Using Full manifold vac adv

[Chippa]

Ok so i've trawled thru all the threads i can about this subject thats been done many times over, i've looked at all the ported timed vs full manifold vac adv discussions on many different forums.

Some ppl have obviously had some success utilizing full manifold vac to dizzy and it seems the consensus is ported is the way to go...i guess its trial and error for some.

To paint some sort of picture i own a 69 with 440 and 727 trans, 3.23 rear,Edelbrock performer intake,750 holley vac sec and headers that has a fairly healthy size cam i think (around 11" vac at idle and drops to 7-8" in gear) with a MP electronic dizzy.I currently have the vac advance disconnected which has helped with some light throttle pinging that i was incurring.
I have it set at 36 deg total @2600 and have 10-11 deg initial so obviously not really the best in regards to mechanical, i can adv at idle and vac climbs up to about 15" of vac but has over 20 deg when i checked. I know i need to sort the dizzy out and get it re curved or weld up etc but for temporary purposes would it help me at all if i was to hook up to full vac and dial in/limit say an extra 5 or so deg on the vac adv to bump up my initial or am i wasting my time/worth the excercise
All thoughts and advice appreciated .....i'm also going to make myself up a spark plug piston stop and just verify TDC incase i'm out of wack a little on the harmonic balancer..
Cheers Chippa 

[b5blue]

  I had Don at FBO set up a dizzy for my 440 and it made a big difference with idle advanced off full manifold vacuum for my six pack.   

[mhinders]

I bought a low compression (below 8:1) 383 with a somewhat hotter cam (XE274H, http://www.compcams.com/Company/CC/misspeaks/Details.asp?cs id=708&Sb=0), giving even lower dynamic compression and low vaccum signal (about 8-10). Spent significant time trying with the vaccum advance to see if I could make it run better at idle and in gear.
But no, it was futile, I had to set the initial timing at 24 degrees, limited the mechanical advance to 12 degrees, that was the best setup in my case.
I believe the extremely low compression was my main problem, and to some extent the lobe separation.

[Chippa]

As I didn't build my motor I don't know what comp ratio I have it's a 77-78 block with 906 heads. The motor has about 8-10k miles on its a healthy engine still and compression is good

[Cooter]

Depends on your Compression as to how much Total timing you can run at any given load.

Some more, some less.

Most aftermarket/reman distibutors (Stock Chrysler) came out around the earliest 1973, which usually have a total advance mechanically, of around 30 degrees, PLUS whatever you set base timing at, then add in another 15-20 For Vac. Advance.

SO, with it totally screwed up, you could have base timing set at 15 degrees, have mechanical adv. of another 30 degrees, and another 15-20 in Vac. adv. for a total of as much as 60 degrees if not careful.

[Chippa]

Yeah Cooter i agree, from what i have read up on the later model 440 engines were lower in comp but i'm hoping that my C/R is more than 8-9:1 as its been rebuilt and definately not stock and i also run the highest pump fuel we have here in the West Oz which is 98 RON.
As stated i have around 24-26 deg of mechanical which i probably need to reduce somewhat so i can bump up my initial or like my original post play around with Full mani vac to see if it will help me out.

[BSB67]

... or like my original post play around with Full mani vac to see if it will help me out.

Why not try it?

I prefer changing the mechanical advance.  It is quick and easy.

Also, you should do a cylinder prssure test.

[Cooter]

As stated i have around 24-26 deg of mechanical which i probably need to reduce somewhat so i can bump up my initial or like my original post play around with Full mani vac to see if it will help me out.

Remember at WOT, you have not as much Vacuum in the intake. So, "Full" manifold vacuum won't really help WOT Timing.

[Chippa]

Yep i understand that once i crack open the throttle plates vacuum drops and i start using mechanical so where i have set my total should be fine as thats what it should be working off anyways... i guess when i'm cruising at highway speeds and the plates close up i will be adding a bit more adv thru the vac but if i restrict it to only say an extra 5 deg at idle then i'm assuming thats all it will and into it.

BSB67  bump up intial which will give me a higher total timing?? and see if it pings or like you have stated just give it a go with the full manifold....i guess my motor will soon tell me if it doesnt like it.
How hard is it to muck around restricting the slots on the older style MP electronic units??? i'm confident enough to pull out the dizzy and dismantle it, i'll start looking for some threads on "how to" change the mech adv.

Thx for the replies so far everyone 

[BSB67]

Keep your total time at 36 degrees and just try the full manifold vacuum.  If it likes it, check the timing too.  Then give it a drive.  Might be too much timing at cruise.

Just weld the slots part way up and file them back to what you want.  I have a chart if you need the slot dimensions for different timing advance.

[Chryco Psycho]

This has never made sense to me at all , connecting the vacuum to the manifold makes it work opposite to the mechanical advance so it reduces the timing curve , I would just weld the slots & make the mechanical advance work giving you the idle timing at both idle & up the rpm band

[Chippa]

Chryco i have been waiting for you to chime in hehe....yes i understand your point of view, for me i dont get how some ppl it seems to work and this is the part i dont get but like i said some ppl seem to swear by it.

