i recently got a timing light with a dial allowing me to fully checkout my timing....

i have it set on 14 initial and 36 total by ~2500 RPM

i have tested the distributor vacuum port on my new edelbrock qjet carb and the ported vacuum source works correctly (tested with a vacc guage)....

so then i went to test at different RPM's with and without the vaccum advance connected and the #'s were the same for both tests!!!!

so this means my vacuum advance is obviously not working...(can is bad...)

my question is.... i already have the advance curve i want.... is the vacuum advance a necessary function???


:confused: :confused: :confused:

 

You have the advance curve you want for your mechanical advance, but the vaccum advance will provide additional advance under acceleration. This is something you want. That is why you set up your mechanical advance with the vaccum unplugged. I would fix the vaccum advance.

 

You want the vacuum advance to advance the timing under part throttle cruise to boost fuel economy and throttle response, so you want to use full manifold vacuum not ported vacuum. The accleration curve is set by the mechanical curve and is not affected by the vacuum amount since the vacuum will drop during acceleration and whatever vacuum advance you had will now start to retard. The cars in the 60s all ran full manifold vacuum, the ported vacuum thing derived from the emission conscious engineering of the 70s as the extra cruising advance is bad for emissions. Sounds like you got your mechanical and intial set right, just get a functioning can and set it up for full vacuum and hopefully it won't ping.

 

Allen71 has it correct. I ran for a while with just mechanical advance and no vacuum advance. The car ran fine. I put in a vacuum can to pull in 14 degrees more advance at a cruise. The MPG went up slightly and there is no difference in accelaration. I would have the vaccum hooked up if you drive it on the street.

 

But, Allan, isn't the purpose of the ported vacuum precisely to supply vacuum under part throttle (and no vacuum at idle)? :confused:

-Pedro

 

Here, this should answer ALL your questions:
http://www.corvetteforum.cc/techtips...=115&TopicID=3

This is Lars' paper on the subject. As far as I'm concerned, that's pretty much the final word around here.

 

Yes, I think that is what the engineers had in mind and this same thinking also applied to the TCS system, namely to reduce or (in the case of the TCS) eliminate idle and very slight part throttle vacuum advance. It is a fact that reducing the ignition timing at part throttle will reduce emissions so I am reasonably sure that was their thinking. But this is not best for performance and economy. Now you defintely don't want your mechanical curve starting at idle, but nothing wrong with vacuum advance at idle. Daily Driver has got his mechanical curve and total set up pretty good and he probably has it set to idle decently now. Once he gets his vacuum advance kicking in, his idle speed will go up and will need to be lowered, but the intial advance settings are not to be changed. Any part throttle timing advance you get from your vacuum cannister is added on top of your initial and mechanical curves, and cruise total timing will be much more than the 36 degrees. At higher rpms, you don't want that much advance but as your rpm increases your vacuum can will retard the timing and you will now be running on just your mechanical curve and initial and will be back to the 36 or 38 degress that you originally set it up for.

 

Thanks, Allan! :)

-Pedro

 

I hope you don't mind if I dissagree. Vacuum has always been set to increase timing when you open the throttle plates. Before a ported vacuum connection was available possibly in the 60's, manifold vacuum was used to retard the timing for idle, and when the throttle was opened allowed timing to advance due to the decrease in vacuum. You could use manifold vacuum if you set it up properly, i.e. set timing to like 18º then plug in manifold vacuum to bring it down to like 6º, but the vacuum signal designed into the Q-Jet is ported vacuum. Please see my posting in Tech Tips at http://www.corvetteforum.cc/techtips...D=86&TopicID=3

 

Daily -
If you let me know if you're running point-style or HEI distributor, I can give you a part number recommendation for a good vac advance control unit that will compliment your 36-degree @2500 mechanical curve. And do check out the link 79Mako gave you to the vac advance tech paper - it's technically pretty accurate.

 

Vacuum advance retards the timing under load- less vacuum. It is used so that more timing can be used. The engine needs varing amounts of timing and both the mechanical(centrifugal) and vacuum advance change the timing curve to affect this need.
Non- vacuum advanced cars - racing cars, don't need it because they aren't driven on the street, so they don't see the bottom end like a red light.
The ported vacuum only works at idle. As the throttle blade is cracked open this uncovers the port in the bore ( you can see it) and delivers full vacuum to the canister. (full advance) If you can't tell the difference with it operational you should use it.
As to the 2,500 rpm check. Depending on the varying curves that were used this could go as high as 5,000 rpm.
2 other options not mentioned are a limiter on the vacuum advance mechanism. This allows you to decide on how much advance the vacuum supplies- and releases.
An adjustable can. This has a screw in the hose nipple for adjusting the spring pressure, for high compresson cars this can help you get the advance that you need/want and at the same time prevent pinging.

Remember if you retard the timing you can eliminate pinging. The vacuum advance retards timing. It can be adjusted to do this at higher gauge pressure thus retarding the timing at an earlier point. This is a real pain to get adjusted, you have to floor it and see if it pings, adjust, check, adjust.

The limiter reduces the throw of the mechanism so that if you have 25* of vacuum you don't have to reduce your static to almost nothing. You can reduce the vacuum to 20* or so picking up an additional 5* mech. or allowing a no ping driving experience.

Another thing the vacuum advance allows is a low timing value for starting. If total was 42* and only done by static and mech. this could cause hard cranking. With some value of the total timing supplied by the vacuum you can have less static timing for starting.


[Modified by Techno, 6:57 PM 12/7/2001]

 

Vacuum advance does NOT retard the timing under load. It adds advance at light throttle to improve mileage.

