ported or full vacuum port?

[BUXMAN66]

i ran a search on the board here 'bout this subject an probably know the answer but was wanting some clarification.i have a edelbrock performance power package,cam is slightly larger in degree than a stock cam.204-214 @.050 lift .420"-.442",...... not a radical cam at all.so i was wondering if i go to the full manifold vacuum port to hook to my dist.,i wont need to go to a lighter spring in my distributer will i ? w/ this easy of a cam i wouldn't think i would need to,but......? i did notice a big difference in accelerator pedal responce and a noticable increase in rpm's.will i need to adjust my timing to compensate for full initial manifold vacuum?sorry for beating the he!! outta a dead horse(subject)! thanks for any and all replies!:p

[72K10]

Bux, my setup is identical to yours (except heads, probably), with a mild cam, and mine runs a lot better off full manifold vacuum. Mine ran OK until I advanced the timing from 4 Before to 8 Before. Then I had problems under load and at idle. Also, you should reset the idle speed (lower it), but timing will stay the same, as you set the timing with vacuum disconnected. Hope this helps.

[mikep]

I run the same cam but with 1.6 rockers. My heads a WP SR torkers. I run about 15 degrees initial without the vacuum connected. Once I connect the vacuum it kicks up to about 34 degrees and I turn the idle back down. Gives me about 36 degrees under mechanical at wide open throttle. My compression is about 9.5:1 . If I dont use 92 octane I will ping like all hell. So I use 92 octane.

[Truckstr]

Ported vacuum is the correct one for hooking up vacuum advance. Otherwise, if you hook it up to full manifold vacuum, it will get advanced all the time (except under near full throttle conditions when even manifold vacuum drops to near zilch).

Just remember that initial advance PLUS centrifugal advance PLUS vacuum advance EQUALS total advance.

The lighter the springs in the centrifugal advance, the sooner in the RPM range that it comes in. Initial advance should always be set with the vacuum advance plugged.

Use a timing light and a timing tape on the harmonic dampener, in conjunction with a tachometer, to see what effect the various "advances" are on the actual timing.

Also, do yourself a favor and make sure that your timing marks on the harmonic dampener truely line up with TDC (especially before installing a timing tape on the dampener). I recently pulled a dampener off of an engine where the inertia ring had "slipped" and the mark no longer lined up where it should have been.

Sorry for the long post, but sometimkes the subject warrants it.

BTW, Happy New Years!

[72K10]

It seems like you know what you're talking about, Truckstr. So my question is, if an engine runs poorly at idle because of a cam, hesitates under load, etc., and the manifold vacuum boost solves that problem, then is there any harm in running full manifold vacuum? If I run ported, my engine bogs down and misses under load and at idle. When I run full manifold, it runs really well. What gives? Appreciate your thoughts on this. Chris

[crazy longhorn]

BUXMAN66, your best bet is to recurve the dist. If you have an HEI, the vac can will be putting out too much to run manifold vacumn(about 20-22 degrees). I use a crane adjustable advance(also has springs & instructions). As a starting point, I would suggest around 10-12 initial, & 32-34 total(vac can unhooked & plugged).....all in @ 3000 rpms. I would limit the can to 10-12 degrees, & hook it to manifold vacumn source at the carb. This should give in the range of 20-24 degrees of timing @ idle with the can hooked up, & 42-46 degrees @ 3000 with the can hooked (no load on the eng). The spring tension is also adjustable on the can, so it can be set to back off faster if part throttle ping is heard. Under part throttle conditions, the eng will handle more timing,as you have a slow burn going. Then when you open the throttle, the vac advance backs off & you will be running on initial + mechanical advance. Just follow the inst with the crane kit, you will have to play with it a little to get the best setting but its worth the time. Good luck.....crazy AL

[Hooter]

Hook up the line to the carb, otherwise you will have some undesirable effects

[mikep]

Weve had this discussion before but timed vacuum is the way to go if you just want to keep your emmision controls. It doesnt help performance in any way and does cause a loss of performance off idle and will cause a detonation problem at WOT as the timing does not retard . Try it for yourself. WOT is when you want to drop timing a bit to avoid detonation. Running manifold vacuum will allow you to have higher timing at idle with a bigger cam and close the throttle plates more to get a better signal from your idle circuit which in turn gives you a better transition signal to WOT . The last time you want to give yourself more timing is at WOT. There is nothing at all wrong with having 35 or so degrees of advance at idle with the vacuum connected. It greatly improves throttle response and as long as you arent trying to use the cheapest gas made will cause no problems with detonation. When you give it gas the vacuum advance drops off so you avoid detonation. at cruise you might be getting closer to 48 degrees . Cruise is a completely different condition than WOT and that much advance will help fuel economy a lot. You wouldnt want that much advance at WOT and guess what ... when you give it gas your vacuum level drops and with it your advance to a rate that is beneficial to WOT.

Timed vacuum is the vacuum that increases as your throttle plates open. You get zilch for advance at idle . Thats great for nitrous oxides emmisions but doesnt do crap for the way your engine idles or accelerates. Its also the way to go if you are running an EGR valve. Are you ?

