Distributor Vacuum Advance?
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Thread back from the dead. TWICE. lol
Is 32-34 degrees on magnum heads also because of better quench ?
MopaR&D
Nerd Member
I haven't seen that member on here for a while unfortunately. But yes Magnum heads require less ignition advance because of the closed chambers with dual quench pads. Provides faster combustion than old-school open-chamber LA heads so the 'fire' doesn't need to be started quite as early on the compression stroke.
Regarding vacuum advance I run it on any engine that takes it which is every engine I've worked on lol. Only time I'd ever bother NOT trying to run it would be a high-compression engine on the edge of pinging or a dedicated race car that won't see any street use. I've found recently as well that manifold vacuum can and sometimes does work better than ported vacuum; both my Duster with a 450-hp 360 and '72 Dodge truck with mostly-stock 360 run better after I hooked vacuum advance to full manifold vacuum. I just set the initial timing first to where it was happiest then hooked up the VA. Idle speed didn't change a whole lot but it is more responsive off-idle and is also smoother when going from coast to part-throttle.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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Agree
Almost all street driving, with a street car, that might have 3.23s to 3.73s, is or will be, done at under 3000..... because in second gear, your rpm is about 2000@32mph, give or take 200 rpm for gearing. Most of the time you will be just cruising or lightly accelerating in traffic.
Without the vacuum advance working, your A/F mixture will NEVER finish burning in the cylinder. Instead of making peak cylinder pressure at the optimum transfer point in the crank's position, the pressure pulse will follow the piston down, and some of the fuel will remain still unburned. When the exhaust valve opens it will exit lazily and keep on burning in the exhaust system, destroying any scavenge signal that may exist in the header. Not only does this waste energy, but the engine is not making decent power, so you have to drive deeper into the throttle to maintain even your modest speed.
Try this;
In N/P set your idle speed to 2000 rpm without the vacuum advance connected, then read your Ignition advance. Next hook up the Vcan. Your rpm should jump up. Read the advance again. Finally tug on the distributor, and bring in more advance, until the rpm no longer rises. Then read the timing again. What you have there is the amount of advance that the engine actually wants at that rpm and load setting. What did your timing light tell you? Compare the final amount to the starting amount.
Don't forget to put the timing back to where you found it, lol
Rapid Robert
Well-Known Member
UTG swears by ported & 4secondsflat swears by manifold. someday when I have time I am going to try/tune both on a project & see what develops
Rat Bastid
Dunamis Metron
It depends. If you build the engine with enough compression and the correct cam timing you most likely will never need manifold vacuum. Too little compression and a big cam...then you need to crutch that with a bunch of extra initial timing. And a properly curved distributor makes a big difference.
If you have a motor with low vacuum at idle and you want it to idle really bad here's one way to do it. Set the mechanical advance curve so it has already started advancing at idle speed and then attach manifold vacuum to a vacuum canister that is only partially applied at idle speed. Then to top it off put an auto transmission behind it.
RR,
MVA has it's benefits...........................PVA is useless & we got it because of emissions [ like lower comp ratios that made engines inefficient ]; engines that used PVA had MVA in the background to come to the rescue & MVA do so until the end of the carb era.
There are some combos that will not benefit from MVA......buuuuuuuuuuut there are very few.
MVA is not a crutch, no more so than using higher ratio rockers to increase HP. The engine needs a certain amount of ign timing advance at idle for best power & MVA is one method of achieving it. Another method would be a locked dist. However, this method may cause detonation because the timing is no longer load sensitive; it is load sensitive with MVA.
The problem with MVA is that there is no procedure [ that I have seen ] to dial it in.
The instructions that come with the adj VA units such as Crane, Accel, Mr. Gasket etc are written by people who are clueless to what MVA was designed to do.
D. Vizard reviewed the then new Crane Elec Dist in Nov 04 of PHR magazine; note that Crane went to the expense of adding VA curves, not just centri curves.....
