same fuel-volume delivered as before. Attempts at signal-intercepting and massaging the incoming air-flow signal to fool the computer into thinking less air is flowing than actual to reference a leaner spot on the fuel-maps is clumsy. It also only work for 55# injectors because there's only one more data-cell leaner than idle and it's only 33% leaner, which kinda matches 55# injectors. All larger injectors will always run too rich at idle because no matter how low you change the voltage, the data in that last fuel-cell is only 33% leaner than stock.
On the GURU chips, we have a scale-back function that reduces the duty-cycle by -37%, -45%, -54% across the board on the entire fuel-map for 55/65/75-lb/hr injectors (no signal massaging necessary). The same flowXrpm look-point is used on the maps to maintain proper ignition values. That means you can just pop in larger injectors and GURU chips, select the injectors to use and that's it. The larger inejctors will deliver exactly the same amount of fuel as before, but at a lower duty-cycle. That means instead of maxing out your stock injectors at 100% and 250-260rwhp, you can now use only 46% with 75-lb/hr injectors for a lot more overhead.
"There are rising rate FPR's available where the rate of increase is adjustable as well, though I don't know if anyone has used one on a 951."
Yeah, there's this funky alien-looking device from Bell Engineering:
It can adjust for initial-pressure and rate-of-rise as well. However you have to custom-map a chip for this FPR due to the different flow characteristics. Too much work really. Just use larger injectors and a standard rising-pressure regulator.
"Danno, do you have a chart of where each size injector maxes out, power-wise?"
No, but I can run it off the top of my head. All at 3.0-bar initial fuel-pressure (3.8-bar pressure will be less for 65# injectors and above):
80%=230-240rwhp, 100%=260-270rwhp for stock 34.6-lb/hr injectors
80%=310-325rwhp, 100%=360-370rwhp for 55-lb/hr
80%=335-345rwhp, 100%=375-385rwhp for 65-lb/hr
80%=365-375rwhp, 100%=395-410rwhp for 75-lb/hr
80%=380-390rwhp, 100%=420-435rwhp for 83-lb/hr
Anyone want to buy a set of 160-lb/hr injectors???
Last edited by Danno; 01-21-2004 at 05:57 AM.
01-21-2004, 09:54 AM
#21
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you guys are good. 
OK Danno, my understanding of the injector ratings was incorrect. So what does the # rating actually mean? Does that number represent the point at which the injectors will deliver the max flow? IE, stock injectors are 36ish, so at 36# they deliver the most fuel they will deliver regardless of pressure? Does the increased 3.0bar regulator 'stress' the open close of these stock injectors, or are they simply at their max volume output?
Also, just because you have 55# injectors, it doesn't mean you have to run 55# of fuel pressure. I never realized that before. All very interesting.
Thinking on this I have a question.
If you put in say 75# injectors but only ran 2.5bar of pressure you'd get good volume into the cylinder, correct? But is the fuel some how atomized by the injector so that it combusts better? IE, is there a minimum pressure setting for a given injector in order to make the fuel most combustible? This would seem to be necessary otherwise we would all run 2.5bar regulators and 75# injectors. right?
OK Danno, my understanding of the injector ratings was incorrect. So what does the # rating actually mean? Does that number represent the point at which the injectors will deliver the max flow? IE, stock injectors are 36ish, so at 36# they deliver the most fuel they will deliver regardless of pressure? Does the increased 3.0bar regulator 'stress' the open close of these stock injectors, or are they simply at their max volume output?
Also, just because you have 55# injectors, it doesn't mean you have to run 55# of fuel pressure. I never realized that before. All very interesting.
Thinking on this I have a question.
If you put in say 75# injectors but only ran 2.5bar of pressure you'd get good volume into the cylinder, correct? But is the fuel some how atomized by the injector so that it combusts better? IE, is there a minimum pressure setting for a given injector in order to make the fuel most combustible? This would seem to be necessary otherwise we would all run 2.5bar regulators and 75# injectors. right?
