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Join Date: Apr 2017
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1980 911sc Euro with 964 Turbo XDI twin plug ignition

Is there any information on the web about XDI twin fire ignition system adjustments. I have a 964 turbo with twin plugs. Just changed them out and the car starts right up and runs strong, however I feel that there is a slight miss fire with one or two of the plugs. Is there a way to use an voltmeter on the XDI box to determine which plug is acting up? Or any suggestions short of pulling them all back out again. When starting cold it has a slight popping like its not firing on all the cylinders. When it warms up it appears to be less pronounced and since there are 12 plugs it is hard to detect from all the power it puts out. But no one in Kansas City seems to have a clue as to sharp shooting the XDI system. The cars starts right up the first time every time but doesn't seem quite right ... The power band isn't smooth and quick. Please help...
Old 10-28-2017, 12:27 PM
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what is your gap and boost ?

unplug the bottom plugs one at a time while idling see if there is a change
same for top plugs
Old 10-28-2017, 12:59 PM
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xdi twin plug ignition

The boost is 1.05 bar spring... originally had a .8.............

The plugs have the standard gap for the plug trying to remember what is in there. They are two difference sizes between the top and bottom. The builder of the motor put I believe a slightly smaller diameter on bottom when engine was built
W-4CS 14mm upper plugs and the bottom ones are XR-4-CS 12mm all with same gap.. Think they came gapped.. recommended not messing with them due to the fine wire.
Old 10-28-2017, 02:38 PM
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here are the recommended spark plug gaps from the xdi manual

Stock Street Engine 0.045”-0.060” (1.1mm-1.5mm)
• High Performance Street 0.030”-0.035” (.75mm-.9mm)
• Alcohol High Compression 0.025” (0.65mm)
• High Power 75 -115 HP per Cylinder 0.025” (0.65mm)
• Over 115 HP per Cylinder 0.022” (0.55mm)
• Over 12:1 CR or Over 14psi Boost 0.022” (0.55mm)

found here:

Installation Instructions : Clewett Engineering, The complete solution for ignition & fuel injection

remember Air is a good insulator
Old 10-28-2017, 03:34 PM
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Spark plug recommendation for 964 Turbo

Getting ready to pull the plugs on the motor and start over with the same spark plugs that were used when the motor was rebuilt at Stuttgart Southwest Motors.
Their build sheet showed

The top plugs are:
14mm Bosch W-4-CS with gap of 24


The bottom plugs are:
12mm Bosch XR-4-CS with gap of 28

Should I use the XDI recommended gap of 25 on all the plugs ?????
Old 11-15-2017, 12:54 PM
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