[WCWhitey]

Gentleman, Thank you for all your information, I learned alot from everyone. It is very nice to see gentleman discuss a topic with class and intelligence.

[grandville455]

Yes excellent read guys!!!!Good to see people can discuss and not resort to name calling and all that..

[momo ´70 formula]

My engine is a 462 with a 041 ceane copy. 6x heads and stock manifold.
My carb has been modified by Kenth and the distributor set up after his recommendations(Pontiac stock specifications) Stiffer springs and the total comes in later makes my engine run a lot better.
I ordered a distributor from Ignition man (rebuilt points distributor to HEI, my distributor would bounce and run very bad over 2500 rpm switched to my stock ditributor with firm stock springs and my car runs jsut great. (no stumbles or hesitations like before)
I use a crane adjustable vacuum can that I gor from Ignition man. Now I use direct vacuum for best idle and best in drive idle.
The drop from park to drive is not big and my engine idles beautifully!!!
I agree with Kenth and Geoff use manifold vacuum and make sure your carn is properly (modified or adjusted for your engine/cam setup).
I have however gotten som very helpful tips from Cliff as well during the time of rebuilding my engine. So all I want to say is find your way that works for your combo, these gentlemen all have great knowledge about our pontiacs and I would just like to say thank you all because you have all helped me along the way and thaught me a whole bunch of things I would never have known if it wasn't for you all... (I just moved to Alabama but I picked up on the "ya'll" thing really quick .

Rock on Rochester!!
From A Swede in Alabama

[Darrell]

SHUT UP GRANDVILLE YOU DWEEB!!!lol Just laugh Bud!!!

Thanks guys this was great reading. One thing I learned here is that alot of tuning can be done through that vac advance unit, be it off ported or manifold. It make sense to try racing with the vac advance off. Man 2 tenths would be nice!!!

[buwalda]

There is an excellent article in in January 2004 HPP magazine by Rocky Rotella, on installing an adjustable vac adv unit. Rocky recommends the Crane unit, because it is the only one that comes with an adjustable plate so that both rate AND total [travel ] vac adv can be adjusted. { While something can be rigged up to limit the travel, the Crane unit makes it easy.}
The engine combo was a 455 HO, 8.4:1 CR, 222/232 @ 050 cam & 2400 stall converter; a fairly typical Pontiac performance engine.
This is what he reports:

No vac advance: 10 degres initial timing
Average 8.63 mpg
Coolant temp mid 180's at x/over


Vac adv connected to manifold vac [ Crane adj unit]:
Idle speed increased by about
300 rpm on start up. Reset to
750 rpm.
Initial timing now 22 degrees.
Engine coolant now 166 degrees
at x/over.
Average 10.26 mpg.
Quote from p. 74:
"....with approx 22 degrees of spark lead, idle quality had noticably improved.
The initial test drive had shown a better low-speed throttle response, but part-throttle cruise was most affected. Minimal amount of throttle angle was required to keep the car at a steady speed & light pedal pressure provided smooth acceleration similar to that of modern fuel injection."
Another quote: " When combined with static initial timing, [ manifold ]vacuum advance provides additional spark lead for better mixture burn & increased idle quality."
The full article is 4 pages long & very informative.

[Skip Fix]

I'll throw my 0.02 in here. When I used to run NMCA Top Stock and we had a vacuum rule almost EVERY racer ran a vacuum advance on manifold vacuum to bump up the idle vacuum. Also most had mechanical curves so almost all advance was in VERY early. Yes most of these had lumpy cams.

Next example my son's new 400 9:1, Summit 2801 cam. my old trusty Q jet with a good idle circuit. After breaking in the cam and setting total timing at 36 and no vacuum hooked up to idle the throttle blades were open so much the motor "ran on" when turned off. Hooked up the vacuum advance and could turn the throttle blades way down and idles great, very slight lope. If it didn't have such a tight convertor for the lock up TH350 probably wouldn't have a lope. So this small cammed motor liked additional advance at idle.

[Dryseals]

This has been a very good thread, lots of good points made here, but I've been waiting for some one to bring up distributor phasing, and it has not risen its ugly head.
I use the same method that Cliff does. Limit the vacuum advance movement, add more initial timing and tube to the timed port and it all has to do with phasing. So what you end up with is more of a mechanical set up than a vacuum setup. I have even gone so far as to modify the design of the weights to increase the mechanical movement.
Take one of those distributors in your garage and examine the different movements, it may change your thoughts.
This is for performance type cams, not stock.

[Cliff R]

Since this thread was getting so much coverage I spent a few minutes today doing some testing. I just finished a late 70's Pontiac q-jet headed for a 455 sporting a pretty big cam. The carb was placed on my 455 for testing. After operating temperature was reached the curb idle was set to 750rpm. This yielded 13.5" of vacuum, initial timing at 10 degrees, ported vacuum to the advance. The mixture screws were set at exactly 2 1/4 turns for best idle quality. The rpm's dropped to 700rpm when the trans was placed in gear and the vacuum reading dropped to 11.5". "Patting" the throttle and allowing the engine to return to idle did not induce any tendencies for the engine to die out or become unstable at idle speed. Just a slight "lope" at 700rpm in gear, idle quality was strong and steady.

