which vacuum port?

[72K10]

Wondering which port on my Edelbrock 1406 to use for distributor vacuum line. I've been using the passenger side, but today I switched to the driver's side, and the engine picked up considerable rpm's. Not sure if that means it's better, but got me to wondering which side I'm supposed to be hooked up to. Does it make any difference?

Thanks,

Chris

[Slammed67]

Driver's side is correct. Do you have the owner's manual for the carb? If so, I belive it explains the differences of the two ports.

[72K10]

Slammed, I checked out your website. What a beautiful truck!! Man, you have done some really great work there. Let us all know when you finish it!

I somehow lost the owner's manual for the carb, which is a real pain. Why driver's side? Does your manual say why? Thanks,

Chris

[Gordo]

I have found using the ported vaccum on the pass side works best for a daily driver looking for fuel economy. No need for full vaccum advance at an idle on a stocker.

[crazy longhorn]

For the most part, hot cams & fat carbs like full manifold on the vac advance.... to set them up usually requires a little recurving & an adjustable adv can. The chevs ran full manifold vac to about the mid 60's, till the carburation was leaned out going into the 70's . Most of the factory vac cans put out way too much vac timing, & were too conservative on the intial/mechanical advance for performance usage. even my old plow truck (stock 355, with a crane 266) is running full manifold on the vac advance. I would say try it both ways.......run it how it works best

[mikep]

OK heres the deal. When you hook up your distributor to manifold vacuum ( ported ) you give it full idle vacuum which boosts your timing at idle. That allows you to set it at 10 degrees static or so for easy starting with a stock starter. Once it is started you develop vacuum and your timing jumps up to 25-35 degrees or so . That gives you a smooth idle with anything over a stock cam. It also gives you excellent throttle response off idle. When you gas it your vacuum level drops since your throttle plates are opening up and with it the amount of vacuum advance you are gaining. So your vacuum advance goes down while your mechanical advance begins to take hold . That allows you to avoid detonation under full throttle while still giving you higher than normal advance under cruise conditions. I run a very mild cam and high compression for a street motor. With 92 octane I can run almost 45 degrees of total timing under cruise conditions. That drops to 32 or so under WOT so I avoid detonation .

For a motor you want to run good. Use 12 or so degrees timing with the distributor disconnected . Hook the vacuum line to the distributor up to manifold ( vacuum at idle ) vacuum.

If your goal is to maintain low NO2 levels at idle then by all means hook it up to venturi ( timed ) vacuum. Thats all it is good for.

[72K10]

I've gotten an education here - thanks much for the replies and information. I'm going to give the full manifold vacuum a chance.

[Longhorn Man]

While agreeing with the aformentioned explination, (and learning from it myself) but I still say that once an engine has been modified, you need to test and play with it looking at things such as idle quality, MPG, and performance.
I know mine likes having the vac disconnected all together.

[our64]

ported side is best for a a mild engine

[72K10]

switched to the full manifold vacuum side today and got a big improvement in performance. I was having some bogging under load at low rpms; this cleared it up nice. I will have to agree with the advice - test out both sides or neither to see what works best. Thanks all for the help.

[mikep]

One more thing I forgot. If you have a big cam , using ported vacuum will allow you to run a lot of advance at idle. That will let you close your throttle plates more than you normally to maintain idle which has the added benefit of letting you actually tune your idle mixture with the idle mixture screws. That way you get a smoother transition from idle to primary feed circuit so you dont get that nasty bog action and you spend less time monkeying with the crutch AKA the accelerator pump.

[breeh]

My understanding is that the two types of vacuum available are full manifold vacuum and ported vacuum. I think that MIKEP is talking about full manifold vacuum but calling it ported vacuum?? Anyway, the benefit of ported vacuum was used when EGR systems were used in the 70's and into the 80's. I have had great success in using the full manifold vacuum. I pulled the info in the following post from the Camaros.net site. There was a serious ignition guru over there.

http://64.246.42.114/vboard/showthre...threadid=31344

[mikep]

Ported = manifold

timed = venturi

[crazy longhorn]

Those terms can be confusing at times, due to the way they are listed, or used...... To look at a Holley diagram, the nipple on the metering block is refered to as "timed spark"(above the throttle plates). The nipple in the base of the carb is refered to as manifold vacumn(below the throttle plates). now go look at the edelbrock site on the exploded veiw of their type carb......#51 is listed as "fulltime vacumn port", & #50 is listed as "ported vacumn port"! I have always refered to timed spark as ported......as there is a small port drilled above the throttle plates to eliminate the vacumn signal @ idle.......AL