Re: Calculating compression ratio - have I got this right?
Posted: Thu Apr 25, 2019 8:17 am
Your method of timing up the engine seems OK to me. SU carbs are not my thing so I don't know if the vac system is in operation at idle. After many experiments on my engine I have found that it needs a lot of ignition advance at idle, from memory it is around 32 degrees. My vac system makes up about 16 degrees of the idle timing because it is in operation at idle, the other 16 required to make the 32 is obtained by the static timing. At idle my engine is running with 32 degrees but if I boot the throttle it will back off to 16, of course the mechanical advance will start adding some timing as the revs rise. My total advance at WOT is 32 degrees but only above 5500 RPM. My total advance when under high vacuum (cruise) is 48 degrees.
I guess if the noise that you hear can be made to come and go depending on the all in timing figures then that would indicate that the noise is probably pinking.
I guess if the noise that you hear can be made to come and go depending on the all in timing figures then that would indicate that the noise is probably pinking.