55lb injectors - adding to the confusion ...
03-02-2005, 12:58 AM
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55lb injectors - adding to the confusion ...
There's a big list of fuel injectors and applications at http://users.erols.com/srweiss/tableifc.htm
As it says, search the page for <<944 turbo>> by using the brower's EDIT --> FIND command otherwise you'll be there all day.
Then there's http://www.clarks-garage.com/shop-manual/fuel-12.htm This lists both Porsche and Bosch part numbers and the spec that states oem 951 injectors are 4.5 ohms, but where the author goes to pains to state that Bosch would not provide help or information over confirmation of the specs.
Then there's Racetronix comment:
"944 owners run our Siemens 3102 injectors along with a resistor pack from an import. The resistor pack has four 6 ohm resistors which wire in series with each 2 ohm injector making the total resistance per channel 8 ohms.
The other option is to run our Siemens 3172 which does not require any resistor box as the injectors are 12 ohms each. The 3172 will work faster than a 3102 with a series resistor".
The comment "will work faster" is interesting. More fuel chucked down the exhaust? Turbo becomes gas turbine? better cylinder filling?
Then there's my 'How to do electronics' book that reminds me that resistors in series are additive. So, Racetronix resistor calculation is correct, but if we want 4.5 ohms and the basic injector (Siemens 3172) has a 12 ohm coil, we need to add 7 ohms in parallel (this actually works out 4.2 ohms). 1/R total = 1/R1 + 1/R2. Now... I'm really getting confused.
Is there anyone out there who really has got to grips with this?
David Benjamin
As it says, search the page for <<944 turbo>> by using the brower's EDIT --> FIND command otherwise you'll be there all day.
Then there's http://www.clarks-garage.com/shop-manual/fuel-12.htm This lists both Porsche and Bosch part numbers and the spec that states oem 951 injectors are 4.5 ohms, but where the author goes to pains to state that Bosch would not provide help or information over confirmation of the specs.
Then there's Racetronix comment:
"944 owners run our Siemens 3102 injectors along with a resistor pack from an import. The resistor pack has four 6 ohm resistors which wire in series with each 2 ohm injector making the total resistance per channel 8 ohms.
The other option is to run our Siemens 3172 which does not require any resistor box as the injectors are 12 ohms each. The 3172 will work faster than a 3102 with a series resistor".
The comment "will work faster" is interesting. More fuel chucked down the exhaust? Turbo becomes gas turbine? better cylinder filling?
Then there's my 'How to do electronics' book that reminds me that resistors in series are additive. So, Racetronix resistor calculation is correct, but if we want 4.5 ohms and the basic injector (Siemens 3172) has a 12 ohm coil, we need to add 7 ohms in parallel (this actually works out 4.2 ohms). 1/R total = 1/R1 + 1/R2. Now... I'm really getting confused.
Is there anyone out there who really has got to grips with this?
David Benjamin
03-02-2005, 01:48 AM
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I remember some time ago there were posts on tests made with high impedance injectors and the results were that they did not act as fast or flow the same as low impedance injectors. If this is true, you would have to re-map the DME to get the correct AF ratio.
As far as resistance goes, you don't need to add a parallel resistor to lower the resistance. The high impedance injector takes less current so the driver will be sinking a lower amount of current. The reason people add a resistor in series with the injector is to lower the current so the drivers don't overheat/fail. At the frequency the injectors are working at, there is no need to "tune" or impendence match the circuit. Just need to worry about how much current you are sinking to insure you don't damage the injector drivers.
As far as resistance goes, you don't need to add a parallel resistor to lower the resistance. The high impedance injector takes less current so the driver will be sinking a lower amount of current. The reason people add a resistor in series with the injector is to lower the current so the drivers don't overheat/fail. At the frequency the injectors are working at, there is no need to "tune" or impendence match the circuit. Just need to worry about how much current you are sinking to insure you don't damage the injector drivers.
03-02-2005, 04:58 AM
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beab951
Brian
Many thanks for that info. I see you have two 951's. Are they running standard oem injectors? If not, what are you using and how do you rate them?
David Benjamin
Brian
Many thanks for that info. I see you have two 951's. Are they running standard oem injectors? If not, what are you using and how do you rate them?
David Benjamin
03-03-2005, 12:08 AM
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I am running 52 and 72 Lb injectors. The 87 is almost stock and runs 16 Lb boost so it has the 52#. The 86 needed a little more fuel for the bigger turbo. Both run well, but I wouldn't get the 52 again, I would go for the 55 instead. More common, flow better. How do I know, I put in the 52 with a set of 55 chips, ran out of fuel at higher RPMs. Did one one run and the WB was showing lean and it started to miss.
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03-03-2005, 01:22 AM
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The manual is showing that the average value of a 951 injector is 4.5 ohms. The average value for a aftermarket, like siemens, is 2.35 ohms.
http://www.racetronix.com/3102FM.html
http://www.racetronix.com/3102FM.html
03-03-2005, 02:06 AM
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Originally Posted by beab951
The manual is showing that the average value of a 951 injector is 4.5 ohms. The average value for a aftermarket, like siemens, is 2.35 ohms.
http://www.racetronix.com/3102FM.html
http://www.racetronix.com/3102FM.html
03-04-2005, 11:07 AM
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If you do buy the high impedence injector, you should have a WB monitor to check the A/F ratio or go to your local dyno shop and tell them to take it easy for the first couple of runs. Don't want to blow the head gasket etc.. Anyone running high impedence injectors in a 944?
03-04-2005, 02:31 PM
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Originally Posted by beab951
I am running 52 and 72 Lb injectors. The 87 is almost stock and runs 16 Lb boost so it has the 52#. The 86 needed a little more fuel for the bigger turbo. Both run well, but I wouldn't get the 52 again, I would go for the 55 instead. More common, flow better. How do I know, I put in the 52 with a set of 55 chips, ran out of fuel at higher RPMs. Did one one run and the WB was showing lean and it started to miss.
If you do buy the high impedence injector, you should have a WB monitor to check the A/F ratio or go to your local dyno shop and tell them to take it easy for the first couple of runs. Don't want to blow the head gasket etc.. Anyone running high impedence injectors in a 944?
Tomas
03-04-2005, 03:29 PM
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One question: Can you run too large injectors? In other words,if 55lbs injectors are just at the limit (at 80% duty cycle) and you install 72lbs ones instead,can you get yourself into trouble with idle or something else?