HEI ignition secrets

[1969c10joe]

i should be able to do this at idle im curious what kind of adjustment can be made .Seen the thread what did i do to my truck today? I FLOORED IT

[1969c10joe]

I just cant help it thanks do you mind if i call you richard?

[Big Kev-O]

If you would like some good info on timing read the link below. I would never run vacuum advance off the ported connection. The reason GM and Summit state to use ported is for legality/SMOG green tree hugger reasons.



http://www.jalopyjournal.com/forum/t...ot-time.54221/

[VetteVet]

http://www.superchevy.com/how-to/97438/


This website gives a lot of great information about the subject of this thread. Here is a quoted part of it by me. Everyone should read it



Where should the vacuum canister be routed?
There’s been a lot of debate whether or not the vacuum canister should be plugged into a ported or direct vacuum source. Internet forums are rife with opinions on both sides of the argument. However, there is a right and wrong way. And it’s not an opinion; it’s just a fact.
Plugging your vacuum advance into a direct source will allow it to engage at idle, which is good for a number of reasons. Much like cruise conditions, engines run leaner at idle than they do under load. Again, this means the mixture burns slower and needs an earlier spark to optimize the burn. Ensuring that the mixture has a complete burn before leaving through the exhaust port also helps the engine to run cooler at idle. All carbureted cars were set up with direct vacuum to the distributor before more stringent emissions requirements reared their heads.
Ported vacuum sources are a result of emissions laws and manufacturers doing whatever they could to get big V8 engines to pass smog before the incorporation of the catalytic converter. The idea was that by using little to no spark advance at idle, the exhaust gas would leave the cylinder still-on-fire and help maximize the efficiency of antiquated air injection systems. Engines from this era often ran very, very hot, were prone to warped exhaust valves, cracked cylinder heads and all other manner of issues. Using a ported spark advance will still allow the vacuum advance to do its job at steady cruising, but all of the benefits of idle cooling will be lost.
Here’s a quick experiment you can try on your car. Plug the vacuum advance into a ported vacuum source and check the idle rpm. Now, switch the vacuum advance to a direct source of vacuum and again check the idle rpm? We’ll bet money the RPM increased. Why? Because the additional ignition timing provided by the vacuum canister and full manifold vacuum source allowed the engine to more effectively burn the air/fuel mixture. It therefore produces more power (even at idle) and rpm rises as a result.

Maybe us Old Geezers know what we're talking about.

[RichardJ]

>>The reason GM and Summit state to use ported is for legality/SMOG green tree hugger reasons.<<

That simply is not a true statement. GM used ported vacuum long before they started with the serious smog systems.
PCV System was the first smog system used on automobiles and it was used to replace road draft tubes.

Early 50s 235 six with Rochester B. The two metal lines to the carb are fuel and vacuum advance line connected to ported vacuum next to mixture screw.



Here is a mid '50s 283 with 2GC and again ported vacuum.



From your link in jalopyjournal.
>>The number one main reason you want full time vacuum is that you need to run a considerable amount of advance at idle.<<

need to run a considerable amount of advance at idle ? What need?

>>Running the engine at idle without the extra advance provided by the vacuum advance system will have the engine running hot in almost all cases.<<

I guess lack of air flow through the radiator at idle has nothing to do if whether an engine heats up or not at idle?

>>engine needs quite a bit of advance at idle since the fuel/air mixture is lean and it takes longer to burn the mix at a low engine rpm<<

If I was worried(?) about a lean mixture taking longer to burn at idle, I'd just turn the mixture screws out a little.

He goes on to mention that manifold vacuum provides Vacuum Retard, but only in reference to engine cranking conditions. He needs to take a closer look at cranking rpm, low vacuum during cranking and the simple fact that ported vacuum would provide 0"Hg and 0* advance while cranking.

[RichardJ]

>>All carbureted cars were set up with direct vacuum to the distributor before more stringent emissions requirements reared their heads.<<

Again the bologna about vacuum source before emission systems.

In the '70s, manifold and ported vacuum were often both used on the same engine and routed through various temperature and electrical controlled devises to determine the best timing to make the engine run like crap.