Next the vacuum source was changed to manifold. The idle speed increased immediately to 900rpm and the engine developed a noticable "skip", just as one would see if the distributor were turned to a high advanced position while setting the timing. It was lowered to 750 rpm requiring right at 1/2 turn less on the adjustment screw. The mixture screws were re-adjusted, best setting was found at 2 turns each. This produced 15" of vacuum. The trans was placed in gear, idle speed fell to 650rpm, vacuum fell and hovered between 12-13". The idle mixture screws were backed out just a tad, this kept the vacuum gauge more steady around 13".

The car was driven using both methods of vacuum to the advance, no difference other than a tad more drop in rpm when placed in gear using the manifold source. There was no tendency for the engine to heat up, even when left idling in gear for 15 minutes. During the entire test the temp was fixed at 180 degrees.

The biggest difference that I can see during the test was the amount of "lope" from the engine at idle speed. The tone is slightly deeper without the extra timing and "chops" just a tad more. When the extra timing is used the idle is relatively smooth, but the engine does "skip" just a tad. With less throttle opening at idle it drops about 50 rpm more when placed in gear, but the idle quality is steady. In addition, the "skip" does not occur when a slight load is on the engine.

Just thought some real world numbers would help. For my particular application, no REAL difference other than the "lope" and sound from the engine at idle speed. Which for my particular liking is the rougher note from the two settings.

I would add that this test was not possible before adding the Rhoad's lifters. Any attempt to use a manifold vacuum source to the distributor was met with very negative results. It was difficult if not near impossible to get the engine to idle when placed in gear as the needed signal to the advance unit was lost. I have an adjustable unit on the distributor, but it is not accessable due to being pointed at the intake so no testing was done it that area. As mentioned extensively in this thread I could have lowered the vacuum setting and probably got much better results.....Cliff

[Malky]

Kenth,
Shouldn't you be giving Doug Roe full credit for your May 29,12:26 PM posting? Since every word of it is lifted from his Rochester Carb book.

[Doug Christie]

Just bought and installed a Crane 99601-1 adj. vacuum advance on the weekend. Installed the limiter plate a the 5th notch. Intitial timing is at 12*, mechanical advance is 36* all in at 2500, timing at idle is 28* with vacuum adv., and total timing appears to about 50-52* when reved up to 3000 rpm with no load.

Idles smooth out of gear, and idles with a little bit of lope in gear. Hopefully I'll pass aircair now. Thanks all.

[Doug Christie]

I think you spoke to soon Darby.

[grandville455]

Hey Doug what weights are you using : #'s and what springs? 50* seems like alot of total with the vacuum? Hows it feel more responsive?
thanks

[Doug Christie]

Darby, only tested it at idle, and off idle in park. Haven't had it out for a blast yet. At WOT the timing will only be at 36* as the vacuum will be non-existant. Last year when I made my fastest ET's (12.12,12.3,12.4) it was with vacuum adv hooked up. I know this shouldn't have an effect on ET (or not), but the car did pass Aircare and go it's quickest with the vacuum adv hooked up. Distributor weights and springs are from a B&M kit using the light springs.

[Kenth]

Malky, done.

........Kent

[GONZO]

Kenth, great information. I see youv'e been to this site : http://www.prestage.com:81/ShowPost.aspx?PostID=66011

The exact same debate and identical posts were originally posted there. Some of you may find some useful information there as well.


BTW, I personally prefer to use the crane adj vacuum advance to help smooth out idle problems. ( I think I'm just lazy)

Also the idle dropping to very low levels has ALOT to due with the stock convertor being used with big cam motors. I don't know if this was covered. (too lazy to check)

[Ty - 68]

Cliff - why would changing the source for the vacuum advance to manifold from ported (and lowering the idle speed accordingly) create a need to adjust the idle mixture screws? I did this on my car yesterday after I saw your post and the best setting for the mixture screws remained the same regardless of ported or manifold source.

[Sun Tuned]

Sometimes the sourcing for the vac adv will depend on the size of the cam. That's no big news. What IS news is the reason why.

There are times when the cam is on the edge of one or the other. The end choice will depend on the idle quality and that in large part depends on what the hydrocarbons and the CO readings are with one or the other sources.

I'll leave Cliff to explain this but it illustrated itself in his last example.

I like to run ported on most instances for the simple reason that when you rework the dist to utilize say 10-14 degrees initial, then use a 15 degree vac can hooked to manifold vac, will in most cases cause the hydrocarbons to climb due to too much parked on the balancer at idle. Lots of times this will cause a slight miss due to too much timing at idle.

This can be worked with by using a reasonable number for the initial setting and going for a ported source to get the vac adv economy part of the deal and well sciencing out the carb circuits to reach the desired idle quality you are looking for.

