5 deg ATDC with vacuum connected vs 5 BTDC without

[jwakil]

I was looking at the various idle timing specs for the various porsche engines, and I noticed that some were spec-ed at 5 ATDC and some 5 BTDC, however all the ones spec-ed at ATDC said keep the vacuum retard hose connected when setting and those at BTDC said remove the hose. This leads me to believe that the vacuum retard probably retards by 10 degrees so that you go from 5 BTDC to 5 ATDC. So basically they are the same condition: (5 ATDC with hose = 5 BTDC without hose). My car is spec-ed at 5 ATDC (with hose), but I normally drive without the hose since I want to lower idle temps. Therefore, I believe I should I set at 5 BTDC without the hose? Is this correct?

[stlrj]

Easy to verify. Just disconnect the vacuum line at 5 ATDC and see where the timing results. My guess is that it comes in pretty close to 5 BTDC.

[BSNMOE]

You can remove and plug vacuum line, then time at 6000RPM and let it idle where it wants. Idle most likely will be a little high so you may have to adjust idle down.

[BURN-BROS]

Originally Posted by jwakil

I was looking at the various idle timing specs for the various porsche engines, and I noticed that some were spec-ed at 5 ATDC and some 5 BTDC, however all the ones spec-ed at ATDC said keep the vacuum retard hose connected when setting and those at BTDC said remove the hose. This leads me to believe that the vacuum retard probably retards by 10 degrees so that you go from 5 BTDC to 5 ATDC. So basically they are the same condition: (5 ATDC with hose = 5 BTDC without hose). My car is spec-ed at 5 ATDC (with hose), but I normally drive without the hose since I want to lower idle temps. Therefore, I believe I should I set at 5 BTDC without the hose? Is this correct?

The CIS cars have 15deg of distributor advance(30 deg crank). Depending on your induction the spec for a USA car with the vacuum disabled is 0 deg @ idle. If you are running carbs and compression is not too high, 3-5 deg @ idle.


You must verify total @ 6000 rpm.