bigbadchev84
1/2 ton status
can someone tell me were to hook my vacuume advance up to a ported source? I am running a q-jet. i looked briefly but all seemed to be full vacuume at idle.
CK5
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ported vacuume?
- Thread starter bigbadchev84
- Start date
It's been too long since i've played with a carb but check the fitting on the lower corner of the carb near that red vacuum cap. I also thing i remember there being a ported vacuum on the passenger side near the choke coil but i don't see it in this pic.
chulisohombre
1/2 ton status
looks like the yellow nipple cover is covering the vacuum advance.
bigbadchev84
1/2 ton status
that yellow one is full vacuume at idle
Give it manifold, see if it likes it. 
Jason4x4
1/2 ton status
I believe it is the black one under the fuel inlet line.
Why would you want to put the vaccum advance on a ported source? Pretty much totally defeats the purpose of the whole device! The vac advance bumps your timing up to around 20 odd degrees at idle which smooths it out, and lets it run far more efficiently. When you hammer on the gas, the timing drops back to your initial timing setting, then carries on using the mechanical advance weights.
Once you are cruising, and the engine is under very little load again, the vaccum levels rise once again, and your timing is again advanced a great deal which allows you to run a leaner more efficient fuel mixture on the highway too
Once you are cruising, and the engine is under very little load again, the vaccum levels rise once again, and your timing is again advanced a great deal which allows you to run a leaner more efficient fuel mixture on the highway too
Why would you want to put the vaccum advance on a ported source? Pretty much totally defeats the purpose of the whole device! The vac advance bumps your timing up to around 20 odd degrees at idle which smooths it out, and lets it run far more efficiently. When you hammer on the gas, the timing drops back to your initial timing setting, then carries on using the mechanical advance weights.
Once you are cruising, and the engine is under very little load again, the vaccum levels rise once again, and your timing is again advanced a great deal which allows you to run a leaner more efficient fuel mixture on the highway too
Almost every chevy has its vacuum advance hooked to a ported vacuum source from the factory.
Most *emissions* regulated chevies do. You won't see it nearly as much on HD trucks and 60's stuff.
It took EFI to allow GM to not have to compromise timing. No way for them to run 20* vac advance at idle and knock that back at cruising speeds while still maintaining decent mechanical advance.
You won't see it nearly as much on HD trucks and 60's stuff. It took EFI to allow GM to not have to compromise timing. No way for them to run 20* vac advance at idle and knock that back at cruising speeds while still maintaining decent mechanical advance.
That is coming from the same company who puts EFE valves on one side of the exhaust, effectively choking off half the engine, along with half a zillion other crazy "emissions" systems that only work for about 5 minutes when new
GM put the vacuum advance on a ported vacuum source merely for emissions. When you run the advanced timing at idle, your emissions aren't quite as good as when you have the timing retarded.
However, you'll have better throttle response, idle quality, and off idle pickup with the vacuum advance hooked up straight to manifold vacuum.
Cowboy_80
1/2 ton status
Do you have to do anything with smog in Oregon?
pauly383
Daddy383
Ported for part throttle cruise at light load .
I had posted these in a thread a couple years back . Find thread and it relates to my numbers . I am running some emissions , but no ported switches . And even with a 305 and emissions , it works fine . And its also a beat up wheeling rig , so I don't care if its not "pretty" .
I had posted these in a thread a couple years back . Find thread and it relates to my numbers . I am running some emissions , but no ported switches . And even with a 305 and emissions , it works fine . And its also a beat up wheeling rig , so I don't care if its not "pretty" .
Not sure if I understand your point correctly, but ported or manifold=no difference at cruise. In other words, at cruise both are providing the same amount of vacuum to the vacuum advance cannister.
Your differences there (if any depending on how it's set up) would come from mechanical or base timing.
Your differences there (if any depending on how it's set up) would come from mechanical or base timing.
pauly383
Daddy383
The port it comes off of should be vacuum at part throttle ,while cruising with the pedal down a little bit . Not idle , and not at WOT .
