The Vacuum Advance Debate!

Some say manifold vacuum...some say ported. As far as we're concerned, there's only one right way to power your distributors vacuum advance. Here's a quick demonstration that shows why.

[corbinhunter]

This is the first time I’ve ever seen this explained well

[mikeymike]

Uncle Tony's Garage: Great video Tony. Retired licence mechanic here. I used two vacuum gauges, one on manifold and other on ported to teach apprentices the difference of the two. Very useful when adjusting the spring pressure and max vacuum advance. Cheers.

[BUILTOFFICIAL]

this guy is the best classic engine teacher on the internet. I've learned so much here

[andy]

Man, this is why you're a genius instructor! Who else would think to take a real distributor in the car to show what's happening?

[FBobby]

I love how you make the explanations so very simple by just physically showing it actually doing what it does.

[wheels-n-tires]

Love the sounds of an old Dodge...the door latching, the hood groaning...I can almost smell the OEM musty smell..👍😂

[robzee]

Great work as always Tony, clear and concise answers and demonstrations that clear the air and give actual information that is physical and tangible for everyone to see and more easily understand the concept. Nicely done sir

[Ltmonte]

it’s all the same except at idle and WOT (wot has no vacuum anyways so it’s a moot point). Manifold has higher timing advance at idle, lower EGT’s and not as good emissions, then pulls timing as the throttle blades are opened. Ported vacuum has no advance at idle, because it sees no vacuum at idle. As the throttle blades are opened, the vacuum rises and it adds timing advance. The way that you determine which is best is the way your distributor curve is set up, and your application. Some guys will swear by ported only, others by manifold only. If you have the ability to use a dyno I’m sure there’s a negligible difference in part throttle power, because in the middle of the part throttle operation, with the manifold vacuum advance dropping and the ported rising, they intersect at the same amount of advance. So, to sum it up, they achieve the same effect, just one adding timing and one taking timing out. The engine will run differently on each due to the way they add or remove timing. Remember, this is only idle and vacuum cruising, wide open throttle doesn’t matter for vacuum advance.

[UnityMotorSportsGarage]

Vacuum Advance plays a huge part in Fuel mileage... I have found on my tunnel ram setup I get better driveability from Manifold vacuum.. you can use adjustable vacuum pots so you can dial in your vacuum advance and have your cake and eat it too....

[kwilliamson]

Switching to manifold vacuum cured almost all my problems. New rebuilt 350 stock except for an RV cam. New holley 600. new MSD distributor the list goes on and on.. Never could get it to idle correctly no matter what I did. 14 degrees btdc. Finally read that article, switched to manifold vacuum and its like a new car. It runs and drives so much better its unbelievable.

[chrisreynolds]

I have not run across an engine yet that performs better with ported vacuum than manifold vacuum. Ported was an emissions era crutch from the early 70s. All the 60s engines used manifold vacuum.

[bretthall]

Uncle Tony, Thanks for tuning me up! It’s been a while since I’ve been in this deep. I’m doing a major rebuild on my ‘79 F100 (bought it new at age 19). Since most emissions parts are obsolete I’ve had to reimagine as best I can. OE vacuum diagrams showed the vacuum advance working off manifold vac, trying to set timing, adjust idle; and it wasn’t making any sense. I think I’ve got it now.
I owe you one, or more.

[Tommy_Mac]

Great video! I agree, ported, only, ever! I had a great Vocational Auto instructor. We had a Sun distributor machine. He taught me how to use it and to set up and recurve distributors. Must have done 100 over the years. I loved using that and a Sun engine scope, he also trained me on. Fun stuff. A lost art these days.

[maxyundt]

I read an article by one of the engineers involved with designing these systems, he knows what he is talking about and says you always want to use full manifold vacuum. It keeps your engine cooler at idle due to the added advance, and has other benefits as well. Ported was something that was used later on to attempt to help emissions.

[edwardpurks]

I thought the only difference between full vac and ported was having extra advance at idle. Ported timing waits until throttle is just off idle to work. So it makes no difference either way. The power and fuel economy is the same. The only change from the exta timing at idle is smoother idle, cooler temp, better vacuum assist to power brakes, ect. depending on your engine combo, it may run better at idle. The higher vacuum signal lets you lower the throttle plates even more to gain better results from your idle mixture screws. Remember, we always set our base timing and and check max timing with the vacuum advance disconnected anyway. WOT doesn't use vacuum advance, so there should never be a change in performance. Anyone who disagrees must be setting their base timing with vacuum advance connected. This is wrong and will affect many things negatively. Love your videos, Tony!

[dr.donscience]

Absolutely correct. The biggest problem is that the off-the-shelf distributors have too much mechanical advance for ported vacuum. I had DUI build me a distributor with 12 degrees mechanical advance and a 14 degree vacuum canister. I start with 24 degrees initial timing which using ported vacuum is your idle timing. But I vary that from about 22-26 degrees as needed. So this gives me 24 degrees at idle, 36 degrees total timing and 50 degrees light load cruise which is exactly what a first generation SBC wants. My engines all run perfect.

For Ported Vacuum

Idle 24
> 12 mechanical advance
Total 36
> 14 vacuum advance
Cruise 50

[stevenbongiorno]

I’ve been always using ported vacuum. I’ve had to listen to so many people telling me to run manifold vacuum, but I read something long ago about which port to use, and I’ve stuck with it! The only thing that I’ve done is try to limit the amount of advance, and keep my initial timing up. It’s worked so far, so good! 😉

[chasevans]

Sweet demo rig. I've never thought to try that. Really interesting to see a real world experiment. cheers Unc. My spare sbf points dizzy recently got rigged with a battery, coil and a plug, held to the workbench in the shed by a chunk of railroad track and spun up with a skateboard wheel with a silicone hose tyre mounted in the drill. Mission was to calibrate the new old been in bits custom tach. Works a treat. Dead nuts on to the digi timing light tach after tweaking it. Some people would have rigged up a signal generator circuit but analogue with dangerous fizzing taser sparks way more fun. Even the wife was impressed.

[berserk]

IMO, vacuum advance was originally hooked up to manifold vacuum. Later, the manufacturers hooked it up to ported vacuum for emissions purposes. The big downside of ported vacuum is the engine tends to overheat on hot days in traffic. Ported vacuum will cause the engine to idle with less advance than what it wants. Idle loves advance. It runs cooler and more efficient.

From what i've heard, never seen one myself, In the late 60's, in California, they started to install a second vacuum advance that was activated by a temp sensor. I think this was an attempt at fixing the overheating problem. So, when the engine started to overheat it would add additional advance from the second canister and cool the engine. Could be wrong. Just a theory.

[josephesposti]

Thank you for all the great info and breaking everything down step by step, and for free to everyone who wants to learn the right way and understand how and why for every video you make. Thank you