just read somewhere else on here it would be better to use the manifold as a refrence for the vac advance in a DSII is this true? i have a carter carb and i have noticed if i watch the vac advance arm when i open the throttle it will move then go back to its original position. as of now i am using the carb for vac. is this right?
where to locate vac advance point carb or manifold for dsII?
- Thread starter karfer67
- Start date
Im running direct manifold vac on all my cars. Most people I know are doing it that way. There are some people here that feel strongly that that is bad but I will let them explain that.
It can vary carb to carb and year to year but in general higher up ports on the carbs are 'ported', 'controlled', or 'timed'. That means they are not just straight manifold. There can also be manifold ports on a carb. Manifold vac is what ever vacuum the engine is creating. When the throttle is closed vacuum is high because the engine is trying to suck more air than the throttle is letting in. WHen you open the throttle vacuum drops because all the sudden the engine can get all the air it needs. If the engine is not loaded the engine easily then speeds up with that extra air (and fuel) and gets back to that point where the throttle is holding it back so vacuum goes high again. If the engine is loaded the vacuum stays low because basically you keep the throttle more open to try and get more air (and fuel) in to try and increase the speed which increases the air (and fuel) demand. The ported, timed, or controlled vacuum ports on the carbs do something to alter that signal. On some the ports are actually covered by the throttle blades so they only come on at certain throttle positions. Some are just simply above the throttle plate so they are not exposed to vacuum unless the throttle blades are open. The later you get the more of those type of ports there are for various emissions gadgets. In your case it sounds like you have a ported source with is not on at idle which is typical. This means you dont get any advance at idle which is good for emissions. As soon as you open the throttle the vacuum port opens so the dizzy is exposed to vacuum which pulls in the advance. If you had a load on the engine the vacuum would not have been very strong so the advance would likely have not moved much until the load reduced then it would pull in some advance. It helps if you think of it as a vacuum retard. The engine can take lots of advance under light or no load where it cant handle it is under heavy load. Now if you think about how the vacuum reacts. No or light load means vacuum is high (because the throttle is basically holding it back) so we get advance which is good. High load vacuum is low and also when the engine cant handle the advance so it get retarded. Kinda neat huh? And they dont need a complicated computer to figure it out with a million sensors like we have now.
It can vary carb to carb and year to year but in general higher up ports on the carbs are 'ported', 'controlled', or 'timed'. That means they are not just straight manifold. There can also be manifold ports on a carb. Manifold vac is what ever vacuum the engine is creating. When the throttle is closed vacuum is high because the engine is trying to suck more air than the throttle is letting in. WHen you open the throttle vacuum drops because all the sudden the engine can get all the air it needs. If the engine is not loaded the engine easily then speeds up with that extra air (and fuel) and gets back to that point where the throttle is holding it back so vacuum goes high again. If the engine is loaded the vacuum stays low because basically you keep the throttle more open to try and get more air (and fuel) in to try and increase the speed which increases the air (and fuel) demand. The ported, timed, or controlled vacuum ports on the carbs do something to alter that signal. On some the ports are actually covered by the throttle blades so they only come on at certain throttle positions. Some are just simply above the throttle plate so they are not exposed to vacuum unless the throttle blades are open. The later you get the more of those type of ports there are for various emissions gadgets. In your case it sounds like you have a ported source with is not on at idle which is typical. This means you dont get any advance at idle which is good for emissions. As soon as you open the throttle the vacuum port opens so the dizzy is exposed to vacuum which pulls in the advance. If you had a load on the engine the vacuum would not have been very strong so the advance would likely have not moved much until the load reduced then it would pull in some advance. It helps if you think of it as a vacuum retard. The engine can take lots of advance under light or no load where it cant handle it is under heavy load. Now if you think about how the vacuum reacts. No or light load means vacuum is high (because the throttle is basically holding it back) so we get advance which is good. High load vacuum is low and also when the engine cant handle the advance so it get retarded. Kinda neat huh? And they dont need a complicated computer to figure it out with a million sensors like we have now.
I have two different carbs I have used. One has a timed port and a full vacuum port in the throttle body. The other has only a timed port up on the body of the carb. The first one operates on throttle blade position in relation to the vacuum port in the throttle body and the second apparently operates on venturi pressure differences. My engine will operate on either one BUT the throttle blades must be opened a bit more using the timed ports vs a full vacuum source. I prefer the full vacuum source.
It only takes a little "Try n See" as to which will operate for you and your engine.
It only takes a little "Try n See" as to which will operate for you and your engine.
it is just an interesting idea i wonder if it would help cure the engine's low end bog at the track. i launch get some tire spin out about 8 feet then when it hooks it is like the timing is not there and for a second the engine falls off then picks back up like the timing catches back up.
I have never tried it myself but I know some guys that have put a valve in the vacuum line to restrict (slow down) the response. If I remember correctly he got it at a fish tank store. I guess they are used to regulate stuff on the air pumps and are cheap. Some vacuum cans also are adjustable.
DupleDreux
Active member
I run (ran?) straight manifold vacuum, but use a Crane adjustable vacuum canister, I think it was about $70. just follow the timing tuning instructions, basically turning a little allen screw inside the canister until its perfect.
ok tried it out and omg what a difference. i removed the line for the wipers and put the vac advance line in with a small plastic T and it is night and day. it idles perfectly now, throttle response is better and low end power is better too. not saying it will help on everyone's car i would think it depends on the carb they are running but at least with mine it works very well.
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