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Old 11-20-2006, 03:31 AM
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Timing advance issues...

Okay... Heres the deal, I have a SBC 305 .030 over low compression, World Torquer 305 heads, Weiand 144 Supercharger with a 625CFM Carter AFB, Crower supercharger hyd. cam designed for up to 10lbs boost, Crane roller rockers with guide plates, fresh double roller timing chain, MSD 6AL, MSD Boost Timing Master, HEI w/ Vac advance...

I have two main issues:

1)My truck is a pain to start on cold mornings...when its cold sometimes it backfires through the carb, doesn't do it warm. Tried messing with the electric choke, its still the same. The distributor was dropped in correctly set in the No. 1 Position with the No.1 position at TDC, so that tells me its not my rotor phasing and I double checked the timing set to make sure its not a tooth off. Timing is set at 8 BTDC. I recently purchased an Innovate LM-1 Wideband Air fuel meter and the backfire is not caused by a lean condition. The distributor has some miles on it, so I ordered a new one which will be here in a couple of days... Im thinking the distributors mileage might be a contributing factor. What can be some other possible causes to the backfire through the carb?

2) I haven't been able to find anything online pertaining to running vacuum advance with a blower. I had it hooked up as usual to a full vacuum port on the carb, but when the motor sees boost, it seems like the motor is working against itself and the top end suffers. I figured out that I would have to run the vac advance off the manifold because the carb is constantly seeing a vacuum signal so it constantly advances the timing when hooked to the carb. When I have it hooked on the manifold, its even harder to start but seems to run better when decelerating and as a plus,the top end is brutal, however the bottom end is really weak. Im thinking the bottom end is weak because the vac advance is seeing boost and not advancing, so I thought about getting a lockout plate for the Vac Advance...But that in itself presents more shorterm problems being, how can I limit my centrifugal advance so I can increase my initial timing to make up for the lack of timing w/o the vac advance? I guess Im just really interested in what others with a similar set up are running for a distributor or what they did to fix these issues.

I eventually plan on getting a Holley TBI projection setup and using that to dictate my timing and fuel issues, but until then I would like to get this set up dialed.


Any ideas or help would be appreciated.

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Old 11-20-2006, 03:50 AM
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I don´t know about a blower situation but I think you should connect to a vacuum source above the throttle blades.
You don´t want the vacuum advance coming into play before you open the throttle.
CHP article
more here
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Old 11-20-2006, 08:12 AM
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Why would you not want vacuum advance at idle? VA at idle certainly makes my car run smoother.
When is idle not idle? If I raise my idle from 600 to 800 rpm and the ported slot is uncovered, am I not idling anymore? Is there damage being done?
Do modern computer controlled ignition systems reduce the timing at idle? Is this a smog thing?
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Old 11-20-2006, 08:16 AM
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IIRC, a friend of mine has told me about some factory type applications (think Superchargerd T-Bird of the 80's) that used a vacuum advance that was a little different. It could BOTH advance and retard the timing based on the pressure that it saw.

So, at cruise, if under vacuum, it gave more advance than base, then under part throttle, some boost, it would retard the timing a little from base, and at WOT, it would retard the timing even more, to avoid detonation as the cylinder pressures and temp. got higher while under max. boost.

I think there are factory Ford vac. advance cans that do this. I have no idea how to figure out what ones do and don't. I'm sure someone around here will.

If not, look for some T-bird sites or something and ask them.

If it was me, I would run a vacuum advance for part throttle conditions, and use my Mallory Hy-fire ignition with the 3 bar MAP sensor and set the boost retard up - and its easy to program. Then you would have control (separately) of both the advance side and retard side independently. FYI, that Mallory box (I think it's called the Hyfire VI) has a list of features that far surpasses the comp. and costs the same as the ol' faithful 6AL. And it works great. Digital controls, start retard, nitrous retard, boost retard, two stage rev. limit. It's awesome for the money.

Course when you go to Projection or Commander 950 you won't need that anymore.

But hey, that's just my $.02.

Good luck.
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Old 11-20-2006, 08:46 AM
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When you set the base timing do you not disconnect the vacuum can ?
At idle, I use 750 rpm, there is no need for spark advance, but when you open the throttle and the revs go up the spark has to keep up with the piston.
Revs go up, timing advances.
Revs come down, timing retards.

On computer controlled vehicles to set timing you disconnect the distributor from the ECM.
Example, my Camaro, was then set to 0º BTDC.
The computer when re connected took over the advance of the spark.
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Old 11-20-2006, 09:37 AM
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SuperchargersOnline.com might give you some insight.
There is a paragraph on timing down the page.
Found some blower/timing info here
again down the page.
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Old 11-20-2006, 09:43 AM
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Quote:
Originally Posted by malc
When you set the base timing do you not disconnect the vacuum can ?
At idle, I use 750 rpm, there is no need for spark advance, but when you open the throttle and the revs go up the spark has to keep up with the piston.
Revs go up, timing advances.
Revs come down, timing retards.

On computer controlled vehicles to set timing you disconnect the distributor from the ECM.
Example, my Camaro, was then set to 0º BTDC.
The computer when re connected took over the advance of the spark.
Whether or not you disconnect the VA when you set the base timing has nothing to do with whether or not you should use it at idle. You disconnect it to get a true reading of the mechanical advance curve.
If there is no need for spark advance at idle, why do you set the initial timing to anything other than 0°?
Why would you use vacuum advance at all if your timing is solely dependent on piston speed? Does your timing curve continue to advance the timing all the way to the maximum rpm of the motor?
When the computer takes over, what does it do? Does the timing stay at 0° at idle?
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Old 11-20-2006, 09:54 AM
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When you set initial timing there is no mechanical or vacuum advance.
You may set you initial timing at 8/10/12º or whatever to give the spark time to initiate the burn process before the piston gets to TDC, then as it hits TDC
the full force of the burn is at the right moment, not before not after.
Off idle the vacuum gives advance until the mechanical weight system takes over, then when all the timing is in around 36º there is no more need to advance.

