Thread: About vacuum sources and timing

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About vacuum sources and timing


What we’re dealing with here is in effect a variable venturi. At least it is as far as ported vacuum goes. The variable venturi bit due to carburetor throttle blade position.

I got curious about a comment I heard about manifold and ported vacuum going to zero at WOT (Wide Open Throttle) and ran a little experiment.

The car - 32 roadster - weighs 2400#, engine is an overbored 455 with 462 cid, 9/1 compression ratio, Edelbrock Performer intake, Carter 750 cfm competition carb with electric choke added later and a Crower Compu-Pro #1 cam which has about 262 & 266 degrees advertised duration intake and exhaust with 112 degree lobe centers. It’s a smooth cam and the car when warm idles @ 19" vacuum.
The dash carries a large (2 5/8") S-W vacuum gauge which indicates the same as my vacuum/pressure test gauge.

Advance is 8 degrees initial and all in at about 2600 rpm with a total of 32 degrees. Vacuum advance is about 16 degrees and sourced from Manifold Vacuum (MV).
The car runs very well on 87 octane in summer and winter and does not overheat in traffic.

Firing the car from dead cold and on the elec choke, MV reads 18-19" and idle is around 900-1000 rpm.
Ported Vacuum (PV) read 12" on startup.

Once the engine is warmed up and off the chokes fast idle cam with choke blade open, MV reads 18.5 - 19" and PV reads zero at about 500-600 rpm.

Cruise at 40 mph with a light throttle setting on a flat road gives you 18.5 - 19" MV and just about the same on PV.
Rolling the throttle in about half way shows 8 - 10" of vacuum on both MV and PV during light acceleration.

Once at 60 mph MV read 18 - 18.5" vacuum (keep in mind this is a very light car) and PV read
10".

Flooring the throttle at 40 mph or 60 mph brought the MV down to 1" or so and PV to zero.

The key thing is, at idle with a fully warm engine, MV reads 18.5 - 19" and PV reads zero.

The lack of additional timing at idle is what creates an overheating problem in the GM engines.
It takes time to burn the lean idle mixture and additional advance is required to get the process underway early and avoid overheating.
Exactly the same thing (overheating) would happen with the timing severely retarded in an engine under load at a higher rpm level.

There’s a lot of confusion out there about timing, both centrifugal (mechanical advance) and vacuum advance as well as the vacuum sources to use.

The key thing is to realize they are two different systems that work together to give optimum spark advance for a particular condition and key on rpm as well as load.

To my way of thinking perhaps there would be less confusion if the vacuum advance cannister was called the vacuum retard cannister.

I’ve been amazed at the lengths some go to, to cure an overheating problem that can be solved in most cases simply by selecting the correct vacuum source.
Granted, most of my experience has been in cars with small engine bays and many times not the biggest radiator in the world, but I note, the bigger cars have the same amount of timing and overheating problems as the small car guys do and for some reason many car owners avoid doing something as simple as swapping vacuum sources to cure overheating and prefer to throw money at the problem.

As far as spinning up a little experiment, I’m not trying to prove anyone wrong here, just got curious, had some free time and those are the results I came up with.




An additional note; for those of you who live at a higher altitude than where these tests took place, you’ll find that your vacuum levels at no-load (idle) rpms will read lower.
To the tune of a 1" vacuum loss for every 1000' of altitude.

The tests took place at 350' altitude and manifold vacuum at idle read 18.5 - 19".
After moving to Sunny Arizona and ending up at 3300' altitude the manifold vacuum now reads 15.5".
Highway figures and under load vacuum levels remain the same.




This combined article was written from research on timing figures and real world experience.

You can learn a lot by taking the time to set up some inexpensive instrumentation and taking the time to run a few simple tests.
What you’ll gain is a better understanding as to what’s going on with your engine and gain a small bit of education about the particular thing you’re researching.

Learning from books is one way to do it and there’s nothing quite like taking advantage of what smart, experienced and educated people have done and written down for your educational pleasure.