If you want maximum performance and fuel economy set your timing with the vacuum disconnected to give around 35 degrees total at 3500 RPM so you are sure all your mechanical advance is in. THEN hook up the vacuum advance to manifold ( ported is manifold. Timed is also known as venturi vacuum ) Then reduce your idle setting and you are done. You'll get way more than 35 degrees total but you'll have outstanding trhrottle response off idle and no detonation at WOT if you are using good gas. Plus you'll get better gas mileage. If you change your advance weights to limit the amount of mechanical and vacuum advance then you can give yourself even MORE advance at idle. Advance is a good thing. Dont be afraid of it.

[crazy longhorn]

Ported vac port, or full manifold port? Ported vacumn is timed vacumn. Regardless of where th signal is picked up, timed vacumn or manifold vacumn to the dist can both drop out under WOT. I prefer to run mine on the "full manifold port", rather than the "ported vac port" (timed). I have run them both ways, as well as the high initial(18-20 degrees), with the mechanical limited to give the proper total(nasty hot cams setting). I run 14 initial, 34 total @ 3200 on my 383, + 8degrees on the vac timing(crankshaft) & am plugged in to manifold vacumn on the dr side of my edelbrock.

[BUXMAN66]

thanks to all for the responses!geez,mike what the he!! are you, a guru? sounds like you've worked w/austin coil (j.force's c.c.)!! i think my brain can handle this?! i just dont unnerstand how you get a TOTAL of 35-40 deg. advance.i only got so many lines on my stock damper to go by!forgive me for bein' a moron 'bout this,but i still don't get how i can have that much total advance.okay,i unhook my vacuum advance and plug the nipple right? then i set my timing to between 6-8 atdc,right? at what rpm should i do this?then i reattach my vacuum advance line which will increase my rpm's.then i re-adjust my idle rpm's, to down 'round 8 to 9 hunnert ,right?thanks for helping guy's!

[mikep]

No guru. Just got a family full of engineers and really good paint and body guys and some of it rubs off. Dad used to be Mickey Thompsons crew chief way back in the day and Moms family was into top fuel at the same time. I spend ALL my time working on engines and "restoring" cars and trucks. Its more obsession them profession. I used to be a ASE mechanic years ago but now I work on industrial packaging machines. Pay is a hell of a lot better than auto mechanics and its a lot cleaner.

Anyway... Buy a stick on timing tape at the parts store. Costs about $7 . You peel it off the backing and stick it to your balancer. I have an advance timing light that allows you to check the total advance without a special tape although the tape method is more accurate.

[Hooter]

Timing always confuses me. I usually just play around with everything till it runs just right. Adjust the distributor, go out for a drive, do it again until it runs the way I like it to. I never even touch the vacum opd on the dizzy because I'm always worried I'll screw something up. Even though I probably should

[crazy longhorn]

Buxman, I think you may have missed a post or 2.......if you want to make some power, I would try setting the timing about 12 BTDC, & spend the cash /time recurving the dist. I wouldnt EVEN try pulling close to 40 on the total(with the can unhooked)......unless you have that block decked & some d@mn good pistons! Total timing is figured on initial + mech advance.......you can run an aditional 10-12 degrees on vac timing(more on some, & less onsome)......depends on the comp ratio, quench,& the cam. My sb is running right @ 10 to1 on pump fuel.

[BUXMAN66]

crazy i had read your post but it just seemed a little too technical for me!no offense intended!! i just had noticed that when i mash the throttle a bit,it tends to lag somewhat b-4 it really kicks in.i put a fuel regulator on to cure my carb prob (too much press.) but i was just messin w/it to see what might be lurking in there to give it more immediate reponse.my cam ain't radical,... nore is the bottom end.actually i don't know about the bottom end,i bought it w/ the knowledge that it need better induction,and a cam to better suite the stock converter.so i went w/ the total pkg. to accomplish this w/in what i knew it did have.this is not a racing type engine but a cruiser/every day driver set-up.although it is definitely not driven every day!i have printed this thread off to take in to work and when my machines are running i can sit down and figger this tech. talk out !!!that's kinda odd, but i do my best thinkin' w/ a bunch of machines singing 'round me!

[crazy longhorn]

Buxman, do your self a favor, & pick up a crane adjustable vac can. It has instructions to help you set up & tune. The 1st time will take a little time, but after that you will have a good understanding of the timing specs. as a rule on a sb, 34-36 total(with the can unhooked & plugged) is about right. You will be checking with a dialback lite, or balancer tape @ 3000-3200(where the advance is "all in"). Initial timing is set @ idle speed, then buzz the eng to check the total. Once you have the total set, & have made a couple full throttle runs,& no spark ping@ full throttle..........then you hook up the vac advance & check for part throttle ping & adjust as needed. Good luck....crazyL

[Spoof]

What's WOT ?
My Suburban has a built up 402, maybe a little over cammed. For the longest time I had a severe detonation problem, even with 92 octane. Turns out I had the carb hooked up to manifold vacuum, moving to timed vacuum solved the problem immediately, this was after about a year of suffering with it.

[crazy longhorn]

WOT.....wide open throttle

[crazy longhorn]

Spoof, most of those factory cans pull in the range of 20-22 degrees timing(at the crank). From what I have seen, that is too much when set up on manifold source. that is where the adjustable can comes in.......cut the vac timing back to 10-12 degrees, & you can run a faster curve on the mechanical advance, with a higher initial setting