I've been wondering for a while, should my dizzy's vac advance be on the ported vacuum source or on manifold vacuum??

Its an MSD pro billet and im running 10* advanced at idle should be 36* maxed around 3k i think.


IIRC it's supposed to be on the ported vacuum which has no vacuum at idle nor at WOT but has vacuum at part throttle

 

which ever floats your boat...

 

um well are there any wadvantages to using either way??

 

well i dont think they are really advantages... but with the ported you arent getting any vacuum at idle by design... therefore no vacuum advance... which is okay with manifold you are getting vacuum so you are getting that vac advance at idle.... which can also be ok it will still go away under WOT like ported... there really isnt a better one maybe on paper but try it both ways and use which ever you like best

 

well my question mostly roots around whether its more beneficial to have a ton of advance that bleeds off when when im cruising or having little advance the increase when cruising...

isnt there better MPG to be had by having alot of advance while cruising

 

eitherway the advance will be there when cruising. or atleast it should be there. the port is located right above the throttle blad so the moment you crack the throttle it now has vacuum. so it's more of does the car handle the extra vacuum advance at idle or not

 

You should have full vacuum at idle, some at part throttle, and none at WOT. That should give you better mileage, and also better throttle response. But if your engine is heavily modified (above 400HP), you won't really see any benefit from the vacuum advance setup.

 

Try it both ways and see what it likes. Different combos like different things. Just remember that from about 5-10% throttle on up, they both work the same.

Some (big) cams don't pull enough vacuum to bring the vacuum advance "all in" at idle. That leaves the timing to sort of "float around" at idle- and that will definitely cause an unstable idle. That's one situation where I always go with a ported vacuum source. Also, some engines with a lot of cylinder pressure (small cam, high static compression ratio) can have light-throttle detonation problems and problems with "run-on/dieseling" at shut-down if hooked up to a full manifold vacuum source.

 

If you've got the MSD vacuum can you can set the amount of vacuum it takes to pull all the timing in. That might help get around that first issue damon mentioned.

I don't think you have enough duration on your cam to need more timing at idle (ie manifold vacuum advance), so you're probably better off with ported. But give each a shot, it's quite easy to change. Listen for detonation. Hopefully you have one of the vacuum advance limiting tabs so you can choose how much timing the vacuum can gives too.

 

lol i miss my lunati cam... even though it scrapped within a month...

It was way more aggresive... really lopey but no PBrakes.... doh..

this cam is a torque monster but no lope... not at all..

i'll see which she likes more... runs fine now so prolly run fine the other way too....

i was playing with the idle mixture screws today adn screwed my idle up ( too fast ). Lean best mixture method used, but it was idle too low b4 so i put it up It started raining so i couldnt set the idle down.