01-21-2004, 01:19 PM
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Danno . a correction must be made for the flow volumes you quote. i got these numbers directly from Bosch
1)The OEM pump Bosch #61421 flows 130l/h or 34.34gal/h
max pressure 300kpa or 43.5psi
2)Lindsey pump flows 128l/h or 33.86gal/h
max pressure 500kpa or 72.5psi
3)928gts pump #61457 flows 147l/h or 38.83gal/h
max pressure 400kpa/58psi
4) racing pump floes 172L/h or 45gaL/h
max pressure 700kpa/102psi
1)The OEM pump Bosch #61421 flows 130l/h or 34.34gal/h
max pressure 300kpa or 43.5psi
2)Lindsey pump flows 128l/h or 33.86gal/h
max pressure 500kpa or 72.5psi
3)928gts pump #61457 flows 147l/h or 38.83gal/h
max pressure 400kpa/58psi
4) racing pump floes 172L/h or 45gaL/h
max pressure 700kpa/102psi
01-21-2004, 06:16 PM
#24
Race Director
Highlander944, very good questions, and you brought up something a lot of people don't realize.
"my understanding of the injector ratings was incorrect. So what does the # rating actually mean? Does that number represent the point at which the injectors will deliver the max flow? IE, stock injectors are 36ish, so at 36# they deliver the most fuel they will deliver regardless of pressure? Does the increased 3.0bar regulator 'stress' the open close of these stock injectors, or are they simply at their max volume output?"
Yes, the flow-rating on injectors is the maximum static-flow rating. The lb/hr rating on injectors is based upon several standard parameters:
1. Fuel-pressure at 3.0-bar (43.5psi)
2. Static flow (100% duty-cycle, ON all the time)
To get this rating, you hook up an injector to a fuel-rail set at 3.0-bar on the FPR. Then apply 12v DC power to the injector to hold it open full-time, then collect the fuel-injected and measure vs. time. Don't have to run for a complete hour, just 2-5 minutes would do.
So the stock injectors are rated at 34.6 lb/hr @ 3.0 bar. This is from imperical testing presented in this thread: WTK- Injector requirements for 400rwhp . You can also rate the same injectors as 31.6 lb/hr @ 2.5 bar or 38.9 lb/hr @ 3.8 bar. As you change fuel-pressure, the max-flow volume delivered by the injectors changes (within flow-limits of fuel-pump). But the standard is to rate injectors at 3.0-bar.
"If you put in say 75# injectors but only ran 2.5bar of pressure you'd get good volume into the cylinder, correct? But is the fuel some how atomized by the injector so that it combusts better? IE, is there a minimum pressure setting for a given injector in order to make the fuel most combustible? This would seem to be necessary otherwise we would all run 2.5bar regulators and 75# injectors. right?"
Fuel-atomization is a function of the injector's tip. There are many types of injector tips with various patterns of spray. In general, small nozzle holes atomize better than big nozzle holes. That's why I prefer the Delphi injectors used on our GURU Racing products:
The six small holes atomize fuel better than one big hole. Especially in the larger sizes. However, the 160-lb/hr injector we sell is a Bosch unit with a single hole. The fuel practically comes out in a stream with no atomization at all because the hole is so big and it has to flow so much volume.
Pressure is another function in atomization with the higher the better. I would never drop fuel-pressure below the stock 2.5-bar rating. That's because injectors need a minimum pressure for atomization, or else the fuel just dribbles out instead of spraying. There's also the fallacy of dropping fuel-pressure to use larger injectors. What's the point of going to 2x larger injectors if you're going to drop fuel-pressure by 1/2? The overall flow will be EXACTLY the same as before! Well, actually, you'd need to drop it to 1/4th, but that defeats the purpose of going to larger injectors. There are people running around with 65#/hr injectors @ 12-16psi fuel-pressure, ridiculous! A 65#/hr injector (@3.0bar) running at 12psi is actually a 37#/hr injector, just 10% larger than stock, why bother? No, keep fuel-pressure at stock levels or higher.
I've also seen several SAE papers where they tested a high-pressure injection system. It was around 100-125psi and purpose-made pumps and injectors are needed. But the atomization is much better and you can place the injectors close to the intake-valves for maximum performance. With the lower-pressure injectors we're using now, the best location is actually 6" to 10" away from the intake-valves to give enough air-volume and time for the injected fuel to fully atomize in the intake-stream. They are only placed close to the intake-valves to ensure all the fuel makes it into the cylinders at idle for emissions control.
