Dave Ray would like to have a word with them.

 

My genuine GM workshop manuals say for manual transmission use ported and for auto transmission use manifold vacuum .
IMO
the M/T is loaded as soon as it accelerates . Ported allows advance to slightly later and prevent pinging .
the A/T is not loaded heavily till convertor stall is reached so the manifold connection might help the transmission pull thru the convertor .. This only occurs on initial acceleration.

The manuals I have apply to pre emission vehicles .

 

Ported vacuum adds advance at part throttle and full throttle but not at idle. This actually works as a timing retard. It retards the timing at idle resulting in higher engine temperatures and higher exhaust temperatures to burn off excess emissions. Under cruise and full throttle it does nothing and allows the regular 36 or so degrees of timing with the vacuum advance. So you get higher idle and low speed engine temps so you overheat in traffic and no idle / part throttle timing advance for better driveabilty.

 

Not arguing ported v/s full manifold.

I am wondering why the two big distributor manufacturers want ported vac to the can?
The Pertronix "Tech" told me that's why my vacuum can failed, and that the new can he is sending will fail too if I connect it to full manifold vacuum.

 

The tech does not know what he is talking about. Cans fail, but that is not the reason. The distributor manufacturers go by what GM spec'd the vacuum hookup was because of government emission regs. As others stated, there is no performance gain between ported and manifold vacuum, the difference is what it does at idle.

 

I always had mine hooked up to ported vacuum. No specific reason. That's just the way my cars were when I got them. I never actually heard of hooking them up to full manifold vacuum until I joined this site.

 

Once the throttle plates open, manifold/ported are the same.
Ported just doesn't add anything at idle.

 

Because the early 70s cars went ported advance with very retarded timing for emissions purposes, I wonder if there is some sort or legality as to why these distributor companies insist on ported vacuum.

 

That's what I was wondering, guess it makes since that they "tow the line" of what the mfg's recommend.
But the guy was arrogant and rude, I realized right away that nothing I was going to say was going to change his recommendation. It's not my job to educate him, just send me my frickin' can.

Thing that made me think was that he was stating that the failure of the vac can was directly related to me having it hooked to full manifold, it wouldn't have happened if I "hooked it up correctly" . I only have 1500 miles on that vacuum can, that's a pretty quick failure.

 

Interesting how mfg's instructions change'd once emission controls were put into effect, I'm gonna stick with manifold vacuum, and as to Pertronix techs' remarks....is his vacuum can smart enough to know the difference between ported and manifold vacuum ? Suggest try calling Pertronix again and see if different tech gives same answer

 

" Why would two large distributor manufactures say to use ported when there is so much that supports using full manifold? "

So very simple, they cannot sell those distributors to persons that do not adhere to EMISSIONS settings.

Anybody actually know the real identification for a " vacuum advance ", and where it came from?

Ford Flathead I6 and V8. Those engines only had the vacuum adjusted ignition curves, NO mechanical until 1935 or so, and some right up until 1957 or so.

Ford called as it was / is, a " LOAD COMPENSATOR ". More load, less timing, less load, idle, cruise, more timing.

That calls for full manifold vacuum

EMISSIONS settings require hotter combustion chamber and exhaust manifold temperatures to help burn off more unburned hydrocarbons, which use PORTED vacuum for the Load Compensator..

This is why I don't get along with most supposedly top techs, most are totally ignorant of how it really works, and include in that group, dyno heroes, ones that have NO idea as to carburetor jetting, and ignition timing for a street driven engine, ONLY A TOP FUELER / FUNNY CAR for the street. Most of which, make power in the stratosphere of rpms, and not enough power / torque to back the car off the rollers.

The OP had it right, and proved it to himself. Then, the Top Tuner PerTronix and MSD "Techs" proved not to know much about things timing.

I see, hear, and fix their insanity EVERY DAY.

 

" Why would two large distributor manufactures say to use ported when there is so much that supports using full manifold? "

So very simple, they cannot sell those distributors to persons that do not adhere to EMISSIONS settings.

Anybody actually know the real identification for a " vacuum advance ", and where it came from?

Ford Flathead I6 and V8. Those engines only had the vacuum adjusted ignition curves, NO mechanical until 1935 or so, and some right up until 1957 or so.

Ford called as it was / is, a " LOAD COMPENSATOR ". More load, less timing, less load, idle, cruise, more timing.

That calls for full manifold vacuum

EMISSIONS settings require hotter combustion chamber and exhaust manifold temperatures to help burn off more unburned hydrocarbons, which use PORTED vacuum for the Load Compensator..

This is why I don't get along with most supposedly top techs, most are totally ignorant of how it really works, and include in that group, dyno heroes, ones that have NO idea as to carburetor jetting, and ignition timing for a street driven engine, ONLY A TOP FUELER / FUNNY CAR for the street. Most of which, make power in the stratosphere of rpms, and not enough power / torque to back the car off the rollers.

