Mopar performance advance dissy springs

[Talkwrench]

I have a set of mopar performance dissy springs that I am going to install, however I thought I would check and see what people think as I was going to install both springs? Do you install both? Below is what I have now in the way of timing [ no vacuum adv].

RPM                  Timing                   Timing variation                   Vacuum
800                   14                                                                17.72
1463                  23.5                     .7                                      19.85
2800                  33.5                     .8                                      20.54
3040                  35                        .8                                      19.85

Im just trying to get some nice early timing.
I will add some vacuum advance and any thoughts on manifold vacuum over ported?
Cheers!

[Chryco Psycho]

it is really trial & error , try both light springs but it may come in too fast & create pinging if so install one of the stiffer springs

[beekppr]

I am told a total advance of about 37-38 degrees is best but with your current curve, it seems to be all in a bit late to me. The lighter spring should do the trick but you will spend hours playing with different combos to get it just the way you like.

Out of curiosity, why do folks always hook the vacuum advance back on after getting the initial and mechanical advance just right?

My initial is around 15 with total of 38 at around 2500-2800 rpm. Seems to work well under all conditions with no pinging but if I hook up the vacuum advance, my total advance becomes 50 which cant be good. Plus it is a bear to get everything readjusted well and running halfway decent. I keep trying to use it but find it's better without for some reason.

By the way, those are really high vaccum readings. I'm jealous.

[UKcuda]

Out of curiosity, why do folks always hook the vacuum advance back on after getting the initial and mechanical advance just right?

My experience on small block Mopars is that it's really just an economy thing.  If the rest is set up alright then attaching the vacuum should not affect driveability but it will have a marginal effect on economy.  If it messes up the driveability then there is too much vacuum advance coming in and you need to back it off (which on most you can do with an allen key into the vac port).

On 351C Fords however a good street tune does seem to need the vacuum advance to drive right - not that you are interested in that.

[Marquis_Rex]

In my experience there is no reason NOT to have vacuum advance.
By NOT having it- you're simply throwing away efficiency- it can make between 25-35% difference in fuel economy.
Hot rodders and old school racers used to drop vacuum advance: the reasons they did this were a few
1) dropping vacuum advance minimises spark scatter at full load (WOT is what racers are primarily concerned with)
2) The vacuum numbers were very low- due to agressive camshafts
3) alot of racing engines had individual port throttles per cylinder very near to the port of the cylinder-in which case it was very hard to plumb in the vacuum advance

A modern engine in a performance state of tune with fully mappable ignition timing would NEVER carry a WOT advance curve at part load- it makes no sense.
Unfortunately alot of people who don't understand engines have run away with just not having vacuum advance on road engines- probably thinking that it makes the engine some how sporty.

On engines with an incredibly fast burn at full load (a pent roof chamber needing say 15-18 degrees BTDC all out)- there would be a real danger of burning out your exhaust valves if these figures were used at part load. As it is with our collossally slow burning Mopar wedges however- at least that danger is unlikely.