Advice on A/F Ratio on Dyno Sheet....
11-12-2004, 11:58 PM
#16
Addict
Rennlist Member
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No, back to my point, do your research on the stock fuel system. If the plumbing was static your point would be true, but the stock plumbing is not static - there is a dynamic pressure regulator in it that limits pressure - the higher the pump pressure, the more pressure is relieved. Fuel rail pressure stays the same regardless of pump.
Sam
Sam
11-13-2004, 12:10 AM
#17
Three Wheelin'
Since there seems to be some serious confusion on the topic of fuel injection systems, I would suggest that the following website is worth a look: http://www.icarumba.com/icarumba/res..._fuel3.asp#top
As quoted from this site:
"These systems operate at a relatively high pressure (usually at least 30 psi). To control the fuel pressure, a fuel pressure regulator is used. As engine load increases, more fuel pressure is needed. This is due to the richer mixture (more fuel needed) and to overcome the increased air pressure in the ports. Any unused fuel is diverted back to the fuel tank using a return line."
Basically, you can cram the biggest pump in the world into the fuel tank, but as long as the fuel pressure regulator is in place, you are still only going to see 2.5 bar at the fuel rail. This is further demonstrated in the aftermarket chips that include an upgraded 3.0 bar fuel pressure regulator, not a new fuel pump.
-Jon
As quoted from this site:
"These systems operate at a relatively high pressure (usually at least 30 psi). To control the fuel pressure, a fuel pressure regulator is used. As engine load increases, more fuel pressure is needed. This is due to the richer mixture (more fuel needed) and to overcome the increased air pressure in the ports. Any unused fuel is diverted back to the fuel tank using a return line."
Basically, you can cram the biggest pump in the world into the fuel tank, but as long as the fuel pressure regulator is in place, you are still only going to see 2.5 bar at the fuel rail. This is further demonstrated in the aftermarket chips that include an upgraded 3.0 bar fuel pressure regulator, not a new fuel pump.
-Jon
11-19-2004, 07:07 PM
#19
Instructor
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Just an N/A guy here, but it would seem that a larger pump (higher pressure/volume) still has to flow through the same dia pipe and is limited in pressure at the injectors by the fuel presure regulator (that's what it does, limit pressure, the flow is limited then by the pipe dia.). The only way a larger pump can produce more flow (gpm or gallons per minute) at the injectors is to change the inside diameter of the pipe throughout the system including the pressure regulator and injectors resulting in more flow at the same pressure
OR
change/adjust the fuel pressure regulator. If you raise the pressure at the regulator less fuel is by-passed back to the tank and increases the flow through the injectors. It really seems that the only things that will allow more fuel through the injectors is (in no particular order):
duty cycle (they stay open longer)
pressure (more fuel in shorter amount of time though a given orifice)
injector size (larger opening)
OR
change/adjust the fuel pressure regulator. If you raise the pressure at the regulator less fuel is by-passed back to the tank and increases the flow through the injectors. It really seems that the only things that will allow more fuel through the injectors is (in no particular order):
duty cycle (they stay open longer)
pressure (more fuel in shorter amount of time though a given orifice)
injector size (larger opening)
11-22-2004, 03:01 AM
#20
Burning Brakes
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.....I'm still learning on porsche technology so excuse me. Are there after market road scanning software platforms available for the porsche ecu as in numerous imports and domestics.
We dyno test and tune but also road scan and tune for load factors. The Dyno jet dynomometer a/f clearly shows tuning issues before boost (rich). At boost you have the leaness issue which is fuel delivery, either pump or injectors..(fmu)
Road scan this vehicle and check duty cycle of injectors under wot first. IMHO
Tripp
We dyno test and tune but also road scan and tune for load factors. The Dyno jet dynomometer a/f clearly shows tuning issues before boost (rich). At boost you have the leaness issue which is fuel delivery, either pump or injectors..(fmu)
Road scan this vehicle and check duty cycle of injectors under wot first. IMHO
Tripp
11-22-2004, 06:34 AM
#21
Nordschleife Master
Stock fuel pressure for a 944Turbo is 2.5 bar which in terms of modern fuel injection is about the lowest I would run. In general, you can increase the fuel pressure to increase fuel flow, however, the following conditions apply:
a) there is a diminishing return as you increase fuel pressure. (The formula is new flow rate = old flow rate * sqr root of new pressure divided by old pressure)
b) I don't like to run Bosch injectors over about 75psi dynamic which means if you have your static fuel pressure set at 55psi and are running 15psi of boost, you have 70psi of dynamic fuel pressure. I generally like to run about 3.8bar (55psi) static on stand alone systems.
