HEI ignition secrets

[1969c10joe]

I have a goodwrench 350 with accel distributor and adjustable vacuum advance edelbrock 1405.I would like your ideas on custom ignition curve right now im running conservative 12 static 20 distributor 16 vac my starter doesnt like 13. Ihave almost 20 in. hg people doubt me when i say that.This my daily driver work truck

[geezer#99]

So what's your timing curve like now?
Timing at 300 rpm increments from 900 rpm to when it's all in is needed.
Then we can make suggestions.

Post a pic of your truck idling with the vac gauge hooked up and with a tach and everybody will believe 20" of vacuum.

[1969c10joe]

You are doubting my honesty Im wondering if the gauge is off.Why would I lie about vacuum? like thats going to impress you guys? besides everybody knows that pic wouldnt prove anything unless tach and rpm was in pic.

[1969c10joe]

i meant in a video

[1969c10joe]

is there a way to calibrate my budget dial back timing light?

[tdangle]

You can get an idea if the timing light is close by dialing it back to 0 look at your timing (say 12 BTDC) then dial in 12 degrees in your timing light and look at the timing again. It should read 0 on the timing index plate

[1969c10joe]

thankyou for your reply yes i did that once in a while i have a good idea what about at 3k rpm the light doesnt hold steady kind of dances around feel free to assume i was born yesterday...ha ha never owned adjustable vac advance before

[geezer#99]

Originally Posted by 1969c10joe

You are doubting my honesty Im wondering if the gauge is off.Why would I lie about vacuum? like thats going to impress you guys? besides everybody knows that pic wouldnt prove anything unless tach and rpm was in pic.

I don't care what your idle vac is!!
What is your vacuum at wide open throttle?

What's your idle rpm?

[1969c10joe]

for right now any info on adjustable vac advance is helpful is my battery weak if it has trouble turning over at 13 initial?can vacuum advance increase timing while engine is cranking if its hooked up to manifold vacuum I wouldnt think so but......even one degree more matters over the years ive noticed there is a two degree window sweet spot ill post my timing numbers geezer sorry cant do it today though im gonna have to retest and write it alldown

[davepl]

Geezer won't like me saying it, but your vacuum advance should be connected to ported vacuum (above the throttle blades), not manifold vacuum. And when properly connected like that, it will not cause any advance during cranking.

If your truck runs "good" at 12 initial, keep it that way, but you might need to have your distributor recurved to lock out some centrifugal, or you could wind up with too much total advance.

If you do run manifold vacuum to the vacuum advance it'll advance at idle (and during cranking a little too) and you'd run less initial advance. I would postulate that 12 initial PLUS vacuum would be way too much.

[1969c10joe]

Originally Posted by geezer#99

I don't care what your idle vac is!!
What is your vacuum at wide open throttle?

What's your idle rpm?

please give me a list of the info you guys need and I will post it.Dont know what vacuum is when throttle is open as i watch where im going when i floor it.

[1969c10joe]

davepl you have a knack for explaining things thankyou for your help. I really appreciate the education im getting I wish i could trust my vacuum gauge and centech dial back timing light

[1969c10joe]

i wont be able to check timing numbers today but i will retest write everything down should i question my vacuum gauge ?is 20 in hg really that unusual?

[1969c10joe]

having trouble with internet connection guess ill go out and floor it and check vacuum gauge.If i get pulled over Ill tell the cop GEEZER told me to im sure ill get off with a warning. thanks guys

[geezer#99]

Originally Posted by 1969c10joe

having trouble with internet connection guess ill go out and floor it and check vacuum gauge.If i get pulled over Ill tell the cop GEEZER told me to im sure ill get off with a warning. thanks guys

I asked that question to have a base of your knowledge!
Obviously it's limited!
Appears all you know how to do is type multiple posts and answer no questions.
Thanks for the entertainment!!

[1969c10joe]

I using a phone with a tiny screen hence short posts if you give me a list of info you need i will do my best to provide ACCURATE info i am concerned about the accuracy of my diagnostic instruments I dont remember last time I floored it is my work truck doubt if i ever exceeded 4k rpm Im glad im entertaining. YES i have limited knowledge compared to people like dave pl but i wont joke with you anymore and im sorry i offended you thanks for giving me a reason to floor it now thats entertainment

[tdangle]

your idle vacuum as long as it is steady and depending on the cam installed somewhere between 15 and 20 inches is good. Don't worry about it. You will not see your lowest vacuum reading flooring it with no load. The higher the volumetric efficiency of the engine the closer you will come to 0 inches of vacuum. Most street engines are in the area of 70 to 75% VE. In a street engine I doubt you will see lower than 3 inches.

