Thanks Tony. I think I have a plan of attack for this weekend. Just in case someone searches this post in the future here's some related info from an article I came across that jives with what you're suggesting.
How can I reduce ignition noise?
Electromagnetic Radio Frequency Interference (EMI or RFI) from the engineâ€™s ignition can be a major problem when trying to capture sensitive electronic instrument data adjacent to a high compression spark-ignition engine. RFI problems worsen as the engineâ€™s throttle is opened (increased cylinder pressure raises the peak ignition voltage) and may more severe as RPM increases (raising the RFI frequency). RFI symptoms include erratic DYNOmite tachometer and/or servo operation, Personal Computer lock-up, spikes in the data.
Taking one or more of the following preventive steps will generally allow testing of even the wildest combinations. These problems are not specific to the DYNOmite, so your ignition systemâ€™s manufacturer may have other suggestions you can follow.
1) Run all ground leads to either the engineâ€™s battery (or block if no battery is used) or to a clean common junction connected by a heavy-duty ground cable to the engineâ€™s battery. Ideally this battery's negative terminal and/or dynamometer's frame itself should be connected directly to a quality earth ground (e.g. buried copper rod) via a large gauge wire (for a very low resistance connection). Avoid multiple ground points as this often creates a â€œground loopâ€ (where RFI can get onto the circuit).
CAUTION: Always hook up all the ground leads before hooking up any of the power leads for the DYNOmite Data Acquisition module, its accessories, or the engine. Failure to follow this precaution can lead to circuit damage not covered under warranty!
2) Try using resistor sparkplugs (or caps) and also run graphite ignition cables (or genuine MAGNECOR brand Race Wires). These components tremendously reduce the level of RFI generated by the ignition. Killing the noise at the source is much more effective than chasing down every dyno harness lead that might be receiving these unwanted random signals.
CAUTION: Spiral wire wound â€œsuppressionâ€ ignition cables are not anywhere near as effective (at reducing EMI) as true graphite core cables. The cheapest set of graphite wires will â€œout-suppressâ€ the most expensive wound metallic wires. The only effective EMI suppression wires (that are not pure carbon or graphite core) used in the high end automotive racing world are manufactured by MAGNECOR. For some straight talk about the pro's and con's of various type sparkplug wires, read "THE TRUTH ABOUT IGNITION WIRE CONDUCTORS" or "A Brief Overview of the Performance Aftermarket Ignition Wire Industry in the USA".
Tip: Keep an inexpensive set of graphite core cables in the dyno cell for troubleshooting problem ignitions. You donâ€™t need expensive high temperature 8+ mm silicone jacket wires with high quality boots to survive on the dyno. Even installing just a graphite core coil lead may do the trick. New graphite cables will not reduce the engineâ€™s power.
3) Keep the ignition wires (and coil wire) as short as practical and avoid routing the DYNOmiteâ€™s harness leads parallel or near to the ignition wires.
Tip: The EGT probes have extra shielding on them to allow operation near the ignition. Conversely, the Engine Temperature leads are quite sensitive to RFI, keep them away from the ignition if you can.
Tip: Temporarily iinstall braided metal shielding, such as Aeroquip stainless braid hose (grounded at one end to the engine block) over all the sparkplug (and coil) wiring. This trick was used on early Corvettes and many marine applications where the fiberglass engine cowling provided poor RFI suppression.
4) If RFI is interrupting communications between the DYNOmite Data Accusation Computer and your PC running DYNO-MAX (via a RS-232 serial port connection) the use of Land Sea's special shielded and filtered cables may help. Typically such interference is much more troublesome as the throttle is opened (increasing cylinder pressure and spark intensity). Also, avoid using USB adapters as they just add another point of EMI sensitivity - instead use a computer with built-in RS-232 ports (or add an RS-232 port bus card) so that there will be an uninterrupted grounded shield from the DYNOmite to the PC..
Tip: In extreme cases fiber optic RS-232 serial "cabling" is available from several manufacturers. Such hook-ups use a converter at each end that translates the normal hard-wired connection into a light signal that runs along the fiber optic link. However, be sure to select models that do not require power from the PC or DYNOmite RS-232 signal to operate!
5) Since many models of PCs and laptops are very poorly "hardened" against the levels of EMI present near a spark ignition engine, try other models (or brands) to find a more robust computer. Most modern PCs are no longer grounded via their power cords or chargers. In certain cases running a large gauge (#12 or 10) wire to ground the PCs case helps - try it both ways. Moving a problem PC further away from the engine is always a good idea. Sometimes, simply reorienting the position of the PC, its power cord, etc. may uncover a less sensitive position (just as moving a radio's antenna effects its ability to pick up a weak station).