What is the difference between ported and manifold vacuum?

 

Not exactly sure how it functions but he goes, the ported vacuum is pulled through the carb and the port is opened when the throttle is cracked a certain amount while manifold, or unported, is a direct link either from the manifold or through the carb that is always open and pulling vac when the eng is running.

 

Manifold vac. as it relates to your carb. is the lower of the two ports, it pulls vac. from beneath the throttle plates, therefore always on. the ported vac.is located above the throtte plates, therefore no vac. till you come off idle and expose this port to manifold vac. Run your vac. advance can off the ported vac. port. Hope this helps, Bob

 

The EGR should be connected to manofold vacuum?

 

Manifold vacuum is below the throttle plates - high vacuum at idle (throttle plates closed) and decreases as plates open. Ported works the opposite - pretty much no vacuum at idle, increases as plates open.

Paul

 

At WOT you should little or no vac from either but at cruise you should get a high vac signal from both, right? Your desc makes it sound like the manifold/ported vac would have less vac at let's say 3000 rpm's cruising than at idle but I would think it migh even have more than at idle.

Just trying to clarify.

 

If you have a Q-Jet then this may help:

https://www.corvetteforum.com/techti...uumFitings.jpg

This was taken from the original carb connections when I got my car.

As was explained manifold vacuum is below the carb, thus when the throttle plates are closed (foot off the gas) the manifold vacuum is high. Ported vacuum is not all the same. There are several ported vacuum nipples on a Q-Jet and they all draw at different times. For ignition advance there should be little or no vacuum at idle and the vacuum will increase at a steady rate throughout the throttle plate range until it is equal with manifold at WOT, so the ignition vacuum signal should be a straight line. For EGR there should be a similar signal, however under WOT I think there should be little or no vacuum so the EGR signal should be a bell curve. At least that is how it should work in theory.


[Modified by Rockn-Roll, 9:54 PM 6/30/2003]

 

Think I'll hook up a vac line and gauge to the diff ports on my holley dp and see what readings I get at diff throttle positions and rpm levels plus cruise vs WOT.

 

Yeah, I'm going to the parts store for a 6' long vacuum line and T so I can watch the vacuum signals while I drive. Let's post the results in this thread so we can see what it going on.

 

:thumbs:

 

OK, I ran a 8' vacuum line from the carb vacuum outlets one at a time into the cab to a vacuum gauge. I determined that the pic I have here is the best vacuum signal, though not ideal in my opinion:



The other outlets had a signal very much like manifold vacuum. I didn't bother with the details of each port, but I did determine that the one on the far left and bottom was the best signal. It has no vacuum at idle, then increases as the throttle is opened; however, vacuum will drop if the throttle is opened wider than just a bit more than cruise opening. In other words if the throttle is opened just a little bit then the vacuum will increase and hold steady while the rpm's catch up. In this way vacuum signal will increase with rpms until it is equal to manifold vacuum at redline. But, if the throttle is opened a bit more before rpm's catch up then vacuum will drop!?!?! Also, when held at WOT the vacuum signal stays at zero even at redline!?!?! Plus, when letting off the gas the vacuum jumps up to equal with manifold vacuum!?!?!

I think there is something very wrong with this. Maybe Lars could shed some light on why this is the case. I was under the impression that vacuum and thus ignition timing should increase/advance as the throttle is opened.


[Modified by Rockn-Roll, 10:13 PM 6/30/2003]

 

That sounds right to me. Unless there is a one way valve, the vac reading for ported or unported feed off eng vac so the signal should drop until rpm's start to level off and the eng can produce vac. At WOT you want no vac signal so your timing drops to yout max power producing timing adv of about 34-38 degress. When running the eng is basically a pump and the carb operates like a valve that regulates vac, when wide open it allows are to rush in and vac drops untill the eng can catch up.

 

Hmm, the way I see it the quantity of charge in the cylinder directly effects the amount of time required for that charge to burn completely. At idle the charge is low enough that it can burn completely if the mixture is ignited just a hair before TDC...6º for my L48. But, even if the charge density remains constant the ignition would need to be advanced due to RPM so that the amount of time would remain the same. This advance is accomplished by the weights in the distributor. However, the amount of air/fuel charge can be increased by opening the throttle. This increase in charge also increases the amount of time required for the mixture to burn completely independent of the RPM, thus an increase in ignition independent of RPM is required. This increase is designed to be accomplished through the vacuum controlled system.

So, ideally with vacuum plugged the ignition advance should climb slowly as RPM's increase throughout the full range of the operating RPM's. Then when vacuum controlled advance is added then ignition advance should ALSO increase throughout the full range of the throttle opening. Obviously the system is not operating as is required from an engineering perspective.

I believe this is one of the extended benefits of computer controlled spark systems which increase ignition advance depending on a sensor located in the throttle bodies of EFI systems via either mass air sensor or throttle plate position sensor. Oh well, if I want to run a stock system then I may have to put up with the slop. Or, find someway to hide a throttle position sensor and vacuum regulator to get the timing that I want.


[Modified by Rockn-Roll, 5:37 AM 7/1/2003]

 

I agree FI is the way to go to squeeze out max hp and economy, reason I will be converting in the near future. Seems to me with the vac adv hooked up there is sometimes a brief pause before the timing drops back to the adv 'sweet spot'. When I run without it there seems to no lag, just instant throttle responce. Not a big deal I guess, trading fuel economy for a little lose of throttle responce.