barnett468

ported vacuum is almost EXACTLY the same hg as manifold vacuum after the throttle is opened around 1/16th - 1/8th of the way.

Gun Jam

I think I would run it off full manafold vac...one of the main advantages of vac advance is to clean up the idle by advancing timing at idle...if its ported you miss out on that.

If I can improve economy, get better part throttle performance and maintain peak HP with a vac adv style dizzy...it seems like a good idea

That damn electronic dizzy is actually looking like a better ot

barnett468

ok,

i'm happy to help as others are but the dist vac, can cause some heated discussions that i want no part of.

i will simply say the following:

in general, if you can not get it to run properly with a dist that does not use a vac can, you will not get it to run better with a dist that has one.

in general, a dist with a vac can is harder to set up. if you can't set up your existing one with my help, i can not help you with a vac type, others here might be able to. i know how to do it but i simply won't be able to help.

you might improve perf at 1/16 - 1/18 throttle with dist vac but you might also create a ping/detonation just off idle. i have had this prob several times.

in general, using dist vac might reduce an off idle hesitation slightly. it might not.

using manifold vac will not clean up the idle. all it does is advance the timing. you can do this with the dist alone however if it gets too much initial timing it will become hard to start

using man vac, you can have 10 deg initial but when you turn the engine over it creates vac that in some cases is enough to advance the timing before the engine starts. i have seen this. if this occurs there is little to no difference between using man vac to get 20 deg idle timing and simply turning the dist to get it.

also, engines can occasionally continue to "run on" after you turn the key off with high initial timing.

the glorious "engineering report" some unfortunately find and view as the holy grail is incomplete in its "facts" to say the least. the problem is that people that know nothing or very little about vac advance and how it works don't realize the errors or lack of additional info the report should contain. this info is also grossly lacking in many online sites discussing the subject.

some people that read these types of reports think that the info they contain is all there is to know when it simply isn't. the prob is that because some info is missing, they will not know it is missing and if they know it is missing then they were informed enough that they did not need to read the report.

60's and later chevs did not use manifold vac for the dist as the report inaccurately states. hmmm?

in the cases i have seen, a properly set up dist vac will increase mileage by an average of 1/2 mpg.

if getting an additional 1 mpg or less was a concern for me, i would not build a perf engine and if i had one i would be installing a smaller cam etc.

i have set up hundreds of dist with a vac can and have run them on hundreds of cars so i have nothing against them but i would not intentionally replace a non vac dist with a vac type, others would.

increasing comp to the optimum level will increase hp, tq, mileage, throttle response and reduce hesitation. yours is a bit low as i mentioned earlier.


happy holidays!

Gun Jam

That's a very helpful write up.

You are one of the fist people to suggest that a non vac "race" dizzy is equally as suitable for street use as a vac style. Im all for avoiding the vac can if you think I can get the same results with what I have.

At 18 deg initial advance I think it ran pretty good I was happy but never drove it much because Im sure total adv was way over 34 deg.

I wanted to get the foursecondflat limit bushing set it to 20 run a 14 deg adv for 34 total and go about my business. The dude was pretty pissed off at me for running a non vac dizzy with a my mild cam and street car and wanted to sell me a nice vac adv one...so I didnt know wtf to do and I was like damn....let me think about it.

maybe I should just run 18 deg adv and 18 limit bushing for 36....I have one for my MSD I think.

thanks again and happy holidays to you too

-Gun

barnett468

hi,

yeah too many experts. if you read my post again you might find he fits the category i mentioned.

i am not the foremost authority on the subject.

i have done this for many years on many distributors.

there is no such thing as a race dist. there is vac type and non vac type and they are all aluminum.

they call then race dists because they are shiny and have a pretty red cap and are a bit easier to get parts for and adjust.

i personally, would not run more than 14 initial. others will. its too much typing to explain why, but my previous post will help, sorry.

i would also check my timing mark at 0 to insure it is still accurate. this is a bit hard to do but i doubt yours is.

if you ran a true 30 deg initial with no stutter in your driveway, it strongly suggests to me that either your compression might be below around 130 or your hearing might not be as good as some others.

quit shooting the elephant gun without ear muffs.

set initial it at 12 - 14.

check timing at 1200, 1800, 2400 and 3000.

i or another can tell you what to do after that but i already explained it in my previous post.

Gun Jam

I have been running 12 deg adv before this thread. At 12 deg idle was actually smooth but if set slightly more rich than peak rpm it would smell a bit rich not a big deal this is pretty common anyhow...the big deal was the engine would not run as smooth as it did at part throttle cruise when timing was set to 16 or 17 initial. It seemed pretty happy around there. I'll continue following your previous write up, check for starter bog and keep total timing down about 34.. I'll let you know what I find.

Thanks again

-Gun

Gun Jam

And on a side note wouldn't 16 to 18 initial adv still be quite conservative compared to a standard vac advance dizzy? If I had 12 initial and was at 1500 rpm 19inhg I would have say 12 initial + maybe 10 to 15 vac advance for 22 to 27 total with maybe even a a deg or 2 of mech advance Vs only say 18 adv with my mech only dizzy?

Wouldn't running at 12 deg adv with non vac dizzy be the same as vac advance dizzy with ruptured adv diaphragm?

-Gun

barnett468

hello


i hate advance, lol.

unfortunately i din't have the time to erxplain it.

if you do what i suggest it will work.


i suggest you read my posts again. i have answered all these questions in them.


vacuum advance will not ever, ever, ever help part throttle. tthere is bo advance from the dist above 1/8th throttle.

cruise is more than 1/8.

as soon as you hit the gas, your vac advance goes to 0 which in yiur case would be 12 etc.

it only works when you let off the gas pedal. it does not work under acceleration.

barnett468

ok you can install lighter springs in your dist but you said your rpm suddenly increased as you rotated the dist so you already have light springs.

nothing can be determined until you get the timing specs i suggested.

you can certainly run a vac distributor, if you want to run a vac adv dist, just buy one and don't mess with the one you have.

part throttle means nothing to me.

what rpm are you talking about?

how far down is the gas pedal depressed?


possible vac adv dist set up for your app.

disconnect dist vac

set initial to 12

set mec adv timing curve for around 24 total at around 1800 rpm and 34 total at around 2800

test drive and check for ping under acceleration from 0, 25 and 35 mph. if no ping go to next step.

set dist can for max advance by turning screw all the way

install a vac adv limiter on max setting.

test drive

Gun Jam

part throttle cruise say 3 gear 40 mph 2000 rpm 18" of vac...I think thats pretty accurate.

I'll re-read everything and see what improvements I can make

thanks again