Ported vacuum advance or manifold. Let's debate!

[Photon440]

Total timing is simply the total timing seen by the engine at a given time under a given set of conditions. It doesn’t specifically refer to WOT and doesn’t dismiss vacuum advance. If manifold vacuum is zero at WOT, then it obviously wouldn’t contribute to the number calculated as total timing. Most would probably be surprised at what their manifold vacuum is at WOT throttle as the assumption of zero is not always correct.

Exactly. And if the carb is too small, vacuum will start to increase at higher rpm. even at wide open throttle. Like just about every two barrel carb ever installed.

 

[4406bbl]

Exactly. And if the carb is too small, vacuum will start to increase at higher rpm. even at wide open throttle. Like just about every two barrel carb ever installed.

Correct, it is wise to consider total timing as what the engine can take without damage, in a car at max load, wide open throttle . For a bb mopar that will be 35-38 degrees total if the fuel can support it. When you are cruising at speed with a light load it will tolerate more timing, from the vacuum can, maybe 10-12 degrees. For the big blocks with stock heads 38 total and 12 more in the can is all I have ever been able to run.

 

[70bigblockdodge]

Manifold is great, until your over cammed, low compression, near stock TC gets slipped into drive. Vacuum drops car stalls.
Ported and mechanical advance limited, initial @23° and total @36° car drives reasonably well, while returning 12.8-13.1 mpg on average highway. Again no magic bullet.

 

[Kern Dog]

Well, one should not use a large cam, low compression and a near stock torque converter together anyway, right?

 

[Wietse]

Well, one should not use a large cam, low compression and a near stock torque converter together anyway, right?

Unless you have too much time on your hands and want to try to make it work

 

[4speed68rt]

I am only looking at this from the 4 SPEED, 11:1, medium duration "mystery" cam point of view.

 

[747mopar]

I too believe its engine dependent. If you are looking to get a high initial timing on a hot street engine without having a hot start issue with too much initial binding on the starter, then manifold vacuum makes the most sense. You can use the manifold vacuum at idle to add sometimes 10 degrees of intial and still have a good start without kickback on the starter. For your everyday run of the mill engine with 14-18ish even initial timing, then I think you are fine with ported.

This is exactly why I'm running manifold vacuum. I did a lot of trial and air and eventually came to the conclusion that it starts great at 16° but is a complete dog off the line unless it's at 34° (it's been awhile so I'm going on memory). I've been driving the car for several years now and haven't touched the timing or carb tuning, it runs flawlessly, gets 19mpg, runs cool (I have a stuck thermostat at the moment and it stays at 150°) and best of all never lets me down. This is on a 451"stroker with a 501 lift cam, nothing radical.

 

[Dragon Slayer]

Total timing is simply the total timing seen by the engine at a given time under a given set of conditions. It doesn’t specifically refer to WOT and doesn’t dismiss vacuum advance. If manifold vacuum is zero at WOT, then it obviously wouldn’t contribute to the number calculated as total timing. Most would probably be surprised at what their manifold vacuum is at WOT throttle as the assumption of zero is not always correct.

I disagree in that you have your own definition of "Total Timing". Not what most folks would call it. Why do you have to call it total in your definition. Just call it engine timing.

Let's also remember vacuum doesn't have to be zero for 0 vacuum advance either. Based on spring preload, the vacuum can timing curve doesn't start adding timing until a certain vacuum level. That level can be set at a pretty moderate level of vacuum too. So at WOT with some manifold vacuum the vacuum advance can still be zero.

I do think the convention is that Total Timing is defined as Initial plus max mechanical. I doubt you would go to your dyno guy and tell him you want him to pull a test to max rpm and tell him to set it up with Total timing of 55 deg. His definition would be different than yours.

 

[Geoff 2]

The poster in post #93 clearly states for the purpose of the numbers he is posting that total timing includes vac adv. Probably did that to avoid confusion.

 

[Hemirunner]

I disagree in that you have your own definition of "Total Timing". Not what most folks would call it. Why do you have to call it total in your definition. Just call it engine timing.

Let's also remember vacuum doesn't have to be zero for 0 vacuum advance either. Based on spring preload, the vacuum can timing curve doesn't start adding timing until a certain vacuum level. That level can be set at a pretty moderate level of vacuum too. So at WOT with some manifold vacuum the vacuum advance can still be zero.

I do think the convention is that Total Timing is defined as Initial plus max mechanical. I doubt you would go to your dyno guy and tell him you want him to pull a test to max rpm and tell him to set it up with Total timing of 55 deg. His definition would be different than yours.

You are correct. The convention is that timing is set with the vacuum advance disconnected and that is initial plus mechanical.

