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ported or full vac advance?

2K views 15 replies 6 participants last post by  diamond mike 
#1 ·
so after reading everything i could on ported vac. ( to hook distributor to ) i went and hooked my dist to full manifold vacum. now i read a post the other day where a guy was getting crucified for doing this? the motor is a 30 over 454, 10.1 comp. normal aspiration,800 eddlbrock carb, hei ignition, and has yet to fire. any thoughts on this? also, ANYONE running 427-396 heads? the "290s"? i still am searching for the correct plug.....thanks, mikeoh, there 69-???70? closed chamber
 
#2 ·
You are correct, hook it to full vacuum. Ported vacuum was only an emissions engine crutch from the auto manufacturers to force the engine to run hotter for lower emissions back in the dim old days of the 70's.

If you heads take a gasket seat plug use AC R44 or R44s(extended tip). If they use tapered seat plugs, use R44t or R44ts(extended tip).
 
#3 ·
cool, i knew someone knew which plugs i should get. goodmorning, thanks for the reply. i was pretty sure on the vac advance deal, but really UNSURE about the plugs. i think ill just get one of each to see which is better, i allready got 2 complete sets of new plugs that dont work. well one set is super close ( autolite 405s) but something just kept telling me to investigate further. ( they just seem to thread only 4-5 turns then stop, long threaded ) the other set isnt even close ( autolite 26s ) anyway, off to earn more $$$$ for my truck!!! merry xmas, mike
 
#4 ·
Tapered seat 5/8" hex plugs (peanut plugs) will thread into the plug hole of a gasketed plug, but won't seal very well and the heat transfer will be inadequate. I don't think that the 13/16" hex of a gasketed plug will fit down inside a peanut plug hole, but if the threads were long enough, the plug might start to thread.

The 3999290 were made '68-'72, so may have either type plug. Also, I see there's a 3964290 head from '69-'70. I seem to remember the break-off point being '70- up for the swap to tapered seat plugs, so these 290 heads may be either plug type as well- but not sure.

Best bet is to take a good look at the plug seat of the head to see just what's up, unless you already know which type to use, and you just needed a heat range recommendation.

Good luck.
 
#6 ·
manifold vacuum is best. (if you have enough vacuum). you may need a vacuum advance with a lower activation level.

ported vacuum advance is an emissions device.

10:1 is high for a 454. hope you are using a good size cam. 290 to 310 degrees advertised.

plus 454's tend to need lots of timing. 40-42 degrees total mechanical, and 15 to 25 initial (depending on camshaft).
 
#7 ·
Zimmo said:
The main problem with hooking up your vac advance to a full time source is that it will bring up your idle rpm.
This same 'problem' is also what makes it so useful for engines running a lot of cam. Because it does bring up the revs, the throttle plates can then be closed to compensate. This alleviates the "no idle screw control/too much transfer slot exposed" problems that plague large cammed engines. ;)

It also will often give a smoother idle w/crisper off idle response.
 
#8 ·
I guess if it works for you then go for it. I had mine hooked up to a full time source & I was having problems controlling the idle speed, because the vac with my cam was erratic causing the advance to fluxuate. Starting was also sometimes an issue until it pulled a good vac.
 
#10 ·
cool, i got what i needed to know, im just leaving it hooked to manifold vac. yes, i got 292 cam, thanks for the advice on initial time ( 15 ) i did need somewhere to shoot for tommorrow. i wish i knew how to put videos on here ( fireing the beast FINNALLY tommorrow!!!!!!!!!) just went and plopped down some hard coin for 102 fuel, but ill tell you, i was HAPPY to finally be doing it. ( i dont need to drive i live right next to work, so this hot rod is just for fun driving ) about 40$ for 5 gallons
 
#11 ·
hey c10454, i see you say i may need a lower start for vac advance, your talking can-weights? i got a weight set ( with springs ) i just havent put it in until i see where its all at tommorrow . are you running lighter springs ? ( did i say that right? ) if so, what color? thanks, mike
 
#12 ·
no, not weights. you can get a vacuum advance canister with a lower vacuum activation point (8 inhg). a 292 cam should have low vacuum (less than 10) so a regular vacuum advance canister with not work.

And your engine should run on pump gas with 10;1 and a 292 cam (no need for 102).

you can also get a vacuum advance that has an adjustable amount of advance. stock vacuum is around 20. I prefer to use 10 to 12 in the vacuum advance and use more in the mechanical.

use zddp oil additive with your cam (if it is a flat tappet cam).

use at break in and at every oil change. zddplus.com

break in the cam with only the outside spring if you are only a dual spring setup.
 
#13 ·
springs

hey buddy thanks for the reply, so if i am getting you, i should use just ONE of the mechanical springs? ( under rotor cap, right?) and nothing on the other side?? it is just a stock hei.....so new vac can huh? well ok, that shouldnt be too hard......man thats real good news about the race gas, i was really hopeing it wouldnt need that ALWAYS! merry xmas, mike
 
#15 ·
He said:
454C10 said:
break in the cam with only the outside spring if you are only a dual spring setup.
You replied:
diamond mike said:
hey buddy thanks for the reply, so if i am getting you, i should use just ONE of the mechanical springs? ( under rotor cap, right?) and nothing on the other side??
I believe he was referring to the valve springs, NOT the mechanical advance weight springs. My suggestion for the mechanical advance weight springs would be one light and one medium. This should get the maximum mechanical (centrifugal) advance all in around 2500 RPM.
 
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