Longgone
John/68 Barracuda & Dart
Is anybody running a vacuum advance on their performance engine? If so what timing are you running, initial, mechanical, and vacuum? Total timing on moderately built 440-6?
Vacuum advance
- Thread starter Longgone
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dgc333
Well-Known Member
Vacuum advance has no impact on the timing in an engine at idle or WOT. It only adds timing under light throttle cruising conditions to improve the economy of the engine.
If you don't drive on the street or don't care about gas mileage then there is no reason to use it. If you do then you should spend the time to sort it out for your combo.
Initial timing is going to be a function of how much compression and how big your cam is and can run anywhere from 5 to as much as 20 degrees before top dead center. You find your initial timing by adjusting for max vacuum at idle.
Total mechanical timing is going to be a function of the design of the chamber and ports in the head. A 440 with open chamber heads is going to want something on the order of 36 degrees. The only way to get it right is on a dyno or with many back to back runs on the strip.
Vacuum advance should be able to add another 15-17 degrees to your total mechanical. The amount is fixed in the can and is stamped on the arm (double the number for crank degrees). You dial it the point at which it starts to advance by the allen screw inside that is accessed though the vacuum line connection. You dial this in by driving the car. You do not want any detonation at throttle tip in.
If you don't drive on the street or don't care about gas mileage then there is no reason to use it. If you do then you should spend the time to sort it out for your combo.
Initial timing is going to be a function of how much compression and how big your cam is and can run anywhere from 5 to as much as 20 degrees before top dead center. You find your initial timing by adjusting for max vacuum at idle.
Total mechanical timing is going to be a function of the design of the chamber and ports in the head. A 440 with open chamber heads is going to want something on the order of 36 degrees. The only way to get it right is on a dyno or with many back to back runs on the strip.
Vacuum advance should be able to add another 15-17 degrees to your total mechanical. The amount is fixed in the can and is stamped on the arm (double the number for crank degrees). You dial it the point at which it starts to advance by the allen screw inside that is accessed though the vacuum line connection. You dial this in by driving the car. You do not want any detonation at throttle tip in.
demon seed
The Original Demon Seed
On the 383 i"m running 14 deg initial and 36 total with no vacuum advance and all in at 2000 rpm. Seems to like it. Starts easy, goes like snot. :burnout:
Longgone
John/68 Barracuda & Dart
I`m currently running 17* initial, mechanical 18*, set with a vacuum gauge and tach @idle and 3000rpm . If I run the vacuum advance I`ll end up running total advance in the 50`s. The car seems to like a total around 36-38*. I run the car on the street. Should I just cap it? Is there any advantage in running it?
dgc333
Well-Known Member
Low 50's with vacuum advance is just where you want it. Remember the vacuum advance only adds timing at steady state cruise when there is high manifold vacuum, it adds no timing under WOT acceleration when the manifold vacuum is near zero. The additional timing is needed to light the mixture early enough to get a complete burn when you are not getting complete cylinder filling.
If you have a vacuum advance and you run on the street you should be using it. It can give you up to 2 mpg additional fuel mileage.
If you have a vacuum advance and you run on the street you should be using it. It can give you up to 2 mpg additional fuel mileage.
C130 Chief
Mechanical Genius
Well, try hooking it up and see what happens. Best way to know how much help it will be. Make sure you connect to PORTED and not manifold vacuum or you will have way too much advance at idle. If you run on the street at all, you want to use this.
Longgone
John/68 Barracuda & Dart
I`ve played around with using the vacuum advance and then not using it. A fellow at the local speed shop said that they`ve dyno`d many a 440 and that hp actually drops off after approx. 38*. Others say run the vacuum advance on the street for better gas mileage and driveability. I`m having some problems with spark knock using the vacuum advance with a 50/50 mixture of 93 and 110 octane. I`m still experimenting!
dgc333
Well-Known Member
For the thrid time, if your engine makes it's best power with 36 or 37 or 38 degrees of mechanical advance then that's what you set it for. The vacuum advance will have no impact on that, it only works at part throttle under light cruising conditions. If you were to dyno the engine the vacuum advance will not have any impact on how much power the egine makes because it will never add any timing.
If you get detonation at throttle tip in or under steady state cruise then the vacuum advance is coming in to much to soon. There is a allen head screw down inside the vacuum pod that is accessable via the port on the pod. Adjusting this either increases or decreases the preload on the spring inside the unit. This will adjust the amount of vacuum needed to start adding vacuum advance and the amount of vacuum necessary for all the advance (a number is stamped on the unit telling your the amount of advance, double this number for crank degrees). The other thing to check is to make sure the advance returns quickly when released, you do not want it hanging up.
You should get a vacuum gauge and characterise what the manifold vacuum is under different driving conditions then get a mighty-vac and characterise the advance unit. Then you can start to make educated adjustments to optimze the settings.
BTW, you should not be needing to mix 93 and 110 gas on a "moderately" built 440, seems like you may have too much compression to start with.
If you get detonation at throttle tip in or under steady state cruise then the vacuum advance is coming in to much to soon. There is a allen head screw down inside the vacuum pod that is accessable via the port on the pod. Adjusting this either increases or decreases the preload on the spring inside the unit. This will adjust the amount of vacuum needed to start adding vacuum advance and the amount of vacuum necessary for all the advance (a number is stamped on the unit telling your the amount of advance, double this number for crank degrees). The other thing to check is to make sure the advance returns quickly when released, you do not want it hanging up.
You should get a vacuum gauge and characterise what the manifold vacuum is under different driving conditions then get a mighty-vac and characterise the advance unit. Then you can start to make educated adjustments to optimze the settings.
BTW, you should not be needing to mix 93 and 110 gas on a "moderately" built 440, seems like you may have too much compression to start with.
Longgone
John/68 Barracuda & Dart
[If you get detonation at throttle tip in or under steady state cruise then the BTW, you should not be needing to mix 93 and 110 gas on a "moderately" built 440, seems like you may have too much compression to start with.[/QUOTE]
You are right ......too high C.R..
You are right ......too high C.R..
Longgone
John/68 Barracuda & Dart
The car had some very nice pistons that I just couldn`t see replacing. With the proper cam and combustion chamber size they came in at 11-1 C.R..
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