 

[AR67GTX]

Run what you're comfortable with.
I do find the big shift from manifold to ported occured when people were looking for fuel economy and manufacturers were looking for better emissions numbers.
My Savoy has no vac advance. That's a new wrinkle for me, but it'll give me something new to study. Any increase in knowledge is beneficial, sometimes even fun.
I come mostly a sport car and Corvette background. One of the tuning gods in the old Corvette world, Lars Grimsrud says to go manifold vacuum 99% of the time in these old cars we love. And let's face it, the engine doesn't know if it's in a Corvette, a Yugo, or a kikass Max Wedge Plymouth. I tend to agree with Mr. Lars.
I'm running 52* total advance in my 1965 396 Corvette. I define total advance as Initial (on the balancer), centrifugal (from the distributer weights), and vacuum (from the can on the distributer). I know y'all are knowledgeable enough to not need that, but many folks refer to total as just initial and centrifugal. Never saw much sense in that, as it leaves a significant variable out.
I also run 52* total on a 400ci small-block Chevy engine I built. It's about 500-550 horse and torque. Runs great at that level of advance. Still, I'd love to have an extensive dyno session to really dial it in (in all aspects).
Right now, my Savoy needs that worse.
So, run it how you're comfortable, and satisfied with the performance, whether you measure that by power, fuel economy, emissions or any other metric.
Most importantly, enjoy your car. Enjoy every part of it. Driving. Wrenching. Racing. Cruising. Showing. All of it.

Carter

Same here. My L88 head/ intake, 256/266 mech cam, 11.7 CR, 4.11 427 66 Vette is streetable mostly because of manifold vacuum advance. However I did have to limit it to 7 degrees as I had a constant light miss at cruising highway speeds from an over-advanced situation that brought all the mechanical and vacuum advance into play. Reducing it to 7 deg, it went away. Each engine can respond differently. But that extra 7 deg at idle along with the 16 - 17 deg baseline advance lets it idle somewhat, I can ease through neighborhoods without running 30 mph in 1st and it never over heats.

 

[Dragon Slayer]

For the folks that are watching this subject to try to understand the how and why of ignition timing go back to post #99 and read the Chrysler training link. Very informative and may have been missed by the volume of post. Couple that with your service manual that gives your distributor specs so you understand how much mechanical and vacuum adv it has. Then you can go experiment to determine what works best for your actual combo.

 

[Manzolds]

There are some interesting YouTube videos on the subject.

Take a look at videos put up by Tall John's Fun shop, IMO he has a firm grasp of the subject matter, and he is excellent at explaining the fundamentals.

Also, Uncle Tony's Garage has some good video on the subject as do many others.

 

[hunt2elk]

Thought I would post a recent result I have had. I have run manifold vac on cars in the past, but it has been awhile. The 493 with a mild cam in my Super Bee has been running quite good (hope I didn't just jinx myself), except for 1 issue. I couldn't get rid of dieseling/run on when shutting it off. Timing was hooked to ported. 20 initial, 34 total, and 44 all in with the vac hooked up. I swapped the hose over to manifold vac. Initial timing is now 30. I had to drop the idle speed back down to 850 from the 1200 I now had. Also had to turn my idle mixture screws in 1/4 turn to get my idle A/F back where it was previously. Pretty hard to tell if there is a difference in performance, BUT the engine run on when turning off is now eliminated.

 

[CoronetDarter]

Thought I would post a recent result I have had. I have run manifold vac on cars in the past, but it has been awhile. The 493 with a mild cam in my Super Bee has been running quite good (hope I didn't just jinx myself), except for 1 issue. I couldn't get rid of dieseling/run on when shutting it off. Timing was hooked to ported. 20 initial, 34 total, and 44 all in with the vac hooked up. I swapped the hose over to manifold vac. Initial timing is now 30. I had to drop the idle speed back down to 850 from the 1200 I now had. Also had to turn my idle mixture screws in 1/4 turn to get my idle A/F back where it was previously. Pretty hard to tell if there is a difference in performance, BUT the engine run on when turning off is now eliminated.

Dropping the idle to 850 is probably the biggest reason why the dieseling stopped. Sounds like the engine wanted more initial if you had to also lean out the mixture. Did you dial back the mechanical after increasing the initial?

 

[hunt2elk]

Dropping the idle to 850 is probably the biggest reason why the dieseling stopped. Sounds like the engine wanted more initial if you had to also lean out the mixture. Did you dial back the mechanical after increasing the initial?

I had it idling at 850 when on ported as well, so that didn't matter a bit. I didn't touch the timing. Thought I may have to, but it runs great where it is, so I am leaving it alone.

 

[diesel_lv]

@hunt2elk , low initial timing causes engine to run much hotter at idle, which in some instances leads to dieseling/run on when shutting off. Hooked to manifold, your timing goes up due to vacuum and reduces heat at idle. Just a thought.

 

[33 IMP]

None of my hotrods has a distributor with provision for vacuum advance.

 

[hunt2elk]

@hunt2elk , low initial timing causes engine to run much hotter at idle, which in some instances leads to dieseling/run on when shutting off. Hooked to manifold, your timing goes up due to vacuum and reduces heat at idle. Just a thought.

I agree. Although my initial timing was at 20, and in my mind that is not low. If I went any higher, it would turn over very hard when hot. So now it cranks and starts at 20, and then goes immediately to 30 and idles there. This engine seems to like it setup in this manner.

 

[RemCharger]

None of my hotrods has a distributor with provision for vacuum advance.

Can't be true. How would they run.