Manifold OR Ported Vacuum Advance?? There is a case to be made and I side with David Vizard!

[@strokermaverick]

Thank you, for taking time on your vacation, to talk to us!

[@goldsgarage8236]

Thanks for watching and commenting Stroker. AG

[@johnh6532]

Good video and article. My understanding was that GM used manifold vacuum until emissions became more of a thing and around 1970 they went to ported for that reason. I use manifold. The engine idles better, runs much cooler at idle

[@goldsgarage8236]

Thanks John, i agree. AG

[@joharp]

Look up the article Ported Vacuum vs Manifold Vacuum by John Hinckley - Retired GM/Chrysler Engineer. He explains that ported vacuum (along with the right mechanical advance curve) was used to have really late timing at idle to increase EGTs to burn hydrocarbons and get the catalytic converter up to temp. So ported vs manifold come down to emissions and where the timing is at or near idle.

[@kevinclemence4661]

Exactly.

[@goldsgarage8236]

Thanks for the reference Jo. AG

[@thisisyourcaptainspeaking2259]

Correct, trying to coax the manifolds into burning fuel.

[@saratoga123321]

Vizard is the undisputed master. Never argue with a person who’s done it all hundreds of times over with research, data, and application…

[@goldsgarage8236]

U agree Saratoga. I was recently honored when DV endorsed my channel. AG

[@kkyler9582]

The widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it. If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more. What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone. Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam. For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively.

[@goldsgarage8236]

Great technical information. Thanks so much for watching my videos and contributing to my channel KKyler.

[@UncleVom]

Now continue with your vacation with a clear conscience. Thanks for thinking of us.

[@goldsgarage8236]

Thanks you Uncle. Its also good to be back home and back to work, lots to do. AG

[@WayneB-rv1xs]

my 350 ported and manifold vacuum lines each come together through separate check valves to a tee, that line has an aquarium air valve used as a partial vent so the vacuum can drop when the engine is shut off, works great I have the best of both.

[@goldsgarage8236]

Sounds complicated but good if it works for you. Thanks for the info.AG

[@SteveZumpf-bd4hb]

I took used to back off my valve springs, lol. What a pain. I saw a video, I believe from Eric Weingartner, about backing off springs. He tested springs and found no evidence of spring degradation. Thank you for reinforcing the timing debate and myth busting. Best wishes.

[@goldsgarage8236]

Thanks Steve. over time we will replace myths with engineering and science. AG

[@fuelbasti]

After too many years of running on ported vacuum I finally use the manifold port and after right timing the engine runs cool enough with proper vacuum and steady idle. It is kind of crazy that even Holleys instruction talks about using ported. And even the "experts" mechs in the garage I use to visit for repairs did not know and tried to fix the timing.

[@goldsgarage8236]

Thanks for the comment Fuel, it is confusing out there for sure. AG

[@kevinclemence4661]

My experience with manifold vacuum came about as a result from trying to tune a somewhat large cammed carbed 355 SBC. The cam is question was Comp's XR288HR. With 71° of overlap and less compression than I would have liked (10.2:1 SCR, 8:1 DCR) and weak cranking compression at 160 PSI, it liked a lot of idle timing. It came to 32° which was 16° initial and another 16° from vacuum. Since it was street/strip deal (mostly street back then) locking out the distributor wasn't really a workable option. Keep in mind that working with manifold vacuum results in a lot of back and forth as more advance results in greater vacuum which gets more advance and so forth. I tied the centrifugal advance weights into a closed position and used a handheld vacuum pump to dial in the vacuum via an adjustable can (for both the onset point and the total provided). Initially I used Crane's limiter plate in the distributor to tailor the amount of advance but eventually found an adjustable vacuum can that had all of the attributes I was looking for. Idle vacuum was in or around 10" and I tailored the curve to be all in at 9". Under light load, the vacuum drops and pulls timing with it. FWIW, that car, as cammed as it was, drove like it had EFI. Admittedly the loose converter helped but there was still plenty of sub 2000 RPM putting around. Thanks Allan. Hope you're enjoying your vacation.

