Vacuum advance. To port or not to port?
- Thread starter Blew86442
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I have the same cam as you, and I find it likes a lot of timing at idle (initial), which can put your total above where you would like it to be and cause detonation. Say for ex. if you found your motor liked initial timing at 20 btdc and your dist. had 26 degrees of mechanical advance, that would put your total at 46, which is pretty high. You can easily fix this with a re-curve kit and limit the mechanical advance. But you could also use your vacuum to help you out, and that is the benefit of being able to choose between manifold and ported. Having the vacuum advance connected to full manifold vacuum will give you that extra timing at idle without having to raise you total timing when at full throttle. So you can set your total at 36 but then at initial it will drop down to 10 btdc and your engine might run a little rough, but when you connect the advance to full vacuum it will add more advance at idle and bring it up to say 20-22 depending on how much advance it has.
If you dont need the extra timing at idle then you can connect it to ported. Say your motor likes to idle at 10 btds, so 10 + 26 degrees of mechanical advance = 36 btdc which is what most people usually aim for, so there is no need to run full vacuum advance so you can use ported.
Hope this makes sense
If you dont need the extra timing at idle then you can connect it to ported. Say your motor likes to idle at 10 btds, so 10 + 26 degrees of mechanical advance = 36 btdc which is what most people usually aim for, so there is no need to run full vacuum advance so you can use ported.
Hope this makes sense
jrm81bu said:
I understand this but what you're describing the exact same thing as what happens when using full manifold vacuum. Ported is the same as manifold vacuum except at idle ( and I'm assuming very light throttle positions ) where it's zero.
Bonnewagon, ported vacuum is the same as manifold vacuum at WOT which is zero ( or close to it ). It's not higher. Using a vacuum gauge hooked up to many carbs in the past has easily confirmed this. Are you not confusing ported vacuum with venturi vacuum ( which many carbs don't even have a port for and I'm not very familiar with )? I am not familiar with that vacuum gizmo, but it looks very similar to the ones on Olds 307's but has a very different function.
The only 'advantage' ported vacuum has over manifold...or so I've heard...is that when you stomp on the gas quickly, ported can provide a split second 'spike' in vacuum ( which would provide you more advance ) which could aid in throttle response. I have read this is not always the case though and/or it doesn't make any noticable difference.
First you gotta understand what's going on inside. Manifold vacuum port is UNDER the throttle plate. Ported vacuum port is ABOVE the throttle plate. Motor running, throttle closed, manifold vacuum is high- pistons sucking like a hooker- producing high vacuum. When the throttle opens, atmospheric pressure rushes in, vacuum drops. When you floor it, throttle opens wide, manifold is open to atmosphere, vacuum drops to nothing. As rpm increases air flowing in restores vacuum signal, but the manifold vacuum port is actually seeing a version of ported vacuum at this point, air flowing past. Motor running, throttle closed, ported vacuum port sees no vacuum signal at all, the air slipping past the throttle at idle is negligible. Open throttle, air flowing past the port creates a pressure drop (vacuum) that gets bigger as the throttle is opened up more, vacuum signal goes up. Ported vacuum goes up more as throttle is opened and at WOT it equals manifold vacuum as both are seeing the same thing, air flowing past the ports. Remember I said "near" WOT.
My experience, my friends experirnce, and numerous articles I have read show different results that what you're saying. The way you describe it, ported vacuum ( which we can all agree is zero at idle ) steadlily climbs up and up as you give more throttle until it suddenly matches manifold vacuum at near WOT where it suddenly goes to near zero. Various vacuum gauges on various cars have shown me personally that ported vacuum does behave like that or should I say not to the extreme you're saying. Of course not every ported vacuum on every carb was calibrated to work exactly the same way and that could be one of the reasons my experiences don't jive with yours.
Now here's something I've never heard anyone do; what if you ran both ported and manifold? You'd always get the vacuum that the higher of the two would provide at that instant. Probably wouldn't be able to get the curve right to run with both but perhaps one could.
Now here's something I've never heard anyone do; what if you ran both ported and manifold? You'd always get the vacuum that the higher of the two would provide at that instant. Probably wouldn't be able to get the curve right to run with both but perhaps one could.
Well DoubleV you got me curious so I did some reading. Pretty much the only difference between using ported or manifold vacuum is you don't get any advance at idle with the ported. Just like you said. Apparently they started using ported to increase the exhaust temp at idle for emissions. So it still comes down to how you set up your base and mechanical timing. If you don't want/need a lot of timing then you may be better off with manifold vacuum to help gain some at idle. If you are already using a lot of advance at idle you will most likely be better off with ported vacuum.
Thats also what I have read and heard .... but my vacuum advance is too much...so Im using the ported...and the car stumbles sometimes and idles rougher when cold...so, Im gonna get another vacuum canister that doesnt add so much to my total and hook the vacuum line back up to the manifold port.jrm81bu said:
bill said:
I think the key to using manifold vacuum advance is choosing the vacuum canister that opens at the correct rate and correct vacuum. I think, perhaps, most distributors come with canisters that work for ported vacuum but not for manifold vacuum due to opening rate and vacuum.
depending on your set up. its like what some of the others said. do some testing. With what you have listed here, id run off the carb like you got. If your carb is working right, the port that your vacuum advance is hooked up to runs off of metered vacuum, Basically means that it only pulls vacuum when your engine reach higher rpm's. this gives you better MPG and performance when you want it. if your not worried about MPG, than run it off the intake and have the advance whenever you want. Its up to you and how YOU want YOUR set up to be.Blew86442 said:
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