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  #16 (permalink)  
Old 08-09-2008, 10:30 PM
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Yeah, clear as can be. Especially when MSD claims you cant go beyond 36* ever, no matter what kind of setup you have. At least with that answer I dont have to wonder if I should put vac back in for highway driving because Im locked out at full and not missing any additional advance.

Before I had the MSD control on the timing I had a bicycle "brake line" hooked up to where the vac advance was supposed to connect to. I'd push the line to retard the timing and get me started and then return it to full advance. The vac advance does increase timing but only when actually used. It does the same as physically turning the distributor to advance it

As I see it, 36* or 38* is the max timing under all conditions (depending on how far you have the distributor turned). I think. lol. I just cant see vacuum coming in at highway speed.

Did myth busters tackle this one yet?

One of you guys must have a dyno where you can get the car to highway speed with the hood still open. Thats 100% of an answer there.

EDIT: according to summit racings tech database....vac advance is additional to total timing putting it above 36*

EDIT #2: To make things more confusing for me: MSD has a digital E Curve distributor WITH a vac advance connection. When you install that distributor you lock the distributor at total timing of 36* and adjust from their. The vac advance on that distributor does NOT add to 36*.

FINAL EDIT: Taken from SUPERCHEVY.com

All the aforementioned applies to an engine operating at wide open throttle (WOT) with the advance curve being totally rpm related. (That's what we would have called mechanical or centrifugal in a regular distributor.) At less than WOT (idle, cruise etc.), the amount of air entering the cylinder is reduced which in turn reduces the compression pressure. As manifold vacuum increases the amount of advance needs to increase. At idle and low-speed operation, the amount of advance required to most effectively utilize the air and fuel entering the engine can be as much as 50 to 55 degrees. This is handled by the vacuum advance; a function many hot rodders believe is not needed because their favorite drag racer does not use it. Now is the time to listen up and listen up good. A functional vacuum advance is the single most effective camshaft tamer you can get. By taking the time to hook up the vacuum advance to a manifold vacuum source you can get a big cam to idle as if it were about 20 degrees less than it really is. Conversely, if you are looking for a decent idle the use of vacuum advance will allow you to use a cam of, at the very least, 5 degrees more duration/overlap than would otherwise be the case.

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Last edited by bubbahotep; 08-09-2008 at 10:49 PM.
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Old 08-10-2008, 09:55 AM
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Quote:
Originally Posted by bubbahotep
Holder350 - Ive read that post on this website MANY times over the years Thus my confusion since it goes against most of what Ive been told or read.

F-Bird: I understand what you're saying but please show me how Im wrong in this statement: What is the difference between "cruise speed" on the road and the car sitting in the driveway at the same rpm as "cruise"? The engine doesnt know its going 70mph with no load. At 70mph on the open road its the same rpm as sitting in the driveway. No? So timing in the driveway (which never goes over 36*) should be the same at highway too.

I do agree 100% that if you can get the vac advance canister to "advance" at higher rpm you will increase your total timing beyond 36*. But, as rpm goes up your vac goes down so you lose the advance. No?

Vacuum advance is load based NOT RPM based.
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Old 08-10-2008, 11:45 AM
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Agreed. I still dont see where centrifugal AND vac coming in at the same time though causing over 36* advance. If anyone has actually seen this with a timing light, at highway speed, Id agree 100% that its correct.

Couldnt this be tested in a driveway easily? Warm up a car (an older non computer controller car), put it in drive with the brake pressed down, and rev the engine to highway rpm. Or is that amount of "load" not the same?

Theres so many conflicting "theories" that I dont know which way to go. theres no doubt to me that vac is better for idle and stop and go traffic since load is changing all the time but that still doesnt answer the idea of getting more than 36* total timing on a motor.
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Old 08-10-2008, 03:11 PM
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--------------------------------------------------------------------------------

There are 2-3 current threads happening regarding distributor curving and vacuum advance cans, Ill try and get the links and edit here.

http://www.hotrodders.com/forum/dis...-1-a-59033.html

http://www.hotrodders.com/forum/tim...ion-142521.html

Here's a good write up by Steve Davis of Performance Distributors (DUI).
http://www.gofastnews.com/board/tec...simplified.html
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Old 08-10-2008, 08:42 PM
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In real world driving, you'll never get total mechanical advance and total vacuum advance added together except in one instance; under decelleration at speeds around 70-80 mph. At this speed the mech advance is maxed out, and decelleration will cause the vacuum advance to max out. But....there's no performance gained since the vehicle is slowing down. There may be some engine braking gained, and maybe less pollution output, but no power or driveability added.

Under normal driving conditions the mech and vacuum advances work together, and give near optimum timing under nearly all conditions, assuming the distributor is 'tuned' correctly. Much better than locked timing, IMHO. With timing locked, there'll always be an rpm range where the timing is less than optimum, in the lower rpm range. With a high-torque stroker engine, it might not be apparent, but it's there.