" At idle & low speed operation, the amount of advance reqd to most efficiently utilize the air & fuel entering the engine can be as much as 50-55*. This is handled by the vac adv: a function many hot rodders believe is not needed because their favorite drag racer does not use it. Now is the time to listen & listen up good. A functional VA is the single most effective camshaft tamer you can get...."
And closer to home, Mopar related, MM magazine Ovt 2015:
" The initial timing went from 15 to 26* while adding an inch of vacuum. Total timing didn't change, but a dramatic gain in low rpm throttle response was realized. If tweaked correctly, you'll see your idle vacuum creep up, as we did here."
MVA has it's benefits...........................PVA is useless & we got it because of emissions [ like lower comp ratios that made engines inefficient ]; engines that used PVA had MVA in the background to come to the rescue & MVA do so until the end of the carb era.
There are some combos that will not benefit from MVA......buuuuuuuuuuut there are very few.
MVA is not a crutch, no more so than using higher ratio rockers to increase HP. The engine needs a certain amount of ign timing advance at idle for best power & MVA is one method of achieving it. Another method would be a locked dist. However, this method may cause detonation because the timing is no longer load sensitive; it is load sensitive with MVA.
The problem with MVA is that there is no procedure [ that I have seen ] to dial it in.
The instructions that come with the adj VA units such as Crane, Accel, Mr. Gasket etc are written by people who are clueless to what MVA was designed to do.
D. Vizard reviewed the then new Crane Elec Dist in Nov 04 of PHR magazine; note that Crane went to the expense of adding VA curves, not just centri curves.....
" At idle & low speed operation, the amount of advance reqd to most efficiently utilize the air & fuel entering the engine can be as much as 50-55*. This is handled by the vac adv: a function many hot rodders believe is not needed because their favorite drag racer does not use it. Now is the time to listen & listen up good. A functional VA is the single most effective camshaft tamer you can get...."
And closer to home, Mopar related, MM magazine Ovt 2015:
" The initial timing went from 15 to 26* while adding an inch of vacuum. Total timing didn't change, but a dramatic gain in low rpm throttle response was realized. If tweaked correctly, you'll see your idle vacuum creep up, as we did here."
I agree with what you said however I disagree with the statement that "ported vacuum is useless" ported vacuum functions the same as manifold vacuum once the throttle blades are opened. Which is most of the time you are driving your car. So if manifold vacuum is beneficial at times when the throttle is opened so to is ported vacuum. They are the same once the throttle is opened. So in those cases where manifold vacuum at idle is not beneficial ( including not having the proper canister for the application such as with engines that have low unsteady vacuum at idle) then ported vacuum may used to achieve nearly all the benefits of a vaccum advance.
There are compromises sometimes that are made when tuning an engine for various reasons. Budget, parts availability, application and time are just a few. Sometimes you have to do the best you can with what you have to work with. Making an argument that that it has to be one way leaves you without options. Mechanical advance, locked or unlocked and vacuum advance, manifold or ported are all tools you should keep in your tool box. Use the tool that is appropriate for the conditions you are working under.
abodyjoe
Well-Known Member
i ran a fbo dist in my mostly stock 360. it really ran well set up like he set it.. did every thing he said it would do..
No. This belief was being promoted by some guru a few years back.
Venturi vacuum was used as the timing control on some Fords after the war. It's totally different system than the one used on our cars.
from 1959 page 10 (Session 136) of the Chrysler MTSC.
Illustrated here
from 1969 Ignition System Analysis (Session 259), Master Technician's Service Conference
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abodyjoe
Well-Known Member
Ported vacuum source, Constant manifold vacuum source explained, the difference between ported and manifold vacuum.
The Mystery of the Vacuum Can?
Ported or Constant?
Let me try and give you an explanation in layman's terms.... Generally, emissions equipped vehicles use Ported and they are connected to a computer, OBD1 or 2 to retard the timing based on information gather from a multitude of sensors in the vehicle.