[@goldsgarage8236]

Thanks Kevin, and thanks for inspiring this video. AG

[@thisisyourcaptainspeaking2259]

Depends on the vacuum canister, manifold cans are all-in with just a few inches of vacuum. Whereas ported vacuum cans are calibrated for about twice that much.

[@cs2-llc]

Manifold vacuum 100%. DV knows what he's talking about. Ported vacuum was created for emissions. Period!

[@goldsgarage8236]

Good point. AG

[@PaulCTownsend]

I'm going with ported vacuum in most cases. But if I have a big cam and I'm looking for more vacuum at idle to run the brake booster trans vacuum modulator I'll use ported.

[@goldsgarage8236]

Thanks for the info Paul. Merry Christmas.AG

[@anthonypaules7893]

Never gave it much thought, so i put my 454 / 850 brawler on the ported . Saw your other video and switched over to manifold. Works better.

[@goldsgarage8236]

Thanks for the feedback Anthony. Glad it was helpful. AG

[@larryw5429]

Full manifold vacuum with recurve kit so weights fly out quicker and adjustable vacuum canister or with limit kit!

[@goldsgarage8236]

Right on Larry! thanks AG

[@davidparizek6741]

Alan,you don’t look that old 😂😂😂 Enjoy your well deserved Vacation and the Warm Weather

[@goldsgarage8236]

Now that is what I call psychic income, thanks so much for the compliment David. AG

[@itseithergonnaworkoritaint7852]

There is a great thread about distributor timing on v8buick. Has all the tricks and tips, it is a pinned thread in one of the categories to choose from, can't recall which one though. Has a step by step how to dial in your curve and how to limit vacuum advance when running an aftermarket cam and much much more.😎👍

[@goldsgarage8236]

Thanks for the info and reference IEGTWOIA. AG

[@billlittle4285]

With a big cam and using vacuum advance when you put it in gear with an automatic with low stall converter the vacuum drops and will lower the idle considerably and stall the engine, ported vacuum will work much better, an adjustable vacuum can can help, hp engines need no more than 15 degrees with 36 to 40 degrees timing, if you get surge at light throttle back of the vacuum timing, just my take on it and 40 years of experience

[@goldsgarage8236]

Thanks Bill. That makes sense. AG

[@yepitsme431]

a big cam with a low stall converter is the wrong combination of part; right from the start is not a proper build, period

[@glennbokma6182]

Hello. Great videos. My cars prefer manifold vacuum. My question is why do we limit mechanical advance to 34 degrees at 3000 rpms? Why not continue the advance sweep up to 6000 rpms? Ty.

[@yurimodin7333]

cylinder pressure will spike and cause a kaboom........moar isn't always better

[@randywl8925]

Picture the piston orientation during the process. At bottom dead center, the compression is zero. Top dead center is 180 degrees. Sobadicslly, you only have 180 degrees to start building compression then ignite it. If you could fire it at 60 degrees before TDC with mechanical advance, it would surely push the piston back down where it came from. The engine RPM, the compression ratio, the flame speed and physics simply determine maximum limit. I'm making up stuff, but it's sorta like that. I'm not an engineer. 😁

[@randywl8925]

News to me about the cam break-in period and not having to break-in the cam in one shot. I never knew that. I had this silly idea the cam lube was still there doing it's job 5 minutes later. I'm thinking now, it washed off rather quickly. 👍

[@cabotbroughton7458]

Spark timing is optimized to produce the best combustion pressure levels during the expansion (power) stroke. At rpms up to ~ 3000 the time measured in milliseconds from the spark to the peak pressure is ~ constant so the ignition timing (angle in degrees to the peak pressure) at 3000 rpm needs to be ~double that at 1500 rpm (in degrees) to get the peak pressure at the same optimum crank angle. However, the time for combustion starts to becomes increasingly shorter at about 3000 rpm and above due to much greater turbulence in the cylinder such that the combustion speed is increasing proportionately to the reduced time available due to additional rpm so the timing in degrees becomes constant. On the dyno, tests are usually done to see what is the best all in spark timing should be and the optimum mechanical advance curve, of course sometimes less than the optimum timing is used to give some buffer against detonation should lower than needed Octane fuel is being or planned to be run.

