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This is for my 1981 where I FINALLY removed the E4ME carb and ECM controlled distributor!
I had a not to local carb shop rebuild a 1978 Rodchestor Quadrajet that was sitting under my bench for decades...he rebuilds then runs them on a smal lblock test engine (Bowen Carburator in Bristol Pa)
The Carb shop told me to use "ported vacuum" for my new vacuum advance HEI and pointed to the lower left (see white paint) port...but I believe that is Manifold vacuum because it is UNDER the throttle blades. I thought... a Vacuum advance distributor does-not want vacuum at idle...getting confused over here.
Feedback, please!
This has been covered quite a bit here. I believe Lars has a write up on ported vs manifold.
The short answer... full manifold vacuum for vacuum advance on a street car. Check your timing with/without advance to see how much it is advancing.
-Stroke
EDIT: The long answer would literally just be me quoting that entire article. You may as well go read it. I’ll see if I can find it and link it for you.
Last edited by Strokemyaxe; 10-13-2018 at 12:15 PM.
I used manifold vacuum, and was pleased with the results. I also followed lars' instructions (on the forum, send him an email for his instructions) on timing, targeting total advance, and that made a huge difference. I had to change out my distributor, and replace every adjustable component, to get it where I want it, but it was well worth it. There are lots of diagrams on the internet, or you can check at idle with your finger, or a vacuum pump/gauge.
If it has suction at idle, it is a manifold vac port. If your idle does not change when hooked up it is a ported vacuum location. There are emissions reasons for ported vacuum but we try to run manifold vacuum when possible.
That choice also has to deal with how you set your timing. If you set it per the GM 'method' at idle with vacuum removed advance can (and plugged off), you most likely will run better using manifold vacuum. If you set timing for 'performance' (again using one of Lars' papers), you should select vacuum source which provides the best idle and low-throottle start quality. My '71 SB L-48 engine is timed per Lars' performance guidelines and it still operates best with manifold vacuum going to the advance can.
P.S. The ONLY difference of operation between 'manifold' and 'ported' vacuum to the advance can is during idle and low-throttle acceleration. With cruise and higher throttle acceleration, manifold vacuum and ported vacuum are essentially at the same vacuum level.
Run ported vacuum if you need to pass an emissions test or if you are very concerned about global warming and think your contribution will make a difference. If you want to have fun with your car and enjoy best performance, run manifold vacuum. The lower left port you have identified in your photo is ported vacuum, normally used for the EGR operation on your engine and on the '78 engines. You should used the port in the forward, driver's side of the float bowl (you have a long, black cap on it) - that port is manifold vacuum, and it's the one that GM used for vacuum advance in 1978.
If you want all the info, e-mail me for my timing papers and I'll send you everything you need to know.
Run ported vacuum if you need to pass an emissions test or if you are very concerned about global warming and think your contribution will make a difference. If you want to have fun with your car and enjoy best performance, run manifold vacuum. The lower left port you have identified in your photo is ported vacuum, normally used for the EGR operation on your engine and on the '78 engines. You should used the port in the forward, driver's side of the float bowl (you have a long, black cap on it) - that port is manifold vacuum, and it's the one that GM used for vacuum advance in 1978.
If you want all the info, e-mail me for my timing papers and I'll send you everything you need to know.
Lars, thanks for that;
referencing to my original very top picture...are you stating the LARGE drivers side-pointing port is manifold vacuum? (that went to my charcoal canister) or, the smaller caped port under it? That originally went to the top of my thermostat housing.
(it appears I already attached the vacuum hose to the wrong port-I thought a port below the butterflies was manifold vacuum) As seen the the pic below... the port I believe your referencing to went to the dual-vacuum hose (gizmo) ontop of the thermostat housing...I also want to delete those hoses too. I just want really good idle quality, smooth running engine, fuel efficiency, drive-ability...not looking to hug any trees or...visit the 1/4 mile drag-strip.
Normally, distributor vacuum for manifold vacuum will be attached to the port shown in your photo above with the hose on it at the far right in the photo. That's manifold vacuum on your carb. That same port is ported vacuum on earlier model carbs. You can also hook it up to the port just to the right (passenger side, or left as viewed from the front) of the power brake fitting in the back of the carb, or to any other port other than the bowl vent nipple (large nipple top front) or EGR nipple (bottom forward passenger side corner of the baseplate). The EGR nipple, which is the one you identified with white paint in the first photo, is ported vacuum. It's the only ported vacuum nipple on your carb. The location of a nipple on the carb is completely irrelevant as to what type of vacuum the port produces - it will produce vacuum based on where the port originates in the throttle bore, which can be anywhere depending on how the port is drilled and routed.
I used manifold vacuum, and was pleased with the results. I also followed lars' instructions (on the forum, send him an email for his instructions) on timing, targeting total advance, and that made a huge difference. I had to change out my distributor, and replace every adjustable component, to get it where I want it, but it was well worth it. There are lots of diagrams on the internet, or you can check at idle with your finger, or a vacuum pump/gauge.