Non-Emissions Hot Rods and Street Muscle Cars Pre-emissions use Constant. Many vehicles were built with specs that designated Ported, that was fine in 1975 when we used real gas and we could set the total advance to 25 with 5* initial and have the vac can pull the timing to 34 under load and fuel economy, formulations and costs were not an issue. These cars ran horrible and got terrible fuel economy, more gas ran out the tailpipe than got burned in the combustion chamber.
GM always used Constant Vacuum and built over 300 different set-ups for all there product lines until computers were introduced and everything changed to meet EPA emissions regulations.
So Your Hot Rod gets connected to CONSTANT manifold vacuum and the distributor needs to be set up properly to burn today's fuel formulations. The Vacuum Can needs to be adjusted to pull enough timing in the motor to allow it to burn the today's fuel and limiters installed to set the part throttle cruise timing numbers so they don't pull the timing too high and cause a lean miss at cruise, it's a delicate balancing act to get it all correct.
Your carb has 4 Venturi's, as the air speed or velocity increases with throttle position a venturi creates high pressure in the center to fracture the fuel into small particles and "Opposite and Equal Reaction" is negative wall pressure.
The negative wall pressure is what draws the fuel through the booster and feeds the engine based on throttle position or air velocity. A carburetor is a simple metering device that delivers fuel and mixes it with the air to create clean combustion, the higher the velocity the more negative pressure the more it delivers, it's a pretty simple device.
Ported vacuum: With that thought in mind consider if the distributor hooked to Ported Vac, as air speed increases the ported vac activates and starts to pull more and more timing in the motor as velocity increases. So if you set you total timing to 34* at 3000RPM that's when the ported starts to do it's job and advances the the timing to the total stroke of the vacuum can arm, usually 12-18* (without Limiters) so now have your total of 34* PLUS the stroke of the Canister arm of say 16* net result=50* of total timing under hard acceleration and your motor WILL "Detonate". If you had a OBD1 computer it would pull that timing back to the 34* as all the sensors feed information to it.... and no detonation.
Now to Constant: At idle/part throttle cruise you have high vacuum, the carb is nearly closed causing a restriction which creates the high vacuum level. Under this light load condition and lean AF ratios the motor needs more timing to burn the fuel (Lean Mixtures take Longer to Burn than rich mixtures) so you need more timing at idle and cruise to burn the fuel correctly and completely. When you stomp the throttle you have NO manifold Vacuum so you have NO vacuum timing and at NO time under high load will it ever advance more than the mechanical "All In" numbers. Stop pointing your finger at the carb for rich idle and top end lean conditions.. It's In Your Distributor Tuning! If your Buddy tells you to hook your Hot Rod distributor to Ported Vacuum, find a new friend because that guys advice is going to blow your crankshaft through the oil pan. Same goes for the guy who say's to disconnect it, they obviously have no idea of how it works or what it does or why we use it.
This not hear-say or an opinion its engineering, based on physics formulas, calculated by people who are a lot smarter than me and I know that so I do what they tell me. They come up with the numbers, we set them up accordingly.
Rule of Thumb: If the Motor Makes 8" of vacuum at Idle with 30* of timing in it then it needs a properly calibrated Vacuum Can, we have Mopar and GM vacuum cans that will read 7" of vacuum. most stock or aftermarket Vac cans will only read down to about 15", the odd one (1 out of 100) will read to 12"
We manufacture Mopar vac cans that will read to 7", our Lane Choice Gold Plated VC-1 BB or VC-1 SB
There's all sorts of unqualified guy's on the "Internet" and so called "engine builders" who will try and argue this point with me, I have given up trying to argue with these folks. They have no understanding how a vac can works or what it does, if they did they would realize how ridiculous they sound to us, Jim and I account for over 100 years experience with well over 10,000 happy customers.
Your "Engine builder" may be the best in the world but he's an Engine Builder and very seldom do they know anything about the fine tuning of the vacuum can or distributor mechanical advance system
The Mystery of the Vacuum Can?
Ported or Constant?
