What is the correct amount of vacumn advance?

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jefflock

jefflock

69 Dart 408 10.08 best pass so far
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I was reading the instructions that came with the my electronic conversion kit from Mopar. They call for 50 degrees of total advance. I thought 34 to 36 was all you wanted. What is correct? This is the reading for max performance? Any thoughts? Thanks Jeff
 
50 degrees probably includes all of the vacuum advance. Initial and mechanical should come in between 32 and 36 depending on your engine.
 
I run a total with the vacumn advance hooked up at 2500 rpm of 38 degrees. With 15 degrees initial on the the balancer. It looks like I can add more vacuumn advance then. Is this correct? Thanks Longone and Adam.
 
Intial and mechanical are set with the vacuum plugged as far as I understand it. Vacuum advance is for economy. The mechanical is what is built into the dizzy and is controlled by rpm. I think the OTB MP ignition kit says all in by 2000-2200. You should be in the 32-38 range depending on you engine and pump gas.
 
Depending on your set up, initial timing could be let`s say 15*btdc and a mechanical advance of 18* for 33*advance. Vacuum advance could be 17* for a total of 50*. It`s relatively rare for all things to come together at the right time that your timing would ever advance to 50* like that, but if you do a lot of around town driving it`s more likely. The vacuum advance will help with your gas mileage at low rpms around town. At WOT the vacuum advance doesn`t come into play so your 32* or whatever you choose is right where you want it. I still run my vacuum advance and with all in I`m at approximately 52*.
 
Here's an easy way to determine the corrrect initial advance for your particular setup:

Start the car and let it warm to operating temperature then shut it off.

Advance the timing 2° and let it sit for 3 -5 minutes to allow the temperature to spike.

Fire it back up. Did it pull the starter down like the battery was weak? If not, repeat the process until it does. Back the timing off 2° and lock it down. This is the maximum advance that your setup will handle and that is what you want.
 
Vacum advance only comes in at part throttle and thats why some cars have around 50 at part throttle combined with 30 to 38 total mechanical advance. But at wide open throttle you have no vacum advance so if you have a total mechanical advance of 38 thats all you will have at wide open throttle. On my street/strip cars I dont run any vacum advance because I run a quick mech advance and have the full 38 in by 2000 rpm. Ron
 
2shelbys said:
Here's an easy way to determine the corrrect initial advance for your particular setup:

Start the car and let it warm to operating temperature then shut it off.

Advance the timing 2° and let it sit for 3 -5 minutes to allow the temperature to spike.

Fire it back up. Did it pull the starter down like the battery was weak? If not, repeat the process until it does. Back the timing off 2° and lock it down. This is the maximum advance that your setup will handle and that is what you want.
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That is not the best way to determine initial advance. Warming the engine up is correct but you need a vacuum gauge connected to manifold vacuum. Initial timing is a function of the duration of the cam and the design of the chamber. Assuming the the heads don't change then it's just a function of the cam. Cams with more duration than stock will require more initial advance for good idle and off idle throttle response.

Increase the timing 2 degrees. If the manifold vacuum increased reset the idle speed and increase the timing 2 more degrees. Continue this until there is no longer an increase in manifold vacuum.

Now if your engine kicks back against the starter (it shouldn't until your initial is 20 or more degrees BTDC) you will either need to turn it back until it doesn't (now no longer ideal for the engine) or if you are using something like an MSD add a timing retard for starting.

The total mechanical advance is a function of the design of the chambers and intake ports. Without dyno work the typical recomended numbers you here for sb 35-36 or bb 36-38 are the best you can do. NOTE: magnum heads becuase of their efficient chamber and port design like less, 32-33 degrees BTDC.

Once the intial an total mechanical is dialed in you go after the vacuum advance. You want as much vacuum advance as you can get at part throttle cruise that does not cause part throttle tip in detonation. The sum of total mechanical and vacuum should add up to around 50-52 degrees BTDC.
 
Perhaps there a few different issues being confused in this thread - I think they are:
initial (starting) timing
total mechanical advance
vacuum advance.

Doing this justice would require pages but let me try the "highlights".

Most folks in this forum would probably be most concerned with total advance as that is what your engine sees when under medium to high rpm load - it is what directly impacts HP and torque. Others in this thread mentioned 32 to 36 and that is general correct but there can be fairly significant power differences between these numbers. It is set by disconnecting the vacuum line and reving the motor to engage all of the mechanical advance that you have dialed into your distributor (usually ~3000 RPM gets it done). Another way would be by subtracting the mechanical advance (usually ~18 degrees) from your desired total (say 32) to get the the idle or start advance (32-18= 14 degrees at idle). I prefer the first approach but many use the second.

