im having trouble with my curve for vacuum advance. i know what i want in In Hg vacuum, but the measurements on the graph are in PSI. i don't know what values i should put down. the stock graph went from top left to bottom right, the way i want it in vacuum, it goes from bottom left to top right... what's going on?

 

find a kPa to PSI conversion calculator online and start from there

 

well i figured that since 1 in hg is 0.4912 PSI i would do that, but i want to be sure since my curve looks exactly opposite of the factory one. why would they have 20 degrees advance at 0 psi/in hg?

 

typical conservative tuning for F-Body's

 

20 degrees vacuum advance at wide open throttle? vacuum advance should be zero at wot.

 

0 PSI is 30 inches of vacuum. High vac is what you see at idle/low load cruise.

With the engine off the MAP sensor should read atmospheric pressure, roughly 15 psi. Motor heads call that 0 inches of vacuum. Run the engine and pressure in the manifold drops. A vacuum gauge measures the difference between manifold pressure and atmospheric pressure. As vacuum goes up, MAP pressure goes down.

Got it?

I use to have a link to a table saying stuff like 12 inches vac = 6 PSI absolute, but can’t find it anymore.

MSD should have an option in their software to do an approximate conversion.

 

that's the info i needed. as a carbureted, non computer guy i've never had to deal with PSI. thank you.

 

I've begged MSD to re-version their Graphview to allow multiple scales for vacuum (PSIA & vacuum) along with altitude correction. Still no answer... Maybe it's time to "gang up" on them!

 

http://www.carcraft.com/howto/ccrp_0...arts_cost.html

page 2 has the conversion info. Hope this helps

CONVERSION TABLE
Pressure in psi absolute Inches of mercury
5 10.2
10 20.3
15 30.6


Ignition Curve Basics
Basic ignition timing can be a bit confusing, so let's dig into the meat of all this to make it less intimidating. We'll start with initial timing. This is indicated on a timing light on the harmonic balancer with the engine at idle. For most performance engines, initial timing is around 10 degrees before top dead center (BTDC). Next, we'll address mechanical advance. Most mechanical-advance distributors can generate around 25 to 26 degrees of total mechanical advance. This is created by a pin moving in a slot in the advance plate of the distributor controlled by a combination of weights and springs. When you add an initial-timing figure of 10 degrees with 26 degrees of mechanical advance, you get a total advance of 36 degrees.

 

i dont need to know what initial, vacuum advance and mechanical timing are. my questions have been answered, thank you.

 

anybody else try locking the timing? i have my timing set at 6 degrees at 500 rpm and 32 up after that, car runs well this way.

 

that's how mine is. that's how many people set the curve up on a mechanical distributor as well, but they don't have the luxury of the real low initial timing setting unless they have a start retard.

 

Is it just me or is CC's table backwards? It's certainly backwards from what I have my MAP set for...

 

the graph you showed is how the map vacuum advance is supposed to be set. supposed to start on the left high, and as map increases (vacuum decreases) the timing should be pulled. like this:


http://www.carcraft.com/howto/ccrp_0.../photo_07.html

 

Sorry, I still think CC has their table backwards. Look here at this chart (it's been posted here before) from a respectable outfit. It shows 14.7 PSIA as being 0" vacuum and 0 PSIA as being 29.92" vacuum. That's how I'm setup and it's working as expected.

 

gonna have to do this soon myself and honestly dont have a clue....i'd like to see several map curves posted with combos...(maybe i'm dreaming! lol)

 

that is correct

 

dude look at the picture i send you again. look at the TOP graph, not the bottom. the bottom graph is rpm, the top is MAP. to the left of the graph is 0, to the right is 15. it starts off high and goes lower as MAP increases...

 

Heart, my eyes aren't good enough see the charts in that lead pic. If you click on the 5th picture listed, it will blow up to this diagram, which shows a back-assward MAP curve. Just trying to keep someone from trashing their parts...

 

o yeah that one there in the pic u showed is totally wrong. thats how i had mine setup before. thankfully it was just idling under no load.