SC Timing Question.
 

Need help with my timing. Have 83 SC and my timing at idle is set at 12-15 degrees btdc. I live at 5000' above sea level in Denver and she runs awful if I set timing at the standard 5 degrees btdc. Is there rule of thumb to advance the timing due to my altitude? Should I just leave it be since she runs fairly nice at the current setting? Thanks, Tim

Whats the total advance? Whats the spec? Warren

Rule of thumb due to altitude is to lean the mixture to compensate for lower oxygen content and leave the timing alone at standard 5 degree btdc with vacuum line disconnected (reconnect when done).

If the 12-15 btdc is working for you and you are not experiencing any detonation, i would have no problem running it like that. I prefer more idle advance on my car as well, as it really improves idle vacuum and throttle response. You may have to retard the timing back to 5* btdc to pass emissions, however.

Just talking here - I don't know nuttin' and I can prove it......

I did some google sniffing and read where on fellow said rule of thumb is 1 degree advance for every 1000 ft elevation. What's it worth, I don't know. Probably little as Porsche did have vacuum ports on WUR's to compensate for altitude - fuel impact only - not timing.

tkmoore,

You understand the vacuum retard/advance hoses and their impact? If all is working properly at idle, when you plug the retard hose back on timing should retard at least 5-10 degrees - so you would be firing at idle around 5 BTDC instead of factory TDC up to as much retard as 5 ATDC.

Vacuum advance starts working off idle.

Nice thing is SC's are wimps at total advance. Factory 18-23 BTDC at 3500 (or 6,000 for that matter - they play out quick)

For common sense reasons I would do some math and figure out roughly where 25 BTDC is on your pulley with the marks. It's a PITA and you need a flexible ruler or tape with the distance on it. Divide circumference of pulley by 360 then that tells you the length of each degree on a circle.

Saying this as these distributors are not spring chickens. If something was bound up or hung up in your mechanical advance this would tell you if you have a problem that needs to be addressed to get full functionality from the engine.

Factory engine at idle is 5BTDC (not counting retard that goes away off idle). Full advance is say 21 (split the factory specs). That should put you with all lines connected at 3,500 rpm at about 26-30 BTDC plus or minus. If you can get this advance and car runs well, your hard parts are fine. I would feel better closer to 25 than 30 though with 87. IF you are at 30 use premium.

You got me there Bob! I have one of those pollutin' Euro trash 911's that don't use a vac retard!

I had a full 25* mech advance in my distributor (factory) and I would get detonation above 5500 rpm at WOT when set at the spec 5* BTDC idle/30* total in the summer running A/C, even on premium gas. If I took 5* out of the timing to get rid of detonation (0* idle/25* total), it had no bottom end power, especially running A/C, and hard to start hot. I adjusted the high rpm mech weight stop to take 5* out of the mech advance and now I run 5* BTDC idle/25* total in the summer (summer tune), and 10* BTDC idle/30* total in the fall, winter, and spring (winter tune).

Guess which tune I like better?

Back when I was going up to the San Jaun Mts of SE Colorado every year for an annual fullsize Jeep shindig, my Holley carburator Jeep would start running like poo poo at about 10K ft. I'd pull over and advance the timing until it ran good again. Much easier than trying to re-jet a Holley on the side of the road!
http://i906.photobucket.com/albums/a.../1stbrkdwn.jpg

Josh,

I am sorry. I meant to put the OP's name in there and put yours in by mistake. I edited the old post.

Good info you provided. (winter was more fun :D).

I would like to see tkmoore check total advance to make sure he is not leaving something on the table OR that he is not too far advanced - nothing beyond that. Also, he may know more than the both of us together. Never know with new guys.

tkmoore could have euro spec as well?