c) your fuel system must be capable of supplying the fuel (944T systems are very robust)
d) you increase fuel throughout the range which means you WILL be running too rich in the off boost areas. You need access to the main fuel tables to make adjustments accordingly (Check with Danno here, he does good work)
The stock 944T injectors are peak and hold injectors which means the BOSCH injector drivers are capable of supplying about 6 amps of current. You can safely change to larger injectors and the ECU will continue to power them appropriately. You will need to have all three fuel tables modified (idle, part throttle, and full throttle) just the same as if you increase your base fuel pressure.
a) there is a diminishing return as you increase fuel pressure. (The formula is new flow rate = old flow rate * sqr root of new pressure divided by old pressure)
b) I don't like to run Bosch injectors over about 75psi dynamic which means if you have your static fuel pressure set at 55psi and are running 15psi of boost, you have 70psi of dynamic fuel pressure. I generally like to run about 3.8bar (55psi) static on stand alone systems.
c) your fuel system must be capable of supplying the fuel (944T systems are very robust)
d) you increase fuel throughout the range which means you WILL be running too rich in the off boost areas. You need access to the main fuel tables to make adjustments accordingly (Check with Danno here, he does good work)
The stock 944T injectors are peak and hold injectors which means the BOSCH injector drivers are capable of supplying about 6 amps of current. You can safely change to larger injectors and the ECU will continue to power them appropriately. You will need to have all three fuel tables modified (idle, part throttle, and full throttle) just the same as if you increase your base fuel pressure.
Last edited by Geoffrey; 11-28-2004 at 09:53 AM.
11-25-2004, 02:06 AM
#24
Burning Brakes
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Originally Posted by boosted951
thanks Geoffrey... can I adjust those fuel tables with my Apexi SAFC II?
11-28-2004, 03:05 AM
#25
Race Director
"thanks Geoffrey... can I adjust those fuel tables with my Apexi SAFC II?"
Actually no. The same fuel-tables on the chips are still in place. What the SAFC-II does is massages the incoming air-flow meter voltage to "fool" the computer into thinking that more or less air is flowing than actual. Then it will look up a different point on the fuel-map, one that corresponds to more air-flow, and will thus inject more fuel. Or it will look up a point with less air-flow and inject less air.
Signal massaging is fine for minor changes in air-fuel ratio, but you don't want to rely on it for large adjustments. That's best done in the chip-maps themselves with programming in different duty-cycles.
Your original dyno-chart looks like it doesn't need too much tweaking. However, the sudden lean tendency that occurs in a straight line sloping upwards after 4000rpm seems to indicate that some sort of flow-limit has been reached, either injectors or fuel-pump. The stock injectors typically are maxed out at 100% around 280rwhp with the higher 3-bar FPR I supply with my chip-kits (9% more total flow than stock 2.5-bar). Looks like you've hit the 100% duty-cycle limit on the injectors. You can try all you want with the S-AFC-II to increase fuel above 4000rpm, but it won't make one bit of different because you can't have the injectors open more than 100%.
ALso it appears that an attempt has already been made at this. The sudden rich-dip at 3500rpm shows evidence of signal-massaging attempt at increasing fuel, as shown by the rapid increase in richness from 3750-4000rpm. But once the injectors hit 100% at 4000rpm, you're maxed out and the linear rising air-fuel curve towards lean is due to a fixed volume of fuel flow but an increasing volume of air with RPM.
Actually no. The same fuel-tables on the chips are still in place. What the SAFC-II does is massages the incoming air-flow meter voltage to "fool" the computer into thinking that more or less air is flowing than actual. Then it will look up a different point on the fuel-map, one that corresponds to more air-flow, and will thus inject more fuel. Or it will look up a point with less air-flow and inject less air.
Signal massaging is fine for minor changes in air-fuel ratio, but you don't want to rely on it for large adjustments. That's best done in the chip-maps themselves with programming in different duty-cycles.
Your original dyno-chart looks like it doesn't need too much tweaking. However, the sudden lean tendency that occurs in a straight line sloping upwards after 4000rpm seems to indicate that some sort of flow-limit has been reached, either injectors or fuel-pump. The stock injectors typically are maxed out at 100% around 280rwhp with the higher 3-bar FPR I supply with my chip-kits (9% more total flow than stock 2.5-bar). Looks like you've hit the 100% duty-cycle limit on the injectors. You can try all you want with the S-AFC-II to increase fuel above 4000rpm, but it won't make one bit of different because you can't have the injectors open more than 100%.
ALso it appears that an attempt has already been made at this. The sudden rich-dip at 3500rpm shows evidence of signal-massaging attempt at increasing fuel, as shown by the rapid increase in richness from 3750-4000rpm. But once the injectors hit 100% at 4000rpm, you're maxed out and the linear rising air-fuel curve towards lean is due to a fixed volume of fuel flow but an increasing volume of air with RPM.
11-28-2004, 09:55 AM
#26
Nordschleife Master
Have Danno burn you a chip, I've worked with him in the past and he has a good grasp of what 944s need. This is a much better route than a piggyback system because you are using an integrated system (factory), not two separate systems which can at times compete against each other.