Where vacuum advance really shines is during off-idle, part throttle accelleration, especially with a straight-shift transmission. It makes for much smoother accelleration and shifting.
Now racing's a different matter altogether, when the throttle's either fully open or fully closed......
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Old 08-10-2008, 09:14 PM
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Quote:
Originally Posted by F-BIRD'88
Yes you'll get more than 38deg. Vacuum advance is independent of mechinical advance and independent to your MSD timing box. The vacuum diaphram rod moves the point plate, magnetic pickup plate and mag pick up around the distributor shaft to advance the timing according to manifold vacuum.
You are way to tied up in therory. Get a timing light and a vacuum gauge and see for yourself how the distributor mechanial and vacuum advance actually functions.

its very simple, no (erronious) theories nessesary to cloud your mind.
Hot Rodding is a hands on experience.
Go have a look see and it will be very clear to you.
No argument there. I mentioned in an earlier post that I used to advance the timing via the vac advance connection manually on my car.

I understand "how" they function fully (I'll have to accept the atmospheric pressure facts as they are) but still have not seen a timing light on a timing tape show more than TOTAL timing when the car is in the driveway. I wouldnt even question any vacuum theory is MSD themselves hadnt told me that "vac advance never increases total timing". I specifically said to the tech "if I set my timing at 38* total, you're telling me that a vac canister will never add more timing to 38*?"

Im not saying MSD is correct. Im no one to say either side is correct. Im just a tad bit lost on the facts. All of which can be proven by a timing light and a running engine.
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Old 08-10-2008, 10:13 PM
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ok......just had a thought....havent pulled the old hei off the shelf to see... but does the vacuum advance have any effect on the timeing till the mechanical is above the vac cans limit? so.....you would never have OVER 36* but it would all come in at diffrent times.

Otherwise when coming off WOT at high RPM you would have all your mechanical and all your vac in at the same time.

Edit: Guess I said that wrong....I should have said "vacuum advance have any effect on the timeing till the mechanical is BELOW the vac cans limit?"

Last edited by Holder350; 08-10-2008 at 10:30 PM.
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Old 08-10-2008, 10:46 PM
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You can put light springs on the centrifugal advance so that you have full mechanical advance as early as approx. 1600 rpm. (Let's talk points since I am old and that is what I learned first). As the engine speed increases, the mechanical advance advances the rotor and the points cam ahead of the distributor shaft causing the points to open sooner. When the points open sooner, the coil sends the spark pulse into the distributor cap sooner, thus advancing the timing. The vacuum canister advances the points in relation to the cam when vacuum is applied. Thus causing the points to open sooner and so on. I looked for tables on vacuum advance units. The best I could find was that some units are full advance at 9 - 11 Hg. Depending on the engine, running down the road at 1600 rpm, 9 -11 is possible. So, if you have light springs and have the full mechanical advance at 1600 rpm AND the engine is not under load, you add them all up, initial, mechanical, and vacuum.
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Old 08-11-2008, 03:28 PM
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let me end my topic. Summit says that vac advance will increase total timing at some point but also said that some digital distributors might not acknowledge this.

Msd today told me that they 'think' their ecurve will also add vac advance to the total timing under certain conditions.

I'll have to accept it and maybe someday I'll see it under the hood at highway conditions.
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Old 08-11-2008, 08:44 PM
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Quote:
Originally Posted by bubbahotep
let me end my topic. Summit says that vac advance will increase total timing at some point but also said that some digital distributors might not acknowledge this.

Msd today told me that they 'think' their ecurve will also add vac advance to the total timing under certain conditions.

I'll have to accept it and maybe someday I'll see it under the hood at highway conditions.
Bubba, I think both those statements are correct, as is Redsdads and several other posts.
But you can't end this topic, it's too much fun!
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Old 08-11-2008, 09:08 PM
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Quote:
Originally Posted by Holder350
ok......just had a thought....havent pulled the old hei off the shelf to see... but does the vacuum advance have any effect on the timeing till the mechanical is above the vac cans limit? so.....you would never have OVER 36* but it would all come in at diffrent times.

Otherwise when coming off WOT at high RPM you would have all your mechanical and all your vac in at the same time.

Edit: Guess I said that wrong....I should have said "vacuum advance have any effect on the timeing till the mechanical is BELOW the vac cans limit?"
Holder, on my Ford distributor, the mechanical and vacuum advances are entirely separate, one turns the rotor, and the other the stator (or points plate), so they are independent of each other.

Otherwise when coming off WOT at high RPM you would have all your mechanical and all your vac in at the same time.

This statement is correct. That's the only time all mech and all vac would come in all at once. And it would only be under decelleration, so would have no effect except maybe for engine braking or better emissions.
As soon as the throttle was opened, the vacuum advance would immediately retard because of lesser manifold vacuum. Under normal driving, both types of advance work together in the normal timing range.....
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Old 08-12-2008, 10:23 AM
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GM is the same. Vac turns the bottom wheel thats normally only moved when you rotate the distrib.

This whole subject seems like Sasquatch to me. Everyone knows about, no ones seen it. With youtube and web images I'm surprised theres not 1 image of timing going above total. Not that I have found anyway.

I know normally after you set total mech advance you reconnect vac and go for a drive to listen for me. People tell me that the ping Is because vac is adding too much to total any going too far advance (above 50 for example). My impression is that the vac canister is wrong got that car and isnt 'removing' its idle advance soon enough and you got mech advance coming in at the same time. You might be at 30* partial advance when you only need 25*

For now I'm staying locked at 38* and letting the msd do the idle and advance curve
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