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  #16 (permalink)  
Old 09-11-2003, 04:39 PM
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Dan - that explanation agrees with what I have always been lead to believe. Crank in as much initial advance as possible and still start and idle well, and crank in as fast a centrifugal advance as possible without detonation. Then, use an adjustable manifold vacuum advance pot and have it all in at or below maximum manifold vacuum, again based on surge or detonation. I believe that this is the optimal performance spec for any engine. I can't imagine how ported vacuum advance has any advantage performance-wise over manifold vacuum - in fact all I can imagine is negative effects. Seems to me that ported vacuum is used as you say to honor pollution requirements at the expense of performance. As you say, tree huggers like the engines running hot with retarded timing. My son just changed a defective thermostat in his '2001 Ford and we were surprised to see that it was rated at 220F! Doesn't even crack open before that.

Can someone explain how ported vacuum could be a performance enhancer compared with manifold vacuum? In what situation is it desireable to have no additional advance at idle and part throttle cruising but have high extra advance on hard acceleration? Inquiring minds want to know!

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Old 09-11-2003, 05:43 PM
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I used an MSD Billet Ready to Run ignition on my ZZ4 motor. In the instructions it still says to use ported vacuum for the vacuum advance. I've tried both and my motor seems to like the ported better than manifold vacuum, so maybe it would depend more on the application.
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  #18 (permalink)  
Old 09-11-2003, 05:46 PM
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I've always assumed that at idle the rpms are low.. So you can have the timing at say 12 btc.. And it will spark and ignite all the fuel... But as you rev it up it seems to me that you would want the spark sooner to be able to burn the extra fuel being pumped into the engine.. There for advancing it at hard acceleration. Plus wouldn't manifold vacuum just keep the vacuum advance pulled in at all times? Or does manifold vacuum decrese at higher throttle?
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Old 09-11-2003, 06:35 PM
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Quote:
Originally posted by willys36@aol.com
Can someone explain how ported vacuum could be a performance enhancer compared with manifold vacuum? In what situation is it desireable to have no additional advance at idle and part throttle cruising but have high extra advance on hard acceleration? Inquiring minds want to know!
[/B]
Willys I don't understand where you are going with this. You say no additional advance at part throttle cruising yet that is incorrect. Ported vacuum allows full advance at part throttle accelleration. You also stated in your earlier post

Quote:
Since it is desirable to have max vacuum advance @ idle and part throttle cruising, it appears that ported vacuum can perform properly in neither case.
This is not totally true. The article said that extra advance at idle can help a radical engine or an engine that runs hot at idle even with a good functioning cooling system. If you have neither of these situations you don't need the additional advance at idle. As far as part throttle cruising you have advance at part throttle cruising from the ported source.

The biggest thing is how everything is set up. You can tailor most engines to work either way by changing total mechanical advance, spring weights (how fast it comes in), and amount of additional advance suplied by vacuum can. I run my car (and many before it) on the ported source. I like to keep her timing down at idle since it makes my cam sound a bit healthier. As the timing advances it tends to take some of the 'thump' out of the cam at idle. By keeping her at base timing at idle it sounds like I have a bigger cam than I do because the retarded timing makes for a slightly rougher idle. The second I crack the throttle she pulls in the extra advance from the vacuum can. This keeps her responsive when the light turns green. Since I like the way she idles at base timing and she never runs above 180deg. all I have to do is tailor the distributors centrifugal and vacuum advance to achieve maximum performance. In my opinion in most cases either way can work but the distributor must be properly 'tuned' one way or the other.

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Old 09-11-2003, 08:46 PM
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Thats the way Chevrolet designed it.

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Old 09-12-2003, 12:24 PM
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Thanx, that all makes sense.
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Old 05-01-2005, 09:43 AM
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I see you've had many informative responses by much more experienced
mechanics than I. But I thought I'd respond, as I had the same question
last year with the purchase of a new Road Demon Jr. I had lots of
conflicting answers from my gearhead friends, so I called BG. They said
absolutely use the timed port on the carb for vacuum adv for a stock or mild
motor and my 350 chevy seemed to agree.
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Old 05-19-2005, 04:21 AM
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Let's set the paramiters of this discussion board:
1. This is the HOTRODDERS Board
2. We need to assume that everyone here is driving a Hot Rod
3. We are not discussing timing events for anything other than naturally aspirated engines.
4. Emmission control devices are not in the equation

So now we have established the bases of the discussion let me try and simplify the timing control systems.

