Josh R

iTrader: (1)

I have an MSD streetfire HEI distributor and it has a tunable vacuum advance. There is a diagram of what the timing is at what vacuum but i really dont understand how it works. I thought you want more advance when flooring it and at higher rpms. Initial is supposed to be TDC.

Heres the diagram. the"Turns out" just means the adjustment screw in the vacuum solenoid thinger.

Atilla the Fun

Centrifugal advance normally starts out at like 6-14 degrees initial, building to like 30-38 derees by 3000 rpm, depending on cam and heads and other things. Vacuum advance adds to that for idle, and cruising for mpg. If it wants you to set initial at zero, then it is going to add the 6-14.
A good starting point for something like an L69 would be about 26 degrees at idle, maybe a little more. You first need to find out how much initial it's adding to your zero, then turn the screw until your dial-back timing light is showing you at least 26.
The vacuum advance goes away at full throttle, so with an older HEI on an L69, (since I don't know your engine combo), we could set initial at 6, then add 20 degrees of vacuum advance.

Josh R

iTrader: (1)

wow wwait what??? I have an LG4 but L69 close enough.

SO your saying that your timing goes closer to TDC when you floor it??? I thought you want an advanced timing for performance? And at idle i thought it should be TDC and then when you floor it it changes to the 26 or so degrees BTDC (advanced).

Sorry to be bothersome. i just don't want to eff anything up haha. Thanks in advanced for your help... haha.

Edit:

OHHH wait i think i understand now. So vaccum advance should be adjusted based on initial advance WHICH should be based on how much overall advancing you want with the Centrifigral Advancing... OOkk. Please correct me if im wrong. So lets say at 5k rpm it makes 35 degrees advanced mechanically on top of the intial (lets say) 5 degrees. So floored at redline i will be 40 degrees advanced. And the idle will require the vaccum advancing in this case about 20 degrees BTDC overalling in a 25 degree advanced idle timing.

Does that sound like i figured it out??? Those numbers are just guestimates. I have to look up how much advancing the distributor makes at certain rpms.

Atilla the Fun

now you understand the goal, but exact settings will depend on the rest of your combo. A stock LG4 will like about 36 degrees total advance at full throttle, all in by 3000 rpm. You might want more like 28+ degrees if initial with the vacuum.

Atilla the Fun

oops, I mis-typed. that should read OF initial, not if initial. sorry.

Josh R

iTrader: (1)

soo why do they say our initial timing should be at 0 (TDC)???

Atilla the Fun

GM sometimes specifies zero initial because the computer adds the initial./ IF the MSD is also specifying zero, then they put the same thing in the distributor you bought, which seems pretty likely if you bought that E-Curve version. If you got the cheap $160 version, then you'll need to forget zero, and set the initial yourself to about 8-10 degrees to start with, then tune from there.

Josh R

iTrader: (1)

its the 170 dollar one so no i dont think theres any E-Curve thinger.

OK so heres my next question! How do you know how many degrees advanced you are. All i see is a 0 (TDC) on the timing mark. There are a few pointers next to it but how much is each degree?

Atilla the Fun

Now either post a pic of it, or measure across your damper to find the diameter then buy a timing tape to match and stick it on there, or use a dial-back-to-zero timing light.

Josh R

iTrader: (1)

ill get a pick hold on dont sign off!

Josh R

iTrader: (1)

The first pic is the timing mark and the second is the first one with it outlined so its easier to see. Thanks.

Atilla the Fun

peak to peak, on the little teeth, is 2 degres. peak to valley is 1 degree

Atilla the Fun

unless that's a 6.75" damper with a GM tab.

Atilla the Fun

The GM tabs have bigger teeth, so are 4 degrees between the little peaks.

Josh R

iTrader: (1)

no it should be the 8" stock chevy one or whatnot. So if i tune it for 10degrees BTDC it will be off the timing mark by a good 1/2 inch???

Atilla the Fun

We don't tune by the numbers, we tune by the results. The numbers are just a tool to help us be sure we're in the range we should be. Set your initial, with the vacuum advance disconnected, with the mark on the damper still somewhere under the tab, and tune from there.

Josh R

iTrader: (1)

ok yeah i got the advanced disconnected. Its about 3 degrees advanced. Im gonna crank it a little farther. Then tune the vaccum advanced. Then tune the carb for lean-best idle and theenn the idle. Ill let you know how it goes. Thanks again.

Josh R

iTrader: (1)

hey atilla the fun, not sure if your gonna get this thread alert or not but i did get the 305 running good.

Then i got sick of it and put a 350 in my car.

So now i was wondering if you could give me some base timing settings that my 350 would like. Im sure its similar to the 305.

Its a 350 60 over with about an 8.5 - 9:1 compression ratio
It has a Comp Cams DE 275 cam with peaks at about 6000rpm
It has ported 882 heads
Edelbrock Performer Intake
Edelbrock Performer 600cfm carb
Exhaust and what not

It probably makes peak power around 52-5500 rpms. Rev limiters set for 5850.About.

What do you think some timing settings should be?

Not sure if u can help me here but i just need an estimate.

Thanks!

Atilla the Fun

Try 14 degrees initial, and 38 degrees total.