The OP had it right, and proved it to himself. Then, the Top Tuner PerTronix and MSD "Techs" proved not to know much about things timing.

I see, hear, and fix their insanity EVERY DAY.

I was waiting to hear what you had to say David, I knew this one would get your attention

He had an attitude right away and told me the new one will fail too if I hook it to manifold vac.
Guy pissed me off and still hasn't responded to my email confirming receiving the receipt and info I emailed yesterday. It will be interesting to see how long it takes to get my vac can.
And it's a bummer the distributor has to come out and apart to replace the vac advance can.

 

This thread amplifies why I never rely on vendor's tech guys. I never even call them anymore. Better info right here on this forum and elsewhere on the interweb.

 

Using ported vacuum is an old school emissions helper. There is so much folk lore out there on how things used to be that disregards progress over the decades! Heck, if your cam is big enough it might not make any difference anyway

 

Rule #1: The customer is always right.

My take is that it depends on the combo and the operator. Meaning whichever camp you were raised in. But that part of the discussion was irrelevant, a leak is a leak and a vacuum advance unit that doesn't hold is bad. They don't make em like they used to. Blah blah, prompt replacement is what matters.

Seems to me like plain stock ones work for like ever, while new adjustable ones are too often bad in the box or after a few miles. Who wants adjustment?

I am from the ported camp, I only have ever advanced as far as mild stock iron head junk. But I got a badly overcammed 400 once and it liked full vac. It depends, but you just can't have a leak. As Gomer said about tires vs gas n oil... thats another matter entire.

 

They want the approval of the California Resources Board so they can include that silver sticker that says the part is legal to use in California and other states that use California smog regulations.

 

^^^^ Boom there it is

 

This topic has been discussed a lot over the years, I've ran it both ways. My experience, for a true street car, with good idle vacuum (ie 14"+) full manifold vacuum advance at idle is great, you set the initial timing, run a conservative mechanical advance curve and get a nice idle and driveability. For a street/strip car I disconnect the vac advance at the track, if running full manifold vac, it would leave me with an engine that won't idle. Or you leave the advance connected to keep the idle and get a full timing retard at hit of the throttle. I found the best compromise for the way I built and drove my cars was to mod the distributor to allow me to dial in approx. 22-24* initial timing and set total timing in the 34-36* range and run much less vac advance on ported vac.
Personal preference has been VC1852/AR22 can (10* vac advance at 12-14") for HEI.
I know this is not text book correct but neither is running an engine on the street with a cam that only produces 12" or less of vacuum at idle. Which is what a lot of us deal with.

 

I'm in the manifold vac camp, no cars before the emissions era had a ported advance (okay, maybe the old flathead Fords, but you know what I mean). And that's why I'm in that camp. You get into the 80's era where performance starts to come back where HEI swaps and other things are the deal -- like timing retard boxes used at idle, locked out timing, etc. and ported vac becomes THE THING to do The magazines all promote the story. The factory HEI's never had a fast mechanical performance curve in that era. They do now, because of the ZZ distributor that came out on a crate motor. If you really look at it, the performance distributors of the era didn't have a fast curve either (Unlite, MSD, Accel).

I've told the story here before, but I got into the debate with a couple of guys originally from Central Indiana. Good friends, one a Mopar guy, one a late GM fuel injected guy (computers and tables, not springs and weights). We were arguing over the idle-ability of my 327 Chevelle with Eddey top end kit, which wasn't running well at the time. Their point was that manifold vac was incorrect and change it to port and it would fix it. My reaction was that it ran well like this before, it should now. This friendly debate went on for a few weekends of bench racing. At some point, I mentioned Dave Ray as the GM carb guru who states manifold vac is the way to do it and those guys got pretty quiet pretty quick. It was a fuel filter that fixed it.

 

Dave Ray emailed me instr
how to Properly set up my 454 with large cap HEI and I followed the inst., engine runs like a top, great 👍👍

 

A well set up GM Delco dist in the street is really tough to beat, right curve, right vacuum canister, right cap / rotor combo...points or Pertronix/Crane..your choice, HEI's...gotta like the "no point" features, higher voltage and longevity between services, they have had their issues at high RPM, that's where something like a crank trigger unit becomes worthwhile. Different solutions for different applications. Very easy to spend a lot of money for un-needed parts.

 

66cayne, Yes, sir,, " Load - A - Matic ", very different, indeed.

 

From John HInkley - Retired GM/Chrysler Engineer...

Ported vs. Manifold Vacuum

Decoding Chevrolet VIN, trim tags, cowl tags, engine, engine block casting numbers, cylinder head casting numbers, intake manifold casting numbers, transmission, interior codes, and paint codes.

chevellestuff.net

 

That is a great article. Note that it covers why the stock vacuum can will not work well with bigger cams. I have found that many people run ported vacuum because of the erratic idle from a stock vacuum can on a modified motor.


