vacumm advance with dual mikunis.

[Braden]

so after driving my new engine about 200 miles i have been tuning and playing with the carbs a bit and since i cant run my vacumm advance its been bothering me, i have played with my timing a bit and found 15 degrees to be about where i want it. so what effect on the power curve does not runnign the vacumm advance have? i guess what i am asking is somebody explain to me or show me a chart the effect of advance on the engine at all rpm levels, i would assume that the more advance the better at high rps but there has to be a point where the advance is hurting not benefeiting power output. what is the average best total advance? i know its different for each engine depending on the type of setup your running. i know my question is a little rough around the edges but please try and help me to the best of your ability. if you want to talk super complicated thats fine im sure i can get my head around it if i read it a time or two.

[datzenmike]

Very simply gas and air are compressed and the spark plug fires this mixture in theory at TDC and the hot expanding gasses push down on the downward moving piston, (again in theory about 17 degrees ATDC for maximum effect) and useful work is extracted from the engine. Everyone can see this and agree. Three things:

 

1/ Because it takes a finite time for the fuel to burn, in other words it is not instantaneous, the ignition must be started slightly sooner than TDC in order for the expanding gasses to reach peak pressure at that 'sweet spot' at about 17 degrees after TDC. At a 650-750 RPM idle the ignition for an L series motor should start at about 12 degrees BTDC. Remember that static timing is set and does not change and must be added to all mechanical and vacuum advance to get total timing advance.

 

2/ As the engine revs up, the ignition timing must be advanced further and further before TDC to retain the 'sweet spot'. This can be done mechanically with spinning weights that are flung outward by centrifugal force acting on a cam that pulls the distributor plate clockwise and this will advance the ignition timing. This is an advance that is totally dependent on engine RPMs and is tailored to advance the ignition precisely to retain that "sweet spot". Mechanical advance starts at zero and begins to work by about 1,000 RPMs and maxes out (does not go higher) by 2,500-3,000 RPMs and should reach some where around 20 degrees total advance. Remember: Mechanical advance is working all the time and is totally RPM dependent.

 

3/ Engines don't run at full throttle all the time and are almost always either idling or part throttle so the above is totally true for full throttle as in racing. If the RPM never drop below 3,000-4,000 RPMs you don't even need mechanical or vacuum advance and can lock the dizzy. But in the real world engines operate at and above idle and seldom see 4,000 RPMs. Now what happens when the throttle plate is closed and the cylinder does not get a full charge of gas and air mixture? Indeed only a half cup of fuel and air may squeeze past the throttle (perhaps only a few teaspoons) and enter the cylinder at idle. It is compressed and fired and provides just enough energy to keep the crank turning. This means that there are very few air and gas molecules with lots of space between them even when compressed compared to a full throttle. So what happens when this weak mixture is ignited? Well it actually takes longer to burn completely than a full cylinder at full throttle. I guess it takes longer for the flame to jump from molecule to molecule. At low throttle use therefor, the timing should be somewhat advanced and slowly retard as the throttle is opened toward full open. Still making sense? The less the cylinder is filled the more advance you need because of the slower burn and still retain the "sweet spot". How to do this? Well manifold vacuum is dependent on throttle opening. At idle it is quite high and as the throttle is opened it drops. So a vacuum advance can be added to the distributor plate to advance and retard totally on a vacuum signal which is also a good indication of engine load too. Idle and low throttle gives higher vacuum which gives more advance. As the throttle is opened and the motor is under increasing load, more fuel and air enters the cylinder, vacuum signal drops and the vacuum advance retards too match it. Vacuum advance totals about 10-15 degrees. So.. Remember: Vacuum advance is totally dependent on throttle opening. (or load)

A word on vacuum advance: All motors prior to around 1970 ran a vacuum advance directly off the intake manifold. You may have timed an old motor and it was recommended to remove the vacuum advance hose and cap it, then set the static timing, then re-connect it back up. Motors in the late 60s began to install carbs with a vacuum port above the throttle plate. This would produce no vacuum advance at idle but only when it was opened. Cars with this ported vacuum can be times with the hose left on as there is no vacuum present. Why? Because it was found that emissions can be reduced by retarding the idle advance. It probably does but as described above in 3/ the mixture is very weak at idle and the motor needs more advance to retain the "sweet spot" so these smog carb engines will have poor idle characteristics and tend to run hot because more of the fuel is still burning as it leaves the engine. Because of the late ignition the hot gasses transmit this heat to the exhaust ports and into the cooling system. So keep in mind that on your L and Z motors that at idle the timing is only the static setting of 12 degrees. As soon as the throttle is opened vacuum reaches the vacuum port and advance jump up to where it is properly needed.

 

So what happens when we start driving? We step on the gas and vacuum is applied to the dizzy and it advances (more for light throttle, less for heavier throttle) and this advance is added to the 12 degree static advance set with a timing light. Lets say light throttle... we have 12 degrees static and maybe another 10 of vacuum for a total of 22 degrees and the motor feels strong and responsive because we are in the "sweet spot" getting everything out of the fuel used.

 

So we step on it... more cylinder filling, less advance needed, vacuum drops, advance retards to match. Maybe 4 or 5 degrees vacuum plus the 12 static for a total or say 15 degrees total... car pulls strong because we are kept in the "sweet spot".

