ported vs manifold

[IrishCJ6]

I know it's been talked about before but I'm toying with the idea of switching my system from ported to manifold vacuum. It's an AMC 360, MC2100 all the smog kit has been removed and I've added a hei dizzy.

Question is what initial timing is everyone using, and what total amount of timing.

 

[Old Dog]

As a general rule with manifold vac. you want the timing to be all in with 32-36* @ 2500-3000rpm's with no Vac hooked up. (Depends on your engine)
With that in mind I have about 12* with no vac. at idle. With the manifold vac. hooked up it's around 22*.

 

[IrishCJ6]

Well had a play today, found that the vac advance gives me 25*, so to get a max of 32-36 meant pulling the initial back to 0. Seems to run good at that. Have to recheck the total again and could maybe advance a couple degrees more on initial. 25* seems a lot.

0* initial - vac disconnected and plugged.
32* total at 2800
25* at idle (750 RPM) with vac connected.

What you guys think

 

[harleydragon]

explanation of ported vs. manifold vacuum advance

Ported Vs. Manifold Source: Vacuum Advance - Corvette Action Center - Corvette Forum

 

[IOPort51]

the hardest part to get your head around is the idea of an increase in Vacuum, or in other words, more of nothing.

Manifold vacuum drops on acceleration and ported increases. less of nothing with manifold and more of nothing with ported.

 

[gert]

Well had a play today, found that the vac advance gives me 25*, so to get a max of 32-36 meant pulling the initial back to 0. Seems to run good at that. Have to recheck the total again and could maybe advance a couple degrees more on initial. 25* seems a lot.

0* initial - vac disconnected and plugged.
32* total at 2800
25* at idle (750 RPM) with vac connected.

What you guys think

At the end of the day it has to be what your motor likes best sounds pretty good so far. You should have an allen screw inside the vacum canister on your dizzy if you ant to add some initial mechanical timing in it

 

[Old Dog]

At the end of the day it has to be what your motor likes best sounds pretty good so far. You should have an allen screw inside the vacum canister on your dizzy if you ant to add some initial mechanical timing in it

sounds like your vac. advance might be pulling to much timing, I set mine to only pull 10* but that's what mine liked.

 

[thistle3585]

I have a NLVR (Non Linear Vacuum Regulator) which attaches to ported and manifold vacuum then goes to the distributor. As I understand it, this was an emissions control device. I was thinking I'd just disconnect it and go from manifold direct to distributor and cap everything else.

 

[IrishCJ6]

My total advance is incorrect, I didn't do it properly so ignore that figure,
Question is, is 25* vac advance normal, can this be reduced if so how? I know that once the throttle is applied the vac advance will reduce to zero and the mechanical will set in but if for argument sake I set the initial to 12* and find that I achieve 32* at full RPM (say 2500-3000 rpm), if I reconnect up the vac to the canister I will be at 37* at idle (12*+25*). Is that ok or too much?
I'm running on of these dizzys
AMC Jeep CJ5 CJ7 304 360 401 V 8 Hei Distributor 6512 R | eBay

 

[Old Dog]

Yes 36 at idle would be to much.
Your vac. can should not be pulling 25, there should be a way to adjust it to only pull about 10. Some you can adjust with an allen wrench and some you can adjust the rod.

 

[IrishCJ6]

I had another look at this today, I was right 25* vac advance at idle, so I disconnected the vac line and plugged it. Timed the engine to 14* initial and then ran it up to 2900 RPM to see what total I got which was 30*. So I'm getting 16* mechanical advance.

So after that I set about fiddling with the vac canister, I tried adjusting it (both ways) to see what change it made on the amount of advance. I got 20* one way and 25* the other way so still no where near where it should be. I opened up the dizzy to see what was going on and saw the vac advance lever and its adjustment. I then tried turning the canister adjustment and found then only thing that changed was the amount of vacuum required to operate the lever, I way softened it the other hardened it. I had seen on the interweb that you can buy/make a limiting plate and attach this to the dizzy. So that's what I did, it was very much trial and error on the length of the plate but eventually I got to 13* advance. (I had 8* on an early try and took 0.5mm off and went to 13* so very sensitive).

Here is a quick snap of the plate



So at the mo these are the figures:

14* initial
30* total (2900 RPM)
16* Mechanical
13* Vac advance at idle

The Jeep seems to run great, haven't been off roading but seems smooth and pulls happily, question is I'm still off the recommended 32-36* total, but if I advance the initial any more I'll be advancing the Vac too. Ideas?