Which is better?

 

Both are good.......depends on your combo...

 

not a matter of which is better - on a STREET motor both are good and both are needed.

the mechanical advance and the vacuum advance are two completely separate systems that work independently but also together to provide correct spark advance under all driving conditions.
the mechanical advance works based solely on engine RPM while the vacuum advance works based solely on engine LOAD.
the two together work to give the motor correct spark advance.

 

Ditto to what Barry said.
Without the vacuum, you would loose the added advance at idle that helps the motor run cooler. You would loose the added advance at cruising speed that enhances economy. You can run without it, but unless you have a dedicated race car what would be the point?
Without the mechanical (or some form of rpm related advance) I don't think a motor would run too far above idle.

 

Mechanical.

 

I don't run a vacuum can and one reason is I don't like the idea of my motor at 50+ degrees advanced with 11:1 CR ratio, can you say "ping" ?

 

Compression has nothing to do with whether or not you should run VA. If your engine has to idle or cruise,. i.e., run the street, you need the can. Under these 'no-load' conditions your engine can accept a lot of timing,..yes, 50* and more. She'll run cooler, get better mileage, and you'll enjoy better off-idle performance. But you need the right can and the correct MA settings. Also, do a search on this site,.."manifold vacuum ported vacuum".

Good luck!

 

Oh OK thanks for setting me straight, no vacuum advance added at part throttle acceleration then ?

 

I may be doing some work in this area before the end of this semester. I'm taking thermodynamics.

Timing is determined by the amount of time required for all the fuel to burn for maximum power during the power stroke. At very low RPM, for example the timing COULD be set at about 0ºTDC and all of the fuel will have been burnt before it gets to 1ºTDC. But, as the engine RPMs increase the fuel will not completely burn until 2º, 3º, etc. which means that there will be less available power to push the piston down.

To compensate the timing is set to ignite the fuel BTDC...at idle RPMs engines need about 10º or so...depending on heads, valve timing, etc. I'm not sure of the exact time at which the fuel is completely burnt, but it takes longer to burn as RPMs increase. The fix is for there to be a mechanical advance which increases timing BTDC as the RPMs increase...ideally it should be a full curve from 0º at idle to a engine specific amount at max RPM.

OK, that takes care of the timing changes due to RPM. However, it doesn't compensate timing if more fuel is crammed into the intake like when the throttle is opened more at any RPM. More fuel requires more time to burn...exact numbers I don't know for now...hopefully I'll get them before the end of the year. The fix is the vacuum advance unit. When the thottle is opened there is a vacuum port which temporarily increases vacuum...the exact opposite of manifold vacuum which temporarily decreases when the throttle is opened. This additional vacuum applies additional timing BTDC so that the extra fuel during the lower RPMs will completely burn...as the RPMs increase then the vacuum will drop off until just the mechanical advance is maintaining proper timing at constant RPMs.

Final word? You need both.

 

some, but how much will depend on your motor, what vacuum your motor produces, what vacuum can you have installed, and what the load is at the time.

assuming you have vacuum can connected to a full manifold vacuum source than your greatest amount of vacuum advance is going to be at idle and than at easy steady cruising such as a constant 60mph down the highway - these are light loads on the motor therefore the motor is producing the highest level of vacuum and the vacuum advance is adding in more advance to the spark timing which is perfect. Why perfect? At idle or under light loads such as easy cruising the motor is working very easily and the fuel/air mixture is lean. A lean mixture (more air, less fuel) burns slowly. A greater amount of advance helps to ignite the mixture and you will benefit from it by better throttle response, better fuel economy, and the motor typically runs cooler.
Under loads such as acceleration and at WOT your motor's vacuum level drops and therefore so does the amount of vacuum advance. Again this is good because as the load on the motor increase such as during acceleration the fuel /air mixture becomes more RICH. A rich mixture (more fuel, less air) burns more quickly. With a quicker burning mixture you want the spark timing to be retarded back and under these heavier load conditions as the vacuum decreases and therefore the amount of vacuum advance decreases it's retarding back the spark timing.

The amount of advance the vacuum system provides will always vary under part acceleration depending on the exact load the motor is seeing at that precise second therefore the amount of vacuum the motor is producing. The heavier the load the less vacuum so less vacuum advance.

You can watch this yourself if you wanred by getting a long length of rubber vacuum hose line and connecting it up to a full manifold vacuum source and hooking a vacuum gauge to it. Make the vacuum line long enough so you can place and read the vacuum gauge inside the car with you as you are actually driving and watch how various driving conditions affect the vacuum level of the motor - easy driving/light loads vs acceleration or WOT/heavy loads - and as you watch the vacuum gauge react under these different condition you will get an idea of how the vacuum advance system is always changing to adjust spark timing.