Let me try and give you an explanation in layman's terms.... Generally, emissions equipped vehicles use Ported and they are connected to a computer, OBD1 or 2 to retard the timing based on information gather from a multitude of sensors in the vehicle.
Non-Emissions Hot Rods and Street Muscle Cars Pre-emissions use Constant. Many vehicles were built with specs that designated Ported, that was fine in 1975 when we used real gas and we could set the total advance to 25 with 5* initial and have the vac can pull the timing to 34 under load and fuel economy, formulations and costs were not an issue. These cars ran horrible and got terrible fuel economy, more gas ran out the tailpipe than got burned in the combustion chamber.
GM always used Constant Vacuum and built over 300 different set-ups for all there product lines until computers were introduced and everything changed to meet EPA emissions regulations.
So Your Hot Rod gets connected to CONSTANT manifold vacuum and the distributor needs to be set up properly to burn today's fuel formulations. The Vacuum Can needs to be adjusted to pull enough timing in the motor to allow it to burn the today's fuel and limiters installed to set the part throttle cruise timing numbers so they don't pull the timing too high and cause a lean miss at cruise, it's a delicate balancing act to get it all correct.
Your carb has 4 Venturi's, as the air speed or velocity increases with throttle position a venturi creates high pressure in the center to fracture the fuel into small particles and "Opposite and Equal Reaction" is negative wall pressure.
The negative wall pressure is what draws the fuel through the booster and feeds the engine based on throttle position or air velocity. A carburetor is a simple metering device that delivers fuel and mixes it with the air to create clean combustion, the higher the velocity the more negative pressure the more it delivers, it's a pretty simple device.
Ported vacuum: With that thought in mind consider if the distributor hooked to Ported Vac, as air speed increases the ported vac activates and starts to pull more and more timing in the motor as velocity increases. So if you set you total timing to 34* at 3000RPM that's when the ported starts to do it's job and advances the the timing to the total stroke of the vacuum can arm, usually 12-18* (without Limiters) so now have your total of 34* PLUS the stroke of the Canister arm of say 16* net result=50* of total timing under hard acceleration and your motor WILL "Detonate". If you had a OBD1 computer it would pull that timing back to the 34* as all the sensors feed information to it.... and no detonation.
Now to Constant: At idle/part throttle cruise you have high vacuum, the carb is nearly closed causing a restriction which creates the high vacuum level. Under this light load condition and lean AF ratios the motor needs more timing to burn the fuel (Lean Mixtures take Longer to Burn than rich mixtures) so you need more timing at idle and cruise to burn the fuel correctly and completely. When you stomp the throttle you have NO manifold Vacuum so you have NO vacuum timing and at NO time under high load will it ever advance more than the mechanical "All In" numbers. Stop pointing your finger at the carb for rich idle and top end lean conditions.. It's In Your Distributor Tuning! If your Buddy tells you to hook your Hot Rod distributor to Ported Vacuum, find a new friend because that guys advice is going to blow your crankshaft through the oil pan. Same goes for the guy who say's to disconnect it, they obviously have no idea of how it works or what it does or why we use it.
This not hear-say or an opinion its engineering, based on physics formulas, calculated by people who are a lot smarter than me and I know that so I do what they tell me. They come up with the numbers, we set them up accordingly.
Rule of Thumb: If the Motor Makes 8" of vacuum at Idle with 30* of timing in it then it needs a properly calibrated Vacuum Can, we have Mopar and GM vacuum cans that will read 7" of vacuum. most stock or aftermarket Vac cans will only read down to about 15", the odd one (1 out of 100) will read to 12"
We manufacture Mopar vac cans that will read to 7", our Lane Choice Gold Plated VC-1 BB or VC-1 SB
There's all sorts of unqualified guy's on the "Internet" and so called "engine builders" who will try and argue this point with me, I have given up trying to argue with these folks. They have no understanding how a vac can works or what it does, if they did they would realize how ridiculous they sound to us, Jim and I account for over 100 years experience with well over 10,000 happy customers.
