alsant
Well-Known Member
For vac advance , seen a lot of debate about this subject. Who on here uses manifold vac ?
Ported Vac or Manifold Vac
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Deltatech
Well-Known Member
Nope.
Only if you drive a Ford.
It's totally build dependent. Maybe you could enlighten us.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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My 11/1 Scr, 367 has had three different cams from 248 to 223 to the current 230 @050.
I've tried it, both ways, and prefer ported.
My system has been optimized for fuel economy, so the Vcan has 22 degrees in it. The Idle-Timing is set to 13 * plus/minus 1. Therefore, the manifold vacuum set-up would have 13+22=35 which is fine, until I put it into gear. At this setting it is IMPOSSIBLE to synchronize the mixture screws to the T-slots without the rpm climbing. and when that happens, my engine makes way too much Idle-Power to drive slowly, with the manual trans. As I begin to ride the brake to slow down, every compression cycle transfers into the chassis, and the entire car begins to feel it, to the point that if allowed to continue she will start to buck like rodeo-bull.
If you have an automatic trans, this bucking could get absorbed by the convertor.
To avoid thatchit, I use ported vacuum, at 13* timing, and when I want to drive really slow, I retard the timing from the Driver's seat with a dash-mounted, dial-back, timing-module, that I have set for a max of up to 9 degrees retard/6* of advance . I use the advance to optimize Cruise-Timing on long trips. Thus, with my power-timing set to 33@3400, my total timing range is from 5 degrees to 33+22+6=61 degrees, at 3400rpm. And no, I have never tried cruising at 61* timing; But, geared at 75= 2100, this is about 13idle+10distributor +22VA+6dial-back=53*grand total cruise-timing, which is where my 223 cam made best fuel economy.
so like Rusty says, your choice of vacuum source, is totally application specific.
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And BTW,
1) my booster worked just fine with the 248cam, except at first start in the morning...... But I have a Manual trans so, a couple of blips on the throttle and by the time I had backed out of the carport, she was ready to go.
2) and yes, she idled just fine at 550rpm with just 5* of timing advance. Even pulling herself on hard flat ground.
3) I believe that any factory Mopar Hex-chamber VA can be modified for a max advance number between 22>24 degrees, just by filing off the external stops, to get what you need. I got 22 out of mine/Trailbeast got 24 out of his. If you have a selection, the arms are engraved with distributor degrees, which is doubled to get crank advance, ie; an engraving of 7, means 14 crank degrees.
I've tried it, both ways, and prefer ported.
My system has been optimized for fuel economy, so the Vcan has 22 degrees in it. The Idle-Timing is set to 13 * plus/minus 1. Therefore, the manifold vacuum set-up would have 13+22=35 which is fine, until I put it into gear. At this setting it is IMPOSSIBLE to synchronize the mixture screws to the T-slots without the rpm climbing. and when that happens, my engine makes way too much Idle-Power to drive slowly, with the manual trans. As I begin to ride the brake to slow down, every compression cycle transfers into the chassis, and the entire car begins to feel it, to the point that if allowed to continue she will start to buck like rodeo-bull.
If you have an automatic trans, this bucking could get absorbed by the convertor.
To avoid thatchit, I use ported vacuum, at 13* timing, and when I want to drive really slow, I retard the timing from the Driver's seat with a dash-mounted, dial-back, timing-module, that I have set for a max of up to 9 degrees retard/6* of advance . I use the advance to optimize Cruise-Timing on long trips. Thus, with my power-timing set to 33@3400, my total timing range is from 5 degrees to 33+22+6=61 degrees, at 3400rpm. And no, I have never tried cruising at 61* timing; But, geared at 75= 2100, this is about 13idle+10distributor +22VA+6dial-back=53*grand total cruise-timing, which is where my 223 cam made best fuel economy.
so like Rusty says, your choice of vacuum source, is totally application specific.
