Alright guys, I'm having some serious brain blocks and i'm hoping ya'll can help me out. I've heard that 36-38 should be the total timing for a 390 correct? As of now, I have my initial set at 10 using the 13L slot which puts me at 36 total right? Ok my question is where would the vacuum adv come in if I'm already maxed out by my initial and centrifugal? Also yet another possible dumb question, why does mallory have a dizzy that says it comes with the choice of mechanical or vacuum advance? Does the vacuum adv dizzy come w/o mechanical advance? Sorry I just don't get it. Maybe it's too late to be thinking about these things. Oh well...

Mallory link- http://www.jegs.com/webapp/wcs/store...48627_-1_10679

 

Even the vacuum models have centrifugal advance. It's that just if you have enough vacuum, you can use either model. The vacuum will be able to pull it advanced, even though you don't have enough rpm yet to get centrifugal.

 

Mechanical advance only, for race engines. Mechanical and vacuum units for street motors looking for a little extra gas mileage.

 

Well it's just weird how they word it i guess b/c they say choose vacuum or mech advance. To me that meant if you choose vacuum that you won't have any mech adv but anyways. Ok, just cruizing down the highway, are you alowd to run your timing higher than 36-38 by vac adv since your engine won't be under a load and subject to detonation? say have it pull it to 42 or so? and then when you romp on it its back to 36 from the loss of vacuum. Is that at all how it's supposed to be set up?

 

That's the jist of it in simple terms

 

Ok now the fogs clearin a bit, so you CAN have more than 36-38 degrees of timing as long as you're not under a load? If thats the case then whats the limit of how high you can go there before you start smokin valves? All i know guys is that the last time i took my heads in b/c my valves sunk and my machinist told me either i had a lean condition or my timing was advanced way too high. Well i know my 360 wasn't runnin lean b/c i was gettin roughly 6 mpg! I honestly thought it was b/c i didn't have the hardened seats put in it. Now I know these numbers are all relative but theoretically you could have something like 10-12 intial, 26 mech, and say 5-8 vacuum? 40-42 total under cruise?

 

What year was your 360? Even if the heads did have hardened exhaust seats, over time, the hardening goes away and you're left with just cast iron again. Induction hardening is where they use a magnetic field to heat the iron. It's not like Ford put real hardened inserts in the heads. And induction hardening will weaken if it's hit enough times (high RPM).

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With "ported" vacuum, the way the stock stuff was setup, you only get vacuum advance when you're lightly on the throttle. Now, at high RPMs and high air-flow, you will get a weak vacuum signal at wide-open-throttle, but not enough to care about.

What I'm trying to say is, at the RPMs required to max out the mechanical advance, the vacuum advance won't be getting any vacuum.

Unless you do the dreaded "reverse vacuum advance" setup like I did - which some people think is the biggest sin since, well, ever Which is used as a band-aid for big cam, bad idle, low vacuum.

If you're running a basically stock engine, the regular ported vacuum advance will work fine for you.

Some things to think about... if you have too big of a cam, and/or you need to crank open the throttle plates to keep an idle, you will uncover the vacuum port in the carb venturi for the vacuum advance. Which means the vacuum advance will be triggered at idle. Not good. Well, sometimes good, USUALLY not good

 

Ya when i talked to Scouder, he told me that there's a passage in the venturi of the carb that needs to line up with the throttle plates and only .030 of that passage should be showing. Can't remember what he called that passage but i guess i need to look at that first. I've gotta 343941 cam w/ stock 4v intake and a holley 600 vac sec. Now when you say run backwards you mean have the vac adv hooked up to constant vacuum?

 

There is the idle circuit, transistion and main meter.
You don't want the transistion slot open to far as you will loose your idle screw adjustability. The way around that is to open the secondary plate to overcome that. Alot of people used to drill holes in the primary plates, but I wouldn't. Simply remove carb, turn it upside down, there is a tiny slotted screw that you can turn IN 1/4-1/2 turn to open the sec. plates. This will allow a touch more fuel and then you can close the primary side and then have your idle screws work.

 

This is a much better method than drilling the primry plates, but shouldn't really be necessary with other than a real hot rodded engine.

 

Yes, but don't go there. With a 600CFM carb, and the '941 Crane, you shouldn't have a problem, I don't think.

Only us weirdos that do things the WRONG way need to run backwards vacuum advance. Right Bear?

 

Would I ever disagree with you?

 

I'm an amateur... never forget that

 

Now that's funny!