Your engine needs lot's of timing at idle and part throttle, I think we've established this from previous comments from qualified technicians.

This is achieved by a vacuum advance unit hooked to MANIFOLD vacuum.

As you load the engine the timing needs to be decreased, this is where the vacuum timing hooked to Manifold drops the timing off and relays the responsibility to the MECHANICAL Advance.

As the engine RPM increases it requires more timing, so the mechanical advance moves the timing up as RPM increases.

This is not an opinion but a fact, this is how they're designed to work.

So let's now look at more details and at some typical issues with distributors.

First off, your stock or aftermarket distributor is not ready to install in your performance engine right out of the box, NO WAY, NEVER, EVER will it not work right. It's no different than a carb or a set of roller rockers, the distributor must be phased, curved and set up to YOUR engine.

90% of carburetor problems are IGNITION related, pick up any basic tuning book and it will read "Ignition First then Carburetor"

I'll use a fairly simple example here:
350 Chevy in a 72 Nova

Example #1

224 @ 50 cam
9.0:1 compression
Iron heads
Headers
3.90 gear
2200 stall convertor
575 Speed Demon
Performer manifold
HEI stock ignition
91 Octane California premium fuel

This car should have a good aftermarket coil, initial timing at about 18* and vacuum timing of 20* at 16" for total timing of 38* at idle and light throttle cruise. The mechnical timing of 18* should come in at about 3000 RPM for a total under power timing number of 36*

Example #2:

Same car
244 @ 50 Cam
10.5:1 compression
Aluminum heads
Headers
4.10 gear
3200 stall convertor
650 Mighty Demon
Air Gap manifold
Pro Billet MSD Distributor
6 AL Box
91 Octane California premium fuel

So now you've deleted the vacuum control (Which is probably hurting you) and the car won't idle, loads up, stinky exhaust, over heats, stumbles...but it runs like a raped ape at WOT.

This car will take 22-24 inital timing (At idle) the mechanical advance should be set to come all in just before the convertor hits so around 3000 would be fine if the heads are efficient, spark plug heat range is right, A/F ratios correct...basically a good tune-up on the thing.

So you read the instructions from MSD and they supply a bushing that will alow you to limit the total mechanical advance to 18* so you add in the 24* of initial and your at a total of 42*???? Detonation City!

So you get out the timing light and back the initial down to 18* for a total of 36* the detonations gone but now it's back to loading up and that fine fragrance of un-burned un-leaded fuel.....sound familiar?.....are you frustrated yet?

You can call Barry Grant techline or MSD and tell them your story, they've heard it 20,000 times and if you get a good tech they'll tell you to call us. We machine bigger bushings to give you 20 and 24* of initial and 16/12* mechanical to stay at 36* total and make it idle.

Let's step back and consider what a vacuum distributor would do for engine #2...

The motor will start at 18* if everything else is correct so the black bushing in your kit will work fine. The vacuum can can be adjusted to give you another 10-15* at 10-12" of vacuum which will bump the idle timing up to about 28-30*. This will give you enough timing to properly adjust the carb to clean up the idle an get rid of that skunk in your tail pipe.

Remember your hooked to MANIFOLD vacuum so as soon as you touch the loud peddle the vacuum timing will drop away and mechanical will take over the job of advancing the timing as RPM increases. The rate (RPM) at which it advances is known as the curve as is set-up with the various springs supplied with the distributor, the total amount of the advance is controlled by the bushings or stops in your kit.

The truth is most of you don't have the machinery or knowledge to properly set up a distributor for your car that will deliver the correct Initial timing, Vacuum timing, and mechanical timing at various loads and engine RPM. We have a data base of over 2500 engine types and combinations from work boats to 8 second Pro street cars, between Jim and I we have over 80 years of experience with rebuilding and curving performance distributors, we've seen it all, we've tried every possible combination, tested every component and aftermarket VooDoo distributor ever made and it really comes down to simply setting it up correctly, curving it right and using good quality parts.

Some advice from a Fat Bald Old guy....

GM Owners-90% of the cars we see with aftermarket distributors will do no more than drain your wallet. A well built stock HEI distributor can be set up to work flawlessly on most applications for 1/2 the money.

