Engine guys... elevation timing adjust C2

[Jerry G.]

I live in Boulder Colorado and am heading to Breakenridge for my first PV test. One of the local guys has suggested that the initial timing set at idle with vacumn advance disconnected should be adjusted upward 4 to 6 degrees for elevation. Mine is a 65 FI that calls for 12 degrees. This would give me 16-18 degree initial advance. What do you engine experts think about adjusting timing for elevation? Jerry

[Duke W.]

Re: Engine guys... elevation timing adjust C2

At 5000 feet I'd guess initial timing could be advanced about four degrees. The limiting factor is detonation.

The initial nominal timing spec for your engine is 12 with a range of 10-14, so you could run up to 18 at 5000 ft. or higher if it doesn't detonate.

Keep in mind that the initial timing on your engine is VERY TRICKY to set. This is because the centrifugal curve starts at about 700 and it is difficult to even keep your engine running below 700.

Since all the centrifugal is in at 2350, the best way to set timing is to disconnect the vacuum can signal line, rev the engine to 2500, verify that the timing mark has stopped advancing, then set the total timing at 36 +/- 2 for sea level operation using a dial back timing light. This is known as the total wide open throttle timing (WOT) since disconnecting the vacuum simulates the timing map at WOT when there is not enough manifold vacuum to add vacuum advance.

Since the nominal maximum centrifugal advance is 24 the initial would be, for example, 36 - 24 = 12.

At 5000 feet you could experiment and adjust the total WOT timing up to 42-44 degrees and see how it operates. - any detonation - does it buck or hesistate at cruise? If so set it back in two degree increments until any bad effects go away.

Since you are doing a PV, there is no point pushing the limit and risking inappropriate engine behavior during the test, so just leave it at 12 for the PV and fiddle around with it later if you like. Also keep in mind that if you do advance the total WOT timing beyond 36 it could detonate like crazy if you drive down to lower altitude unless you are running high octane race gas or avgas.

Duke