Later timing also makes stick-cars with 3.55s, less jumpy, at or below idle speed.By lighting the fire a lil later the peak pressure in the chamber occurs a lil later with the piston already well on it's way down. This smoooothes the power delivery to the crank, and makes the engine a lil lazier to respond to throttle inputs.This makes it possible to run the engine down below idle speed with those hi-way gears, and allow you to perhaps parade the car with 3.55s instead of a much bigger gear. Comfortable walking speed is about 3.5 mph. Just try to drive your 3.55 stick down there. This is 410rpm; it ain't gonna happen.About the lowest my 360 230* cam engine will go before the vacuum is so low that the PV starts messing up, is 500rpm. With 3.55s and a 3.09 low, this is 3.66mph.
At 14* timing @500rpm,the car starts to buck like an old '80s Mustang. At 6* it still ain't smooth enough, so I took the CFII disc out and put in a nice Factory 340 disc.With the softer sprung hub,the reduced timing, and a big fat flywheel, things settled down enough to make it a doable thing, and I sold the 4.30s.
When my car was all shiny and new, parading was the thing to do. Not so much any more. Now I just do it cuz I can,lol. 3.66mph is still kindof a power-walk.
The power-timing window is very small. With iron heads it usually falls between 32 to 38 degrees. With top grade gas,and a well engineered chamber it may fall in a slightly tighter window of 34 to 36 degrees.
But the idle timing and rate of advance can be just about anything. With an automatic and a 2200 type stall, you can run a lot. But when it is just 1 degree too much, stuff makes noises and eventually breaks, so keep the engine quiet. Better it is to be even 5* retarded than to be 1* too advanced.
As to the vacuum advance, that baby is a gem.Yeah sure you can use it to good advantage in extracting every last bit of energy from a lean mixture and drive fuel economy to the max.
But there are other advantages. Remember that this device is almost in lock-step with manifold vacuum. And remember that leaner mixtures are possible when you start the fire early. Not only possible but the leaner mixtures demand the fire to be lit earlier.And the leaner mixtures are often where the torque is at,with these low throttle/high manifold vacuum settings. The lean mixture/early fire, seems to smooth the engine in low-speed, low-load applications such as cruising above about 1800 to around 3200. When I say cruising, I'm not meaning steady-state speed but rather just tooling around in traffic. This is where the engine could be wanting 40 to 50 or more degrees of advance, and there is just no way to get it without the Vcan. The engine will develop more footpounds with the extra timing, allowing you to reduce the throttle opening, and that means, you guessed it, less fuel being dumped into the intake.So here you have a double whammy; more torque on less fuel. What's not to like about that? And remember where a streeter spends the bulk of it's life; hyup, tooling around!
IMO, if you are not running a Vcan on your streeter with a typical street cam, you are either; unaware of the advantages,not talented enough to pull it off, or just don't care enough to pursue it...... I used to say; ignorant, stupid or just don't give a liquidfart. But that's just plain mean, and nobody like a meanie;and I prefer to be liked.
So now, you can no longer claim unawareness, and the talent you can learn.As to caring enough to pursue it, I guess we'll see.
BTW;
The window of operation of the Vcan is pretty narrow.I suggest you put a vacuum gauge on the spark-port and check it out while driving. You will see that from idle to well above,that port remains pretty silent. Then it begins to build vacuum and eventually catches up to manifold vacuum. Then it runs lock-step with manifold vacuum for a while, before starting to drop out. By the time the primaries are open enough to engage the secondaries, the spark-port vacuum should be dropping quite fast, perhaps already dropped right out. What I'm saying is that this spark port vacuum might only be working from about 20%primary throttle to a bit over 50/60% primary throttle, AND it may be highest over a very very narrow window of say 25/30% to 35/40% I'm being purposefully vague, cuz I've never actually measured the throttle opening. But since the spark-port is physically fixed in the throttle-bore, the amount of vacuum measured at the port is always related to where the throttle blade is in relation to it, can never exceed manifold vacuum,always reads highest when manifold vacuum is highest, runs lock-step for a bit and then joins the manifold vacuum in its declining stage, but dropping out faster.
So now, it's up to you to take advantage of this knowledge, and give the engine what it craves, when it is craving. If you start with too much idle timing you may limit the amount of Vcan timing possible. If you don't start with enough idle-timing, you may never reach the optimum timing. Therefore it follows that if you really want to find the optimum cruise timings, you need to start with an optimum idle-timing and rate of advance. To try and help you find that optimum idle-timing, you need to sync the transfer port. I believe this was already mentioned. You can check out the Holley website, or click on my signature down at the bottom of this post.
Do not be overcome if you see the cruise timing start to climb above 40 or even 50 degrees. And as you start leaning the cruise circuit out, you may revisit the vcan and find that it wants even more. Eventually you may find the total timing at a cruise speed of say 60 mph, wanting to be close to 60degrees. In all likely hood your dizzy won't go there. That doesn't mean the engine won't be happier with 60* But most will only go to 36 in the mechanical plus 24 max in the can for a total of 60*. But seldom does this happen simultaneously . More typical is 28* mechanical at 2800 rpm plus 24 in the can =52*; this being about 65 with 3.55s . But if you're running 3.91s and 65mph is 3200 rpm then it is possible that everything is all in and then that would total in the neighborhood of 60*. It's never enough until it is too much. Don't let it be too much, cuz stuff breaks.It's always better to be a little short on timing than a little too much!