Wooooow! So... I set my harmonic ballancer at 10-degrees advance and dropped-in the new HEI with the Rotor pointing at #1 I delete a few more... non-needed vacuum lines, map-sensor line, charcoal canister, unplug more unused ECM controlled items and tidy-up the engine area of all that BS and excess wires. Hooked-up...the Dizzy vac line to the Manifold-Vacuum nipple as (Lars) and other suggested (not ported vacuum) Unplug that darn ECM... and let the plugs hang (fu*k that computer) Put in the ignition Key....and within 5 seconds of cranking (standing outside the car) the Engine lights right up! I notice... (by ear) timing is off just a tad so I rotate/advance distributor a C-hair and it's smooth as silk! Let her idle... for a few minutes and then restarted, one tap of the key and she fired-up. Revved...the engine a little she sounds happy I ran out of time tonight... so tomorrow I'll warm the engine, pull and block the vacuum Dizzy line, hook-up the Dial-Back Timing Light and go for 32-35 degrees total timing, all in around 2800 rpm...I hope the vacuum Can on this Chinese HEI falls-in (not too soon or too late) to where I need it to be...fingers crossed. Atest dive tomorrow...fingers crossed no lean/rich or bogging/stumble issues and I have a good test drive.
*I read all of Lars info on Quadrajets...(thanks for emailing it to me) a LOT to chew-on and digest and while I know the Holley 4150 like the back of my hand for 'me' ... the Q-Jet is from another Planet. *Next obstacle...for highway cruising I would like to get the TH350c TQ to lock-up...computer controlled that (fu*k that computer)
Most of “MSD style” HEI’s come with an adjustable vacuum can. Adjusted by inserting an Allen wrench inside the vacuum port and rotating.
I remember some of the "adjustable" vacuum cans.
No MSD $$$$ here, my distributor is a $50 Billet eBay Chinese special, I'm doubtful it is adjustable.
When I looked inside I didn't see anything to turn. And of course no paperwork comes with the eBay Distributors.
I remember some of the "adjustable" vacuum cans.
No MSD $$$$ here, my distributor is a $50 Billet eBay Chinese special, I'm doubtful it is adjustable.
When I looked inside I didn't see anything to turn. And of course no paperwork comes with the eBay Distributors.
If you find that the vacuum advance is adjustable, this is from my ACCEL 9107 Distributor:
Step 13 – Insert the supplied 3/32” hex wrench in the vacuum canister nipple and rotate the adjusting screw counter-clockwise until it bottoms against its stop, then turn four complete turns clockwise. This is approximately 7” dist. (14” eng.) at approximately 12” vacuum. This is a starting point for the vacuum advance adjustment. If surging or pinging is noticed at cruise RPM, turn the adjustment counter-clockwise until condition is no longer noticed. If more advance is needed, turn adjustment clockwise.
The "adjustable" vacuum advances are only adjustable for the amount of vacuum force it takes to pull them in - all you're doing with the "adjustment" is changing the pre-load on the diaphragm spring. The critical feature - the length of the vacuum curve - is not adjustable. Thus, most of these "adjustable" cans have a vacuum advance curve that is way too long (usually 18 to 20 degrees), and they are not usable. They need to be used with a mechanical limit/stop, or the best option is to replace them with a unit of known configuration as outlined in my Vacuum Advance Tech Paper (available from me via e-mail request).
The "adjustable" vacuum advances are only adjustable for the amount of vacuum force it takes to pull them in - all you're doing with the "adjustment" is changing the pre-load on the diaphragm spring. The critical feature - the length of the vacuum curve - is not adjustable. Thus, most of these "adjustable" cans have a vacuum advance curve that is way too long (usually 18 to 20 degrees), and they are not usable. They need to be used with a mechanical limit/stop, or the best option is to replace them with a unit of known configuration as outlined in my Vacuum Advance Tech Paper (available from me via e-mail request).
Lars
Lars, I have a LOT of reading to do... and hope to comprehend what I needed to know, as for an emergency (90 year old father in the ER) and 'if' my Truck goes down, I need this car on the road.
With HEI Vacuum distributor unplugged from a vacuum source;
I set total advance at 32-degrees at 3000 rpm...initial drops to 12-degrees at 800 rpm
(I hoped that was a done deal...no way)
I hook-up the distributor to 'manifold' vacuum;
800 rpm timing is at 35-degrees
3000 rpm timing is at 52-degrees
I can't drive the Car like that
I try the 'ported' vacuum nipple at the carb;
800 rpm timing is 15-degrees
3000 rpm timing is at 36-degrees
4000 rpm timing is 50-degrees and will continue to climb with more rpm
(how I drive, my engine never really sees 3500 rpm)
Lars, I have a LOT of reading to do... and hope to comprehend what I needed to know, as for an emergency (90 year old father in the ER) and 'if' my Truck goes down, I need this car on the road.