Sent from my iPhone using Tapatalk

 

And, that is exactly why adjustable vacuum advance cans were invented. They did not come form the after market, they came from both Ford and MOPAR, BUT, nobody seemed to adjust them correctly.

The Allen screw in the hose nipple IS NOT fo set the degrees stop, it is strictly to set the vacuum operating range of the engine, and the vacuum created by the altered intake valve closing time

A separate degrees stop is needed, completely divorced frm the vacuum setting, and the bets ones were the Crane 99619-1 scroll plates. Now that Crane is a dead player, I have had a stamping die made to reproduce them, and will do so after I get rehab for my new hip. Pictures below show how to do both stock and adjustable vacuum advances with a couple of different methods.

Way too many people get the wrong info on how to do it right, and end up with the advances not working correctly.

 

Dave, how would you recommend setting up a distributor for a street/strip car. Real world example, 4070# Chevelle. 454, T400 (3000 stall converter), 3.42 gears. Running a 236/244 .610/.632, 112LSA hyd roller. Engine pulls about 13-14" vacuum at idle. This particular engine seems to run best at typical 34-36* at WOT. Current setup is HEI, mechanical advance has been limited to about 12* (via HEI mechanical advance modification). Base timing is set at 22-24*. Out of the box, the HEI was calibrated to 24* at 3000 rpm, initial timing set at 12*, easy deal. The engine wants more timing at idle. We swapped the vac can to VC1852 which gives 10* @12-14" (I assume this could be done with an adjustable vac can as you listed above).

Question is, running a non- modified distributor, if the ignition is set to run manifold vac , we set the timing to 36* total on a degreed balancer, engine settles in at 12* initial, connect the vac advance can for 22* at idle, it idles great and has overall great street driving. Just as you describe. Problem is at the track. Bring the engine up to about 1000 rpm (just enough to take little slack out of the drivetrain), throttle hit is not crisp, it doesn't like the timing retard at hit of the throttle. Disconnect the vac advance can on manifold vacuum completely at the track and the engine won't idle. The engine just wants more timing at idle. What do you suggest?

 

Street, vs Strip are two completely different animals.

Street, 14 initial, 10 vacuum on full manifold vacuum, idle of 24. Mechanical, 22 degrees for 36 total degrees. In the HEI, to get the mechanical, ZZ series weights, 41, and center, 375 will get you right there.

One thing that most do not know, the points distributors had so many different distributors, because the curves were all different for every engine, and the distributors had the different centers brazed to the main shafts. When we got into the large HEI, fixing stuff, one of the things we found was Delco did a good thing, but for the wrong reasons. In the case of the large HEI, they made the mechanical advance stops and the center curves that the points distributors virtually disappear. This was done so there only needed to be ONE generic distributor base for each engine family. Chevy, all, Pontiac, all, etc. The key was, the bodies and guts could be "universal", and only the vacuum advances, weights and center packages, springs needed to be changed to make ONE base distributor work in all that series engines, only needed the right weights, center, spring and vacuum advance package for this engine, different for that one, per application. 350 vs 454, same base, different curve package.

So, what we can=me up with, is literally 437 different stop/limit/rate packages for all the different EMISSIONS engines the HEI was created to service. YES, 437 different packages.

Now, EVERYBODY knows I do not advocate to the moon initial, no vacuum advance for a STREET engine, however, it is exactly what you need for the STRIP.

All those large HEI "curve kits" (weights and center) reduce the mechanical degrees, so the INITIAL can be boosted to the levels you seek for the STRIP, and, they are also to run with the vacuum advance DISABLED. THIS works great for a strip car, but, makes things totally out of control for a STREET engine, creating a very undriveable engine, literally creating a " Top Fueler / Funny Car " for the street.

It isn't that hard to swap the weights, center and springs, and plug the vacuum advance hole, re-time for the drags, then, go back for the street.

 

Thanks for the explanation Dave. Been discussed over the years, possibly somewhere on this site or the internet is a list of the weights and center pieces referenced.

 

On an engine with low idle vacuum, using manifold vacuum on the vacuum advance can aggravate an unstable idle. As the idle speed drops, so does the vacuum, and then so does the vacuum advance, and that tends to drop the idle RPM, just the opposite of what you want. Using ported vacuum at least takes the variable vacuum advance out of the equation at idle. On a friends HEI disty, the supplied springs started giving mechanical advance at high idle speeds. So when the idle drifted higher, the engine got more advance so it idles even higher. And like the variable ported vacuum, when the idle dropped, so did the advance and the idle speed, again opposite of what you wanted. We put in stronger springs to delay the mechanical advance and that helped reduce the crazy idle swings. This was on a lazy, 6 cyl. Land Cruiser engine so your mileage may vary!

 

Thats why it is recommended you use a vac can that operates at least 2" below the vac of the engine. I think I have seen what you are describing referred to as the "Idle death spiral".