Vacuum advance pod
I ended up getting a VC-167. I know there's a spec chart out there for most of the cans, my question is more related to what would be the "right" one for a 340 with a pretty run of the mill cam, I usually have about 15-16" of vacuum at a steady state cruise rpm of 2,000. I have an OD so my RPM's are a bit lower than normal.
TrailBeast, I think, is on it too. He's got a pretty sharp tune going on in his ride for sure.
Firstly I'm gonna assume your power-timing and initial-timing and T-port sync are well-established, cuz the cruise-timing has to be the last timing to be tuned. I always map these on graph-paper, before messing them up. Then if I get lost, I can go back to what works.
Secondly;Detonation must not be allowed to occur period.
And then;
Well at your manifold vacuum and cruize-rpm, the the cruise timing might like to be 42 to 52 degrees. It might even like more depending on the build. But getting more is difficult with the factory parts.
But how do you know how much to give it?
Simple. Rev it up in neutral slowly until she hits cruising rpm, then block the throttle to maintain that rpm.Then start cranking in the timing in 3* steps, until the rpm no longer rises. Then reduce the rpm back to 2000. Now read the timing, and record it. Next, retard the timing until the rpm no longer rises, and read the timing. If it it only slows down, then advance it instead until the rpm peaks again. Don't try to find the teensiest change. If you crank in 3* and nothing significant changes, back it up to the last time it did. Badaboom, that 's gonna be pretty close. It could be over 50 and tickling 60, so you'll probably need a timing tape. If the ignition starts to crap out, it's probably because the rotor phasing is too wide,so you'll have to deal with that first.
Now, you'll have to figure out how to get there, from where you're at.
Furthermore, this Vacuum advance is gonna have to drop out pretty quick when you start applying throttle ,to keep the engine out of detonation, with part-throttle loading.So, inside the Vcan nipple is an allen-head adjustment screw which is used as a sort of regulator. You can crank that screw a little this way and that, to affect the rate of advance.You almost need a vacuum pump and timing lite to witness how it works. If you screw it too far, it will,additionally, start to reduce the amount of timing. If the screw starts jamming up;STOP!, you have reached the end of the adjustment range.
Most of the time, some sort of
sacrifice is going to have to be made.
With the stock parts getting more than about 42* gets difficult.I got this number by taking a centrifugal rate of advance of .8 degree per 100rpm beginning at 1000rpm; plus 12* initial, equals; 8 + 12=
20@2000. Then I added in 22* from the can, equals 42*. I found 12* initial to be sufficient in my combo. Your engine may like more. Probably not less. The T-port sync should have established it.
Of course cranking in idle-timing will upset your power-timing so you always have to keep that in mind, and after testing, you'll have to fix that.
About the very most you can get is say, 16/18 initial with a very fast centrifugal at all in by 2500, which could be say,18*, and so that could be;18 degrees in 1500 rpm or 1.2* per 100,
so that totals 30* at 2000. Add in 22 in the Vcan and that totals 52*. That is probably the max. So now we have established a range of 42 to 52, with a 22* can.
But YOUR combo may not like 30*@2000 under power. It may be wanting to self-destruct with detonation.
Detonation must not be allowed to occur period.
So until the cruize testing is complete, no WOT and easy on the pedal. Now it's road test time. This is a steady state test. It is run on flat level ground, in highest gear,and
at exactly the test rpm, and on a day with little to no wind with zero gusting.
So install a vacuum gauge plumbed into the V-can line, and hung where you can see it, then get her up to speed.Get your throttle set to where the mph remains exactly steady@2000rpm. Record the vacuum reading and the total timing including the Vcan timing. Turn around and go back to the start point of the test.Take out 3* of idle-timing, and repeat the test including reading and recording the vacuum and total advance.If the second test shows a higher vacuum reading, that means you were using less throttle to maintain 2000rpm.That would show up as less gas burned per mile. If it shows less vacuum, then that was the wrong way to go. Finally retest with another 3* change, but in the right direction.
If the vacuum reading differences are too difficult or too small to measure, replumb the gauge directly to intake vacuum, put the timing back to the original number and start over.
Once the cruize vacuum is sorted out, and if it's the same as the very first rev-it-up-in-neutral test, and you at that time reworked your power-timing, then you're done. But if it's not, or you didn't, then now is the time.
After this you will need to do a bunch of roll-ons to see if the rate of vacuum advance decay is agreeable to your engine, and adjust it with the tuning screw mentioned earlier.
If all this seems overwhelming, then just bolt on whatever Vcan you got and that's that. It will still be way better than no can at all.
But either way, the cruize fueling might want a little tweaking. Again rev it up to 2000 and block the throttle at that setting. Readjust the idle mixture screws for max rpm. Return the throttle to idle. Make a note of where the screws are relative to lightly seated.write it down. Now enrichen the mixture screws 1/8 turn, and roadtest.
To improve accuracy at determining the cruise rpm, I fabricate a throttle stop, and adjust it to limit the cruise rpm to exactly my target. Then by changing the timing, the mph will change. If the mph increases, then she liked the extra timing, and so on. My throttle stop is a home made device and is a two-stage, so I can get up to speed normally then back out of the throttle and engage the stop. I'd explain it, but
Rumble's already snoring.
Detonation must not be allowed to occur period.