----------------------------------------------------------
And BTW,
1) my booster worked just fine with the 248cam, except at first start in the morning...... But I have a Manual trans so, a couple of blips on the throttle and by the time I had backed out of the carport, she was ready to go.
2) and yes, she idled just fine at 550rpm with just 5* of timing advance. Even pulling herself on hard flat ground.
3) I believe that any factory Mopar Hex-chamber VA can be modified for a max advance number between 22>24 degrees, just by filing off the external stops, to get what you need. I got 22 out of mine/Trailbeast got 24 out of his. If you have a selection, the arms are engraved with distributor degrees, which is doubled to get crank advance, ie; an engraving of 7, means 14 crank degrees.
alsant
Well-Known Member
Oh ok. So you're not going to tell us about the build, but you want advice on vacuum advance. Ok. I got it. Have fun!
alsant
Well-Known Member
sorry been busy
It’s a 318 .40 over
340 cam
68 heads
QFT 680 cfm
Edelbrock LD4B
4 speed A833
Block is a 73
I’ve rejet the carb for 5000’ , need to do more to the carb
Need to change springs on the distributor , right now advance is coming in till about 3500rpm
Thanks
It’s a 318 .40 over
340 cam
68 heads
QFT 680 cfm
Edelbrock LD4B
4 speed A833
Block is a 73
I’ve rejet the carb for 5000’ , need to do more to the carb
Need to change springs on the distributor , right now advance is coming in till about 3500rpm
Thanks
I agree. A cam that mild and with compression at or possibly under 8:1 you need ported vacuum.
alsant
Well-Known Member
Great advice
Played with manifold vac and she didn’t seem to like it.
Played with manifold vac and she didn’t seem to like it.
Probably because sans for altitude, it's already got a good tune on it.
alsant
Well-Known Member
The car was at sea level , I’m not happy with how it is running. I put a phenolic spacer on for the vapor lock issue , that’s gone now. Will at some point put a regulator on and run a return.
Aslant,
MVA is a method of increasing the ign timing at idle. Chrys missed the boat on MVA when it was most needed, but was forced to use it in the 70s when the emissions junk made the engines run hotter.
MVA, when properly set up, gives these advantages over the generic 6-12* BTDC that Chrys used:
- smoother idle
- cooler running
- more idle vacuum
- better economy
- better tip-in response.
Note people running locked timing are getting the same benefit at idle as using MVA.
Stock or near stock engines will only benefit with a few more degrees if idle timing. Plugging the VA in on a stock engine with a VA unit that adds 25* will likely result in poor running, with MVA getting blamed [ didn't work on my car.....] when just adding 8-15* of VA by limiting the total amount of VA.....would have made a world of improvement.
GM used MVA on their cars, idling at 24-26*.
The factors that will require more idle timing for best idle are: cam duration/overlap & CR. More cam needs more idle timing, & so do low compression engines.
You can determine very quickly & easily if MVA will benefit your engine: warm up the engine, chock the wheels, & put in gear if auto. Loosen dist clamp & SLOWLY advance the timing. If rpm increases, it needs more timing & this can be done with MVA.
MVA is a method of increasing the ign timing at idle. Chrys missed the boat on MVA when it was most needed, but was forced to use it in the 70s when the emissions junk made the engines run hotter.
MVA, when properly set up, gives these advantages over the generic 6-12* BTDC that Chrys used:
- smoother idle
- cooler running
- more idle vacuum
- better economy
- better tip-in response.
Note people running locked timing are getting the same benefit at idle as using MVA.
Stock or near stock engines will only benefit with a few more degrees if idle timing. Plugging the VA in on a stock engine with a VA unit that adds 25* will likely result in poor running, with MVA getting blamed [ didn't work on my car.....] when just adding 8-15* of VA by limiting the total amount of VA.....would have made a world of improvement.
GM used MVA on their cars, idling at 24-26*.
