I am looking for some clarity on this topic. I have been doing mechanical work for the last 30 years and more a remove and replace kind of guy but like to have some understanding of how things work. Lately my understanding of Vacuum advance seems to be at question since I entered the Nailhead arena. I see people post questions on other forums where their cars arent running the best and the do some troubleshooting. Many times I hear them state they have low vacuum on the port they are using for vacuum advance at idle. Which I then ask which port they are using and we discuss if it should be 'Ported Vacuum' or 'Manifold Vacuum'. I keep seeing people post that their cars are on Manifold vacuum and depending on which carb they have this is correct. I checked the service manual for my 65 Riviera and it states it could be either or, granted its generalized for different engine configs in 1965 (300 ci, 401 ci or 425 ci). I have never come across an engine that uses manifold vacuum for vacuum advance. I was always under the impression that the 'ported vacuum' applies more vacuum as the throttle plates open and thus draw the diapragm at the distributor and advance the timing with RPM. If you use manifold vacuum does that not reverse, since manifold vacuum drops with throttle advance? I will be starting up my nailhead sometime over the next month or two and want to make sure I have everything right. I reassembled the car as I found it and will be using the port off the carb that was hooked up to the distributor, but if thats not right I need to know.
the only thing that changes is at idle, after that all is equal. some engines like more timing at idle and go with non ported. non ported is good with a bigger cams, the extra advance at idle will keep the plugs cleaner and smooth out the idle and you will not have to open the the throttle plates as much to reach the same idle rpms. i like the non ported even with stock cams. some engines might not like all that timing at idle so try both ways. really no right or wrong way.
As Joe stated, either is correct depending on application & cam. The ONLY deviation I can add to this is for standard shift cars. If you are using MANIFOLD vacuum as you start to let the clutch out & pushing down on the gas pedal you IMMEDIATELY lose vacuum & since the timing immediately retards, depending on how much limit you may have installed, the engine will hesitate & sometimes backfire. In this case I would use PORTED vacuum. AS you let the clutch out & step on the gas pedal it will have more of a tendency to advance rather then retard. If you've experienced ANY of these problems just try PORTED instead of MANIFOLD vacuum. May just solve a problem. Best part it costs zero $$$$.
On a Chevy, I always run manifold vacuum. On a nailhead, the shop manual for a 63 states ported, but I have experimented with both. Running manifold vacuum will keep the engine cooler at idle. Check out the first reply to this thread. It works. http://www.bangshift.com/forum/forum/bangshift/tech-section/32851-ported-vs-manifold-vacuum-advance
Another consideration is engine coolant temperature. If you have a Nailhead that is initially timed at 2.5* BTDC, and you run ported vacuum, the engine will be idling in traffic at 2.5* BTDC. That is relatively retarded timing, and the engine will have a tendency to heat up in stop and go traffic on a hot day. In that case, using manifold vacuum will boost timing at idle, and it will actually make the engine run cooler. In 1972, Buick used an emission control called the Tansmission Controlled Spark (TCS). It did not allow for any vacuum advance at idle, 1st and 2nd gears. The engines were all timed at 4*BTDC. Because the engines had a tendency to run hot like this, they incorporated a thermo-vacuum switch that would apply manifold vacuum to the vacuum advance canister when coolant temperature exceeded 220*. Here is the description in the Chassis Manual, Being the main purpose to vacuum advance is better gas mileage, it does not matter whether you use ported or manifold vacuum. At part throttle, both will yield the same amount of advance. More engine load will drop manifold vacuum quicker, retarding the timing, and as Tom said, could result in some hesitation or worse in a manual transmission car. The same thing applies in a numerically low rear gear car and a tight converter. My best advice is to try both ported and manifold vacuum. See which one the engine likes better, or what feels better to you.
4 speed cars and '66 Calif emissions had the vacuum port above the throttle plates. Anybody know if Q-jet carbs did the same?
Knowing how this works is always a good thing to keep in mind when deviating from stock. It's such an excellent tool to use while planning the big picture. I tweaked a 75 vette 4 spd that came from factory with advance from direct manifold source. It worked well to set the short curve to total timing and it acted like a start retard, with idle timing being rather high with vacuum, once running. Letting out the clutch didn't drop the vacuum (or timing) enough to hesitate the car.
I have a custom vacuum advance that adds timing at part throttle and retards timing when the turbos create boost. It works great.
So I understand correctly, I should set the base timing to 2.5* with the hose disconnected and plugged and then try both ported and manifold. Both base timings are the same?
Not all Nailheads had that initial timing, but a lot did. The timing specs for your engine are only valid with the distributor it left the factory with. Distributors can be replaced over the life of a car, so unless you bought this car new, and know it's entire history, I would not assume it had the same distributor. Distributors can differ in the amount of mechanical advance built into them. What you want to do is set the maximum wide open throttle timing to 30-34*. You do that with the vacuum advance disconnected and plugged. Vacuum advance doesn't work at wide open throttle. Once you have that set, then you add in the vacuum advance. If you have modified the rate of mechanical advance, to bring it in earlier, then it will be necessary to reduce the AMOUNT of vacuum advance. All of this is explained in my Power Timing thread which applies to Nailheads also. http://www.v8buick.com/showthread.php?63475-Power-Timing-your-Buick-V8
Hi Larry.. I have the lightest springs in my distributor, so my total timing comes in at ~2200rpm at 34*.. I have manifold vacuum, and haven't modified the amount of vacuum advance, and I have no problems.. It's a 401" 65' Maybe it matters that Denmark has octane 95 as standard gas, and the whole country is at sea level?
You should measure the timing at your cruise RPM and see what it is. Stock springs will typically only allow part of the mechanical advance in at cruise RPM. 14-20* more from vacuum advance puts the cruise timing around 40* or so. Getting all your mechanical advance in by 2200 and adding 14-20* gets the timing up around 50*. Some engines will surge and/or ping at that timing. If you have no problems, leave it alone. Id measure the timing just to see where it is.
I actual had problems with the car, stumbling in gear at standstill, and thought at timing right away, but turned out to be the low/high stall electronic arm, that follows the throttle had gone out of adjustment.. Haven't checked the timing at cruise, only idle and total, 10* and 34* as I recall..Vacuum plugged off cause..
Hi Alini, As stated on the other forum...the stock `65 single 4 barrel carb only has a manifold vacuum nipple. There is no ported vacuum nipple on the stock single 4 barrel carb. However, the dual 4 barrel rear carb has a ported vacuum nipple. The reason for the different treatments is the static timing spec. The standard single 4 barrel Nailhead is timed at 2.5 degrees BTC and the dual 4 barrel engine is timed at 12 degrees. As you can see there is a considerable amount of static advance built into the dual 4 setup which never goes away no matter what position the throttle is in. Manifold vacuum drops off very quickly with even small throttle openings (try connecting a vacuum gauge on any car and take it for a ride) so the vacuum advance system, whether manifold or ported, is for the most part, not a consideration when a substantial amount of throttle opening is present. At low rpm full throttle opening the static timing setting becomes the significant factor, not the vacuum advance system. The above is assuming a stock `65 car. If running a non-stock setup the above advice re cam choice, etc is right on. Hope this helps, Tom Mooney
