one92rs

iTrader: (5)

i have been reading and have done plenty myself. i always beleived that the vacuum advance was suppose to be hooked to ported vacuum. that is where i have always hooked mine in the past. but!!!!!!!! after messing with ls1's and 5.3's and 6.0's along with tuning them i have to agree with these post's. as i have read live tuning on many a computer controlled vehicles that the timing during idle and cruise is way up there. so i have hooked my vacuum advance to direct manifold vacuum and reset the idle and timing. i have to say that my off idle dead spot is gone and it runs much better. here are the articles. they are lengthy but very good info.

http://www.nationaltbucketalliance.com/tech_info/engine/Dvacuum/vacuum.asp

http://www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html

Damon

It depends on the engine combo. I've run engines with too much compression and too small a cam that love ported vacuum hookup (prevents dieseling at shut-down and light throttle detonation). I've also run lower compression motors that prefer full manifold vacuum (helps compensate for the lack of cylinder pressure at low RPMs- improves their idle and low RPM performance).

Current example- The mild blower motor in my Malibu likes full manifold vacuum becuase the engine is basically just an over-cammed low compression slug when not under boost. It loves the ~24* of advance it gets at idle (8* initial + 16* full manifold vacuum advance). Frankly, it could take more, but I keep the tuning rather conservative.

Above about 10% throttle either one will give you the same vacuum to the distributor. It's only at idle and very light throttle that there are differences. Above that they work the same.

Rayzor32

iTrader: (1)

It does depend on engine combo, but 90% of the time engines will run best on FULL manifold vacuum.

I've tested it and it's clear to me that FULL manifold is better, slightly faster/crisper throttle response, better off idle response. AND due to the fact that there is more advance at idle the idle mixture screws on the carb can be leaned and the idle screw lowered =better gas mileage and idle quality.

Ported was invented for smog reasons, the fire is being lit later resulting in less emissions and a hotter running engine.

five7kid

iTrader: (14)

It's more tuning approach than it is the engine itself. You need to tailor vacuum advance rate, idle fuel restrictors and air bleeds, transition slots, mechanical advance rate, etc., to all work together.

Full manifold vacuum was the practice until the late 60's/early 70's during the first attempts at emissions controls. NOx was the main driver. Funny they went back to more idle advance with computer controls (CC carb led EFI in that regard).

Fionnla

When I installed the Vortec 350 with the 650 Barry Grant speed demon VS. I was told to hook the Vacumn Advance on the Distributor to the timed Vacumn port and the Trans to the full vacumn port. With this setup I had to set the intial timing at about 16 degrees or I would get a bog at road speeds when I kicked it. A friend of mine, an old Hot Rod Mechanic told me to set the intial timing at factory spec's, 10 degrees with the line unhooked and switch the lines, full Vacumn port on Dist, timed port on trans. The motor runs better, no more bog & the trans seems to shift smoother. Retuned the carb at 850 rpm and a little leaner at idle in park and it runs better and smoother. Just some info for those having the same problem. Took it for a test run and no change in engine heat in 80 degrees weather and it Booga-Loos down broadway.