Now the tough part ... what should the total (32 in this example) be? Best answer is probably a dyno. Issue though is as soon as something changes (like jetting) - you may get a different answer. My current stroker likes 33 and that did not seem to change much as we changed jetting but other motors I have dynoed really required less advance as the motor leaned out.

Second way is at the track.... probably a lot more fun!

The primary issue with initial advance is making certain it is low enough so the motor starts. In this example of 32 total, 18 in the distributor and 14 starting: if the car was still hard to start when hot you could add more advance inside the distributor (there are stops or bushings stops that can be changed in many distributors) to lower the initial or idle advances. Fir example you may go to 20 and therefore only 12 to start the car (32-20).

Vacuum advance comes t play under part throttle - call it economy load. Since more timing can be applied under light loads (when there is higher vacuum) and it helps economy - the distributor advances the timing under those conditions.

Kory
 
Kory, getting the initial timing dialed into what the engine wants is just as important as the total mechanical timing dialed if your car is primarily street driven. If the intial is not dialed in the car will not idle smoothly, you will not have snappy throttle response off idle and it will be very difficult to dial in the carb to work well on the idle and transition circuits.

The method I suggested above using manifold vacuum as the guide to the ideal initial timing is the best approach. But most stock distributors have approxiamtey 24 degrees of advance in them so it will require either modifing the stock distributor or getting an aftermarket perfromance distributor that allows it to be easily adjusted.

For the most part if you have a cam that is truely suitable for street use then the intial will wind up at a value that won't kick back against the starter. If you have a radical cam that wants more than 20 degrees of initial and the starter kicks back then you will need to compromise or step up to one of the igntion boxes that accomodates a retard for starting.
 
I guess it depends what you are after - different strokes for different folks. What is idle? (-:

Have a good holiday

Kory


dgc333 said:
Kory, getting the initial timing dialed into what the engine wants is just as important as the total mechanical timing dialed if your car is primarily street driven. If the intial is not dialed in the car will not idle smoothly, you will not have snappy throttle response off idle and it will be very difficult to dial in the carb to work well on the idle and transition circuits.

The method I suggested above using manifold vacuum as the guide to the ideal initial timing is the best approach. But most stock distributors have approxiamtey 24 degrees of advance in them so it will require either modifing the stock distributor or getting an aftermarket perfromance distributor that allows it to be easily adjusted.

For the most part if you have a cam that is truely suitable for street use then the intial will wind up at a value that won't kick back against the starter. If you have a radical cam that wants more than 20 degrees of initial and the starter kicks back then you will need to compromise or step up to one of the igntion boxes that accomodates a retard for starting.
Click to expand...
 
Hey guys Thanks for all of your reponses. I have been fighting a stumble right off the line. First I thought it was fuel. After replacing the carb I still had the same problem. I then that carb gone thru by one of the local carb experts to check it for problems. None found. I then added a jetplate , notched float and jet extensions. I had one last trip to the strip to check it out. Better but still there along with fuel stravation on the big end. Currently I am replaceing the whole fuel system. New tank, 3/8 pick up, 3/8 line and carter strip fuel pump to feed the Holley 750 vac secondary. My times have been disappionting to me. I know the car should run faster. I wonder now with my timing being set at around 36 total with vac advance hooked up is my problem. My new tank came in today so I can finish it up and get it running again. Of course we are supposed to get another 6" of new snow tonight. Snow is better than NO electricity. We lost it for a week due to an ice storm.
Thanks Again Guys. Jeff
 
Try running more advance and see if you go faster. Keep adding more until you either slow down or hear detonation. Then, hook up vacuum advance to ported vacuum. Increase or vacuum advance until you get a little light throttle ping, and then back it out until it goes away. If you want to be scientific, and really dial in your timing curve, evaluate your mechanical advance in 1000 RPM increments. Obviously this is easier with a manual transmission. A slight uphill grade helps as well.
 
C130 you hit the mark I was waiting for. Just before spark knock is usually the best for mileage and power. Set with the vac disconnected first. Reconnect vac and try again. If you get ping you can try a vac pot from another engine or a delay valve. Old Jeep Waggys have a ton of them. Color coded. Usually the truck or/6 will have less total vac adv pots. If you have a good parts store guy he can look up the specs in the application/crossover catalogs under the counter collecting dust. I collected the old cats when I worked at a parts store. Have the GP Sor ones if you want to try that approach.
 
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