Vacuum Advance

[dennis kelley]

Is your vac advance connected to timed vacuum port or constant manifold port? The reason I ask is because I have a mild cam in my 302 and it pulls crazy vacuum at Idle (15inch) and my initial timing is extemely low 4 degrees. I was thinking that if I switch to timed vacuum I can increase initial and have better response. Any opinions appreciated. If it wasn't so cold out I get the car out but no heater or top no test drive until warmer weather

[Henry Rifle]

Now, there's a subject that generates both heat and light. Do a search on "timing" or "distributor timing" and you'll find a lot of comment.

I contend that most applications work best on ported vacuum. Others will tell you different, but I'm right

[lt1s10]

Originally posted by dennis kelley
Is your vac advance connected to timed vacuum port or constant manifold port? The reason I ask is because I have a mild cam in my 302 and it pulls crazy vacuum at Idle (15inch) and my initial timing is extemely low 4 degrees. I was thinking that if I switch to timed vacuum I can increase initial and have better response. Any opinions appreciated. If it wasn't so cold out I get the car out but no heater or top no test drive until warmer weather

i use manifold vac. most of the time so i can run lower base timing for hot starting purposes. then the vac, adv. will adv. the timing when you start the motor for good response. sometimes it can cause some heating problems in hot weather. try it both ways and see what way you like best.

ps- and dont believe nothing that Henry tells you

[john gemmer]

Originally posted by lt1s10
i use manifold vac. most of the time so i can run lower base timing for hot starting purposes. then the vac, adv. will adv. the timing when you start the motor for good response. sometimes it can cause some heating problems in hot weather. try it both ways and see what way you like best.

ps- and dont believe nothing that Henry tells you

yeah what he said

[lt1s10]

Originally posted by dennis kelley
Is your vac advance connected to timed vacuum port or constant manifold port? The reason I ask is because I have a mild cam in my 302 and it pulls crazy vacuum at Idle (15inch) and my initial timing is extemely low 4 degrees. I was thinking that if I switch to timed vacuum I can increase initial and have better response. Any opinions appreciated. If it wasn't so cold out I get the car out but no heater or top no test drive until warmer weather

Dennis if im reading this right you are looking to increase your response time. you have 4 degrees base timing and when you crank the motor you are idling with about 15 degrees total timing (i don't know what that is, im just using 15 degrees) so you have about 15 degrees timing when you accelerate, then the timing falls off. so you go to ported vac. and increase your timing to 10 degree base timing and when you start the motor, then you still have 10 degrees of timing, as far as im concern you just lost 5 degrees of timing which will hurt your response time. if response is what you're looking for.

[john gemmer]

Originally posted by lt1s10
Dennis if im reading this right you are looking to increase your response time. you have 4 degrees base timing and when you crank the motor you are idling with about 15 degrees total timing (i don't know what that is, im just using 15 degrees) so you have about 15 degrees timing when you accelerate, then the timing falls off. so you go to ported vac. and increase your timing to 10 degree base timing and when you start the motor, then you still have 10 degrees of timing, as far as im concern you just lost 5 degrees of timing which will hurt your response time. if response is what you're looking for.

I will agree with that S-10

[Henry Rifle]

I'm glad we cleared that all up.

[lt1s10]

Originally posted by dennis kelley
Is your vac advance connected to timed vacuum port or constant manifold port? The reason I ask is because I have a mild cam in my 302 and it pulls crazy vacuum at Idle (15inch) and my initial timing is extemely low 4 degrees. I was thinking that if I switch to timed vacuum I can increase initial and have better response. Any opinions appreciated. If it wasn't so cold out I get the car out but no heater or top no test drive until warmer weather

Anytime you go from manifold vac. to ported vac. you gonna hurt your response time. you will gain it back some place else, but your response time will slow down. try it both ways and see.

[lt1s10]

Originally posted by Henry Rifle
I'm glad we cleared that all up.

quote....I was thinking that if I switch to timed vacuum I can increase initial and have better response.
henry, this was the ?, and i dont belive that you agree with what.

[Henry Rifle]

lt1,

This is all I'm saying:

-That having full vacuum advance at idle with a lower initial is not necessarily a good thing.

-That every technical article I've read tells me the same thing.

- That every performance car I've owned - granted, no Ford engines, just Pontiac and Chevy - worked best when connected to ported vacuum.

