Eric wrote: > Can some netters kindly explain why VW manuals give 7 deg BTDC and aero > conversions give 25 to 28 deg BTDC as the timing.?. How does one "tweak" > the VW to use 25 to 28 deg BTDC.?<
The VW manuals give 7 degrees because their distributors have both a vacuum and a mechanical advance system (usually) that will eventually advance the timing to something like 28-30 degrees of TOTAL advance once the engine reaches a higher rpm. Using a regular timing light at idle, 7 degrees is where you'd set it. Aviation conversion engines use a variety of different ignition sytems, so the safe and only accurate way to describe timing is total advance that you'll get at normal operating RPM (say above 1800-2000 rpm). No matter what system you use, you need to make sure that the timing is reaching the max advance (I use about 32 degrees on VWs and Corvairs) when the engine is running at close to wide open throttle. This is done with a timing light. Obviously you need to have your pulley marked accurately. The most certain way to deal with it is mark the pulley at TDC and use an "advancing" timing light set to 28 degrees. Most people fiddle around with timing while they're trying to start the engine for the first time. Knowing how much advance the distributor has allows you to set it will almost perfect accuracy before you ever start the engine (called "static" timing"). Again, you need to have the pulley marked with the proper number of degrees, but for example, my Corvair needs to be set at 8 degrees BTDC because it has 24 degrees of centrifugal advance built in (for a total of 32 degrees BTDC at high speeds), so I set the engine at 8 degrees BTDC for number 1, then rotate the distributor past where I think it should fire, then turn on the ignition and slowly rotate the distributor until the points fire (this is with the cap off, you can both see and hear them spark). Lock it down, rotate the engine in the proper direction twice to make sure you have it right, and now you can forget about timing until after you have it started. Actually, I'm so confident that this works, that's how I set mine, period. I put 200 hours on the Corvair without checking it with a timing light. I finally figured I really ought to, just to make sure, and it was right on the money. I really hate standing behind a spinning prop at wide open throttle, and in some cases, you just can't do it. In those cases, statically setting the timing makes sense, but all efforts should be made to check it with a timing light before flying behind it (do as I say, not what I do...). Mark Langford, Huntsville, Alabama see KR2S project N56ML at http://home.hiwaay.net/~langford email to N56ML "at" hiwaay.net