[@goldsgarage8236]

Good question Glenn. Comments below are pretty accurate so check them out too. As I understand it, if the cylinder has good quench, after 3000RPM or so the quench provides sufficient turbulence, and thus a homogeneous mixture in the combustion chamber to increase the speed of flame travel to the point that more advance is not required. It is also about establishing maximum MEP, (Mean effective Pressure) in the cylinder at just the right time (I think this is about 10-15 degrees ATDC), combined with the connecting rod angle to generate the most torque. Remember also that the pressure decays as the piston moves down. Hope this helps. AG

[@oldvsnew3400]

I can't remember if you mentioned a adj vac canister. I found out mine was advancing the rod too far.

[@goldsgarage8236]

Thanks Old. No it didn't get into this in the video, but they are a good fine adjustment device. Many good aftermarket distributors have them. AG

[@bobbyoshomebuilt2544]

Love your style Mr. Gold. Much to weigh in with, but as usual no time at the moment.

[@goldsgarage8236]

Thanks Bobby, it will be there when you are ready. AG

[@deerslayer5863]

If v ad is hooked to manifold your pulling timing at idle with no vacuum advance at higher rpm in street cars

[@goldsgarage8236]

Correct, thanks for the comment. AG

[@1958johndeere620]

When I set up my fathers Blueprint Engines 383 chevy the tech from Blueprint told me not to hook it up at all. He said the engine we have will make a little manifold vacuum even at full throttle due to the amount of air its pulling in. I have not verified if that is so but did keep it unhooked. It is grumpy at idle, but is a 450 horse small block so I guess it should be. Over the years I have done it ported and vacuum but have no idea what is better. I am lucky if my junk runs at all.

[@SteveZumpf-bd4hb]

Big cams don't make enough vacuum to run vacuum advance consistently. You may be to that point.

[@goldsgarage8236]

Thanks 1958 for the question. i have heard that about BP engines before and i really don't understand it. We have dyno'd 383 engines making 450HP before and they work fine with vacuum advance. My last 406 made over 500HP and it worked fine with VA. No engines have vacuum at full throttle but we had a good 15" on that 383 and more on the 406. unless it voids a warrantee or something, I don't see why you couldn't try it under controlled circumstances. AG

[@1958johndeere620]

@@goldsgarage8236 The way it was explained to me was even with the throttle plates wide open, the engine is so efficient at pulling air into the engine the dyno guys have seen enough vacuum to slightly add some amount of degrees to total timing. They said at very high rpm's it could add enough to cause detonation. If I do hook it up, I think I will take a couple degrees out of timing just to be safe. It would idle better with it,,,, for sure.

[@shane-222]

@@1958johndeere620if the carb and air cleaner are restricted enough you could have some vacuum at WOT. I would measure actual WOT vacuum and play with it…..

[@twgarage-terrywatson1672]

Correct that more aggressive cam timing produces less vacuum. The fix. Use an adjustable vacuum advance. You can adjust the vacuum advance to work with engines that have very little vacuum. Manifold vacuum advance gives you the benefit of a cooler running engine at idle and more timing advance during light loads such as cruise conditions.

[@dadalebreton184]

Who had a digital flow bench 50 years ago? I think that every engine as it's own soul. You can take two same blocks, prepare them exactly the same way, fill them with the same components, time them at the same degrees, same carburators, exhaust and everything, and you wont get the same results. Yes you will get within a certain range of 5..10..20hp? But one block will flex differently, the flow want go at the same speed, the oil passage can be slower, cooling system can affect the block from a few deg. Etc. Etc. The main thing here is to do you best to have results that reflects your expectations, within a range, for your unique application. The moment your drive that engine, the way you treat it also affects the outcome. I think that if your still doing it after 50 years, you must have been doing it right and if not, you fixed it and you learned from it. Who had a digital flow bench 50 years ago? You and all the ones who's still standing after all these years done it with all your hearts and precision for "One" thing. To beat the ice cream out of the opponents ont the streets or on the track. Thanks and enjoy the sun. It will give your body energy from vit.D production. Trust me, Im a nurse! Yes a male one! Hahaha.

