Runs better with vac advance unhooked (Help)

[rich weyand]

Slow down and think for a second? I had this straight in my head 45 years ago, and the auto industry had it down pat 75 years ago.

I know that the mechanical comes in slowly. But base timing dominates at low rpms at high throttle settings. OK, I'll stop thinking in terms of mixtures and cylinder pressures and burn rates and milliseconds of advance, and think in terms of aircraft carriers if it helps. I want that catapult push, and so do naval aviators. Without it, they can't achieve airspeed. That's why the front of aircraft carriers is armored: so they can run over airplanes when the launch fails.

You keep saying that you are through with base timing so fast it doesn't matter. But that's not true. Hook that truck up to a trailer, or put a half-ton of cargo in the back, and you will be in base timing a long time before the mechanical comes in strong.

The base timing is to get the vehicle moving, to generate the most torque possible in the low-rpm regime, and the mechanical comes in slowly as the rpms come up to keep the available burn time up as 15 degrees of rotation means less and less time in milliseconds. So there are two regimes of timing there, with a ramp from one to the other. I understand this very well. I understood it in 1969, when I got my driver's license.

Ignoring base timing and having the mechanical advance come in way early to accommodate for low base timing is not the optimal tune.

I've done this a long time and know what I'm talking about. But we will never agree, so I'm done.

 

[350runner]

I've hooked up trailers hauled and all sorts... and you still don't understand where I'm coming from... you don't have the mechanical start early to compensate. You bring it in earlier to burn more of that lean mix... Not all engines will run off the same jetting or timing. Doesn't matter if they are built the same. There are other factors like altitude and such. When hauling or towing the engine load brings the rpms up and away from the initial that your now seeing vac and mechanical. It doesn't matter how long I've been doing something I'll always learn something new. Even the "auto industry" never ran that much initial... why because they understood the use of vac and mechanical advance. Good may you prosper. Just remember detonation can be silent...

Sent from the dust in front of you!

 

[SkinnyG]

20* base timing at 10.9:1! Whooeeee! Betcha you're running the best gas you can buy .

I are. The saving grace and/or need for such high initial is the large overlap on the cam.

With 8" at idle in drive, your vac advance ought to be set to drop out at 5-6" of vacuum so the timing doesn't hunt at idle. Did you make that mod?

I have an adjustable can turned down pretty much all the way, and it still hunts at idle. Not really optimal, but it makes the choppy idle a bit more dynamic which I find appealing.

When I went ported vacuum, the idle was smoother (no hunting), but I had to add enough idle speed screw that I was getting into the transition ports of the carb.

Idle with this cam is a serious mileage killer, so running manifold vacuum helps out.

Some folks with really rowdy cams just run locked distributors at full timing with no advance at all. Not a fan of that.

 

[rich weyand]

Yeah, the large overlap gives you a much lower dynamic compression at low rpm, which raises the detonation limit.

Yeah, crazy cams can respond well to various timing tricks. I'm trying to remember some of the stuff we pulled back in the day. Mostly, I think we disconnected the vac advance on very big cams.

The reason it didn't hunt at ported vacuum is you weren't running any vacuum advance, so no hunting. And yeah, that will kill mileage. Since the vacuum advance can is not a switch -- it ramps up -- I'm not sure you can dial a can down enough not to hunt at 8" and still have the vac advance all the way out when you get on it. Not a lot of room there.

 

[rich weyand]

I have an adjustable can turned down pretty much all the way, and it still hunts at idle. Not really optimal, but it makes the choppy idle a bit more dynamic which I find appealing.

I've been thinking on this on the mental back burner, and I have a solution.

The problem is that you really only want vacuum advance when the throttle is closed, but you can't do it with vacuum. The vacuum window is too small, and you can't ensure the vacuum advance would be completely out when you were on the throttle, which would be bad.

But, I think you can mod a vacuum mech to run with a solenoid instead of a vacuum canister, and run the solenoid with a throttle position switch. When the throttle is closed, you pull the solenoid into vacuum advance. When the throttle opens, the solenoid springs back into its rest position.

You want the active direction of the solenoid to be advanced, so that if the wiring breaks or the solenoid fails, you don't go into advance. The default position of the solenoid should be no "vacuum" advance.

This setup will not hunt, and will also go into advance when you take your foot off the gas while coasting, improving driveability.

For the throttle position sensor, GM used a switch on the 1969 Camaro cowl induction setup that is pricey as NOS but there is an aftermarket reproduction that's pretty cheap. It contacts the other way -- making electrical contact when the throttle is opened past a certain point -- but could be mounted oppositely, I'm sure. It is the same switch as the 4WD indicator switch for late 70's NP203 xfer cases.

The solenoid could be something like the idle solenoid.

Here's the switch:
http://www.ebay.com/itm/1969-Camaro...1969|Model:Camaro&hash=item3386f2fa51&vxp=mtr

You would have to see how much pressure it takes to depress the switch and if the throttle return spring could handle it. Or you could use a microswitch.

Anyway, that just occurred to me: ditch the vacuum canister and run your "vacuum" advance electrically off the throttle being in or near idle position.

 

[73 C10]

Wow! I thought I would pop in for a quick update, and I had quite a bit af reading to do to catch up.

I'm getting somewhere! I had my doubts but not anymore. I asked for advise and I am trying to follow it.

I have to guess, but I think with the spiral limiter at the starting position, I was getting 15* or more. I spun the limiter around 3/4 of the way and am now getting ~10*. I also changed the springs from blue/silver (800-2800rpm) to blue/yellow (800-2600rpm). The vac can is adjusted fully clockwise. I dropped the base timing back to 12* to start. I need to find out just how much my mechanical is actually. I will be making some timing tape to figure that out still.

It's running better. Almost like with no va but it starts better. I'm not done yet but it is looking like I'm going to win.

Thanks for sticking with me. This stuff has been fun to learn. I'm grateful that guys like you are willing to help a guy out.

Rich, you might like this:
http://www.superchevy.com/how-to/engines-drivetrain/0411sc-crane/

 

[rich weyand]

Rich, you might like this:
http://www.superchevy.com/how-to/engines-drivetrain/0411sc-crane/

That's pretty cool.

A fellow could put this on and try all kinds of stuff without taking anything apart to change springs and weights and cans. Just turn a couple of selectors.

Heck, you could drive it all week on one set of settings, and then dial it in to a completely different tune for the weekend in a couple minutes.