Ported vacuum vs. full manifold vacuum
This is probably more curiosity than anything, but what is the correct vacuum port to hook up a vacuum advance distributor to? I have a SBF 302 with Pertronix ignition, CDI, coil and FiTech 30003. The more I read about the difference between the two, the more I get confused. In the instruction manual for the 30003, its says to use ported vacuum for distributor advance. Other places I have read that it is best to use full manifold vacuum, hence my confusion. Thanks,
Tim
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The main difference is that with manifold vacuum you have full vacuum advance at idle. With Ported vacuum there is no vacuum advance at idle. Different motors like different setups. I use manifold vacuum when I am not using computer controled timing . But I would suggest that long term use the computer control. Billk
Bill,
The 30003 doesn't allow me to have FiTech control the timing. With that being said, should I use in manifold or ported vacuum?
Tim
99% of the time with even a mild performance engine, use the manifold (full) vacuum port for your ignition timing. Generally, a 302 will like about 12-14 degrees advance for a base setting with the vacuum disconnected, and about 32-34 total (depending on compression, camshaft, elevation, etc.) Start with 10 as a base to be safe while your efi tunes, as running lean and too much advance don't make for a happy engine. Ported creates a slower curve by allowing the timing to come in slower, which is fine for a stock engine, but you want that timing there on a modified one. When you are setting base timing, the engine is going to run rough until you hook up the vacuum advance, which is normal and fine. Just something I figured I'd throw in there so you're not surprised.
Thanks for the info! I am using the vacuum advance on my Pertronix magnetic trigger distributor and changed the mechanical advance springs out to advance a little earlier. I think I am running around 12 degrees for base timing right now without any issue. I will take the car out and drive it to see how it feels with the copper springs in place.
For full manifold vacuum, do I just need to run a port from the manifold itself? I have a few open ports I can use.
Tim
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Yes, you can just hook it to one of the nipples on your intake manifold. Just cap off all others that aren't in use.
Make sure to listen for pinging with the light springs in. All timing in at 2000 rpm is a pretty aggressive curve. If you've got your timing "all in" before 2500, you may need to slow it down a little bit. If you do hear pinging, maybe try a copper/silver combo.
Quote from 1972Mach1 on September 5, 2019, 3:57 pmMake sure to listen for pinging with the light springs in. All timing in at 2000 rpm is a pretty aggressive curve. If you've got your timing "all in" before 2500, you may need to slow it down a little bit. If you do hear pinging, maybe try a copper/silver combo.
I just took it for a test drive and everything sounded great. No pinging and it was definitely a little more responsive with that curve. I will need to pickup a hose adapter later to run the vacuum off of the manifold. I will play with it more and look at spring combos or changes after that, but so far, so good.
Thanks!
Tim
You can also use the 3/16 nipple in picture 4 of your original post if you don't have it going to your transmission vacuum modulator or if you are running a manual.. Does the same thing as going directly to the manifold.
I have a manual transmission, but that connector is taken up by a vacuum line that runs to the side of the throttle body. Looks like the one in the pic below.
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