Under load (litle to no vacuum), the vacuum advance can does nothing and you are running mechanical advance. Hence, race cars have no vacuum advance. Also, that is why you have to disconnect the can when setting timing.

Use Lars paper. He helped me set my carb up and checked my timing. Hooked vacuum advance can up to correct port. Results - runs like the proverbial "bat outa he77" and gets 18-20 mpg on the highway cruisng at 3000 rpm.

 

You got it, Steve.
Glad to see the car is still running strong for you - hope you're doing well out there!

 

lars its an HEI

thanks for all the help guys!!

i knew vacc adv can is shot becuase i get same values for it connected and not connected...

no pinging right now...

:)
:cheers: :cheers:

 

Full or ported?

:confused:

I no longer have the TCS solenoid on my '72, and I am currently running full manifold vacuum to my distributor. I plan to recurve the dist. and rejet the stock Qjet.

I was going to change to a ported advance after the mods, but after reading the thread I am now a little confused.

I understand that the ported advance comes on during light throttle. But should I go ported or full to the distributor? Hmmmm.

BTW, currently all stock L48, have 64cc camel humps and Xe268 cam to be installed. Retaining stock intake and Qjet for that stock look.

Any comments on distributor vac. hook up appreciated. :) Also, big thanks to Lars for all the tech papers - great info.

 

Dino 72 I really like the color combo on your car. Looks kind of the same.

 

This is a little off topic, but sounds like you guys are "in the know".

I am running a mechanical advance Mallory Unilite distributor. How would having a traditional vacuum advance distributor change the way the car runs? I was told that this distributor is used in drag racing and not so much on the street (previous owner had it installed)? Car runs well but is a little rough at idle.

Also what is meant by "ported vacuum"? Is this off the carb. as opposed to directly off the intake? Can you run a vac advance dist straight off the intake port?

any help appreciated!!!

Regards, Mark

Regards, Mark :flag :chevy

 

i can asnwer some of them....

ported vaccuum only turns on like after ~1500 rpm or soo... theres no vacuum below that... (yes its off the carb.... a dist or smog port)

yes u can run vaccum advance dist either ported or straight off manifold.///

 

SteveG75, Its a matter on how you want to look at it. Since the addition of vacuum increases timing, the reduction- closer to atmospheric , reduces it or retards it from what it was with the vacuum.

Pull the hose and the timing goes down. plug it in and the timing goes up. If the can increases advance under light load, at some time it has to reduce it.


The use of timing is a fairly long topic.
but simplified.
The faster an engine is turning the less time the fuel charge has to burn, so at high rpms the spark is introduced very early to allow the combustion to do its job as the piston begins its down stroke.
Under load this changes, so the timing is retarded Affectivley disconnecting the vacuum advance. The timing is retarded because an engine under load provides a faster flame front and doesn't need the spark made as far in the pistons up stroke,before top dead center as an engine not under load.

This gets confusing in discussion because we assume unspoken things. The mechanical starts at zero and goes up, while the vacuum starts at full and goes down, in its operation.

Its been awhile but stated from a mythical average. A very powerful engine driving a light highly geared car can use mechanical advance early, and quickly introduced. A short curve that starts low in the rpms
A heavy under powered car with a high way rear end needs a long slow mechanical advance.

On the ported advance, it makes little difference. Its for idling. if the carb is adjusted correctly and your engine can idle with the blades nearly closed it functions. If the blades are opened to obtain a smooth idle then the port is bypassed anyway and does nothing.

After my engine was broke in it would idle correctly, before this the weights flew out and rpm rose to 1200, if I adjusted the idle the weights pulled in and then it was 600, too low. This took awhile before I finally figured this out.

I wrote down all the different advances for the different big blocks and tried to make sense from it. I couldn't so I changed my timing and checked it up to 4700 in the garage. Very loud and scary. trial and test.
The road test was the proof, I went back to the light springs and have full mechanical advance in by 1300? This feels like a kick when it goes in. The more gradual timing didn't work for me( mostly because I liked the kick). After this I adjusted the vacuum advance spring to work without pinging, with the static timing I was using. With the after market adjustable can, I found I was gettng too much vacuum advance and limited this also.

Take note that if you have changed your cam then forget about tuning by the book because you no longer have that engine.

 

The Chilton's repair manual has all the vacuum hose diagrams. Some use manifold vacuum, and some (like my 75) use ported vacuum. If you use manifold vacuum then it must be used with the correct can, thanks to Lars for listing them, but it would also be useful to have the Chilton's manual to see which diagram should be used with what can cause there's a lot of Thermally Controlled Spark valves on several diagrams and some include an air filter where at certain temperatures the vacuum signal is cancelled.

Do this at home: Buy a vacuum gauge usually available at your local parts store for about $30 and connect it to the ported vacuum line on your Q-Jet (I provided a link to a photo of which port this is in my earlier post to this thread) and thread a long vacuum line around into the car so you or a friend can take vacuum readings while at idle, revving while in neutral, holding 2000rpm while in neutral, cruising, and under mild, medium, and WOT. I believe the port opening in the venturi is just above the primary throttle plates and using the vacuum gauge you will see vacuum increase as the plates are opened, and maxed out only when the plates are completely opened, and the vacuum will drop when the secondaries open...I'm not sure how much it will drop cause it's been a long time since I've tested the port vacuum; but, anyway that's how the ported vacuum operates and why I use it...there is no need for any thermal switches cause the choke plates alter the ported signal, while the manifold signal is NOT corrected for cold engine operation.

So, what I'm trying to say is if you use manifold vacuum then be sure to include all the TCS devices or you might have a rough running engine at certain temperatures, or you could just use the ported vacuum which has thermal adjustments built-in as long as you have a properly adjusted choke.