The computer does the same as the vacuum can and the distributor has weights in it as well.
When you time the engine with the computer disconnected to 0º and re connect the ECM then sets the timing to the right degrees BTDC, and advances it as needed as the engine revs climb.

I separated out vacuum and computer advance systems but you seem to be confusing them.

Some more on blower timing etc.
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Old 11-20-2006, 11:43 AM
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Quote:
Originally Posted by malc
When you set initial timing there is no mechanical or vacuum advance.
You may set you initial timing at 8/10/12º or whatever to give the spark time to initiate the burn process before the piston gets to TDC, then as it hits TDC
the full force of the burn is at the right moment, not before not after.
Off idle the vacuum gives advance until the mechanical weight system takes over, then when all the timing is in around 36º there is no more need to advance.

The computer does the same as the vacuum can and the distributor has weights in it as well.
When you time the engine with the computer disconnected to 0º and re connect the ECM then sets the timing to the right degrees BTDC, and advances it as needed as the engine revs climb.

I separated out vacuum and computer advance systems but you seem to be confusing them.

Some more on blower timing etc.
There's no confusion between computer generated timing curves and mechanically generated ones. The engine doesn't know who decided to light the spark.
When you set the initial mechanical timing, you are setting the lower limit of the WOT timing curve. Too much initial and the engine will detonate at low rpm WOT conditions. This is not to be confused with throttled conditions, when the fuel/air mix is much less dense. Timing under these conditions needs to be advanced considerably to get the peak cylinder pressure at the right point in the cycle.
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Old 11-20-2006, 03:06 PM
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Vac Adv...

First off...Thanks for all the responses!

When I set the timing, yes I disconnect the Vac Advance and the idle is at 800rpms but even so there isn't enough vacuum to pull on the vac adv. The problem is even with an adjustable vac advance can, there isn't enough vacuum when first starting it, after firing it up and it fully warms up, builds vacuum, I cant get it to idle at any less than 1000rpm with the vacuum advance hooked to the carb . Another problem with the Vac Adv hooked to the carb is deceleration, it wants to shut down especially around corners because the throttle drops out and the vac advance retards too quickly almost like a "retard spike."

My timing is not yet dictated by a computer, its just your standard HEI distributor wired to run the MSD 6AL and MSD BTM. I have recurved my distributor to advance as fast as possible using the lightest springs since I have an automatic. In N/A form I ran the Vac Adv. from the carb and it ran great, but since its boosted the blower is constantly pulling air through the carb and because of that the carb never sees a "no vacuum" condition and it constantly pulls on the vac adv in turn advancing the timing from idle to wide open throttle instead of functioning correctly as in hit the throttle vac adv comes in and near the mid to top end vac adv disappears and only the initial and mech advanced are used. So because the blower constantly pulls a greater volume of air through the carb, Im constantly under Vac Adv which isnt good, that is why I ran the vac adv to the manifold. The manifold sees boost and vacuum, when it sees vacuum under a cruise condition, its good to go, when it sees boost, it kills my low end but functions correctly on the top end and just my mech/centrifugal and initial are used. It seems like Im damned if I do and damned if I don't as far as vacuum advance goes.
I hope my explanations are somewhat okay to understand, its my theory of whats going on with the vac advance...I could be wrong.

I am going to look into the vac adv that ford makes for supercharged applications as I might be able to adapt it to my set up.

If I increase my initial timing to make up for the lack of vac adv. how can I limit my mech advance? I was thinking about increasing the initial, keeping the vac adv. to the manifold and adjusting the retard limit per pound of boost with the BTM...is this logical am I on the right track? I have heard that retarded timing can burn valves so it worries me to use the BTM too much.

Please forgive me if I forgot someones comments/advice, Im not very experienced using forums.

Thanks again, I look forward to more comments.
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Old 11-20-2006, 03:51 PM
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Here´s an article that explains limiting mechanical advance
You can also get MSD distributors which allow you to vary the mechanical advance.
And some more blown motor advise

Last edited by malc; 11-20-2006 at 03:59 PM.
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Old 11-20-2006, 04:40 PM
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Thanks Malc.... Im going to try and figure out the part number on that Holley distributor. Im also going bust my MIG welder out and play with my mechanical advance, those were very helpful articles.

Hope to hear more ideas or suggestions from others as well.

Thanks!
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Old 11-20-2006, 10:02 PM
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Another thought and question...

Another thought and question...When an engine is naturally aspirated and the vacuum advance is connected to the carb and you floor it from a stand still, the vacuum advance doesnt advance the timing because there is no vacuum to pull it correct?

So...that means the engine is relying just on the mech and initial timing, maybe my vacuum advance isnt really the issue and its more of an initial timing issue?
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Old 11-20-2006, 11:24 PM
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If you spin the distributor fast the weights will fly apart and advance the timing. This vacuum thing only functions from off idle as a transition into mechanical advance.The springs retaining the weights have to be overcome.
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Old 11-21-2006, 03:24 AM
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Quote:
Originally Posted by malc
If you spin the distributor fast the weights will fly apart and advance the timing. This vacuum thing only functions from off idle as a transition into mechanical advance.The springs retaining the weights have to be overcome.
I thought the vacuum advance shifted the whole plate the weights are anchored to, thus adding to the timing when not under wot.

ie - 3000rpm = 36 dg of cent + 20dg of vacuum = 56 total under part throttle or just the 36dg when wide open. (just an example)

Anyway dizzies are so yesterday - all the cool kids are now plugging in laptops
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