Vacuum advance for your dist will only come into play when the engine is running. at engine cranking rpms I doubt you will get any advance so that wont cause hard starting The vacuum advance comes in for cruising at light to moderate throttle allowing the engine to run more efficiently and get better gas mileage (I'm not sure what the optimal amount of vac advance is)

A good mechanical advance for a SB chevy street engine is somewhere about 32 to 36 degrees all in by 3000 rpm. So 12 initial and 20-24 mechanical advance should give you good performance. Any more advance or a quicker advance could lead to detonation which will kill power (and could hurt your engine)

Good Luck with your project.

[1969c10joe]

that makes sense i had vac advance plugged in manifold and played with idle speed timing didnt change. I holed a piston because sae mechanic set my timing never again.Of course if i had pulled over for that ford on that long grade outside of bakersfield....we were both pulling trailers and he insulted my truck im going deaf didnt hear knocking BUT I LEFT THAT DIESEL IN THE DUST.very slight satisfaction in beating him one of the dumber things ive done

[davepl]

Originally Posted by tdangle

Vacuum advance for your dist will only come into play when the engine is running.

I've never looked at the gauge to be honest, but if a motor can pull 21" at an idle of 500, gotta think it can pull a few inches of advance while cranking at 300rpm (or whatever they crank at).

I don't know if it's a material amount of advance or not, I'm kind of picking at nits here, but figured I'd mention it...

[1969c10joe]

thankyou for info and thankyou for assuming im smart enough to understand what volumetric efficiency is havnt heard that in a long time does ve peak at torque peak?

[1969c10joe]

thanks dave nitpicking will save my pistons from disaster

[1969c10joe]

for the record my engine is stock and has the no lift cam i have 500 pounds of tools in bed and pull a trailer that weighs about 1200 pounds

[1969c10joe]

Originally Posted by 1969c10joe

for the record my engine is stock and has the no lift cam i have 500 pounds of tools in bed and pull a trailer that weighs about 1200 pounds

i am more concerned about mpg and reliability

[RichardJ]

The question was ask if there is manifold vacuum during cranking.
Vacuum for a carbureted engine is pressure lower than Atmospheric pressure. In order for fuel to flow from the bowl, through the jets and into the venture, the manifold pressure must be lower than Atmospheric.

Most vacuum advance units don't begin to move until vacuum reaches 6 or 7 "Hg., at which point it should only add a couple of degrees if connected to manifold , so for 200 - 250 rpm cranking vacuum should increase timing very little

Sorry Geezer, but Vacuum at WOT isn't going to tell you a damn thing. Is that WOT in neutral, in gear @ 20 mph, @ 80 mph? Useless information.

Accel and Moroso Adjustable Vacuum Advance units say to connect to manifold vacuum, but I have read that Summit suggest that you connect theirsto ported vacuum. Don't know who makes theirs.

Manifold or Ported?
Lets say you set your base timing at 10* and connected to manifold vacuum, your 20 "Hg adds 15* for a total of 25* timing at idle.
You should all agree (most will) that when you open the throttle the manifold vacuum will drop down close to 2-3 "Hg.

At that instant, the 15* vacuum advance disappears.

Mechanical advance often doesn't start until about 800 rpm(unless you have made adjustments. At 1000rpm mechanical may have added 1-2*

Lets review.
If idle is 600 it has 25* advance
At 700 it has 10 * advance
At 800 it has 10* advance
At 1000 it may have 12* advance
At 1200 it may have 14* plus maybe a couple of degrees from the vacuum advance.
Vacuum advance won't start adding in until vacuum reaches 6-7"Hg and at that only 1-2 * advance. This is a factor of where your right foot is.

Manifold vacuum turns a vacuum advance unit into a off-idle vacuum retard unit.
It becomes nothing more than a Band-Aid to stop off-idle pinging.

[1969c10joe]

thankyou for your reply i have read each thoughtful reply several times the problem is obvious my brain is connected to manifold vacuum. You gave me an idea going to use vacuum pump and gauge find out how fast adjustable vac adv comes in