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-   -   ignition timing (http://www.hotrodders.com/forum/ignition-timing-21887.html)

bad502 08-25-2003 05:05 AM

ignition timing
 
help,
I have been playing the setting the timing on my 502 chevy,
I set the timing at 15 degrees with the vac adv. off this was
based off the chilton book for ref. after I hooked the vac adv.
againe the engine idle raised and then I lowered the idle to 1000 rpms. I have been reading other postings and I keep hearing about full adv. is at 35 degrees. Could someone explane and
did I set the timing the correct way?

Frisco 08-25-2003 07:13 AM

Yes you set your timing the correct way. Total timing is your initial timing + whatever the preset mechanical timing is in the distributor. The vacuum advance is NOT added to these figures.

In your case if you had a built in mechanical advance of 20 degrees and you added that to your initial of 15 degrees (with the vacuum advance hose disconnected and plugged) you would have a total of 35 degrees.

The mechanical advance weights combined with their return springs will determine WHEN the mechanical advance begins to come into play and at what RPM the weights are fully extended. You should have the full mechanical advance in somewhere between 2500 and 3000 RPM for most applications.

bad502 08-25-2003 08:30 AM

thanks for the info,

I will check to see what the mechanical advance will be.

dmorris1200 08-25-2003 09:45 AM

Vacuum advance is calculated into your total advance figures. When you accelerate under mild load your engine will see mechanical+vacuum advance. This has been discussed in older threads and links were actually provided to websites that dealt with this issue.


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Frisco 08-25-2003 10:31 AM

Quote:

Originally posted by dmorris1200
Vacuum advance is calculated into your total advance figures. When you accelerate under mild load your engine will see mechanical+vacuum advance. This has been discussed in older threads and links were actually provided to websites that dealt with this issue.


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While I am aware of what you are referring to, since there is NO vacuum under WOT I respectfully disagree. Also if the vacuum advance was needed to set your timing the manufacturers would tell you to leave it connected when setting the timing. To my knowledge no manufacturer even suggests setting the timing with the vacuum advance connected. They all state that you should dis-connect and plug it when setting the timing. The same is true with where the vacuum advance hose should be connected (ported or full vacuum). I use full vacuum and that works best for me. Someone else may find that the ported connection works best for them. Peace!

dmorris1200 08-25-2003 10:58 AM

The reason you disconnect your vacuum to set timing is because you're setting BASE timing. Base timing means timing with no mechanical or vacuum advance. Also even under WOT as your engine rpms increase so does your manifold vacuum. That is why your accelerator pump only needs to give you a short momentary boost of gas, then as your engine builds up rpms fuel is drawn through the jets because of the engines intakevacuum. If you have a vacuum advance can set to provide advance at a low vacuum signal you will see this advance even under heavy acceleration. Maybe you should read this link HERE were it tells you to add your vacuum advance to your mechanical advance to see your total ignition advance. At 40mph when you step on the gas to pass someone your engine is seeing your distributors mechanical+vacuum advance. Too much of this combined advance will cause that annoying little 'ping,ping,ping'.

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78 monte 08-25-2003 11:12 AM

Crank the distributor around,take it out and step on it,keep advancing the distributor till it just starts pinging a little,then back timing down till it just stops pinging,now lock it down and drive it.
If it starts hard when warm after this then maybe play with advance curve kit.So you can lower your initial timing for hot starts.
But I've never had to recurve a distributor and I allways max my total timing.

bad502 08-25-2003 12:14 PM

so your saying that I should leave the vac adv. hooked up
when I set the initial timing? Should I leave it at 15 deg.
or should I change it.

Frisco 08-25-2003 12:24 PM

Quote:

Originally posted by dmorris1200
The reason you disconnect your vacuum to set timing is because you're setting BASE timing. Base timing means timing with no mechanical or vacuum advance. Also even under WOT as your engine rpms increase so does your manifold vacuum. That is why your accelerator pump only needs to give you a short momentary boost of gas, then as your engine builds up rpms fuel is drawn through the jets because of the engines intakevacuum. If you have a vacuum advance can set to provide advance at a low vacuum signal you will see this advance even under heavy acceleration. Maybe you should read this link HERE were it tells you to add your vacuum advance to your mechanical advance to see your total ignition advance. At 40mph when you step on the gas to pass someone your engine is seeing your distributors mechanical+vacuum advance. Too much of this combined advance will cause that annoying little 'ping,ping,ping'.

http://hotrodders.com/photos/0000145..._121992289.gif

OK I read the article. Below is a portion of it that I have copied and pasted. The section that is bold does not seem to be what you claim the article stated. Did I miss something here?

What is Initial Timing and Total Timing?
Initial timing is that setting you make while your engine is idling with a timing light. This is typically between 4 ATDC (After Top Dead Center) to 16 BTDC (Before Top Dead Center). For performance applications "in most cases" you want as much initial timing as the starter can handle (the more timing the more cylinder pressure that the starter must overcome to crank the engine). Too much initial timing, besides the starter load, can also run the HC (Hydrocarbon) emission levels beyond legal or breathable levels <hint to the smog machine as a tuning tool above>.
Total timing is the calculation of the combined initial and mechanical advance timing settings (max). This "can" also include your vacuum advance though, I like to describe it as "total timing plus vacuum advance". Since vacuum advance is a nearly inconsistent value I like to use it after I have set up the correct mechanical values in the distributor. At times I must use the vacuum advance to generate the timing values I require, but I try to only use it as an "economy" tool to give a street driven vehicle more efficiency and throttle response by this addition.