Mixture and burn time at idle sounds more like an emissions issue.
I guess 50s and 60s engines were never allowed to idle for very long if the had ported vacuum.

[1969c10joe]

Thankyou i didnt mean to start a debate about manifold vs ported vacuum kind of like tastes great less filling. I understand the difference. got to go and make some money ill be back

[geezer#99]

Originally Posted by RichardJ

>>All carbureted cars were set up with direct vacuum to the distributor before more stringent emissions requirements reared their heads.<<

Again the bologna about vacuum source before emission systems.

In the '70s, manifold and ported vacuum were often both used on the same engine and routed through various temperature and electrical controlled devises to determine the best timing to make the engine run like crap.

Mixture and burn time at idle sounds more like an emissions issue.
I guess 50s and 60s engines were never allowed to idle for very long if the had ported vacuum.

What makes you think that just because the vac ports on both the 235 and 283 are up near the top of the carb above the throttle plate that they have to be a ported vac source.
They are manifold vacuum!

[davepl]

My understanding of the matter is that the vacuum advance was moved to ported in order to retard timing and increase heat at idle; this serves to ensure a more complete burn and also assists cars with A.I.R. pumps, as a hot charge makes it easier to finish burning "unburned hydrocarbons" in the exhaust.

I've long been an advocate of setting it up the way the factory did, but the question is whether or not there's a downside to running manifold vacuum on a car designed to be set up ported. I don't have an AIR pump.

Is the mechanical advance curve from an early 60s (manifold) different from that in the early 70s (ported)?

[1969c10joe]

1I wont be adjusting weights or springs anytime soon2My immediate goal is to fine tune adjustable vac advance 3ive tried both ways it is connected to manifold at the moment starts right up idles vac gauge needle looks glued in place. but that could change cuz im always learning. badass engines has an excellent explanation of ignition timing very concise most of you guys already know that stuff

[1969c10joe]

so i will find out when my my adjustable vac starts to kick in i will watch my vac gauge when driving ill add up the advance numbers and report back to you i will test and record manifold and ported results wish i could drop everything and do.it.now

[1969c10joe]

thankyou Mr Geezer for your input due to the fire and ash in the air my eyes are burning(unless that kid turner is idling nearby) i have to postpone my test.

[1969c10joe]

I believe my test will be thorough and accurate and should be very interesting to all members maybe even surprising to some I will test and record results from manifold and ported vacuum.judging from the interest i better do a good job

[1969c10joe]

Mr. Geezer back where i was raised addressing a man with Mr is a sign of respect. Thankyou for your valuable experience and input

[1969c10joe]

and to vette vet thankyou for taking the time to post

[Mike_The_Grad]

Any chance that you were able to resolve your issue?

[demian5]

Originally Posted by 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

Lock out (disconnect) the vacuum advance and time it to 38 degrees total advance at about 2500 rpm (or more, just watch for when it stops advancing), then adjust down or up from there.

[crabman]

I would like to share my experience with timing my 69 C10 396 TH400 bone stock with HEI and QJet. I read several threads and Googled the topic and did some experimenting of my own. My truck was set at 10* initial and 32* total with 20* of vacuum advice thru ported vacuum. The truck ran fine no complaints. I decided to change to manifold vacuum for the vacuum advance and immediately the truck idled smoother. I drive the truck and it was better down low and transitioning to midrange. I brought it back and bumped my initial timing to 12* for a total of 34* timing and left the vacuum advance at 20* and manifold vacuum. After all of that i had to reset the idle speed to 750RPM and the QJet was rich so I adjusted the mixture screws until I got it to idle at 750 RPM and smooth. The timing with the vacuum advance hooked up and idling at 750 is 32*. The truck is much smoother low and mid range and it starts super quick. I am a believer in MANIFOLD vacuum for vacuum advance after this experiment. After driving like this for a while I will try more initial timing and see what it does but for now it is a nice improvement.

Edit: All my timing is in by 2800 RPM

[mike16]

I always used the vacuum advance from a 75 corvette