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Saab Model 99 Bulletin Board
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Re: Correct K Jetronic
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Posted by Gary Stottler [Email] (#1463) [Profile/Gallery] (more from Gary Stottler) on Tue, 30 Dec 2014 09:55:15 Share Post by Email
In Reply to: Correct K Jetronic, alvaro isla, Mon, 29 Dec 2014 09:10:57
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I tend to agree with John, don't second guess the engineers. That said, the main problem you run into trying to increase power is "can the stock fuel system supply enough fuel?". The stock K-Jetronic Lambda system should be good for about 145-150 horsepower (the European spec output), above that you may run into trouble and start to run lean if you increase airflow. Lean running can cause pre-ignition and burned exhaust valves among other expensive problems. If you are doing any kind of performance modification to the engine in search of higher power, then you are going to increase airflow (modified cam, higher boost, etc.)and therefore need higher fuel flow. I would strongly suggest that you install an air-fuel ratio sensor so that you can monitor your mixture. I would think that under any kind of above-stock boost (so 10 psi or higher) you need to run at least 11.0:1 (14.7:1 is "stoichiometric" or Lambda=1.0 which means there's exactly the right amount of oxygen to consume all the fuel). A richer mixture reduces combustion temperature and protects pistons, valves, turbos, catalysts, etc. Lean air-fuel ratio at high flow is very bad (did I already mention that?). The only other "quick fix" I know of to provide a little more fuel flow is to use a 0 438 140 085 Control Pressure Regulator which has a vacuum/pressure port and enriches the mixture some under boost. However, I would still use the air-fuel ratio sensor, because the stock fuel pump, regulator, and distributor can still only deliver so much fuel. I have seen, I think in the factory rally cars, that they used a V-6 fuel distributor with 2 of the ports plugged, so I guess that must have come from an application that could provide higher fuel flow, but I don't know what application that came from, maybe a Porsche. However, doing that would require quite a bit of engineering and it might be difficult to get it to run well at lower speeds and at part throttle.
As far as the airflow sensors, it's hard to say. The naturally aspirated sensor might push the plunger up further for the same airflow, but it would also max out at 118 hp (the highest power of the N/A engine)and leave any airflow above that essentially un-measured. Also, since the turbo fuel system has the lambda trim, you might get into other problems if you change the aiflow sensor - lambda will try to reduce the fuel if the sensor is pushing it richer at part throttle. You are OK adding extra fuel with Lambda at full throttle with the boost-enriching control pressure regulator since lambda goes to a fixed pulsewidth at full throttle using the little microswitch on the throttle.
If you want to get really crazy you could rig the cold start injector to fire at high boost, but you would not have any way to calibrate that - it would just spray a fixed amount of fuel.
I hope that helps!
Gary
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Gary Stottler
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