Your "Engine builder" may be the best in the world but he's an Engine Builder and very seldom do they know anything about the fine tuning of the vacuum can or distributor mechanical advance system
You can go with his system, and for most people it will work out. However, his theory of why it works is not entirely correct. This why for many years his stuff didn't always work with vac advance. I've tried to discuss that with him here but he always disappears.
The idea that GM did not use ported vacuum advance in the pre-emissions era is hogwash as can be seen by looking at some pre-68 high performance Chevy engines.
ps. His example of '1975' is also misleading. Those numbers don't match anything.
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abodyjoe
Well-Known Member
i was only passing on his thoughts after someone else mentioned him.. like i said i had one of his dist in my stock 360 and it worked well and did what he said it would.. i'm sure there is no one fix for all though.
If you really want to know how manifold and ported vacuum work try what I did. Get 2 vacuum gauges a couple of vacuum t's and some extra vacuum hoses. connect one guage to manifold vacuum and one to ported vacuum. You can route the gauges through the door if it doesn't pinch the hoses shut. Put the gauges side by side where you can see them both while driving. It works best visually if they are matching gauges but not a requirement. By the way they can be had fairly inexpensively and don't need to be top of the line for this test. A little duct tape and cardboard is what I use to hold them in place. I have also duct taped them to the outside windshield. What I have found is that as soon as the throttle plates are cracked open the 2 gauges read the same. They read the same at wot which is nearly zero. That has been my experience. I encourage others to gather their own data and find out for themselves instead of taking mine or anyone else's word for it.
92b,
PVA is useless in the sense that MVA does the same as PVA...and more. It adds timing at idle, which helps the engine run cooler & increases idle fuel efficiency. A win-win.
MVA is still with us today, in electronic form, with EFI. The LS engines idle at about 22* & their efficient, high compression, chamber only requires about 27* at WOT. When you extrapolate the numbers on our old dinosaurs that need 35+* at WOT, suddenly 28-30* at idle doesn't sound excessive.
PVA is useless in the sense that MVA does the same as PVA...and more. It adds timing at idle, which helps the engine run cooler & increases idle fuel efficiency. A win-win.
MVA is still with us today, in electronic form, with EFI. The LS engines idle at about 22* & their efficient, high compression, chamber only requires about 27* at WOT. When you extrapolate the numbers on our old dinosaurs that need 35+* at WOT, suddenly 28-30* at idle doesn't sound excessive.
AbodyJoe,
Few errors/misconceptions in your post.
- once the PVA port is exposed, the actions of MVA or MVA are the same. Both are exposed to manifold vacuum. There is no difference.
- your claim of detonation at WOT with the example of 34* + 16* VA=50* is incorrect, because the VA [ MVA or PVA ] drops to zero at WOT. Timing at WOT would be 34, not 50*.
- minimum vac of aftermarket VA cans. Crane & others go down to 6~7" of vacuum.
Few errors/misconceptions in your post.
- once the PVA port is exposed, the actions of MVA or MVA are the same. Both are exposed to manifold vacuum. There is no difference.
- your claim of detonation at WOT with the example of 34* + 16* VA=50* is incorrect, because the VA [ MVA or PVA ] drops to zero at WOT. Timing at WOT would be 34, not 50*.
- minimum vac of aftermarket VA cans. Crane & others go down to 6~7" of vacuum.
So maybe not as useful but not necessarily useless. Agree?
Rat Bastid
Dunamis Metron
Rat Bastid.
Total nonsense by you.
An engine, any engine, requires a certain amount of timing advance at idle. It doesn't care how it gets it, as long as it gets it. MVA is one method of providing of it. MVA has the twofold advantage of providing more timing at idle plus providing additional timing at cruise.
Total nonsense by you.
An engine, any engine, requires a certain amount of timing advance at idle. It doesn't care how it gets it, as long as it gets it. MVA is one method of providing of it. MVA has the twofold advantage of providing more timing at idle plus providing additional timing at cruise.
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