The amount of advance controlled by the vacuum system will than depend on the vacuum can used. different cans provide different amounts of advance and at different vacuum levels so you need to select a can that is appropiate for the vacuum levels that your particular motor produces.

remember, although independent from the mechanical advance, it's also working together with the mechanical advance to control spark timing. The mechanical advance will always increase advance at higher rpms and decrease advance as rpms drop while the vacuum advance system increases and decreased advance as motor vacuum increases or decreases so your total spark timing under a condition such as part acceleration (your original question) will depend at that precise moment on the rpm level and the vacuum level. Under the same amount of part acceleration the rpm level may be a certain amount but the vacuum level could vary lets say based on acceleration on a flat road or the same acceleration on a road going uphill - the hill will create a greater load on the motor so less vacuum - less vacuum advance.

sorry, no real "set" answers to provide because of reason. nobody can simply tell you an answer of "under part acceleration the vacuum advance will be adding in 7º of advance vs 12º at steady cruising" or any other number like that - it always varies.

did that help at all?

bottom line is that for a STREET motor the vacuum advance has many benefits. Can you have a street car and not run a vacuum advance system? sure, many people do as you are aware since currently you aren't running one but there are benefits to running a vacuum advance system on a street driven vehicle - typically better idle characteristics, better throttle response, better fuel milage, and the motor tends to run at a bit of a lower operating temp.
A race car on the other hand it's basically worthless which is why race cars doesn't use a VA system. They spend most of their time at WOT where there is virtually very little or no vacuum in the system so there wouldn't be any VA added in.

 

Motorhead if you havnt already tried then get an adjustable vacumn canister and restrict the advance in the can to 8*-!0* and then adjust the vacumn needed until it doesnt ping.You wont have a great deal of vacumn advance but you might be surprised at the differance it makes.You can adjust it to where as soon as you apply a little throttle at cruise that extra timing disappears.Leave all your power timing settings alone.Its worth a try if you havent already.My latest was an 11 second drag car that was put on the street.It took a little work but it really made it much more streetable and the customer was amazed at the fuel milage differance.(Something to do when you dont feel like cutting the grass )

 

Beleive it or not I know how it works and like I said I don't need any additional timing added to my 36 degrees advanced at any speed or RPM with 11:1 CR I am on the edge as it is. I have never had any detonation problems doing it this way and the motor runs fine.

 

 

If setup properly the vac advance will only advance timing when there is lots of vacuum, which is at idle and cruise. This is to improve fuel economy and it may improve emisions as well but who cares about that. At WOT your vacuum drops and the vac advance should not add anything. If you connect the vac advance to ported vacuum rather than manifold then you will not get vac advance at idle, only cruise.

 

Your final word is correct, but I'm sorry, some of your reasoning is incorrect. A quick summary of ignition timing is to shoot for an advance setting that will produce maximum cylinder pressure at 15* ATDC. This yields maximum mechanical and thermodynamic efficiency. Vacuum advance compensates for the change in flame travel and burn rate caused by differing charge density (pressure) levels in the cylinder. This is indirectly measured in the intake manifold, and labeled as "engine load". The vacuum canister does not compensate for any "extra fuel" that gets injested. Additionally, extra fuel (rich mixtures) generally require less advance, while lean mixtures generally require more advance. Regardless, you don't want either (rich or lean) until you are near WOT (then a tad rich, obviously), as your efficiency, and your wallet, generally take a hit when you're off stochiometric fueling.

my $.02

 

you may or may not be at the edge at 11:1 CR.
my '65 is 11:1 CR with the stock configuration 327/365 motor andtiming set at 12º initial and 36º total before the vacuum advance. The vacuum can adds 16º advance at it's 28º at initial and up to 52º total with the vacuum can connected.
I have zero pinging or knocking or detonation.

If you are happy the way it is, fine, leave it, but just be aware there are added benefits to the vacuum system, that's why it exists.

 

I'm confused. You say you are "on the edge", but also say you have never had any detonation problems. Seems either contradictory, or you have an absolutely flawless timing curve. (I know which one I'm betting on.)

 

You can beat this subject to death....

Bottom line is that the use of vacuum advance depends on your application and needs.

I'm familiar with Moterhead's setup, and I wouldn't run a vacuum advance on his combination - a good mechanical-only curve with a lot of initial timing (short curve) is going to be his best setup, and I think that's what he's running.

Most mild to moderate street engines can benefit from vacuum advance, but the unit needs to be tailored to the needs of the engine. The more radical (cam and compression) the engine is, the more difficult it is to find the fine line needed for vacuum advance. Milder engines have great benefit from the additional timing produced at idle, cruise and light throttle acceleration. But these are all tuning parameters, and you need to evaluate what your engine combo needs and what it can take for timing before making absolute decisions on vacuum advance.

 

I have never had any problems running my chevy small blocks at 36 degrees, this is where I want it so I have not had any detonation problems, "on the edge" is purely speculative based on my knowledge pf small blocks, the compression ratio I run, the gas I put in the tank and a cranking cylinder pressure of 230psi

 

Ey lars, which vacuum port on the q-jet should the vacuum can be hooked up to again? I'm suddenly doubting that I've had it hooked up to the right port now.