 

RPMs are building Say 1,500, so there is less time to burn the fuel, mechanical advance kicks in and adds proper amount of advance to compensate. Say 2-3 degrees plus the 12 static plus the 4-5 vacuum for a total of 18-19 degrees.

 

RPM increase, heavy throttle. Vacuum drops to almost zero, mechanical advance increases with RPMs and maxes out at 3,000 at 20 degrees plus the 12 static for a total of 32 degrees which is perfect for higher RPM (above 3,000) full throttle.

 

So we have just passed a car at highway speed and full throttle and let off the throttle. High RPM so the mechanical advance is maxed at 20 degrees and the static advance is always there at 12 and now the vacuum in the manifold goes through the roof on deceleration and adds 15 degrees or more for a total of 47 degrees. No harm done. Don't forget the throttle is closed so there is very weak cylinder filling needing more advance. RPMs are high so more mechanical advance needed. 45-50 degrees is just fine to hit the "sweet spot" at this speed and prevent late burning exhaust popping down the exhaust manifold as it burns on the way out.

 

So static, vacuum and mechanical advance, all overlapping and adding their effects depending on engine speed and load to hit the "sweet spot" to get the most from the fuel.

 

 

 

To answer your question: Vacuum advance allows the engine to extract all it's potential at part throttle speeds below 2,000 to 3,000 RPM, which is pretty much where all driving is done. Sure it will run without vacuum advance but power and throttle response will be lacking leading to poor fuel economy.

[Braden]

thanks mike i understand that 100 perfectly, now on to those who have had experience with recurving dizzys and playing with the mechanical advance. what have you done to improve top end power only adjusting distributor timing? reason i ask is because when i first ran my new engine i didnt have the engine timed properly. i had probbaly 20 degrees initial advance, then the mechaical advance, and i had the vacumm advance running straight off the manifold (yeah i know dumb but i didnt have a clue at the time). so at idle the truck purred at about 800-1000 rpm, i decided it was time to adjust the timing so i got my timing light out. because the vacumm advance was hooked up wrong i never could get it timed right. (matchobx dizzy and i thought i had something installed wrong on it). so without using the timing light i turned the dizzy effectively advancing the timing but the engine seemed to run smoother. after driving the truck with the vacumm advance hooked up wrong and initial timing set extremely advanced i drove it, it had great power through the whole rpm range and had more to go past 6k rpm. now that i have unhooked the vacumm advance and timed the engine properly (tried 12 and 15 degrees) i have driven the truck and i have noticed a big difference in throttle response as well as rpms past 5k there is a flat spot where the engine feels like it quits (almost like a rev limiter. its so wierd how when i have the timing light out and i retard the timing down to 12 or 15 degrees the engine idles rougher (im assuming because of the cam) but when i advane the initial timing to 20 degrees or higher the rpms increase and the engine seems to run smoother. i just wish i had my power back because i love how it runs after 5k rpms that cam kicks in and she goes!!!! but now i cant take full advantage of it because of the dead spot after 5k rpms.

[Braden]

also anybody on here figure out how to run their vacumm advance with dual 44's???

[datzenmike]

As I stated in 2/, mechanical advance does not increase after 2,000 to 3,000 RPMs and is topped out at about 20 degrees+-. Add to the static timing by timing light and rotating the dizzy to get 12 degrees at idle you should have about 32 to 35 degrees at anything over 3,000 RPMs.

[Braden]

so the flat spot at high rpms is more or less a carb tuning issue at this point correct? i have a mikuni manual but it seems vague about setting mixture, the manual doesnt give starting points it just states set pilot then idle pilot then idle screws. i think i have it closer, i definitely made progress today on it but it takes alot of tuning and driving.

[datzenmike]

I would think so. Use a timing light and slowly rev to 3K with vacuum advance disconnected. Mechanical plus static should add up to about 32 degrees. This isn't written in stone. Some motors might like less or more. Advance should top out between 2,500 to 3,000. Race motors like their advance in sooner.

[banzai510(hainz)]

Mike I ask Ghetto to PM you.

44mm mikunis at leas t meine and maybe Gehtto dont have a vacuum port. So You will never run a vaccum adv I guess.

 

I dont know if it even matter if the canister diaphrame(vac adv) is bad or not. Your not using it.

 

Be honest I would swap in another dizzy as it cheap and fast way to trounbe shoot.

 

Nissan motorsport has diffent adv weigths or springs (I THINK)

 

Ghetto

when you ran of the manifold you have to remember u only ran off 1 cylinder also. So it would be a pulasated effect/ not a nice constance average of 4 cylinders.

 

I did this once with a PCV valve and it would make a noise at the cylinder. Intake then exhaust.

[datzenmike]

You could run manifold vacuum, basically this is what you are doing with the stock carb. The problem is setting the initial or static advance. 12 degrees might be too much when you plug the hose back in. The idle will jump way up and have to be turned back down. It might ping off idle because the vacuum advance curve is for no advance at idle.

 

You could plug in the manifold vacuum line and ignore the static advance and just adjust for best running. This is preferable to no advance at all. Won't cost anything to try.