Scientific Test Results on Timing

Well, driveway results, anyhow. In a previous post, I asked which was better, hooking the distributor vacuum canister to ported vacuum on the carb or direct manifold vacuum. Armed with my trusty timing light and a couple vac caps, I tried to find out. With the vacuum canister hooked directly at the intake, my 350 shows 44 degrees total advance. With the vacuum advance on the carb, it shows 44 degrees total advance. I checked the intake vacuum and had about 16". I hooked the guage to the port on the carb and got about 23". However, the vacuum advance had reached its max at the lower manifold reading. All tests were done at about 3000 RPM. I conclude that there is NO difference as far as ignition advance goes where you hook the canister. The end result is the same. The only advantage I see is a possibility of better mileage with the advance hooked to manifold vac. Cruising at about 1500 RPM, there would be a little more advance, but I don't think it would make a lot of difference. Might try a tank of fuel with the manifold in charge of the advance and see what happens.

 

Isn't 44° a bit much advance?
I'm no gearhead, but I've read 36° is plenty.

 

From what I have read, there is a lot of disagreement here, too. Chevy guys recommend anywhere fron mid 30s to 50. I have always heard 35-38. Mine runs good, does not detonate, starts with a bump of the switch........does everything right. I guess when you factor in timing chain stretch and other wear, maybe it is OK. There is something about dividing something in half to get a true figure. I am running 8 static on the balancer. I think the vac can adds 6 more, and the rest is centrifugal, which would come out to 30.

 

i do know that there can be different places on some carbs where you will have manifold vacuum, and on the other vacuum port you will have hardly any vacuum until you start to raise the rpms . thats the one i usally use . as a rule of thumb you ought to have all of your advance by 2500 to 3000 rpm . but theres allways other factors that come into play, like comp ratios and diff types of ignition ,jetting and whatnot.

 

The ported fitting is above the throttle plates and anything under them gets manifold vacuum. Usually, there is only one ported fitting.

 

In the Ford Duraspark distributor there is a cam advance plate.
There are two slots in it which give the option of two different "mechanical" initial advance settings, usually 13 and 18 degrees, IIRC.
Because the cam rotates at half speed of the crank you multiply that X2 to get the initial advance number.
So 13 x 2 = 26°. Add the static timing of the balancer, say 8° + 26° = 34°.
Then you factor in the vacuum can advance 6° + 34° = 40°, (which is changing due to load/vacuum and rpm).
So yeah the number can get up there into the 40s if you're running manifold vacuum which is high at idle.
Ported vacuum is 0 at idle.
And then the cam could be advanced (or retarded) with keyways on the gear, usually +4°.
At least that's how I understand it, but like I said I'm no gearhead...

But I don't know what your dizzy looks like so that's for a Ford.

 

Mine is a MSD Streetfire which looks just like a GM HEI unit. It starts, runs and performs great with no detonation, so I can't see any problem. I could retard the initial down to 4 which would give me 40 degrees full, or 0 initial which would give me 36 full degrees. Of course, I am using a dial back timing light and I am just assumeing it is accurate.

 

I wouldn't go as low as that, start with the stock setting and go from there, if there's a problem. In my case 8° is stock, but a lot of guys bump that up to 12° or more because it makes the engine "peppier". But then you have to compensate for increase idle speed by adjusting the carb. I run ported vacuum so that's out of the equation at idle, no vacuum timing is necessary at idle. Going down to 4° timing will slow the idle speed considerably, too much I think. Unless your car idles well at 400rpm in neutral.
Oh yeah are you taking the settings/readings in park/drive or neutral? Some specs call for the car to be in gear for tuning.

 

Like you say, the idle can always be compensated by adjusting the carb idle speed. When I hooked to manifold vacuum, I set the carb back. When I went back to ported, I had to speed it up a little.

 

The 36 degrees you've heard about is the total advance at full throttle. The vacuum advance is only working at part throttle (light to moderate engine loads), where the engine needs and can use more timing advance. At full throttle, there is no manifold or ported vacuum, thus the vacuum canister is adding no timing.

The amount of "total advance" an engine likes at full throttle depends on very many factors. The original small and big block Chevys like 36 degrees as a good starting point. After much drag strip testing, I find that 32 degrees is the best figure for my big block Buick (430 cubic inches).

On every one of my non diesel cars, I have the vacuum advanced hooked to intake manifold vacuum. The advantage of manifold vacuum verses ported vacuum is that idle manifold vacuum is higher, cold driveability is better and very low load economy and response is better.

 

"At full throttle, there is no manifold or ported vacuum, thus the vacuum canister is adding no timing."

Turbobill:
I can't agree with you on the above. The higher the RPM, the greater the ported vacuum. If you don't believe it, hook a vacuum guage to the ported fitting. There will be no vac @ idle, but as engine speed increases, the guage will rise. I have about 18" at 3000 RPM on my 350 with a Carter Afb.

 

Ported vacuum starts as the throttle butterfly moves from idle and reaches a peak at about one third to one half throttle. Above that it diminishes to zero as the throttle butterfly reaches wide open.

I have verified this but also think about it. If vacuum remained at full throttle, you'd have the extra timing of the vacuum canister along with the centrifugal advance, resulting in to much full load/throttle advance.

 

Bill........I don't know how you came to this conclusion, but on my car, with the vacuum canister going to ported source, I am getting 6 degrees of extra advance from the canister. All canisters have a maximum amount of advance, and no matter how much vacuum you put on it, that is all you are going to get from it. Mine is set at 6 degrees. The canister does not provide an infinite amount of advance. Also, the ported fitting picks up it's signal from ABOVE the throttle plates, which means the more air flow coming through the primaries, the greater vacuum created at this fitting. At about 3,000 rpm, my ported fitting is providing around 18" of vcuum, but there is only the additional 6 degrees from the canister.

 

I"m not sure what you two are disagreeing about.
Remember that the vacuum is load-dependent, and really only functions during changing rpm and load conditions.
Which is why it is use to improve economy by allowing the engine to respond better to normal stop and go driving. There is no vacuum from either source applied at steady state WOT.

 

I just went out and hooked a vacuum guage to the ported fitting on my carb. At high RPM, I get as much as 23". When I open the throttle completely for just a second, it will drop back down some, but not to zero. It still has enough to pull in the canister, which only adds 6 degrees to the mix. My canister is adjustable. I can set how fast or slowly the 6 degrees come in with an Allen wrench. I have it set pretty light so it will come in quick. I guess the whole discussion was about the best place to connect the canister, to direct vacuum or ported vacuum. As mentioned in a previous post, I tried both ways and could not rteally tell any difference except a faster idle with it hooked to straight vac. I decreased to idle setting on the carb. When I went back to ported, I had to increase it back to where it had been. Seems like it is a little more responsive with it hooked to ported.