Vacuum Advance

[eventhorizon66]

So I was checking my ignition timing the other day after discovering a slightly loose distributor hold down clamp (oops).  I have always had it around 14-15 degrees initial (with 21 degrees of mechanical advance in by 3200) and it has run fine.  Timing was fine but I forgot to hook my vacuum advance back up (very forgetful aren't I).  Well it ran like a turd, very sluggish, and whenever I started to roll on the throttle it would bog down then pickup and accelerate.  Mind you I did cap the vacuum nipple on the carb (so there was no vacuum leak).  Hooked the vacuum advance back up and it runs OK again.

My question is shouldn't the engine be able to run fine without the vacuum advance?  I know vacuum advance aids in combatting low engine speed flame propagation to achieve better cruise efficiency but the difference in how it runs shouldn't be so dramatic should it?

[VileZambonie]

Your vacuum advance is designed to work with the centrifugal advance. Without it you are only getting centrifugal advance therefore you do not optimize spark timing. If your base timing is say 10° BTDC your total advance at 3200 rpm should be around 36° BTDC. So at low engine speed spark timing may be as late as 10°BTDC and combustion continues until say 23°ATDC. At the higher speeds w full advance the burn time is 36°BTDC to 23°ATDC so as you can see there is a much larger spread.

Vacuum advance actually moves the pick up coil in relation to the pole piece. When you stomp on the throttle vacuum dips retarding the timing momentarily. Your spark curve is non linear so it will change based on engine speed and load. So basically with a vac advance distributor you need to have the vac advance hooked up in order to have it work to it's full potential.

[HAULIN IT]

Vile, Please understand, I'm not disagreeing with you. I would think like Even is questioning "should the engine run that poorly?" I also would think not to the extreme he is describing. I have to question if there is, in fact 21* mechanical? If he sets the timing (w/vac off) to 36 (hey! How did you make the degree little circle?) wouldn't there be enough in to make it run Ok at say 2400rpm? Maybe the curve is really slow at coming in? Even, Do you have a dial timing light? I would be interested in what it has at about 2200 or so, maybe not much more than the 15?   Just thinking, Lorne

[VileZambonie]

I would check the total advance, centrifigul advance and base timing over. If the centrifugal advance isn't working time to rebuild the distributor.

[eventhorizon66]

Vile:Thanks for the response.  I got another one for ya.  I have always heard that every street driven vehicle, even with a fairly large cam, can benefit from a vacuum advance.  Only full-on race engines and boat motors can't benefit from them. This is why I went with the Pro-billet HEI (overkill I know, but it will keep up with future mods), so I hope a rebuild isn't in order, LOL.  But I notice that you run a non-vacuum advance distributor on your 454.  Why?  At what threshold of engine performance do you decide to not run vacuum advance?

My timing was set with vac advance disconnected.  Initial is at 14 degrees and total is at 35 degrees (@ 3200).  I confirmed this with an advance timing light.  According to the chart in the MSD instruction manual I would have around 26 degrees @ 2500 rpm (but I haven't confirmed this).

[VileZambonie]

Well there are so many debates over HEI vacuum or no vacuum, how it effects timing etc. Many people say vac advance has nothing to do with total timing, some believe vacuum advance is just for better emissions yada yada yada. Many points are true but timing changes over a broad range of modes of operation. Vacuum advance is a good method of controlling spark timing for a daily driver. It can help ensure better complete combustion and can help when engine loads differ.

Now for my 454 for example, this is not a daily driver. It is strictly a powerhouse that goes from essentially idle to wide open throttle nearly every time I get in it. When I am at cruise speeds it's just that...cruising. I don't care about fuel economy, temperatures, emssions etc aren't as much of a concern. So for a daily driver carbureted non computer controlled vehicle I like vacuum advance and recommend it. I suggest play around with it. Try different weights and springs. See if your vac advance does effect total timing  and I always suggest running full vac to the advance at idle not ported.

[Redneckchevy]

Now for my 454 for example, this is not a daily driver. It is strictly a powerhouse that goes from essentially idle to wide open throttle nearly every time I get in it

lol im glad to hear that you dont baby that 454! 

[eventhorizon66]

Vile: I checked the timing both with and without vacuum advance hooked up and it actually doesn't make a difference to initial or total.  The reason it doesn't affect initial is because it is hooked to ported vacuum, as per MSD instructions.  MSD instructions say that it will "advance up to 10* with 15 lbs (LOL, typo) of vacuum".  Why do you suggest full versus ported?  Where did GM hook them up to factory HEI applications?  I'll try switching over to full vacuum and see what happens.

[VileZambonie]

Set your base timing with it disconnected as a base line. You'll need to compensate your idle speed increase once you hook it back up. I think you'll notice a better quality idle and throttle response. Just try it and see. Ported vacuum helped with more complete combustion for better emissions at idle speeds and higher idle temperatures but throttle response typically isn't as good....Remember I said the word Base line. This only represents a starting point in which to work off of. Many HEI equipped 350's were set to acheive 50° total timing from the factory and around 18° centrifugal. So with such a broad range of "specs" it's best to go by what your engine likes rather than relying specifically on a given number. Once you determine what your best timing is record your readings for your own knowledge and configurations. Sometimes the old advance it til it pings, back it off and make sure it starts easy is the best method as backyard as that sounds.

As far as how the factory hooked up vacuum advance this varied from model to model and year to year. With breaker points I'm fairly certain all of them were straight ported vacuum. When more and more emissions controls were implimented time delay valves were also added to inhibit the vacuum advance. But again I would hook it up full time and see how you like it

[eventhorizon66]

Thanks, I'll try full vacuum.  Gotta pick the right time for a test drive as I'm still running around open headers with no inspection sticker (LOL, haven't gotten pinched yet).  But, I don't see how less advance at idle could reduce emmissions (even though I've heard it several times before).  It seems that as much advance as possible would burn more completely at any engine speed thus reducing emmissions?

[VileZambonie]

No, the engine tends to run richer at idle speeds and requires less advance for more complete combustion and a hotter burn to help reduce emissions. You need to throw some pipes on that puppy....even some header mufflers or something.

[eventhorizon66]

It'll be getting a full exhaust within a few days.  Just a single 3" for my bone stock 350.

[SUX2BU99]

Not all vacuum canisters on the HEI have adjustable advance though, right? Or do they? I have the canister on mine, and the vacuum line is hooked up. Just don't know if it's adjustable or not.

[HAULIN IT]

Sux, No they do not. As Vile mentioned, many HEI's have way too much advance with the vacuum pulled (50*) I have found many need limited in some fashion (weld the slot,get an adjustable plate stop) more like 42*-44* total. Lorne

[VileZambonie]

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