I don't disagree with you, it's just this has always worked for me. One thing we totally agree on is . . . "try it and see which works best."

[dennis kelley]

Hey guys I'm going to the dyno next friday and I'll try it both ways. The problem is that the vac advance at idle with the consaant vacuum forces me to run extremely low initial timing. I appreciate everyones input. I'll let you know how it turns out. the engine is a 302ford with AFR 165 heads, demon 650dp, 1 3/4 headers, MSD ready to run unit with 6AL. I've never used the timed port and I thought because it has no vacuum pull at idle I could increase timing. Last Trip to the dyno 273 RWHP and 298RWT. I'm shotting for 300 hp an 300tq at the wheels. The motor seems to law down above 5000 rpm.

Thanks S10 and everyone for your input

[lt1s10]

Originally posted by dennis kelley
Hey guys I'm going to the dyno next friday and I'll try it both ways. The problem is that the vac advance at idle with the consaant vacuum forces me to run extremely low initial timing. I appreciate everyones input. I'll let you know how it turns out. the engine is a 302ford with AFR 165 heads, demon 650dp, 1 3/4 headers, MSD ready to run unit with 6AL. I've never used the timed port and I thought because it has no vacuum pull at idle I could increase timing. Last Trip to the dyno 273 RWHP and 298RWT. I'm shotting for 300 hp an 300tq at the wheels. The motor seems to law down above 5000 rpm.

Thanks S10 and everyone for your input

Ok dennis let us know, but the truth is each motor combination needs dif. things, but if your problem is above 5000 your vac. adv. wont help you no way. youll be working with the mechanical adv. then. this thing with Henry got started a long time ago and now if anybody ask about adv timing he says ported and i say manifold, which is the way i ran it for 30 yrs. and didnt like it the other way, but he says hes always run ported. most Hi prof. motors dont run any vac adv. so work with it a little and see what you come up with.

[lt1s10]

Originally posted by DennyW
Move your timing to 10BTDC. Install heavy weight springs in the distributor.

hes setting it up on a dyno next week so its no telling what he will end up with. if youer looking for max top end hp ,its no telling what your idle is gonna be.

[lt1s10]

Originally posted by DennyW
He needs to set it up like the old days with a load on it, then set the timing for max pull.

that new dyno loads the motor.

[Henry Rifle]

Here's my point. In this case, Dennis says he has to run very low initial timing because of the high manifold vacuum. When he drops the hammer, ported and manifold vacuum are essentially equal, and he's left with that low initial. If he uses ported, he sets a higher initial, and the immediate drop-off isn't as severe.
The following article was written by Kenneth Steinbach on another forum, and I find it quite logical.

Centifugal advance is strictly related to engine speed. Vacuum advance is related to engine load. Under light load, ie. low throttle angle conditions, cylinder pressure is relatively low and the less dense air/fuel mixture tends to burn slowly. Therefore additional ignition timing is necessary to burn all the mixture, hence vacuum advance. On the other hand, at wide throttle openings engine load is high, manifold vacuum is near zero because the engine is no longer sucking past partially closed butterflies, cylinder filling and pressure are greater, and the denser mixture burns faster. The vacuum seen by the vacuum advance diaphragm is not enough to overcome the spring which opposes the vacuum signal, so no vacuum advance occurs. The issue of ported vs. manifold vacuum is very simple, and 99% of what was said in the stangnet forum is wrong. The main reason for using ported vacuum rather than manifold vacuum is to reduce hydrocarbon emissions at idle. If you have an extra 10-15 degrees of advance at idle you'll gain nothing in performance but will double or triple your HC emissions. Another issue is idle quality. If you have a lumpy cam, vacuum at idle tends to be unstable, resulting in a constantly fluctuating vacuum signal which would cause erratic vacuum advance activity. A third reason to use ported vacuum is idle speed. If you increase timing at idle by 15 degrees, idle speed is going to increase by several hundred rpm. If you try to compensate by turning down the idle speed screw on the carb you may close the throttle blades too much and disturb the relationship between the blades and the idle transfer slot which will result in an off idle stumble. As far as ported vs. manifold at WOT, there's no difference. Neither produces enough vacuum to actuate vacuum advance.

But again, the dyno will tell.

OK, Dennis, it's your responsibility to tell us the results. Don't make us hunt you down.

. . . then, there's daylight saving time . . .