What I’m talking about is the stubbornness and unwillingness to learn from those who’ve been down the road before us.
It’s amazing sometimes to talk to an individual who thinks factory engineers don’t know much.
Thing to recognize is that factory engineers know a helluva lot more than we do and get into sophisticated areas that the great majority of us know nothing about.

Keep in mind too, factory engineers are constrained by the bean counters, the necessity to build a vehicle that is useful to the majority and seldom are let loose to pursue a dream or even an interesting idea.
When they do get the freedom to investigate particularly interesting areas, the results can be astounding.

The name, “Zora-Arkus Duntov” should ring a bell....

Interesting post
Thanks for takeing the time and effort to share with us

I would like to add that the real world is not always the same as what is in the book. There are many variables i/e altitude as you pointed out, and ware on parts.
Thanks again
Charlie

[QUOTE=C9x][SIZE="4"]What I’m talking about is the stubbornness and unwillingness to learn from those who’ve been down the road before us.
It’s amazing sometimes to talk to an individual who thinks factory engineers don’t know much.
[\QUOTE]

Say it isn't so Jay..........say it isn't so!!

Thanks for the good info.

The vet I restored (1) that was enough. I thought I'd like driving it which I did , but I hated getting in and out Sold it after about 2 mounths.

C9X Thanks for all that info. Learned alot from it and thats why I like this forum.

well i tried it tonight and hooked the dist vacume pot to manifold vacume, engine speeds up about 1000 rpm was able to set back idle and also set idle screws in about 2 turns. car idles much smoother down to 400 rpm !! and it seems to respond better while bliping the throtttle against the brake.itll break the tires loose just barely off idle wet outside tonight but will see what it does once out on the road. it sounds a lot better now. running advance is about 25 degrees ,initial 10 deg and mechanical goes from there
ill let you know tomorrow how it turns out re pinging etc

Sorry about the duplicate threads.

I didn't think I'd posted that here.

hey denny i fiddled and fiddled and the timing did not help as much as i thought. i finally changed jets on the quadrajet from 71 to 74 and removed the metering rods. now it works ok but fuel milege had definitly gone down. it now hits pretty good off the line but i still think lean in the lower end until it warms up
i am going to try now to put metering rods back in it.
i think i figured it out. as i modified the intake from a double plane to single and hogged it out and made it into a single plane plus plugged the ex crossover i messed with the intake carb signal making it lean off idle.
i am going to try some of the different metering rod combos i have and see if they improve fuel consunption or not.

so a intake spacer may help speed up the air flow? how so just by getting it back in a straight line? cause a spacer also makes the chamber larger?

i would assume then i should get a closed plenum spacer not a open one?
i was looking at one today and just about ordered a 1 inch open spacer for a q jet

being the woodworker / craftman i am,i sized up one of the carb gaskets i have and am going to attempt to make one of these.
would you suggest 3/4 of 1 1/2 inch spacer as i was going to laminate 2 pieces of 3/4 of plywood together and then start cutting from there.
heck they sell wood ones

hood clearance ? no problem,no hood!1

Originally Posted by canadianal

being the woodworker / craftman i am,i sized up one of the carb gaskets i have and am going to attempt to make one of these.
would you suggest 3/4 of 1 1/2 inch spacer as i was going to laminate 2 pieces of 3/4 of plywood together and then start cutting from there.
heck they sell wood ones

Since you're a woodworker I'm guessing you're going to use a veneer plywood.
No voids with this stuff and it should work ok.

A couple pieces of laminated oak strike me as being a good way to go as well.
Orient the grain 90 degrees to resist warpage and use an epoxy glue.

Another option is to use UHMW.
It's a white plastic that is very strong - about 70% stronger than the Teflon it resembles.
Easy to work with, the only difficulty is getting a smooth edge, but the right sander should take care of that.

i am using good quality cabinet plywood i have here but i was going to use w/s urethane to stick them together as its pretty tough. the 90 degree thing is a good idea thanks.