Vacuum Advance Question

[Flying Bandit]

I am removing a Holley Street Avenger carb and adding a Pro System HP.



Question is I do not have a vacuum line on the HP carb. Does this need to be hooked up or cap it. The car has a billet Summit HEI distributor.

Thank you for any info.

[anthonys 69]

Cap it off. vacuum advance is really not an advance. with out vacuum when the engine is off it’s a retard allowing the timing to slide back to about 8 deg or so to aid the starter motor when starting a higher compression motor. So when the engine is running the vacuum bring the timing back to the place the timing should be in the first place. At higher rpm say wide open throttle the vacuum drops off, but the rpm bring the weights into play to bring the timing back up again. Depending on your compression on your motor you can bring the initial timing up, but you’ll need change the stops on your distributors weights to stop the timing from going to high at rpm. I hope this helps .good luck.

[Flying Bandit]

Cool thanks.

Cap it off. vacuum advance is really not an advance. with out vacuum when the engine is off it’s a retard allowing the timing to slide back to about 8 deg or so to aid the starter motor when starting a higher compression motor. So when the engine is running the vacuum bring the timing back to the place the timing should be in the first place. At higher rpm say wide open throttle the vacuum drops off, but the rpm bring the weights into play to bring the timing back up again. Depending on your compression on your motor you can bring the initial timing up, but you’ll need change the stops on your distributors weights to stop the timing from going to high at rpm. I hope this helps .good luck.

[TheMeat]

Vacuum Advance is an ADVANCE...hence the name. What anthonys69 is describing is a start timing retard function found on aftermarket ignition systems.

Vacuum advance is timing in addition to your mechanical and initial timing during vacuum. At WOT(very little vacuum) conditions, the vacuum advance is not operating and your engine will see what you set your timing at which is usually 34-39* for most SBC apps. Under normal cruise conditions the vacuum advance adds more timing to that 34-39*. How much it adds depends on the can itself and some cans are adjustable.

So you have your initial timing when the engine is at idle controlled by turning the distributor clockwise or counter-clockwise. Then, you have the mechanical advance added on top of the initial controlled by weights and springs inside the cap. And finally vacuum advance for another supplied amount...mine adds about 10*.

IMHO, I would not run a street car without a vacuum advance distributor. You should be able to add/tap a port to the carb once you receive it. Your plugs will stay cleaner longer, better fuel mileage, and crisper throttle response. All sound like things I would want on a street car.

Ray.

[anthonys 69]

It called a vacuum advance because that’s what it does. It adds timing when the engine is at or just off idle. Correct. Initial timing is low to aid with starter and starting only. I’m sorry but vacuum advance is not vacuum advance in addition to mechanical advance timing. The way to check FULL timing advance is to run the engine’s up to 2500 rpm or so and check timing. And yes you should have the vacuum line disconnected when checking. if you rev your engine to lets say 2500 -3000 rpm the mechanical weights are at full timing lets say 34 digress if you add more timing from your vacuum advance on top of that don’t you think you would get detonation. Plus what would be the point of checking full timing with a unknown figure which the vacuum advance would be. the second part of the statement implies your engine may have close to 49 degrees of total timing advancement 34-39 +10 degrees. At 45 degrees of timing your in the next cylinder 360 degrees of distributor rotation divided by 8 cylinders is only 45 deg between cap contacts. the third part of your statement maybe true depending on your motor for e.g. if your motor has high compression and a mild cam and have to keep the initial timing low say 8 degs you would get better gas mileage at say 12-15 deg. But if your motor is not a high compression motor you can start with a higher figure. It all depends on your motor if it starts fighting the starter when hot you have to much initial timing. Here is msd installing a crank trigger they set the initial timing at 30 deg and they say to aid starting you can use a retard starting aid for the msd box. And msd on timing. msd has good clips watch them and make your own mind up
http://www.google.com/url?sa=t&rct=j...65vRHxcjHddmbA
http://www.google.com/url?sa=t&rct=j...NAbRILeG-5g12A
not on timing clip there is no vacuum advance i hope this helps

[anthonys 69]

i hope i didnt make it more complicated than it need be. If you find you have to bring the initial timing down to 8 degrees or so for starting by all means hook the vacuum line to a vacuum port. If your starting of at a low initial timing say 8 degrees hooking the vacuum advance to a vacuum port will bring the timing up to say 18 degrees at idle. if you can start with initial timing at around 15 - 18 deg your golden.if you cant get that kind of initial timing to start with. The vacuum port can have vacuum at idle or have vacuum just off idle. At rpm and throttle opening the vacuum advance drops off and the weight take over.

