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  #16 (permalink)  
Old 04-16-2008, 10:39 AM
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Stokey, I'll give you credit for your patience! LOL

I think some are just razzin ya on the vacuum can. I've called it that for years. For me, "vacuum can for the dist" and "vacuum advance" are the same.

Back to your original question.
There are many many STOCK advances out there that begin moving at different amounts of vacuum AND pull different amounts too.

The aftermarket advances do NOT let you set when it begins to move, but only let you adjust HOW MUCH it moves, by turning the stop screw that is inside the advance port. You can LIMIT the amount of travel, but NOT when it operates.

If you are using an aftermarket adjustable vacuum advance, the easiest way to determine WHEN it begins to pull is to "T" in a vacuum guage between the advance and you. I usually have the advance laying on the bench and "T" in a guage and then suck on the hose and watch the advance for first movement and note the amount of vacuum needed to get that first budge. Atleast then you have a good starting point. You can do the same thing with a timing light and the engine idling. Just "T" into the hose and suck on it till the timing mark begins to move.
You can also continue to suck on it to see what the total advance on the unit is set at. (using the timing light.)
Mine was set to max when I got it. I did not need to add another 15 degrees!!! I adjusted it down so it only adds 5 degrees at max pull. For me, it is borderline pinging but getting by ok.

Mark
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Old 04-16-2008, 10:45 AM
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Strokey
For whatever it's worth, I've go along with Bogie on this. (I usually do anyway.) I think you have a torque converter problem. The converter is just too tight for that cam. I think (if memory serves) that comp says an XE268 is all you can use with a stock converter. So, I've gotta imagine a cam with the timing you've got is gonna demand a lot of idle speed (you're at 1300 now?) and when you drop it into gear, several things happen. A -- The converter places a load on the motor, reducing the idle (and the manifold vacuum) B -- The reduced idle is also pulling down the mechanical advance C -- The reduced advance (mech and vac) contribute to slowing the motor further. This is a viscious cycle that, hopefully, will reach a point of equilibrium, and the system will stabilize. But, in your case that isn't happening. What is happening is that the engine dies. You could tweak the mechanical advance to try to make its cut-in speed a little higher, or change cannisters to make the vacuum demand for full vacuum advance a little lower, but I think that fundamentally, you should be looking at a 2500 or so stall speed for your converter.
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Old 04-16-2008, 11:04 AM
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I totally understand that the converter is wreaking havoc. Heck Comp recommends a 2800 stall for this application.

However at the time I dont have the funds to go out and buy a new converter for this 700r4 and in the mean time I'm just looking get the advance curve a little more squared off.

J Mark - Do you think I should skip the after market adjustable advance.

I've looked over this link that was provided earlier in the thread.

http://www.corvette-restoration.com/...c_Adv_Spec.pdf

Do you think vacuum advance vc1852 AR22 would be a good choice for me?
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Old 04-16-2008, 11:51 AM
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Adjustable Vac. Adv.

I Hope that works for you... But, The Cam Duration, Plays a big part on Vac. @ Idle. & Through the power curv.
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Old 04-16-2008, 12:46 PM
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strokey,
to add to Jmarks info a bit.... here's a graph for how this particular (accell) vac unit works/adjusts....the rate of change only changes in that narrow near vertical line and the actual Hg is only at that Hg range for about one heartbeat...

yes, with a 284 cam if the rpms are less than 1200 there is no advance added possible using manifold vacuum due to very low low Hg so you are at the base 12*....and with no timing the motor makes next to no TQ and it will stall and die due to the convertor drag

the 284 cam needs 18*+ base to make decent TQ and about 1,000 idle speed for decent Hg....confirm that your dist only has 20* centrifugal and set the base to 16- 18*.... and don't use the vacuum advance at all..... or hook it to a ported timed source so timing is added when the Hg is high

click on the 284 chart on this link for idle Hg characteristics on your cam

http://www.compcams.com/Technical/DynoSheets/

the principal function for vacuum advance is added hwy mpg...not...performance tuning a big cam

old bogie. Techron....do you have a link to all of Lars articles....his personal website that I used has gone off the web...so I don't have them anymore
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Old 04-16-2008, 01:12 PM
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strokey,
here's one of Lar's articles on how to set up your timing...incuding vac advance....

http://www.vetteclub.org/technicalti...t%20Timing.pdf

he's calling for 18b*'s+20c*'s+16v*'s=54TT (max practical possible)

my .02
18+20+12=50 is plenty....depending on the curve and car wt and gears

but anything from 46 to 54 is normal depending on the car

and here's a excellent article from Barry Grant's Demon Carb tech section for how cam duration changes base timing and dist set up....

http://www.gnetworks.com/v4files/bar...withimages.pdf

once you have swapped the convertor for more slip...about 21-24* base wiill make the motor launch so hard your neck will get sore!....obviously you need to have the dist re-worked for less cent to stay at about 38* max WOT...
(and with that much actual base timing you can probably have a 800rpms idle)

Last edited by red65mustang; 04-16-2008 at 01:30 PM.
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Old 04-16-2008, 01:34 PM
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hey red, lars hangs out in CFOT, when i run across him i'll update this post with a link. meanwhile here's another cool link:

http://www.lbfun.com/Corvette/Tech/vettetech.html

ibfun is a cool websight...
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Old 04-16-2008, 02:54 PM
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thanks techron....

looks like most if not all of the articles are on that link...

(they sure can save a bunch of typing on these threads)
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