Mopar Owners-Your distributors have difficulties with the vacuum advance system. An all mechanical distributor with a REAL ECU will usually give you 25-75 HP. There's only one ECU that really works correctly, you can find it by searching "Mopar Ignition" in google, pay attention to the Fat Bald Old guy.

Ford Owners- Your distributors need help search "Ford Distributor Curving"

Final words- Don't buy a Race distributor for a Street car, don't call a Proctologist to fix your tooth ache so don't call a Carburetor company for Ignition advice, bright and shiny usually only attracts crows and doesn't do anything for performance.

"Ready to Run" is the biggest lie in the performance industry.
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  #24 (permalink)  
Old 05-19-2005, 07:45 AM
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Cuda; How DARE you confuse us with the facts! Let me see if I understand your post. I am running a late mode "A" engine electronic distributor and MSD5 in my Willys w/ vacuum hooked up and weights and springs to have the timing all in @ ~2000rpm (very light car/ ~400hp). I have the great old eye-burning exhaust from unburned gas @ idle. I set my initial timing w/ vacuum disconnected @ maximum manifold vacuum. Are you saying more initial advance will help clean up my exhaust @ idle?
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  #25 (permalink)  
Old 05-19-2005, 09:36 AM
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We seem to go through this exercise about every six months. A year or so ago, we even had two "expert" articles that reached two different conclusions. One expert even said increased vacuum to the canister reduced advance. Go figure.

I'm lucky to have a friend has one of those old Sun distributor machines. I'm amazed at the test results. I have agree, "out-of-the-box" does not equal performance.

I have to vote for manifold. It makes more sense to me.

Rant over. Back to main program.
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  #26 (permalink)  
Old 05-19-2005, 09:41 AM
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Go to my pictures and look at how i modifyed my sun machine......

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  #27 (permalink)  
Old 05-19-2005, 10:54 AM
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Keith! that reminds me of the Mag machines I used to use in my A&P classes. We had adjustable gap electrodes that we could open up to over 1/2" to test for a weak spark wire. Just don't get close to em when the mag was spinning at full speed, sucker would set you back a few feet!
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Old 05-19-2005, 11:01 AM
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this is an older post, but I rmember when it first started. I decided to hook up a vac guage to my advance and do for a drive, trying manifold and ported and record different driving conditions on each setup. About the only difference between the 2 is at idle. Manifold gives full vac advance, ported gives 0. At part throttle cruising, both gave identical advance. At WOT, both gave the exact same, 0 advance. It was amazing to see just when the advance came it and went away. The vac reading span was quite small in most cases. Most cans have a very short span and only work in a very small curve. I used the big list of advances to find the one that I thought would work on for what I was looking for, based on my vac reading during different accelerations and speeds and dialed it in. Seems to be working very well.

Mark
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Old 05-19-2005, 12:16 PM
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Quote:
Originally Posted by Jmark
Keith! that reminds me of the Mag machines I used to use in my A&P classes. We had adjustable gap electrodes that we could open up to over 1/2" to test for a weak spark wire. Just don't get close to em when the mag was spinning at full speed, sucker would set you back a few feet!
L.O.L........the guy that got me started on building that machine told me to never try and blow on the spark plug while it's running................. He said the spark WILL follow the moisture in your breath and give you a KISS you will never forget... I didn't bother to ask why he was blowing on it but i know for sure if i decide to do it i'll use the air hose!!!!!!!!!!!!!!!!!!

Keith
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  #30 (permalink)  
Old 10-09-2005, 08:58 AM
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Cuda66273
I know this is a old thread, but at 60 I guess i could be considered OLD.
The question I have is bushings on a MSD HEI, am I reading that at full throttle or 3K the max advance should max!I am getting a reading of 43* with a turn back light, I think this is too high right!
After reading the instructions it looks like I need to change the bushing to something that will read only 36* total, correct.
This is on a ZZ4 with a cam change (1000 to 5000 RPM) and a Tremac 5 speed, and 5th. is .72. The speedometer shop calabrated the speedo. and 75MPH I am only at 2250 RPM. The question I have is in OD at 60 -65 with lower RPM's where should my timing be?
Is the change done with weight springs or bushings or both?
I have not stuck a allen wrench in the vacuum can to see if it is adjustable so if it is I guess that is another item to address. What I am trying to not do is have detonation at cruzing speeds
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