Hello:
I read this recently from AJ on an old post (this not a quote): I have seen many folks advancing timing on larger-cammed engines to improve vacuum with the result being the transfer ports are dead.
And that's my question. What is the balance between timing advance and proper transfer slot positioning?
I was fussing with my car the other day, got 2-3" more vacuum at idle with an initial timing going from 10ºBTDC to 18º...but also was a good bit over a reasonable idle RPM. I had the primary side shut down on its adjustment and had started turning down the secondary plates. I'm not done with the fussing quite yet, but it looks like I'll have to back the advance off a good bit to get a reasonable idle...say 850rpm.
A reasonable question: What am I trying to accomplish? I'm just looking for a better tune. I've run 10ºBTDC with a mechanical total of 32º for a long time. The car idles fine (no dithering/hang-ups), off-idle response is excellent. There is ZERO bog/hesitation. What got me started on changing the setup is to get some heat OUT of the exhaust. That relates to hot soak problems, and I DO have that. The change from 10-18ºBTDC certainly cleaned up the smell of the exhaust and (maybe a placebo effect) it seemed the engine bay cooled down a LOT.
My headers are coated inside and out. A btw.
...I should have taken some IR gun header readings. Didn't think of it at the time.
So: Timing the transfer slots comes first, and then get as much 'better' running from initial timing advance as you can get second?