MySwiss, thanks for the flow-ratings of the various pumps. Do you have the pressures at which the flows were rated? Fuel pumps flow different amounts based upon the pressure:
Any flow-rating for a pump should have the pressure at which that volume was delivered. If we look at that Walbro pump above, it can be called a 10-gph or an 80-gph, but without the pressure number, it can be misleading. So really, the Walbro GSS307 pump is a 10-gph @90psi pump or an 80-gph @10psi.
"1)The OEM pump Bosch #61421 flows 130l/h or 34.34gal/h
max pressure 300kpa or 43.5psi"
At what pressure does Bosch rate this pump for 130l/hr? My own testing shows 108-ltr/hr@43.5psh and 72-ltr/hr@55.1psi. From the trend, it may appear Bosch rates it for 130-ltr/h @36.3psi (stock 2.5-bar fuel-pressure). Also we know the pump is good for more than 43.5psi simply because of the rising-pressure regulator which increases pressure with boost. A 951 using 3-bar fuel-pressure running 20psi boost will end up with 63.5psi fuel-pressure (of course volume flow will be dramatically decreased).
One simple upgrade fuel-pump to use is Bosch #0.580.254.942 which was used on the '78 Mercedes 450SLC. I'm playing around right now with a NipponDenso pump that's good for 260-ltr/hr @43.5psi or 300-ltr/hr @36.3psi.
p.s. Gary, those ballast-resistors above will let you use any aftermarket low-imp. peak&hold injectors with the Bosch DME. 
"my understanding of the injector ratings was incorrect. So what does the # rating actually mean? Does that number represent the point at which the injectors will deliver the max flow? IE, stock injectors are 36ish, so at 36# they deliver the most fuel they will deliver regardless of pressure? Does the increased 3.0bar regulator 'stress' the open close of these stock injectors, or are they simply at their max volume output?"
Yes, the flow-rating on injectors is the maximum static-flow rating. The lb/hr rating on injectors is based upon several standard parameters:
1. Fuel-pressure at 3.0-bar (43.5psi)
2. Static flow (100% duty-cycle, ON all the time)
To get this rating, you hook up an injector to a fuel-rail set at 3.0-bar on the FPR. Then apply 12v DC power to the injector to hold it open full-time, then collect the fuel-injected and measure vs. time. Don't have to run for a complete hour, just 2-5 minutes would do.
So the stock injectors are rated at 34.6 lb/hr @ 3.0 bar. This is from imperical testing presented in this thread: WTK- Injector requirements for 400rwhp . You can also rate the same injectors as 31.6 lb/hr @ 2.5 bar or 38.9 lb/hr @ 3.8 bar. As you change fuel-pressure, the max-flow volume delivered by the injectors changes (within flow-limits of fuel-pump). But the standard is to rate injectors at 3.0-bar.
"If you put in say 75# injectors but only ran 2.5bar of pressure you'd get good volume into the cylinder, correct? But is the fuel some how atomized by the injector so that it combusts better? IE, is there a minimum pressure setting for a given injector in order to make the fuel most combustible? This would seem to be necessary otherwise we would all run 2.5bar regulators and 75# injectors. right?"
Fuel-atomization is a function of the injector's tip. There are many types of injector tips with various patterns of spray. In general, small nozzle holes atomize better than big nozzle holes. That's why I prefer the Delphi injectors used on our GURU Racing products:
The six small holes atomize fuel better than one big hole. Especially in the larger sizes. However, the 160-lb/hr injector we sell is a Bosch unit with a single hole. The fuel practically comes out in a stream with no atomization at all because the hole is so big and it has to flow so much volume.
Pressure is another function in atomization with the higher the better. I would never drop fuel-pressure below the stock 2.5-bar rating. That's because injectors need a minimum pressure for atomization, or else the fuel just dribbles out instead of spraying. There's also the fallacy of dropping fuel-pressure to use larger injectors. What's the point of going to 2x larger injectors if you're going to drop fuel-pressure by 1/2? The overall flow will be EXACTLY the same as before! Well, actually, you'd need to drop it to 1/4th, but that defeats the purpose of going to larger injectors. There are people running around with 65#/hr injectors @ 12-16psi fuel-pressure, ridiculous! A 65#/hr injector (@3.0bar) running at 12psi is actually a 37#/hr injector, just 10% larger than stock, why bother? No, keep fuel-pressure at stock levels or higher.