There are two types of vacuum advance functions available:
One uses a PORTED vacuum source that as RPM increases, so does
the vacuum to the port .... this is a BAD connection in Racing, especially
since we typically retard our timing at the upper RPM levels. But it can be
used as a tuning tool so long as you do not exceed the MAX total timing
figure you need. In this case you will be using the vacuum advance as part
of your total timing.
The second uses FULL MANIFOLD vacuum, which is how I prefer to
use vacuum advance (street cars only). Some may disagree, but it works for
me! To do full manifold mandates an adjustable vacuum canister on the
distributor. The way this system works is when you smash the throttle and
vacuum levels go to atmospheric the vacuum advance is stopped. No ping, no
excess unneeded timing that is only good for street car efficiency anyway.

The above is from the article you linked to.

Quote:

Originally posted by bad502
so your saying that I should leave the vac adv. hooked up
when I set the initial timing? Should I leave it at 15 deg.
or should I change it.

Absolutely NO as to hooking up the vacuum advance when you are setting the timing! Hook it back up AFTER you have set the timing. For now leave it at the 15 degrees and try it. If you do not get any pinging you can increase it 2 degrees at a time. Once you experience ping then drop it back down 2 degrees. The best performance will be at the highest degree of advance WITHOUT any ping.

dmorris1200 08-25-2003 12:38 PM

Well it looks to me like you just confirmed my statement and you even copied it for us. "This "can" also include your vacuum advance". As you even stated, depending on how your system is set up the vacuum advance plays a big role in your engines total advance. I, as well as many others, use ported vacuum as a source for my timing advance. My Ford has no problem handling as much advance as both vacuum+mechanical can provide. Depending on how you hook up your advance, full or ported, obviously as you stated affects the engines total timing under different load conditions. So again I say your original statement that " The vacuum advance is NOT added to these figures" you are incorrect. You even posted this yourself here...
Quote:

This "can" also include your vacuum advance though, I like to describe it as "total timing plus vacuum advance".
and also from the same link...
Quote:

Now, when adding vacuum advance to the equation you would have the total (34 degrees) PLUS the additional vacuum advance.
PS. I wasn't looking for a major arguement here, I just wanted to make a point that you can't eliminate the vacuum advance function from the equation when setting up your timing.



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Frisco 08-25-2003 12:45 PM

Unfortunately; while you and I are debating what 'Total Timing' is, BAD502 would like to know what and how to set up his timing. I have stated what I feel is the correct way to do it. What do you have as a suggestion for him?

dmorris1200 08-25-2003 12:59 PM

Quote:

Originally posted by Frisco
Unfortunately; while you and I are debating what 'Total Timing' is, BAD502 would like to know what and how to set up his timing. I have stated what I feel is the correct way to do it. What do you have as a suggestion for him?
I never disagreed with how to set up his timing. I was just pointing out that he needs to understand the relationship between mechanical and vacuum advance and how it could affect his total timing. Also like you said, depending on how and what type of driving he is doing (street or strip) and how his vac. advance is hooked up, his total advance can change under conditions like passing someone on a highway. I know many guys that use as little base timing as possible with their vacuum advance on a ported source so they can pass a state inspection sniffer test (still required in many states). If base timing is too advanced emmission levels usually climb too high for state inspection laws. With a low base timing the mechanical and vacuum advance must then be tailored correctly. There are many different ways to set up an engines timing, I merely stated that you shouldn't eliminate your vacuum advance operation from the equation as it does play a role depending on how you are set up. As I said in my last post, I was not really looking for an arguement here.

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Dave E Shank 08-25-2003 01:08 PM

I would like to chim in with a further question... my sbc 350 runs hot at idle in traffic, about 230 degrees. Local hot rod supplier is telling me the cure is to advance distributor until rattle and ping under load, back off a little and engine will run a lot cooler??? Have any of you seen this in action?. Right now I am running about 8 degrees BTDC. But this sounds so off the wall to me. Any comments would be appreciated.....THANKS....DAVE

dmorris1200 08-25-2003 01:20 PM

Quote:

Originally posted by Dave E Shank
I would like to chim in with a further question... my sbc 350 runs hot at idle in traffic, about 230 degrees. Local hot rod supplier is telling me the cure is to advance distributor until rattle and ping under load, back off a little and engine will run a lot cooler??? Have any of you seen this in action?. Right now I am running about 8 degrees BTDC. But this sounds so off the wall to me. Any comments would be appreciated.....THANKS....DAVE
This would probably make a good new thread Dave, may get more attention as a thread on it's own. This is one of those things where a few different approaches could be done. What your generally looking to do is to see if modifying your engines timing while idling will help your temp concern at idle. Changing base timing will do this, also if you're currently using a ported vacuum source for your vacuum advance you could try using manifold vacuum. Understand anytime you change something you're changing your engines timing characteristics (curve, total, etc.).

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Frisco 08-25-2003 03:41 PM

AHA!!! DMMorris we do agree at last!!!:thumbup:

Increasing your initial timing can lower your operating temps. Experiment to see. Increase in 2 degree increments until you get 'ping' under a light to moderate load. Then back off two degrees.

And Yes, I discovered this when breaking in a cam in the past. I set the distibutor at a point where I knew the engine would start immediately. I then increased the RPM to about 2800 RPM and began to go over the engine looking for any leaks. After about ten minutes of run time I noticed the headers were glowing red at the junction of the individual pipes and the collector. The water temp was about 225. I almost panicked. Without lowering the RPM, I hooked up the timing light, dis-connected the vacuum hose and plugged it and set the timing to 36 degrees. It was about 22 degrees to start. Pipes immediately cooled down, water temp slowly went down to 185. :D


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