del91_305

I am just curious as to how much advance a stock HEI distributor will allow. Basically what im asking is, the vacuum advance on the dist, how many degrees does it advance the timing. At an idle my engine makes about 13 inch vacuum. Just wondering becuase it will help me time it a little better.

sixpackmtrspts

vacuum advance is only there to help with fuel economy, its not a performance deal, since when you go wide open throttle you lose vacuum and the vacuum advance actually retards. when you set your timing just unplug the vacuum line and plug it, set your timing and then plug it back in.
the amount of advance is typically 8 degrees or so

del91_305

So it has nothing to do with performance? Then how come people talk about things like total timing to help performance? Total timing being base timing and the advance form the vacuum. not trying to sound like a smart *** or anything. My engine backfires throught the carb if i set it at 8 degrees

Sonix

iTrader: (1)

I'd have to completely disagree with sixpack.

It's a performance thing as you can get a decent idle in a high overlap cam with a good amount of vacuum advance.
I also think 8* is really low, try closer to 22*.

Let me dig up a file written by "Lars" he's got all you need to know...

Sonix

iTrader: (1)

Wait, you have TBI? And a vacuum advance dist?

Maybe i'm on the wrong page here....


Anyway, here is everything I know about non-cc style HEI timing, and how it works, and what you can do with it....(also check out Damons article on the main page of TGO, non-cc HEI timing)

Folder

I got these files from
http://corvettefaq.com/engine.asp
if you are curious.

del91_305

No, it was TBI but i swapped to Carb along with heads cam and intake swap.

Well, i advanced the timing to a base of about 18-20 and it is doing a whole lot better, no missing or hesitation, and runs really good. Think i should advance it some more?

Sonix.. Thanks for the link, i will check it out.

Sonix

iTrader: (1)

oh, right.... ok, put carb in your info then, that confused me..


Base, do you mean with vacuum unplugged right?

Well, before everyone jumps in here, it depends on the individual car, and person, so set your timing for what you like best, and what makes the car run the best...

Yea, check out those links, decide on using ported or manifold vacuum, and see what 'idle' timing works best, and keep in mind ~36* degrees at WOT will probably give best power, and ~52* at cruise will give best MPG....
Good luck

oh yea, and notice in the article on the main TGO site by damon, he mentions the crane kit (pn 996001 or something...), I would also second that recommendation.

8Mike9

If you disagree with sixpack, you'd be wrong...he pretty much said it "how it is"

I/e

Base timing + mechanical at XXXX RPMs is "ALL IN" at XXXX RPMs.

Vacuum advance is only there (should be anyways) at part throttle/cruise...will go awaty (should anyways) when yuu put your foot to the floor (reason stated above...no vacuum+no vacuum advance)

Economy comes in at part throttle/no/low load...where you have higher/highest vacuum, and more advance is pulled...hmmn, we'll call it vacuum advance

Apeiron

While vacuum advance isn't really a WOT performance necessity, it's a desirable thing on a street car to avoid fouled plugs and miserable part-throttle performance.

del91_305

So vacuum advance plays no part in WOT. Then as mentioned before 36 degrees is desirable at WOT, how do i acheive that?

Damon

You unplug the vacuum advance, rev the motor up high enough that the centrifugal advance is "all in" and won't advance any higher. At that point you make sure your timing is at 36* BTDC. Let it back down, plug in the vacuum advance line again and you're all set. Vacuum advance is IN ADDITION TO the initial + centrifugal advance and, as was correctly pointed out above, only comes into play at part throttle- it won't supply any advance at WOT when manifold vacuum is close to zero.

On a performance carbureted application that has had the initial and centrifugal dialed in for best WOT performance (36*, all in by about 3000 RPMs or less) you don't need much vacuum advance- 12* is plenty. Many stock vacuum advance cans give way WAY too much vacuum advance- 24* or more!

There's an article I wrote in the tech articles section of this board that talks about tuning the HEI distributor for performance. If you've got an HEI, follow the article. If you don't then read what it says about the advance curve and try to mimic that in your distributor.

del91_305

Ok, im gonna have to get me a timing light with a dial on the back

Apeiron

Just mark your balancer instead.

del91_305

What do you mean? I was able to borrow a gun with a dial from a friend but i still want to know what you mean. I check it and i actually had it set way to far, at like 25 degrees base. So i set it at 18 degrees then at WOT its right at 36, so i gonna see how it does now

Sonix

iTrader: (1)

What apeiron meant, was that instead of getting a dial back timing light, then dialing it to, say, 36*, you could just buy a $5 timing tape, put it on your balancer, then you can tell past 12* or whatever the factory tab shows...(using a cheaper standard timing light)
I did this, it works fine, but eventually the tape comes off... use epoxy or something...

Apeiron

Or you can save $5 on a timing tape and just scratch a mark in the balancer.

del91_305

I see, ill give it a try. Ok now next question, after reseting the timing, do i have to re-adjust the carb? Im thinking i do, im starting to get a hang of the carburator stuff. It seems like its running rich now

Apeiron

Set the advance curve up first and then leave it alone while you tune the carb.