|04-16-2008 03:54 PM|
looks like most if not all of the articles are on that link...
(they sure can save a bunch of typing on these threads)
|04-16-2008 02:34 PM|
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:
ibfun is a cool websight...
|04-16-2008 02:12 PM|
here's one of Lar's articles on how to set up your timing...incuding vac advance....
he's calling for 18b*'s+20c*'s+16v*'s=54TT (max practical possible)
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....
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)
|04-16-2008 01:46 PM|
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
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
|04-16-2008 12:51 PM|
Adjustable Vac. Adv.
I Hope that works for you... But, The Cam Duration, Plays a big part on Vac. @ Idle. & Through the power curv.
|04-16-2008 12:04 PM|
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.
Do you think vacuum advance vc1852 AR22 would be a good choice for me?
|04-16-2008 11:45 AM|
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.
|04-16-2008 11:39 AM|
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.
|04-16-2008 11:32 AM|
The Pic. in the Link: Shows only the Diaphragm that makes the lever move the advance base!
trying to fix the link
|04-16-2008 11:22 AM|
|strokey||Once again, speaking of the vacuum advance cannister on the distributor, not a reservoir can for the brakes...|
|04-16-2008 11:03 AM|
Something NEW: Canister for Dis. Adv,?
I never used a Tank for the Dis.Adv.
When the throttle is pressed Quickly, the Manifold & Carb Vac. Port, Experience a Vacuum Force. The Dis. Vac.Adv. Plate Turns to advance the Dis. For the Increase of Fuel & Air Mix.
The Canister I got goes to the Vac. Operated Equip.( Heater Doors,Etc.)
The Brakes: Get the Vac. From The Intake. ( EXCEPT: My Tunnel Ram.)
So I Disconnected my Brakes. Ya-Huuuuu.
I'm going to use an Elect. Vac. Pump.
Add a Electric Oil Pump Aux.
Elect. Water Pump.
|04-16-2008 12:29 AM|
For a stock converter you're probably right up against the stall, although stall speeds are kind of mushy things to describe.
With a hot cam when driven on the street you're looking for something a little different in a high stall converter than a drag car. for the street you don't want such a hard engagement, but unless you're going on Pinks, you don't need a loose converter that doesn't hookup till the torque peak on the cam. All you need is something high enough to keep from knocking your teeth out when you put the tranny in gear or pulling you through an intersection with the brakes set.
My recommendation is that you talk to Comp and some converter folks, as well as lay your hands on a Summit catalog, they always include a little tech article regarding converter stall against cam timing. It's kind of generic, but it's a decent data point and you'll get plenty of those, decent and otherwise.
|04-15-2008 10:21 PM|
|04-15-2008 10:00 PM|
good find bogie, lars emailed me that paper a few days ago, it's one of the references i was refering too, you beat me to it.
here is the other discussion on vacuum advances i was refering to:
|04-15-2008 09:14 PM|
Techron - I'd love to have the link, Ive been reading old posts for hours trying to diagnose.
I think I've finally narrowed the problem.
I spent time blaming my sour performance on the stock stall converter and stock highway gears, then the swap meet 650 holley double pumper with a kit in it.
I bought a brand new box stock holley 670 and noticed much crisper throttle response in park, yet the same completely no idle/stall when in drive.
I'm running a 388 ci sbc with a comp xe284h. The car makes 6-7 hg" of vacuum in gear at 800 rpm's.
Ive been blaming the drive train/running gear for the high idle it needs in park in order to sustain a sufficient idle when placed in gear. Here is what I think my problem is.
The car has a real sweet attitude idling at 1300 in park. This is probably at the point where it is producing a good ammount of manifold vacuum.
When the car is placed in gear the rpm's drop and it immediately falls on its face and stalls.
I think this is because of the non adjustable factory style advance canister. It is providing no advance in drive ( at 600-800 rpm's, again making 6-7 hg" of vacuum).
In a nutshell to even have the car driveable I was running at 1300 rpm's in park. I have now realized that at this point (in park) the vacuum advance is running at full clip providing an additional 15 or so degrees of timing, thus a total of around 27. Then when you switch to drive and the rpm's drop (as well as manifold vacuum levels) you are back to the base timing of around 12 degrees (because the canister is providing little or no advance at this point with such a low ammount of vacuum).
Simply there is no happy medium with this cam and this advance canister with this configuration.
I need an adjustable can badly.
I may be preaching to the quire.
But someone please let me know if I am on the right page.
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