Generic question: Looking to understand why 'hotter' camshaft requires more ignition advance.

[David Womby]

This is really a generic question rather than specific tuning of my car. I think I understand it but would love somebody to give me a simple confirmation or a correction, please.

My car is a 360. Compared to stock (8-10BTDC) , my camshaft ( a P4452761 ) needs much more initial ignition advance (23BTDC). Several people here told me to expect that and tuning the car has confirmed it.

I think it's because the intake valves are opening earlier and longer and so the spark is needed sooner than stock. Is that right?

Thanks

David

 

[R/T Boy 67]

The cyl. pressure is lower at low speed because of poor filling and a late closing intake valve. Iower pressure requires more time to burn completely, so the spark has to happen sooner.

 

[Geoff 2]

R/T boy has covered it well. The mixture is less dense as cam overlap increases. More distance between the molecules; flame front takes more time to spread; hence more timing [ advance ] needed to get the most productive burn. With really big cams, up to 50* at idle may be needed...

 

[David Womby]

Thank you, both, for that. It makes sense to me now.

David