Timing should be heading up not down, is the transition between the two too short to be noticed?? going from say 15-18 deg with the help of full vac at idle only to be taken back to the initial mech of say 11 deg??? then start to climb....i'll play around with it this weekend and see what results i come up with 

P.S on the Edelbrock performer manifold there are some bungs on the drivers side, am i able to get vac from there or is my only option from the Xmas tree fitting at the back on the passenger side??

Cheers   

[Cooter]

Russ just sent me a Chart. It is a MUST HAVE for fine tuning your Mechanical Advance.

[Chippa]

X2 

Thankyou Russ for also forwarding onto me the specs for the slots 

[Chippa]

Thought i would throw up a pick of my plugs as well for the gurus to read....maybe lean/timing??? i made up a piston stop from an old plug and my TDC is good on the marks just to verify it...i thought the plug colours would be a bit more consistant,maybe the motor is possibly not as fresh as i thought it was.....or it just needs some proper tuning all round

 

[b5blue]

Don't overlook finding the correct "heat range" for your plugs also. (As that can effect "ping".)

[Cooter]

Don't overlook finding the correct "heat range" for your plugs also. (As that can effect "ping".)

X2!! More than you would think.

From the plugs, it would appear that Chippa might be running a dual plane intake.

[Chippa]

Well there a BP5S NGK plug which i would say are about right for heat range,and yes dual plane it is a couple of the plugs look pretty good to me and others not...is it best to keep all your plugs the same heat range or can you tailor each cylinder to what you need?? Hope its not a dumb Q..

I've decided to pull my dizzy 2moz and weld up the slots and file back i'm going to start of with say 18-20 crank deg of mechanical as i want to have at least 16-18 deg of initial  to bring me up to the 34-36 range of total...

Well its 12:15 in the AM here so i'm off to sleep.....Thx again for everyones help thus far and if anyone has more to add good or bad in regards to plugs etc please feel free to comment.

Andrew 

[mhinders]

I doesn't look like the 4 "wet" plugs run on the same intake plane, looks like both planes are involved?

I doubt you can fix it by having hotter plugs on the 4 rear cylinders, if it's bad flowing intake runners they may  suddenly start working better on other rpms, and the plugs may run too hot.

Did you check cylinder pressure?

[Chryco Psycho]

What intake are you using , , the Eddy non RPM Performer has horrible fuel distribution . if you had a leak in the power brake booster I have seen the plugs from the cylinder that the runner feeds go wayy lean . You might have a leak under the intake where oil from the lifter valley is being drawn into # 5 & 6 cyl .

[Chippa]

mhinders-no i have not done a comp test yet, i went to do it yesterday and found the flexi hose was no good so i need to buy a new one...

Chryco-This is the intake and i'm gathering its the crappy one from some time ago....This is all a big learning curve for me which i'm enjoying, i've always been around cars and engines to some extent but these days i'm finding i'm wanting to really understand more about whats going on inside an engine and what to look out for...



Just want to get this motor in some good order for what it is, but i know for sure i wont be able to help myself and i'll end up pulling it out and getting a good combination of engine parts that compliment each other and doing everything to my standards thx to ppl like yourselves

Ok time for some breakfast then off to the garage to start on the distributor  

[Chryco Psycho]

OK good guess , that explains the 4 clean plugs at the front & the 4 bad ones at the rear although 7-8 are better than 5-6, Anything is a better intake .The engine is probably fine !!

[Chippa]

Ok well first update...I only have one spring on the advance weights!! So looks like someone has played around with it..I will have to either make the changes to the tee slots and put it back together and hope the curve isn't too fast or try and put some other type of spring in there to help slow it down.. Short of taking it to get re curved I will have to map it out every 200 rpm
It has a 13L tee plate which confirms my 26deg of mech adv... Back to the shed I go

[Chippa]

2nd update....

Dizzy back together, timed up at 34-36 deg @ 2600 when i let it idle it was sitting on 19-20....i backed it off to 18

curve map

1000- 18
1200- 18
1400- 19
1600- 19
1800- 20
2000- 26
2200- 28
2400- 30
2600- 35-36

3000- 35-36
3200- 35-36


So not real flash-it didnt do much till it hit 2000 rpm but bit seems to have to have a better transition from off idle circuit to primarys..i guess maybe its time to get curved properly...or source the right springs etc...enough for today anyway  

[firefighter3931]

Nice work Andrew 

The initial & total are fine. The curve looks descent too....i allways try to shoot for total "all in" by 2600-2800 rpm. This usually works with one "light" and one "medium" spring. Your advance slots should be ~ .375in at this point.

I'm guessing that the previous owner removed the second spring to quicken up the curve to help with the big cam. He did 1/2 the job which you are now finishing by shortening up the mechanical advance slots 

How does it drive ? Any signs of detonation ?

The plugs will give you clues ; black or silver specs on the plug are signs of detonation. 

As mentioned above there are issues with fuel distribution and possibly some oil in the combustion chamber.


Ron