Originally Posted by SpyderD

My concern is that if you ARE using the manifold vacuum can to get your timing to 16 or 18 at idle then what happens when you do a hard throttle launch?

Originally Posted by bobs77vet

spyerd , We are talking about initial timing of 16* with the can disconnected....and when it gets connected the initial timing goes up to like 30-32*

Comp, final CR is 9.25: 1 and I use sunoco 93 octane

Originally Posted by SpyderD

I don't know about this, why have an initial of 30? ... But anyways the negative aspects still apply - you tune your carb (idle speed and mix) based on a timing of 30 and then when you launch and go to WOT you timing immediately retards to 16 and all the timing and mix settings go out the window...

Originally Posted by SpyderD

what happens when you do a hard throttle launch? Your vacuum goes down as you open the throttle suddenly and the timing is reduced probably at a greater rate then the centrifigul timing advance as you increase RPM. You are effectevely retarding your timing, if but just for a few seconds

Originally Posted by bobs77vet

.....i bet there is vacuum pulling in at the same time the RPMs are coming up and by the time WOT happens and the vacuum disappears you have past the 2500rpm point and the centrifugal advance is all in....so you have never gone back to 16*

Originally Posted by EDDIEJ82

Yes it does and this is a good thing.

But your forgetting about your mech advance which is still advancing. You don't want that much advance (mech and vacum) on a WOT launch because it would cause detonation.

Myself running 94 octane I have to pull 2 degrees out in the winter and 4 degrees out in the summer to keep from detonating because of my high compression but a regular motor will do the same thing with too much advance in the timing.

Originally Posted by SpyderD

- it may impact if launching FROM IDLE if you tuned your idle speed and mix and pump shot, etc, for that 30 degree timing at idle (and like me when you increase the timing you probably have to decrease the idle speed, lean out the mixture, etc, which is all a good thing except when it then drastically changes at launch) and it drops to 16 suddenly and your car then says "hey, I need more gas, I need more pump shot, I'm fuel starved...".

Again, this is all theoretical.

Originally Posted by SpyderD

I understand that, that happens weather it is manifold or ported vacuum. The difference is that at ported vacuum at WOT your timing is always increasing exponentially from idle but with manifold you have high timing then a sudden drop then and increase again. In those seconds before centrifigul takes over you are loosing power (again theoretically). I have 10.7 compression. I have a neat MSD gizmo on my dash that allows me to advance or retard timing 7 1/2 degrees in each direction. I'm always playing with it. Comes in real handy on hot late-winter days before the gas stations switch over to the summer gas blend. I just dial it back a few degrees. If I am at the strip and use race gas I dial it up a few degrees.

Originally Posted by bobs77vet

ok heres the result of a couple of hours of playing around....i never changed the distributor setting so....i think my base timing would be my lowest reading of 15*.... i think my max centrifugal advance would be my highest reading with vac can disconnected so that would be 25* (40*-15*)....so i have put in the Moroso silver (med) springs since they gave me 19* initial and 36* at 2500-3000, i will stretch my vacuum advance to 16*(right now its at about 12-13*) and will play around with ported vs.direct vacuum....


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Originally Posted by bobs77vet

ok did some more vacuum testing and this is my conclusion....the vacuum tube going from the PORTED carb outlet to the vacuum canister must have been just slightly too large of a diameter and i was losing vacuum at the port connection....hooked up a smaller diameter tube and got at least 6" of vacuum when i hit the accelerator....i'm running 15" of vacuum from the ported side if i hooked the vacuum can to that wouldn't I have the full pull right from the start just like the 9" reading at 1200rpm of 39*? wheres my 17yr old when i need some one yelling out RPM #s...need to talk to that boy about priorities....

edit: Ok just back from the test drive WOW WOW WOW....it was incredible how much of a difference that extra timing makes....this is what i discovered with just me testing it so its at a idle of about 1100 rpm...the vacuum cannister pulls at 5"/extra7*, 7" /14*

my springs are "silver,crv 600, medium tension for full advance at 2800rpm"

so i now have 16* base at 800 rpm, centrifugal advance of at least 12* and vacuum advance of at least 14* and maybe even 20* if it gets a 9" pull.... so 16 +12+14= 42* or 16+12+20=48*....need to see what my highest vacuum reading is at that ported connection when i'm driving around tomorrow. any thoughts???

Originally Posted by L82-JWS

what kind of dist do you have. My HEI is giving me 20* od mechanical advance. Can I reduce this?

Originally Posted by slvrbullit

Very interesting info in this thread. Have you figured out which setup you are happiest with?

Originally Posted by bobs77vet

mine is a GM HEI with a proform build kit.
yes you can but i don't know the mechanics of it....i'm not sure 20* is a problem though the trick is to get all the advance in at the lower RPM range of 2500-3000. so if you had 16 initial+ 20 centrifugal advance +16 vacuum advance i think you would be close to the goal everyone is striving for.. you may need to play with the springs to get it as you like.