[@goldsgarage8236]

Great comments Dad! thanks for sharing your thoughts and motivating me to do more. AG

[@samferguson6839]

Ford used port vacuum in the 1960s then mid 1970s went to manifold vacuum to help meet epa smog regulations. I believe port or manifold vacuum advance depends on the distributor centrifugal advance settings.. Ford electronic ignition distributor has 2 advance limiting grooves 8 degrees and 16 degrees.. Most of what I have seen is port vacuum advance distributor were set at the 8 degree slot and the manifold vacuum advance distributor were set at the 16 degree slot. Guess there is not a wrong way or a right way there is just a which way do we want to use..

[@goldsgarage8236]

Thanks Sam. very good information. There is always so much to learn. AG

[@357fairmont]

Msd instructions say to connect to ported. I didn't but its a confusing subject.

[@brandonpriest3604]

Hope you all have a great Time A.G.

[@goldsgarage8236]

Thanks Brandon, we did but it s good to be back. Lots of work to do. AG.

[@daveshepherd8460]

Old 6 cyl chevs had no mechanical advance and used ported vacuum.

[@goldsgarage8236]

Thanks Dave, I did not know that. Thanks for commenting.AG

[@randywl8925]

It's 42 degrees here on the Oregon coast... low of 34? Envy here ...........but have fun, we will wait. We only have one lifetime and so do you. 😉 You're not losing anyone so dontcha be worrying.

[@WayneB-rv1xs]

When I first saw your degrees I thought you were going to be speaking of timing, OK that is cold.

[@randywl8925]

@@WayneB-rv1xs ...after posting, I noticed that myself. 😁

[@goldsgarage8236]

Thanks Randy, the weather was nice in Punta Cana, back to reality and back to work. AG

[@sqr2024]

I have a 1971 big block corvette and the factory LS6 vacuum advance hookup on the factory Holley carb is on the manifold. There is no provision for a vacuum hookup on the front metering block. GM knows best.

[@goldsgarage8236]

I agree, that makes your decision pretty easy. AG

[@apocolypse11]

Electric ignition n no one has look back.

[@aznplumber280]

Ported

[@alexvendrig9205]

get some sand between the toes and enjoy your vacation

[@goldsgarage8236]

thanks Alex, good to be back! AG

[@patrickwendling6759]

Port vacuum runs hotter then manifold vacuum..

[@rogerstill719]

I think both port vacuum and manifold vacuum disappear under full throttle

[@WayneB-rv1xs]

I think that is correct, and that they are opposite when off idle one drops the other increases, I wonder what would happen if you tee them together with two check valves to get the best of both, now you can have low advanced timing at start up that won't kick the starter, and idle timing jumps up and stays up when you first open the throttle, just a thought. I'm back months later and it works both are teed together with two check valves and a aquarium valve as a vent so it will drop when shut off.

[@goldsgarage8236]

Thanks Wayne, sounds like you have given this a lot of thought. AG

[@wayneries5970]

poor volume.

[@goldsgarage8236]

Thanks Wayne. We are working on it. AG

[@patrickwendling6759]

Thank you for your knowledge and video's USA 🇺🇸 TRUMP TRUMP

[@goldsgarage8236]

Thanks for commenting Patrick. AG

[@peterschmidtchen8560]

Been a mechanic for 35yrs and manifold [below throttle plate] vacuum is for automatic transmission. Set initial timing with this disconnected. Ported [above throttle plate] is for manual transmission. Set initial timing with this connected. Pretty simple. Race engines don't need vacuum advance as they are wide open throttle most of the time.

[@AustinRBa]

I've got a big block 4 speed car with a decent amount of cam, which needs 34 degrres advance at idle. If I run ported vacuum I'll have to set base timing at 34 degrees. I'd then get mechanical advance of 20 more degrees by 3000rpm. I don't think 54 degrees advance at WOT is a good plan. So I run manifold vacuum to get advance at idle, AND at cruise when throttle blades are pretty closed and cylinder pressures are low. Vacuum advance is a hell of a lot more complicated than what transmission a car is running.

[@peterschmidtchen8560]

Your opinion. Not mine.

[@goldsgarage8236]

Thanks for your input guys. AG

[@My_67_Camaro]

Mine runs like garbage and will only idle at 1k rpm on the port and 800 on the manifold. It idles and runs great on manifold.Mine is a manual.