Ironracer
Gearhead
Nice. I had a '90 MP engine book. Loved it. It's long gone now. I don't know who stole it
Vacuum advance information and myths
- Thread starter halifaxhops
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Keep an eye on Mancini's literature section. They sell the MP engine manuals CHEAP when they have them.
Ironracer
Gearhead
Right on Brother, Will Do! Thank You! Right now, I have Way more intense issues to deal with... like tryin' to keep my Address! And what hope I had for that is trashed, reversed and heading down the septic tank. I hate to say it but it's pay to play in Florida if you do not own the land that your home is on and you don't pay exactly what the people claim you owe they can do what's called a default judgment motion and get the judge to grant them possession of your home and get you out of it.... in 10 days. As long as they got a dirty *** lawyer with no morals. And no I'm not knocking lawyers..
subscribed!
(Hi, Ray!)
(Hi, Ray!)
There is a two part video on Unity Motor Sports you tube, on vac adv. D. Vizard appears in part one. Done about two years ago, sorry do not know how to link it. Before & after results/comments are provided, on a tunnel rammed Ford with two 660 Holleys! Worth watching.
I use a different method to set up MVA than shown in the video.
I use a different method to set up MVA than shown in the video.
Rat Bastid
Dunamis Metron
There are certain situations (very limited situations) where you can’t use VA on a street car.
Other than that, you need it.
And, if you NEED manifold vacuum at idle you don’t have enough compression, your cam is too big or both.
If that’s what you have then by all means do it. It’s cheaper than getting the compression up or changing cams or both.
IMO it’s better to build it right the first time.
Other than that, you need it.
And, if you NEED manifold vacuum at idle you don’t have enough compression, your cam is too big or both.
If that’s what you have then by all means do it. It’s cheaper than getting the compression up or changing cams or both.
IMO it’s better to build it right the first time.
TT5.9mag
Two atmospheres are better than one
The biggest problem with VA (either ported or manifold) is the lack of willingness of the owner to understand what is happening and why, and the ability (knowledge and tuning time) it takes to get it right. It’s amazing to me how many people you meet at car functions, (race track, car show) who have no idea how to set basic timing or adjust a curve or don’t even know where their timing is. Me; “what’s the timing at?” Them; “don’t know haven’t even put a light on it yet.” Me; shakes head and walks away.
Yup,
VA is not understood. Not helped by ignorant magazine writers you talk about the 'feature' engine in their article, & mention the MSD or DUI dist they are using for free plug, but make no mention of VA.
VA is not understood. Not helped by ignorant magazine writers you talk about the 'feature' engine in their article, & mention the MSD or DUI dist they are using for free plug, but make no mention of VA.
TT5.9mag
Two atmospheres are better than one
That’s usually because they are writing a magazine article and the only number that matters is the peak number on a dyno. Almost never does the “magazine build” get stuffed in a car and re-tuned for the street
I don’t want to get off topic here, but can you tell me which manuals are the ones you’re referring to? I’ve done some searches, and there are quite a few Mopar engine books available. Are there books published by Mopar that I can get, or just the ones I’ve found that don’t seem to come straight from Mopar and are written by other folks? Here are some I’ve found:
I want vacuum advance for,
Mileage
Throttle response
Cooler running
Idle quality
Best vacuum for accessories
Tunability if engine parts are changed
Did I miss anything ?
Mileage
Throttle response
Cooler running
Idle quality
Best vacuum for accessories
Tunability if engine parts are changed
Did I miss anything ?
Rat Bastid
Dunamis Metron
ONLY IF the rest of the combo is wrong. If it’s right, it will run on ported vacuum.
Again, claiming everything need 50 degrees of timing at idle is as absurd as most everything else you post.
65TerrorCuda
Well-Known Member
Everything You Wanted to Know About Vacuum Advance and Ignition Timing
add cleaner spark plugs to that list
You're describing general benefits of using vacuum advance.
1. Mileage Yes
2. Throttle response It might. Wouldn't guarentee it.
3. Cooler running Yes. Whether it will always be a noticible difference is another story.
4. Idle quality Depends on the distributor setup. If the advance curve is correct for the application, no difference.
5. Best vacuum for accessories. Not really. It uses vacuum as an indicator of load and density. It doesn't create vacuum.
6. Tunability if engine parts are changed. Depends on perspective. For drag racing there are good reasons not to use it. If the engine has been altered in a way that effects the burn at light to moderate throttle, then yes the vacuum advance should be tuned to match that.
Another way to look at it is functional.
When the burn rate is slower due to lower density of the fuel and air, more lead time is needed for maximum pressure during the maximum crank leverage.
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Pressure, Load, and Temperature in the chamber all effect the combustion development.
Changing rpm changes the time available for the pressure to develop before its largely wasted (in the form of heat into the cylinder walls and out the exhaust).
from Chrysler Master Technicians Conference 259
Changing rpm changes the time available for the pressure to develop before its largely wasted (in the form of heat into the cylinder walls and out the exhaust).
from Chrysler Master Technicians Conference
Density of the fuel air in the combustion chamber is another factor.
Important enough to have a method to automaticly compensate for the effect on the burn rate.
Density of the fuel air in the combustion chamber is another factor.
Important enough to have a method to automaticly compensate for the effect on the burn rate.
from Chrysler Master Technicians Conference 259
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Pingity, pingity, ping.
Idle is surprisingly high load, and before emissions concerns, fairly rich. The engine is producing very little torque at 500 or even 600 rpm. A percentage of that goes to overcoming friction. So best to think of it as a high load, somewhat rich and dense situation.
As far as what a factory 383 or 440 4bbl likes, the factory spent quite a bit of time working out timing curves to maximize power and economy. We can check out the acceptable range of mechanical timing and vacuum advance in the 1967 and older service manuals.
I've posted a number these on FABO.
For example 1967 383 standard 2bbl and 4 bbl from the Plymouth FSM plots outs like this
Next one is the average vacuum advance rather than the acceptable range just to make it easier to work with.
Knowing the cruise rpm and cruise vacuum the timing can be figured out.
I think its safe to say that unless the car hits real high wind resistance, at highway speeds and rpm the vacuum on flat road will be above 15"Hg in most cases.
A more general baseline can be had from the older Direct Connection /Mopar Performance guidelines that I posted in the vacuum advance how to.
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440 Mike,
Post #43.
60* of timing at cruise is probably too much for a high compression engine of the 60s.
However, if the engine has a large duration cam, it might require that for cruise & up to 50* at idle, but 25-45* is more common.
Post #43.
60* of timing at cruise is probably too much for a high compression engine of the 60s.
However, if the engine has a large duration cam, it might require that for cruise & up to 50* at idle, but 25-45* is more common.
Fishmen,
It seems you might be unaware that Chrys used manifold connected VA to cool the engine?
For 1973-78:
' When engine coolant temps at idle reaches 225*f, the valve opens automatically & applies manifold vacuum directly to the distributor.....This increases engine idle speed & provides additional engine cooling.'
You want to think about why the idle speed increases.....
It seems you might be unaware that Chrys used manifold connected VA to cool the engine?
For 1973-78:
' When engine coolant temps at idle reaches 225*f, the valve opens automatically & applies manifold vacuum directly to the distributor.....This increases engine idle speed & provides additional engine cooling.'
You want to think about why the idle speed increases.....
I asked in a FB mopar group a while back VA ported or not. The whole thing got pissy fast. I was surprised how many people don't, won't try to run manifold vacuum. 68 was the year Mopar changed over, 65 in Cali I think for emissions. I always run my street cars manifold VA and I know racers that use MV to keep their engine cooler between rounds.
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