Vacuum advance information and myths
Ok here's something for your perusal;
My alloy-headed 367 has had 3 cams in it since 1999. At .050 they were: 249, 223, and 230 now. The only things that I changed between those combos, was the Scr, and the Q, to keep the Dcr at about 9.0, and the pressure to, in the range of 180 to 185psi which I knew ran on 87E10 without detonation.(altho I ran the 223* at 194psi for several weeks).
All of them ran on the same 750DP.
The only changes made to it, was the size of the Idle-Air bypass holes that I drilled in the primary plates.
The only adjustment differences were to the amount of Transfer-slot exposure and a minor change in mixture screw adjustment.
They all ran the same distributor, and altho I tried many many different timing curves; I kept coming back to the same total-timing, the same initial-timing, and the same ported V-can.
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I like to idle thru the A&Ws (etc) at walking speed.
To help with this, I have a 3.09Low manual trans with 3.55s in the back, for a starter gear of 10.97 (this is like having 4.11s with a regular 2.66low trans). To get to 3.5mph, with this combo, requires the idle to be ~500rpm. The only way to do this with a Dcr of 9.0 (or better) without the chassis getting jumpy, is to retard the timing, until peak cylinder pressure occurs way down on the powerstroke, and the engine becomes smoother, and has just enough power to pull itself and none extra.
In my combo this comes at ~5* Idle timing.
But, it's normal idle-timing 12>14, with 20* in the Distributor-cam, for a maximum mechanical timing of 32>34*
The Vacuum advance is modified to deliver 22* .
The timing curve is a two-stage, with a kink in it, like the one Mattax posted.
The rate of advance is .8* per 100rpm, beginning at around 900rpm, and it's all finished advancing on the primary spring, at 2800; the amount being ~14>15 over base. So I usually see 28/29* @2800. From there to all in, then is ~4/5 more.
Knowing this, It is possible to graph this out., and see exactly how much timing the engine is seeing at any rpm. And, It is possible to set the Vcan to add it's timing to that amount. At low rpm with a big cam, the vacuum is low, so it pulls in only a small amount of V-can timing. As the throttle is opened, the engine vacuum rises, bringing the V-can timing with it, until about 2000>2400, where the vacuum peaks. and so my V-can is adjusted to roll in smoothly, and be all in at about that time.
Now then, my combo cruises at 65=2240 rpm in overdrive.
The mechanical timing will be ~11*, add the base 14* = ~25*, add the Vcan 22*, for a cruise timing of ~47*.
2240 in Second gear is 26mph and the Power timing is 25*, and you might think my 367 is gutless, but think again; (your 340 might be, lol that's joke guys). Well cruising at 2240 she has 47*. Dialing in the throttle bit by bit, it falls steadily to 25*. But because the engine makes 180psi CCP at cranking and who knows how much more at 2240, and is running in 1.92 x 3.55=6.82 Second gear, she has gobs and gobs and stinking GOBs, of torque and easily spins the 295s. Do I need more timing? Not hardly...
At 2800 in Second gear (33mph), the timing can be varied from 50>28, by the gas-pedal.
At 3400 in Second gear, (40mph), the timing can be varied from 56>34*
You can imagine my opinion of running a locked distributor ...on the street; and you can imagine my opinion of not running the Spark-Port.
>Oh but you say, it works the same.
Sure it does, but at different gas-pedal openings, because the Spark-port works differently than the Manifold-port. And with a manual trans, and a high-pressure design, it is easy to have too much power at idle/off-idle, and then it gets very hard to drive slowly, with a manual trans..
>To fill-in my modest timing curve, I have a spark-delay box in the cabin with an adjustable range of 15*. I set it in such a manner as to get up to 9* retard and 6* advance. This makes it possible to set the base timing from the cab, from ~14* down to about 5*, and the cruise timing from ~47 to ~53@2240 rpm. The engine likes cruise timing of up to 60* at 2850rpm, but with overdrive, that is pushing 85mph, which is well over the 68MPH max speed-limit in Manitoba.
Sure I could recurve my distributor one more time, and get rid of the Timing Box, but it's working so fine as it is.................
>Finally, the 292/292-108 cam was in my 11.3Scr version of this 367. It was none too excited to idle at 500 in gear and pulling itself. But it did pretty good at 550 (4mph), so I learned to toe the clutch a lil. This is one of the reasons I pulled that cam out after just a few weeks.
> Here's my point:
I say smart people build engines that don’t need 30, 40, 50 degrees of timing at idle to run.
I'll admit that I ain't the brightest bulb in the box,
but my elevator goes to the top. And
I know that a 292/292/108 cammed, 11.3Scr 367, can be tuned to idle at 550 rpm in First-gear , and pulling itself , with just 5 measly degrees of Idle-Timing........ because I had one that did this. and
My 230@.050 will do it at 500rpm@5*
Listen to what the engine is telling you....... Don't force it. Be patient.
BTW:
If you care to try it;
Just run your tach up to 2400 rpm and keep it there. Then, without regard to the actual numbers, advance your timing and return the fast-idle to 2400. Repeat until additional timing does not cause any increase in rpm. When you get there, read the timing at 2400rpm.
Don't be surprised to see numbers in the 40s or even 50s. That is the optimum timing for your engine at 2400 unloaded. Put the timing back.
You can do this at say every 400 rpm from 3600 down to idle, and plot the numbers on a graph, then connect the dots and smooth it out.
Now you have the Optimum no-load timing curve for your engine. Use it as a guide to build up your PowerTiming curve, to cruise-timing; plus/minus say 200rpm. Adjust the Idle Timing up or down to best make it happen. Remember, the closer your cruise timing is to optimum, the higher is your potential for best fuel-economy at that rpm.
What your Idle-Timing is relatively unimportant because it can NEVER be optimum anyway ...... because if you try to make it so, the Idle-Rpm will run away on you, ever higher, and the transfers will be closed or nearly so, and the mixture screws are likely to be WFO, in compensation. Oh sure, she's "idling" all rightee...... but just try putting her into Drive and taking off ....
Anyway
These are my experiences, offered for perusal. What anybody actually does/does not do, with this information is of no concern to me.
Happy HotRodding
