Manifold vacuum gives you engine vacuum through the rpm range. Ported vacuum gives you no vacuum until you uncover, by moving the throttle blades the port that the vacuum is attached. In other words if you have initial timing set at 8 btc it stays there if using ported until throttle blades move. If using manifold vacuum it advances at idle.
manifold vac vs. ported vac?
Little confused about the two. In the past I've always been told ported was better.
Most of you guys say manifold is better. I did switch to man. vac. that made my off idle stumble almost all went away.
My ? is won't the engine loss timing advance once you floor it loses a manifold vac. signal ?
Most of you guys say manifold is better. I did switch to man. vac. that made my off idle stumble almost all went away.
My ? is won't the engine loss timing advance once you floor it loses a manifold vac. signal ?
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Everyone seems to recommend trying both. I have only had luck with manifold. Like you said, you only get ported when the throttle blades are open, so you don't have any advance at idle.
It kinda depends on how your distributor is set up, too (mechanical advance and how much as well as how much vacuum advance your can gives you).
It kinda depends on how your distributor is set up, too (mechanical advance and how much as well as how much vacuum advance your can gives you).
In reality you need to give the engine what it wants, if the engine responds better to full manifold vacuum then give it that.Little confused about the two. In the past I've always been told ported was better.
Most of you guys say manifold is better. I did switch to man. vac. that made my off idle stumble almost all went away.
My ? is won't the engine loss timing advance once you floor it loses a manifold vac. signal ?
The purpose of the vacuum advance is to give your car more timing at lower rpms, which makes your car idle better, and smoother and really drive better. Once you get on the gas hard the manifold vacuum drops and the vacuum advance basically drops off because your vacuum is low, and in reality you don't need it at that point any way.
One of the reasons full manifold vaccum made your off idle stumble go away is because full manifold vacuum is giving your car more timing at those lower rpms compared to not of the ported vacuum.
Whats your timing at? Initial and total?
When experimenting with these two advance options, it's important to determine the vacuum reading at which your canister gives full advance. If you're going to use manifold vacuum, you want to be sure that you're getting 100% of the available advance at idle (both in and out of gear if it's an auto). Otherwise, you can end up with inconsistant idle characteristics.
For example, my MSD unit gives full vacuum advance at 15". I have a mildly cammed engine that produces as much as 15.5-16" at idle in park/neutral with no accessories running, so this would be fine. However, in gear my engine produces as low as 14-14.5" with the electric fan, etc. running, which then creates some instability in idle speed and a much bigger drop from park to gear. If I wanted to retain manifold vacuum, I'd need to change the advance canister to one that's rated lower than my lowest idle vacuum.
I didn't bother with with changing the canister though, because the engine is mild enough that I don't need the extra advance at idle. Driving the car around, I discovered that the road manners were no different with ported vs manifold in my application (but when vacuum advance was totally removed, it definitely had a sluggish feel at part throttle). This did not surprise me, as I didn't expect to feel any real difference between ported and manifold on the road, since I had already watched what happens with both using a timing light and manually working the throttle. The ported advance starts responding as soon as you get above idle speeds, at which point they are the same.
If I went with a more aggressive cam in the future, something that really needed the extra idle timing, then I would certainly consider manifold vacuum (and thus make the proper changes to my advance canister). But for now, based on my tests, it just seems unnecessary.
For example, my MSD unit gives full vacuum advance at 15". I have a mildly cammed engine that produces as much as 15.5-16" at idle in park/neutral with no accessories running, so this would be fine. However, in gear my engine produces as low as 14-14.5" with the electric fan, etc. running, which then creates some instability in idle speed and a much bigger drop from park to gear. If I wanted to retain manifold vacuum, I'd need to change the advance canister to one that's rated lower than my lowest idle vacuum.
I didn't bother with with changing the canister though, because the engine is mild enough that I don't need the extra advance at idle. Driving the car around, I discovered that the road manners were no different with ported vs manifold in my application (but when vacuum advance was totally removed, it definitely had a sluggish feel at part throttle). This did not surprise me, as I didn't expect to feel any real difference between ported and manifold on the road, since I had already watched what happens with both using a timing light and manually working the throttle. The ported advance starts responding as soon as you get above idle speeds, at which point they are the same.
If I went with a more aggressive cam in the future, something that really needed the extra idle timing, then I would certainly consider manifold vacuum (and thus make the proper changes to my advance canister). But for now, based on my tests, it just seems unnecessary.
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