The stock setting of 6 degrees will give a certaing HC reading. Going up to 8 will clean that up a little and going up to 10 will do the same, on some setups the use of 12 degrees will make the reading start to climb again and after 14 degrees initial the HC reading usually exceeds the original reading at 6 degrees. Add the manifold vac source to the mix and "boom" HC reading usually goes off the scale.

There is a certain amount of a balancing act to be followed here and this area of tuning is one place where "only one way to do it" just ain't gonna get it done. When at this level one will need to remeber that BOTH the carb and the dist settings and calibrations can be juggled to help each other. You can't just set one and then "make" the other follow suit. You can, but it won't be nearly as functional as if you paired them as a team to compliment each other. It is this level of tuning that one almost has to have the actual vehicle itself to make work 100% perfectly unless you are dealing with some very accurate combos that one is used to tuning for. catch one that the cyls don't seal just right on and you'll be changing up some things even if only very slightly.

There are reasons for both types of sources (manifold and ported), but generally I have gone for the ported source, cause I usually like that initial up around 12-14 range anyway.

[Doug Christie]

I just took my car out today to see what improvements in performace were gained with the Crane Adj. Vacuum Advance. All I can say is %#@*&%!#@ing piece of $h!t distributor! When initially installed with the vac. can adjusted all the way in, it idled great. Could drop in gear without so much as a hint of a rough idle (not like with the stock advance).

As noted above, at idle the timing would be at 36* with vac. adv. hooked up, and about 50*+ when rpm increased to 2500-3000 rpm. Full mech. advance was all in (36*) by 2500 rpm without vac. adv.. I noticed that my vacuum reading increased to almost 20 in/hg as rpm increased.

On the entrance to the freway I stopped and loaded up hte converter to see if any pinging could be heard. OH YEAH! I thought that under load (freeway driving) this (high vacuum condition) would be nullified by the WOT and the increased timing would abate and leave the mechanical timing to do it's job. This was not the case. The car ran like absolute crap, so much so that I got of the freeway and drove on side roads.

So to make a long post shorter. How do I get the vacuum adv. to "get the he!! out of dodge" at higher rpm?

[grandville455]

Doug get rid of those bm weights and use your factory weights with one light and one med spring and try that.mine is set up with 14 intial and 34 total all in by 2500 and it pulls hard ..with no pinging at all! May try more at the track

[Cliff R]

"Cliff - why would changing the source for the vacuum advance to manifold from ported (and lowering the idle speed accordingly) create a need to adjust the idle mixture screws? I did this on my car yesterday after I saw your post and the best setting for the mixture screws remained the same regardless of ported or manifold source.'

Ty, it only allowed the mixture screws to be turned in (lean) about 1/4 turn until a change was noted in the rpm and idle quality. This was difficult to detect as the engine had developed a "skip" and was idling pretty smooth. When placed in gear I immediatly noticed that this leaner setting was NOT going to work as the engine vacuum fluctuated and the idle rpm dropped to about 575-600rpm. It help some to go back to the previous setting, idle quality improved in gear, but still dropped more between neutral and when in gear using a manifold source vs the ported.

"There are times when the cam is on the edge of one or the other. The end choice will depend on the idle quality and that in large part depends on what the hydrocarbons and the CO readings are with one or the other sources.

I'll leave Cliff to explain this but it illustrated itself in his last example."

With well chosen cams in relation to the static compression ratio, there is no need to run a lot of initial timing at idle. I could illustrate many hundreds of examples, but will use my own 67 Impala SS 327 engine as I have real numbers on it.

The engine is a 67 327cid, true 10 to 1 static CR, 350/300hp cam, all stock otherwise. The cam is rather small, about 194/204,.390/.410. In this engine it idles dead smooth with the initial timing down around stock 6-8 degree settings with a "deep", heavy sound. Adding in any additional timing, either by moving the distributor or plugging the advance will smooth the engine out and you will barely be able to tell it's running. Going past about 20 degrees induces a miss or skip in the exhaust note. Going past about 30 degrees and the engine begins to kick and buck in protest. It doesn't want or need any more than about 6-8 degrees of initial timing. At the stock setting, idle quality is fine and barely any change in or out of gear. When tuning this particular combo, as with any other, I listen to the engine and let it tell me what it wants.

Most of us choose bigger than "stock" cams for our engines and in most cases don't raise the static compression ratio accordingly. This usually has the stock initial timing settings falling short of what is needed. Unless the cam choice is "out to lunch" we can typically just recurve the distributor so we can increase the initial timing to 10-14 or so degrees and not add any more timing with the vacuum advance. It is also helpful, in most cases to improve the carb's ability to deliver idle fuel at the lower vacuum readings.

This is the point in tuning where I take a radically different approach to the others who have posted information in this thread. I don't see any improvements if the initial timing is advanced via the vacuum advance when hooked to a manifold source. I've tried it many hundreds of times and can easily get it to work even if a custom canister must be purchased....but....aside from now having a big cammed engine that often idles dead smooth, I can't see any other real benefits. The running cooler issue is purely theoretical and would only improved things if sitting at a stoplight or stuck in heavy traffic. If your heating up on extended cruising, the source of vacuum to the advance is NOT an issue.....Cliff