I've also seen several SAE papers where they tested a high-pressure injection system. It was around 100-125psi and purpose-made pumps and injectors are needed. But the atomization is much better and you can place the injectors close to the intake-valves for maximum performance. With the lower-pressure injectors we're using now, the best location is actually 6" to 10" away from the intake-valves to give enough air-volume and time for the injected fuel to fully atomize in the intake-stream. They are only placed close to the intake-valves to ensure all the fuel makes it into the cylinders at idle for emissions control.
MySwiss, thanks for the flow-ratings of the various pumps. Do you have the pressures at which the flows were rated? Fuel pumps flow different amounts based upon the pressure:
Any flow-rating for a pump should have the pressure at which that volume was delivered. If we look at that Walbro pump above, it can be called a 10-gph or an 80-gph, but without the pressure number, it can be misleading. So really, the Walbro GSS307 pump is a 10-gph @90psi pump or an 80-gph @10psi.
"1)The OEM pump Bosch #61421 flows 130l/h or 34.34gal/h
max pressure 300kpa or 43.5psi"
At what pressure does Bosch rate this pump for 130l/hr? My own testing shows 108-ltr/hr@43.5psh and 72-ltr/hr@55.1psi. From the trend, it may appear Bosch rates it for 130-ltr/h @36.3psi (stock 2.5-bar fuel-pressure). Also we know the pump is good for more than 43.5psi simply because of the rising-pressure regulator which increases pressure with boost. A 951 using 3-bar fuel-pressure running 20psi boost will end up with 63.5psi fuel-pressure (of course volume flow will be dramatically decreased).
One simple upgrade fuel-pump to use is Bosch #0.580.254.942 which was used on the '78 Mercedes 450SLC. I'm playing around right now with a NipponDenso pump that's good for 260-ltr/hr @43.5psi or 300-ltr/hr @36.3psi.
p.s. Gary, those ballast-resistors above will let you use any aftermarket low-imp. peak&hold injectors with the Bosch DME.
Last edited by Danno; 01-21-2004 at 06:58 PM.
01-21-2004, 07:31 PM
#25
Three Wheelin'
Join Date: Jul 2001
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Ok I am going to try not to look like a fool here
I was under the assumption that we do not gain any fuel flow when the fuel pressure rises with boost because the same amount of pressure is put on the other side of the injector by the air. I thought the pressure was being increased to keep the same flow.
I was under the assumption that we do not gain any fuel flow when the fuel pressure rises with boost because the same amount of pressure is put on the other side of the injector by the air. I thought the pressure was being increased to keep the same flow.
01-21-2004, 08:56 PM
#26
Race Director
Tom you're correct. The mappings on the chips are flow-based and don't take into account boost. They are assuming an engine with 0 to X-CFM flow-rates and inject fuel based upon air-flow X RPM; assuming of course, that fuel-pressure remains constant. Raising fuel-pressure in relation to boost effectively removes boost from the fuel-injection parameters.
The increase in fuel-pressure I was talking about in the post with the Honda fuel-pump was an increase in initital fuel-pressure. Then the FPR still adds pressure on top of that. So if we have a 20psi engine operating @ 36.3psi (2.5-bar) , then the range the injectors would operate in is 36.3-56.3psi. However, if we're desperate because our 55-lb/hr injectors are maxed out at 5000rpm@350rwhp, and we have an upgraded fuel-pump, we can bump up fuel-pressure to say.... 43.5psi (3.0-bar) for an operating range of 43.50-63.5psi for a 9% more total fuel-volume. This might just allow us to get 370rwhp without resorting to swapping injectors.
The increase in fuel-pressure I was talking about in the post with the Honda fuel-pump was an increase in initital fuel-pressure. Then the FPR still adds pressure on top of that. So if we have a 20psi engine operating @ 36.3psi (2.5-bar) , then the range the injectors would operate in is 36.3-56.3psi. However, if we're desperate because our 55-lb/hr injectors are maxed out at 5000rpm@350rwhp, and we have an upgraded fuel-pump, we can bump up fuel-pressure to say.... 43.5psi (3.0-bar) for an operating range of 43.50-63.5psi for a 9% more total fuel-volume. This might just allow us to get 370rwhp without resorting to swapping injectors.