With HEI Vacuum distributor unplugged from a vacuum source;
I set total advance at 32-degrees at 3000 rpm...initial drops to 12-degrees at 800 rpm
(I hoped that was a done deal...no way)
I hook-up the distributor to 'manifold' vacuum;
800 rpm timing is at 35-degrees
3000 rpm timing is at 52-degrees
I can't drive the Car like that
I try the 'ported' vacuum nipple at the carb;
800 rpm timing is 15-degrees
3000 rpm timing is at 36-degrees
4000 rpm timing is 50-degrees and will continue to climb with more rpm
(how I drive, my engine never really sees 3500 rpm)
That looks a little high, but not too crazy off. Vacuum advance is only at cruise, light acceleration. No/little load on engine leaner mixtures etc....
My Curve
13 base
19 mechanical (all in @3k)
12-13 vacuum advance.
44-45 total timing at highway cruise.
Also, fwiw, my Vacuum can is adjusted by setting a lock out on max length the diaphragm can travel. It doesn’t “pre-load” like the ones Lars spoke of. I’d have to check, but I think it’s Moroso.
-Stroke
EDIT: I just noticed that whole response was intended for LArs. Oh well. That’s my input anyway. Lol
Last edited by Strokemyaxe; 10-16-2018 at 12:56 AM.
Lars, I have a LOT of reading to do... and hope to comprehend what I needed to know, as for an emergency (90 year old father in the ER) and 'if' my Truck goes down, I need this car on the road.
With HEI Vacuum distributor unplugged from a vacuum source;
I set total advance at 32-degrees at 3000 rpm...initial drops to 12-degrees at 800 rpm
(I hoped that was a done deal...no way)
I hook-up the distributor to 'manifold' vacuum;
800 rpm timing is at 35-degrees
3000 rpm timing is at 52-degrees I can't drive the Car like that
I try the 'ported' vacuum nipple at the carb;
800 rpm timing is 15-degrees
3000 rpm timing is at 36-degrees
4000 rpm timing is 50-degrees and will continue to climb with more rpm
(how I drive, my engine never really sees 3500 rpm)
Yes you can unless the car is pinging at cruise or light loads. If the car isn't ping you should be fine. If it is pinging, you have four options to try. Switch to premium fuel. May eliminate ping. Reduce initial timing. Easy. Reduce mechanical advance. Reduce vacuum advance. Or a combination of two or three.
Try driving with the vacuum advance disconnected/plugged. If it runs fine then the initial/mech'l advance is probably ok. Now connect vacuum advance. If it pings, you can modify or buy a vacuum advance canister that has a reduced advance. Trial and error but very doable.
Lars, I have a LOT of reading to do... and hope to comprehend what I needed to know, as for an emergency (90 year old father in the ER) and 'if' my Truck goes down, I need this car on the road.
With HEI Vacuum distributor unplugged from a vacuum source;
I set total advance at 32-degrees at 3000 rpm...initial drops to 12-degrees at 800 rpm
(I hoped that was a done deal...no way)
I hook-up the distributor to 'manifold' vacuum;
800 rpm timing is at 35-degrees
3000 rpm timing is at 52-degrees
I can't drive the Car like that
I try the 'ported' vacuum nipple at the carb;
800 rpm timing is 15-degrees
3000 rpm timing is at 36-degrees
4000 rpm timing is 50-degrees and will continue to climb with more rpm
(how I drive, my engine never really sees 3500 rpm)
If the rpm at anything but idle were not loaded they are meaningless. Free-revving doesn't not accurately reflect the actual vacuum/advance the engine sees in real life.
Lars, I have a LOT of reading to do... and hope to comprehend what I needed to know, as for an emergency (90 year old father in the ER) and 'if' my Truck goes down, I need this car on the road.
With HEI Vacuum distributor unplugged from a vacuum source;
I set total advance at 32-degrees at 3000 rpm...initial drops to 12-degrees at 800 rpm
(I hoped that was a done deal...no way)
I hook-up the distributor to 'manifold' vacuum;
800 rpm timing is at 35-degrees
3000 rpm timing is at 52-degrees
I can't drive the Car like that
I try the 'ported' vacuum nipple at the carb;
800 rpm timing is 15-degrees
3000 rpm timing is at 36-degrees
4000 rpm timing is 50-degrees and will continue to climb with more rpm
(how I drive, my engine never really sees 3500 rpm)
You need to increase your mechanical timing - 32 degree total is not enough. Bump your timing up at least 4 degrees so you get 36 total and 16 degrees initial. Your vacuum advance is too long - your numbers show that you have a 23 degree long vacuum advance can. You need to replace it or limit it to 12 degrees.
You need to increase your mechanical timing - 32 degree total is not enough. Bump your timing up at least 4 degrees so you get 36 total and 16 degrees initial. Your vacuum advance is too long - your numbers show that you have a 23 degree long vacuum advance can. You need to replace it or limit it to 12 degrees.
Lars
I originally set it for 35-degrees at 3000 rpm.
But when I hooked up the vacuum and read the much higher numbers I bumped it down in attempts to lower it some...didn't really help.
Looking at the info you sent me to find a GM HEI 'can' with 12-degrees in it (I'll even take 10 degrees) then hoping my local parts store can get it.
thanks
10-13-2018, 12:01 PM
Ported or Manifold Distributor Vacuum?