[TheMeat]

I’m sorry but vacuum advance is not vacuum advance in addition to mechanical advance timing

I'm sorry BUT Vacuum Advance IS timing in addition to mechanical timing.

The way to check FULL timing advance is to run the engine’s up to 2500 rpm or so and check timing. And yes you should have the vacuum line disconnected when checking. if you rev your engine to lets say 2500 -3000 rpm the mechanical weights are at full timing lets say 34 digress

I agree completely. A lot of folks forget to disconnect the vacuum line when trying to set timing.

if you add more timing from your vacuum advance on top of that don’t you think you would get detonation. Plus what would be the point of checking full timing with a unknown figure which the vacuum advance would be.

Yes and no. You would get detonation if your engine still produced a high level of vacuum under heavy load allowing the vacuum can to increase timing. This is the beauty of the design...under load, it does not add any timing. You are simply left with your mechanical advance and initial which will say 34*. Also, it is not an unknown figure. It is very simple to measure using a timing light and you can vary the amount the can adds. And as you stated earlier, you disconnect the vacuum from the can when setting the mechanical advance.

Also, you can have the vacuum advance add timing at idle as you suggest only if you connect it to a manifold port. If you connect it to ported vacuum, like the original post was asking about located on the carb, you will not be adding timing at idle from the vacuum advance.

the second part of the statement implies your engine may have close to 49 degrees of total timing advancement 34-39 +10 degrees

No implication made here. It is a statement of fact. My engine does have that much timing and I drive it a lot! I think my initial timing is set on 20* with another 17* added in from the distributor being all in at 26-2700 RPM. My vacuum can then adds another 10 degrees on top of that for a non-detonation amount of 47* on 93 octane pump gas in a 10:1 SBC. No starting retards used, stock starter for over four years, driven about 25K miles and still going.

Here is an article for you to read....http://www.highperformancepontiac.co...h/viewall.html

And I'll quote the last paragraph..

"
Conclusion
There is one constant generality about vacuum advance that likely applies to every combination: A suitable amount (whether manifold or ported) can noticeably increase part-throttle response without any negative aspects. But because most of our engines are modified in some manner, neither form is necessarily "correct" for any vehicle. Since vacuum advance offers such distinct advantages for any car driven on the street, we suggest experimenting with both forms to determine which works best for your specific application. Once you find the right combination, you'll wonder why you waited so long to perform your own vacuum-advance experiment!"

[anthonys 69]

Good article an interesting read thanks for posting it its hard to argue with that.

[Flying Bandit]

I hooked up the vacuum advance up with a T between the carb and the brake booster to see if it made any difference. It did raise the idle rpm about 200 rpm but does seem to have better manners at slow speed driving. showing about 15 on my vacuum meter.

I am ordering a a fitting form my manifold to see if there is any difference using that port.

[MonzaRacer]

Ok first of all lets get a little timing info out there, you want as much initial as your engine can use, one) without bucking the starter, 2)makes best power. Now centrifugal advance combined with base timing are what you use to tune for best power under load. I have ran my 10.5 to 1 402 BBC, closed chamber heads, zero deck and it ran in the 14 to 16 range, I forget what it ran with the cent also but with vacuum advance it was running in the range of 53 to 58 total at cruise.
Vacuum advance is mileage and driveability in a nutshell. oh and gas mileage. take it away and you have crap for mileage.
I tuned my 402 and had it pushing 14 to 15mpg drove hard and with 3.55/th400 setup and an 800 dp holley, if Ijetted different I might get a mile or two more but it was rattle pron on crap fuel as sunnoco 94 wasnt as easy to find in all areas.

[Flying Bandit]

Is there a difference in the initial timing if the vacuum is coming from the manifold or carb?

Ok first of all lets get a little timing info out there, you want as much initial as your engine can use, one) without bucking the starter, 2)makes best power. Now centrifugal advance combined with base timing are what you use to tune for best power under load. I have ran my 10.5 to 1 402 BBC, closed chamber heads, zero deck and it ran in the 14 to 16 range, I forget what it ran with the cent also but with vacuum advance it was running in the range of 53 to 58 total at cruise.
Vacuum advance is mileage and driveability in a nutshell. oh and gas mileage. take it away and you have crap for mileage.
I tuned my 402 and had it pushing 14 to 15mpg drove hard and with 3.55/th400 setup and an 800 dp holley, if Ijetted different I might get a mile or two more but it was rattle pron on crap fuel as sunnoco 94 wasnt as easy to find in all areas.