The factors that will require more idle timing for best idle are: cam duration/overlap & CR. More cam needs more idle timing, & so do low compression engines.
You can determine very quickly & easily if MVA will benefit your engine: warm up the engine, chock the wheels, & put in gear if auto. Loosen dist clamp & SLOWLY advance the timing. If rpm increases, it needs more timing & this can be done with MVA.
Mattax
Just the facts, ma'am
First thing you need to do is measure and plot the timing vs. rpm.
If the distributor is a factory pre-68 49 state then that's a good baseline.
If its a Chrysler built MP distributor, that too is a good baseline if it hasn't been mucked with. When you have time you can do a search and see what I've found with those, along with the guidance from MP.
If its a Chrysler distributor from the smog era, its hard to know what you'll find, but oiften those have a quick and long primary advance. With those sometimes an effective crutch is to use manifold vac advance to get the initial timing up after start. Dpends on the details of the advance and how well the engine idles.
4spdragtop
Well-Known Member
No but I read up on a good "Best oil to use" thread. A good spin off would be "Best oil filter" thread. Haven't seen one of those for a while.
Do you know which vacuum canister is in the distributor ?
There is a number on the arm . And do you know you can change when it starts advancing? You will need a mighty vac to figure it out.
It is important to know regardless of which port you use . Maybe you have already mentioned this but what is the curve like ?
A properly setup distributor makes a big difference in power and driveability.
There is a number on the arm . And do you know you can change when it starts advancing? You will need a mighty vac to figure it out.
It is important to know regardless of which port you use . Maybe you have already mentioned this but what is the curve like ?
A properly setup distributor makes a big difference in power and driveability.
alsant
Well-Known Member
Yes I need to change the springs on distributor, advance is adjustable.
My next step is lighter springs
My next step is lighter springs
Mattax
Just the facts, ma'am
NOPE !
Why would you change something when you don't know what it does?
Then you'll have a before and an after but you won't know what either was.
Do what I posted, including the search.
Then you can reference these two articles, but only then.
Distributor starting point for a curve
I get a lot of questions where my distributor curve should start this is a great guideline of different engine combinations and a good area of where the mechanical portion of the curve should be. I have been looking for something like this for a...
www.forabodiesonly.com
How To Limit and Adjust Chrysler Vacuum Advance Cans
This comes up enough that it seemed worthwhile to create its own thread. Part 1. Chrysler Vacuum Advance Canisters (pods) Lets start with how two of the three vacuum advance characteristics can be adjusted. This can be done either to bring the...
www.forabodiesonly.com
Some assembly required
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Manifold.... Absolute pressure. 
With a digital advance table that uses the same advance regardless of MAP reading until above 1200 RPM to give a similar behavior to ported vacuum.
My experience with EFI tuning has been that very few engines will give their most stable idle at more than 15-18 degrees of advance, and many work best with less. The 5.0 Mustang guys quit complaining that "Megasquirt doesn't properly support Ford idle control valves" about the same time they quit copying some rando's "base map" that called for 50 degrees of idle timing.
With a digital advance table that uses the same advance regardless of MAP reading until above 1200 RPM to give a similar behavior to ported vacuum.My experience with EFI tuning has been that very few engines will give their most stable idle at more than 15-18 degrees of advance, and many work best with less. The 5.0 Mustang guys quit complaining that "Megasquirt doesn't properly support Ford idle control valves" about the same time they quit copying some rando's "base map" that called for 50 degrees of idle timing.
Woah, back up the bus, the OP has a carbureted 318. Don't even mention fuel injection here.Manifold.... Absolute pressure.
My experience with EFI tuning has been that very few engines will give their most stable idle at more than 15-18 degrees of advance, and many work best with less. The 5.0 Mustang guys quit complaining that "Megasquirt doesn't properly support Ford idle control valves" about the same time they quit copying some rando's "base map" that called for 50 degrees of idle timing.
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