01-21-2004, 09:06 PM
#27
Rennlist Member
Originally posted by slim_boy_fat
For those who cant read, my car has the screw on FPR, NOT the push fit, hence the need to find a screw on FPR!! Try and keep up guys.
For those who cant read, my car has the screw on FPR, NOT the push fit, hence the need to find a screw on FPR!! Try and keep up guys.
01-21-2004, 09:50 PM
#28
Rennlist Member
Okay, slightly different kind of question but still on topic:
I've established that my 52# injectors are close to being maxed out. Via chips, I can do one of two things: a) run 3.25 bar fuel pressure and probably hit 100% duty cycle in the upper RPMs or, b) run 3.75 bar fuel pressure and hit around 90% duty cycle.
I know, both scenarios are not very good and yous are just going to say "why don't you just get bigger injectors?"
Well, I don't feel like going through the hassle of rechipping AGAIN and I'm not on the track doing full power pulls for long periods of time. It's not like I'm running out of fuel; just want to know which scenario would be harder on the components (DME, drivers, injectors, and whatever else)
We cannot use the arguement that the pump will not be able to supply enough fuel in scenario "b" because I have the 993tt fuel pump (don't have the part# ; starts with 964_ _ _ _ _ _ ). Maybe someone can verify this.
Also, slightly OT: curious as how DFASTEST's car can put down that kind of power with apparent 52# injectors. Not doubting anything; just wouldn't mind a logical explanation. Is it possible that some injectors are under rated or there is wide variances? Also, the pumps: except for the one race pump listed above, it would seem (with my calculations) that some of the heavy guns on this forum wouldn't have enough fuel pump capacity (just an observation but I rarely see fuel pump upgrades in SIGs).
I've established that my 52# injectors are close to being maxed out. Via chips, I can do one of two things: a) run 3.25 bar fuel pressure and probably hit 100% duty cycle in the upper RPMs or, b) run 3.75 bar fuel pressure and hit around 90% duty cycle.
I know, both scenarios are not very good and yous are just going to say "why don't you just get bigger injectors?"
Well, I don't feel like going through the hassle of rechipping AGAIN and I'm not on the track doing full power pulls for long periods of time. It's not like I'm running out of fuel; just want to know which scenario would be harder on the components (DME, drivers, injectors, and whatever else)
We cannot use the arguement that the pump will not be able to supply enough fuel in scenario "b" because I have the 993tt fuel pump (don't have the part# ; starts with 964_ _ _ _ _ _ ). Maybe someone can verify this.
Also, slightly OT: curious as how DFASTEST's car can put down that kind of power with apparent 52# injectors. Not doubting anything; just wouldn't mind a logical explanation. Is it possible that some injectors are under rated or there is wide variances? Also, the pumps: except for the one race pump listed above, it would seem (with my calculations) that some of the heavy guns on this forum wouldn't have enough fuel pump capacity (just an observation but I rarely see fuel pump upgrades in SIGs).
01-22-2004, 08:37 PM
#29
Rennlist Member
No more takers on this topic?
Danno, when you say the stock pump can flow 1500cc at 3 bar, does this take into account the boost pressure against the injectors? That would mean that the pump flows some lesser amount than 1500cc at 4 bar (if we assume 1 bar of boost)? This seems awfully low. Even 1500cc doesn't seem right when doing some quick calculations:
1500cc x .75 = 1125 grams of gasoline/min.
1125 x 12.5 (air/fuel ratio) = about 14.06 kg of air/min.
14 kilograms of air per minute is worth about 300 crank horse power. That's it! What am I not understanding here?
Danno, when you say the stock pump can flow 1500cc at 3 bar, does this take into account the boost pressure against the injectors? That would mean that the pump flows some lesser amount than 1500cc at 4 bar (if we assume 1 bar of boost)? This seems awfully low. Even 1500cc doesn't seem right when doing some quick calculations:
1500cc x .75 = 1125 grams of gasoline/min.
1125 x 12.5 (air/fuel ratio) = about 14.06 kg of air/min.
14 kilograms of air per minute is worth about 300 crank horse power. That's it! What am I not understanding here?