If it was a Chevelle with a lumpy old school cam , I would hook it right to the manifold .
If it was a Chevelle with a lumpy old school cam , I would hook it right to the manifold .
I gotcha, specifying WHICH port you are right, there are two timed ports on later Q-jets, EGR and vac advance.
WOT wouldn't matter either, no vacuum.
More advance at idle tends to work best, but can only add so much with a vacuum cannister.
WOT wouldn't matter either, no vacuum.
More advance at idle tends to work best, but can only add so much with a vacuum cannister.
I disagree. Manifold vacuum increases toward idle (very little manifold vac at WOT). Ported vac is the opposite and rises with RPM (zero vac at idle) - there is a little port drilled in the side of the throttle bore, and as air rushes past it faster and faster, the pressure drops.
Soooo, man vac would decrease timing as RPM's ramp up, whereas ported vac would increase timing.
If you want my opinion, with a modern engine and unleaded fuel I would use manifold vacuum.
pauly383
Daddy383
Sorry if I sound like an *** , but using manifold vacuum goes against all the money the manufactures threw at engineering those motors systems .
I'm not offended.
What manifold goes against are the *emissions* systems. At WOT and cruise, it doesn't matter one bit if there is manifold or ported vacuum, there is no difference in the amount of vacuum pulled. Ported is simply manifold vacuum for a specific throttle opening. "Timed" vacuum is a better description. The source for the vacuum is still the manifold, and that is still 100% based on engine load/throttle opening.
GM likely spent millions developing all the emissions stuff, which is why there are things like thermal vacuum switches, secondary lockout and what not, all trying to get the engine to heat up faster, and keep a cold engine from polluting as much. No argument there, you start monkeying with the components you throw the system out of whack. But that doesn't mean it's what the engine wants, or how it will perform best.
If you crank up the timing at idle, doesn't your idle RPM increase? Doesn't the engine smooth out? Sure it does, it's just not helping emissions.
What manifold goes against are the *emissions* systems. At WOT and cruise, it doesn't matter one bit if there is manifold or ported vacuum, there is no difference in the amount of vacuum pulled. Ported is simply manifold vacuum for a specific throttle opening. "Timed" vacuum is a better description. The source for the vacuum is still the manifold, and that is still 100% based on engine load/throttle opening.
GM likely spent millions developing all the emissions stuff, which is why there are things like thermal vacuum switches, secondary lockout and what not, all trying to get the engine to heat up faster, and keep a cold engine from polluting as much. No argument there, you start monkeying with the components you throw the system out of whack. But that doesn't mean it's what the engine wants, or how it will perform best.
If you crank up the timing at idle, doesn't your idle RPM increase? Doesn't the engine smooth out? Sure it does, it's just not helping emissions.
pauly383
Daddy383
I went off and found what I was looking for . The vacuum switch originally on there had ported when cold , and manifold when warm . I have no cooling issues so I use ported full time for part throttle cruise economy and emissions passing .
Tried full time and she doesn't like it .
Tried full time and she doesn't like it .
You don't sound like an arse at all - I just don't agree with you on this point...and I'm an engineer.
It's mostly a question of design drivers at the time the vehicle were new.
During the period of time that quadrajet carbs were used, engine emissions and fuel economy were the key drivers in design - not performance or driveability. This is the same reason those same engineers put low-compression smog heads on almost every truck from the mid 70's thru the mid 80's.
I believe that with modern engines and modern fuel, you can obtain the best performance and driveability results with manifold vacuum. This is based on my own results experimenting with properly timed engines.
It's mostly a question of design drivers at the time the vehicle were new.
During the period of time that quadrajet carbs were used, engine emissions and fuel economy were the key drivers in design - not performance or driveability. This is the same reason those same engineers put low-compression smog heads on almost every truck from the mid 70's thru the mid 80's.
I believe that with modern engines and modern fuel, you can obtain the best performance and driveability results with manifold vacuum. This is based on my own results experimenting with properly timed engines.