[anthonys 69]

Vacuum advance isn’t always the best for you’re car. Power and fuel economy is obtainable easer with fuel injected motor. But not so much in older carbed higher performance motors. I’m not saying it going to be terrible but every car and set up is different. When you snap that throttle open [hard acceleration] vacuum advance isn’t used at all. If timing advancement coming in too fast it can cause pre ignition not always noticeable with free flowing exhaust. too much timing can cause pre ignition Not all cars will like the same initial timing or the same vacuum advance timing. Factory timing on [most] older motor used to be around 8 degrees and vacuum advance would bring the advance up to around 16 degrees or so. Mechanical timing would take over as rpm increase and vacuum drops off. If initial timing is increased to 18 degrees or so you’re already put the amount of timing in that the vacuum advance should. Then you need to check that the total timing is not too high. Some cars like vacuum advance some car don’t because of cam, vacuum, compression, fuel, etc. it’s all in the set up and the tune. You’ll have to experiment. They make adjustable vacuum advance so that you don’t put to much timing in. with your set up with the vacuum advance coming in at start up id probable drop the initial timing back to 10 degrees or so I can’t be very specific trial and era. If you put the initial timing up to 18 degrees and you hear the motor kicking against the starter that is pre ignition. Combustion has reached a point were the power has come in before the piston has reached top dead center and trying to force the engine to turn backwards.

[Flying Bandit]

Yeah I un hooked it this morning and it runs much better at idle and it didn't try to stall everytime i let the clutch out. The car does have a kind of surge or miss at cruising speeds. I pulled a plug and they are gapped at .045 with a summit HEI distributer. Could it be there is just not enough energy and spark to burn the fuel? Maybe MSD may be the answer to my problem.

[anthonys 69]

If you came to my shop I would start with a basic tune up make sure all the plugs, plug wire, cap, rotor etc are all good. I would turn both air mixture screws on the carb all the way in and then back both air mixture screw back out 2.5 turns. disconnect the vacuum advance. Set the idle to 750 - 800 rpm. set initial timing to 8 degrees. Let the motor warm up. Recheck idle speed. Slowly tighten one air mixture in listen to the engine. The engine pitch and rhythms will start to stumble at that point back the screw back a little bit until it straitens out. make a not of where the screw is [number of turns] back the screw back out when listen for the engine to stumble tighten screw back to about center of those two points. adjust the other air screw the same way. Recheck idle. Slightly close the chock or slowly cover the air inlet with say your hand. If the rpm raises the carb is running lean were looking for it to stumble. Then disconnect the smallest vacuum line if the rpm raises the mixture is to rich were looking for the engine to stumble. If its stumbles in both instances the mixture will be good. Reconnect vacuum advance vacuum line. Road test. That is a basic tune. You can disconnect vacuum line and check full timing. From here you can play with your set up. If you want to run without the vacuum advance hooked up, you can raise the initial timing and see how it starts. And make small adjustments from here.

A great investment is a air / fuel ratio gauge it will help with jetting and with accelerator pump settings as well as idle mixture and heck they look cool too.
Tuning a car is like getting to know your girls personality is she cold natured does she purr like a kitten after all she’s your trophy.you should enjoy getting to now her and take your time.

[68Formula]

It doesn't like the manifold vacuum at idle, but throttle tip in is better, yet cruise you have surge. Bottom line is your engine likes vacuum advance, but its adding a little too much. Most aftermarket HEI or adjustable canisters use a 3/32" allen wrench I believe clockwise should decrease your vacuum timing. Try turning one rotation at a time to remove timing and most likely your surging at cruise will go away along with the stalling, without degredation of throttle tip in.

[anthonys 69]

How much vacuum is your engine making and is it steady
I must say that's a pretty big cam 562 intake 580 exhaust. Cams are usually set with a 1.5 rocker excluding today’s ls motors. If you add 1.6 rockers it basically make your cam a 599 intake 618.6 exhaust that’s pretty healthy. The calculation is 562 /1.5x 1.6= 599.4. Are you looking for max hp. If not id drop down to the 1.5 rocker to help with the street ability and slightly more vacuum . Unless your racing.
Good point made above ===by 68 formula
With a big cam like that you should be able to up the initial timing trial and era. good luck