Initial timing-Ported vs Manifold vacuum

I have read the Power Timing post and have a couple of simple questions....I think. my 1971 Skylark has a stock distr w/Pertronix instead of points. I switched over from ported vacuum to manifold vaccuum-idle improved and throttle rsponse improved. Have the initial set at 10* btdc. With ported vacuum the timing does not change at idle(no vacuum advance to change it) but with manifold vacuum it adds 10* of additional vacuum advance.

Question-should the initial timing be set lower to consider the additional vacuum advance when using manifold vacuum? Should I use manifold vacuum or stick with ported(I like the smoother idle)?

My total timing comes in way late and I have the Mr. Gasket advance curve kit w/bushing to adjust the total timing. Have the dial back timing light and am ready to start testing...but am curious about manifold vs. ported vacuum.

Thanks for any input

 

I am curious about this too, I have 12* initial and I have the vacuum advance plugged so I dont over advance it since I haven't limited the amount of vacuum advance from the stock 18 to a more manageable 8 or 10* I never thought of using ported instead of manifold.

 

OEM's have used both over the course of the years.

Try both and decide which you like better. As there are two primary differences. The manifold vacuum one will allow you to crank the engine with lower advance and idle with a higher advance. Down side is there could be a hesitation/stumble off idle as the timing will retard when the vacuum drops (not always the case though). The ported vacuum one will keep the timing during cranking and idle the same (true initial). The ported vacuum source will smoothly start adding vacuum advance as the throttle is opened (less likely to bog/stumble off idle). Down side is if you run a high compression engine and need a lot of initial for idle it'll be hard cranking.

When the throttle is open both sources behave the same.

I find if I want to run lots of advance when cruising on the highway I'll run it off the ported vacuum and set the initial up higher so the vacuum advance adds more to the total from idle. If I want/need to run less I'll run it off the manifold with a low initial so the vacuum advance is setting the idle timing.

 

Great post above!

I like to run about 14-16 degrees of initial timing, manifold vacuum for the vacuum advance and I limit the vacuum advance to 8-10 degrees. I set up the mechanical advance to add 18-20 degrees of timing all in by 2300 rpms. Experiment with the initial setting till you get 32-36 degrees total timing without the vacuum advance... Then once you are happy with that then try adding the vacuum advance, if you then get ping then you will want to limit the vacuum canister to 8-10 degrees.

 

If you are using a modified distributor with lighter springs to get the advance in quicker....you prob are not even really utilizing the vac advance as the goal is to have yout total advance by 2000 or so. Saying this the ported vs. manifold means nothing......
On the other hand with a lightly modified dist curve...you prob will have better response with ported advance as you will have a burst of advance as the port opens and then a curve much the same as the manifold vac.
Using the manifold vac as your primary source will give you the better idle as you are already advanced but will give you a drop in advance at throttle open and then the advance curve.
Using the manifold may remove some ping scenarios and give a smoother acceleration but in my opinion I like using the ported for a crisper throttle.

 

Manifold vac can sometimes give an unsteady idle - as rpm drops, so does vac, then timing drops, then rpm drops more, etc.

 

Makes sense-when I tested out the manifold advance the car seemed to gain more power on initial acceleration-it would finally spin the wheels...I think what I'm going to try is the spring kit w/ported vacuum and get the timing to come in earlier. The Power Tuning post has good info.

Anyone know if the Mr. Gasket Adv kit (927G I think-don't have in front of me) with the brass bushing will limit the total advance?? I'm assuming that is what the bushing is for.

Thanks for the input!

 

all distributors are a bit different....you have to either use a machine or trial and error to get it where you want it ..
you will get it .....just a project

 

You can get away with that much timing? I run about 12* Initial timing (what a difference from the -2* I had before ) And I have lighter springs to bring it in sooner, stock mechanical advance of 18-20* and I have the vacuum advance disconnected because I thought the 18* from the vacuum advance would be way too much timing. I have been running 89 octane to make sure I am not pinging. Am I doing it all wrong? Where should I run a ported line on the rochestor 2gv?

I also disconnected the vacuum advance because I figured 30* of timing at idle is way too much, but if I can get away with having it hooked up I'll try ported

 

I would lmit the vacuum canister to give only 8-10 degrees then you will be less likely toover time the engine...

Like others have said every engine is a bit different. The low compression engines can handle a bit more timing. Typically under full throttle even a 10:1 350 can handle 32-36 degrees of timing... At light throttle an extra 10 degrees from vacuum advance is typically fine.

I have tuned about a dozen 350 cars and every on of them has liked between 32 and 36 degrees of timing plus 8-10 from the vacuum advance.

Be carefull not to over time sine this can cause damage...

 

I completely believe you, I guess its all the Chevy guys putting fallacies in my head:beers2: I keep meaning to get around to either limiting the stock cannister or buying an adjustable one but I haven't been up to taking the distributer out just yet. Maybe I'll pull the initial down to 8 for now so I can run the vacuum advance.

This thread has been fairly informative versus what one typically hears when this subject is brought up!

 

You only have to remove the dist cap and rotor to get at the vacuum canister so it is not a big job...

Yes, this is a great thread and I 100% agree with Larry that the vacuum advance is the last step in the whole timing process.

 

Larry is correct ....Best to get the timing and curve working with NO vac period. That way you can play with the vac asvance,, as good as your dist is built and clean as it may advance, the vac applied is erratic at best.
I like to use no vac and play with the curve by using just the springs. I have had multiple motors that like 18 initial and then 14-16 mechanical to get my total. Still there are alot of factors to consider....comp ratio, camshaft duration, octane rating, air-fuel ratio, engine temp, ambient temp,...and others

 

My car liked to run with 18 initial but it caused the starter to struggle on hot starts.

I had replaced the distributor in the distant past and it came with a bushing that limited mechanical advance. I removed that bushing to increase the mechanical advance so I could run less initial to fix the starting problem but still reach 30 degrees total.

I used the lightest springs and I think they advance a little at idle which lets me start at around 13 initial but increases a few degrees once the engine catches.

I've limited the vacuum advance to 9 degrees, so my highway timing is around 39 which is probably a bit conservative. I intend to fine tune it a bit more next spring.

Even with my conservative settings the change in the engine is remarkable. It starts easier, runs cooler, gets better gas mileage and accelerates better.

My point is it's best to follow the steps Larry suggests. Get the different types of advance roughly into the ranges you want and then drive the car for a while. Find out how it reacts and then work you way from there.

Limiting the vacuum advance seems to be the most intimidating part of this process and it's tempting to do all the rest and skip this step. Don't do it, you can seriously hurt your engine. It's not that difficult. You can take the vacuum advance out with the distributor in the car. Cut out a square of sheet metal of roughly the right size and drill holes for mounting. You can use a caliper or drill bit to determine just where to mount it. Mark the holes and drill them out. It's best to tap the holes with a proper tap but you can usually use a screw if you go carefully and use a different screw once the threads are cut. Screw down the metal with a little loctite and reinstall.

If you've got an external vacuum source you can see how many degrees are in your vacuum advance by watching the timing mark move as you apply vacuum. While this is not required it's good to know the number.

John

 

Here's another one where every engine is different. I run 26* fixed (no mechanical advance) timing on the turbo car with 9* vacuum advance connected to manifold. It idles at 35*..This car runs and idles like a dream. It even starts right up when hot with no issues.. What a nice car..

 

How about this-my 1971 started off as a 2bbl-converted to 4bbl w/stock manifold and 1971 quadrajet. Tune-up specs for the 2bbl are 10* btc. Specs for the 4bbl are 4* btc-idle is the same for both. The car runs crummy with 4* and really likes 10-12* of initial. Why the difference in the factory specs?? It uses the same dist.

When I recurve and get the timing right it seems like you just go with what works best and make sure you don't have too much total timing. Is that kind of the idea-forget what the old manuals say??

 

That is what most of us do... Whatever works and makes the engine happy.

 

I beleive the 2bbl and 4bbl 1971 and 72 cars are identical besides the intake. The specs are probably for emissions reasons:beers2: