Chebbies have 'always' been full manifold vac while Fords are port vac. Set your static timing with it disconnected and the vac hole plugged and you will be amazed, as you say, at the difference in performance.
Dave

 

Every Chevy I ever fooled with has been like this, they do not like ported vacuum. My former 350 with vac advance connected to a ported source made it a complete turd, it would barely burn a tire holding it against the brakes, it had so little power I was afraid to drive it, and it had 041 heads, headers, stealth intake, a small RV crane cam, 3.23 gears. I connected it to a manifold vac source as BOBCRMAN of this site suggested and I couldn`t believe the power difference. But, you`ll likely run into many on this site that are going to argue the fact with you as I have run into in the past. The need for vac advance at idle is because the mixture is lean, and leaner mixtures burn slower, thus the need for more spark advance. Further proof of this is my car ran about 15 to 20 degree`s hotter with it connected to a ported source.

 

On my 70 chevy truck, I used ported vacuum for years but have recently converted to using manifold vacuum. I like it much better! It runs much leaner at idle (doesn't stink anymore) and has better part throttle response.

 

vacuum advance on the ported source is an emissions setup. Makes the timing redard at idle which increases exhaust temperature to burn off hydrocarbons or whatever. Works ok with tiny cams and low compression (80's smogger engines)

 

This info would be good on another site I know of !! Right Dave ??

Got lost in cyberspace did ya Denny !!

Unfortunately that other site is at least temporarily "dead in the water" or was an hour or so ago.

Amazing what we Fordnatics have to do for these Chebby guys isn't it

Dave

 

The mechanical curve and base timing need to work with the manifold vacume, for those running 20 degrees base timing and 20ish mechanical advance there might be problems with running full vacume advance of 15 (now 35@idle) at idle. Gearing and cruize rpm also play in as well as engine size, weight of vehicle, etc...
You can find out what your combo likes at cruize just by playing around with the base timing, then get that amount of timing with the curve and vacume advance. Run some vacume line into your passenger compartment with a guage and check how much vacume is there at cruize on both manifold and ported to determine which will work for your application. The total combination needs to be considered.

 

Under modest to hard acceleration, the vacuum advance drops out completely whether hooked to ported or manifold source. Under steady cruise conditions, it does not matter what source you are hooked to, the advance will be working, all things considered anyways.

Well, that's what I thought as well, until now. I have a mighty Demon 750 carb, and it seems that at cruising speed, the throttle plates were not open far enough to actually get a vac. signal through the port vac. source.

If you have a vacuum guage, it's an interesting experiement to first test your can to see exactally were it starts to pull and at what reading it's at full pull. Then "T" into your advance hose and run the guage inside and go drive it and make notes of where you "should" have advance and where it doesn't work. I did this some time ago on my truck when trying to set up my new HEI. I amazed me just how little throttle was needed to drop the advance completely out. But it makes sense, fat mixture does not need all that advance.

Yeah, I could try this, and I have a datalogger so I could do that too. I have the Crane adjustable vac can, with it all the way loose, since the motor only has about 9" Hg idling on a good day. And I obviously want the can pegged at that vac. level. I actually have to run the idle a little higher than I want to keep the vacuum up far enough that it doesn't drop the vac advance and die.

My 48 Ford with the 355 really loves the full time vac source on it. It runs great that way, but then it has much less cam in it and more idle vacuum so that I don't have to fight to get the idle right.


** On a side note, I can't seem to get a nice clean idle on the camaro now that I have the vac advance hooked up. It's always smoking and smells rich - and I didn't have that problem with the vac advance hooked to port vac.. But I was trying to tune it with the vac advance hooked up - should I have it disconnected then try to idle it down like was mentioned? It seems like I would be chasing my tail on that one. I got a nice square of the transition slots showing and I don't really want to mess with the butterly positions. I just adjust the idle with the mixture screws and the IDLE-EZE doodad.

Any thoughts on the order of tuning for full time vac advance? I seem to struggle with it some.

 

If your idle circuits are working correctly than if you turn in one side of the ilde mix screw all the way to lightly seated it should stumble/kill the motor. If the idle is not responsive to this than it's not pulling fuel off the idle circuit.

It should take you about 34 seconds to dial in the idle on a correctly working carb.

 

If your idle circuits are working correctly than if you turn in one side of the ilde mix screw all the way to lightly seated it should stumble/kill the motor. If the idle is not responsive to this than it's not pulling fuel off the idle circuit.

It should take you about 34 seconds to dial in the idle on a correctly working carb.

oh yeah, it will definitely kill it, tried it last night to make sure. I just don't understand why I can't get rid of that faint smoke. That means rich, so if I lean the screws out more (already at less than 1 turn each) then the motor wants to die. If I add more air with the idle-eze vacuum leak deal, then it gets to be a problem to start unless its super warmed up.

I have already drilled the IABs out to .078 P and .076 S for a leaner cruise A/F ratio for some mileage.

With a cam sized like mine, should I just expect a certain amount of "smoke" at idle?

I just don't recall seeing that before changing it over to full time vac advance.

 

I've read through the posts..........what carb are you using?? cam specs?? and distributor/vac setup??? , John

 

I've read through the posts..........what carb are you using?? cam specs?? and distributor/vac setup??? , John

Mighty Demon 750 with annular boosters on a single plane Team G. Cam is a 240/248 at .050 hyd. roller (Crane 306/314 adv.) .590 lift with 1.6 rockers, MSD 8361 dist. with vac advance, Crane Cams adjustable vac cansister with limiter plate set to 12 deg. vac advance, Mallory HyFire VI ignition box, MSD blaster2 coil.

 

The cam seems fine for this set-up. Obviously there is conflicting ideas on the "ported vs. manifold vac". Manifold vac looks like it may be better for you, though I run mine ported. (personal preference & slightly less on the cam specs). I'm a Holley guy so I think it's best to refer to one of the Demon experts.

Try looking at www.4secondsflat.com, tech articles. Under carburetion there is Demon carb tuning 101. I think you can order it for ten bucks online and they'll e-mail it asap.

If you can solve this problem and get alot of other useful info it's a worthy $10.00 investment. Hope you get this solved soon as it drives me nutso when my engine is not working correctly.

 

hotrodf1,

A couple of things. First it's been pretty common on SBC performance engines to connect the distributor to direct intake manifold vacuum. This increases the timing at idle, and usually helps clean up the idle from a large camshaft. The only downside to doing this is that on some combinations you can get too much timing at very light cruise, but since you have an adjustable vacuum canister this shouldn't be an issue. If you're idle RPM jumped up that much going from the ported vacuum to constant vacuum you're idle circuits are most likely still too rich. What was your Innovate giving you for idle, and part throttle AFR? It doesn't make any sense for the engine to be smoking now with more timing. The increase in timing may have increased the RPM enough to draw more fuel. The first thing to do would be to drop the RPM back down, and then possibly lean out the idle a little more.

 

Maybe it's oil smoke? I seem to recall he said he could get it to lean out by screwing in the screws but it was still smoking. Maybe tranny fluid too? Just a thought.

Well, guess I couldn't rule it out. But the motor only has 3K miles on it. I don't recall it smoking before when i had the dist. connected to port vacuum.

Have a 6 speed in the car, so I don't know how it could be tranny fluid I guess.

 

hotrodf1,

A couple of things. First it's been pretty common on SBC performance engines to connect the distributor to direct intake manifold vacuum. This increases the timing at idle, and usually helps clean up the idle from a large camshaft. The only downside to doing this is that on some combinations you can get too much timing at very light cruise, but since you have an adjustable vacuum canister this shouldn't be an issue.

Yep, have the limiter plate set to 12 degrees adv. I originally had it hooked to manifold vacuum, but wanted to be able to disconnect it at the track, which then would require a retune, then tune again when I'm done at the track and ready to go home.

If you're idle RPM jumped up that much going from the ported vacuum to constant vacuum you're idle circuits are most likely still too rich.

Hmm. never thought of that before. Guess I always thought the idle sped up due to the more complete burn of the mixture.

What was your Innovate giving you for idle, and part throttle AFR? It doesn't make any sense for the engine to be smoking now with more timing.

Well, the LM-1 is out of the car right now. I took it out so that I wouldn't have to deal with it on the Power Tour. Had enough things to think about besides it

The increase in timing may have increased the RPM enough to draw more fuel. The first thing to do would be to drop the RPM back down, and then possibly lean out the idle a little more

I do have to idle the car higher to make sure it produces enough vacuum to keep the can "pegged."
I know that the vacuum at idle is higher with the extra timing than without it(at the same RPM) I gues due to the butterflies being closed more. I was thinking that perhaps the extra vacuum is pulling more fuel into the motor. Is that possible? Seems like it could happen.

It seems like if I try to lean it out, it just looses power, idle becomes more erratic sounding, and if the fan comes on the extra draw will kill it sometimes.

And if I open the idle-eze valve to give it more air (to lean it out presumably), then it just wants to idle faster again, which makes it tend to roll-on when I kill the ignition.

perhaps I need smaller idle feed restrictors? I guess I'm surprised that tuning this motor with the full time vac to the dist. is so much different than with the port vac source. My IABs are already larger than stock to lean the part throttle out, I have .078 and .076 bleeds (.070 is stock). MABs are .029", stock is .036"

Now when I say smoke, I'm not talking lots of smoke. just a very faint "fog", but it smells rich. If my wife can tell me it's rich, then it's rich ya know?

I guess the LM-1 is going back in the car tonight. . .

 

The cam seems fine for this set-up. Obviously there is conflicting ideas on the "ported vs. manifold vac". Manifold vac looks like it may be better for you, though I run mine ported. (personal preference & slightly less on the cam specs). I'm a Holley guy so I think it's best to refer to one of the Demon experts.

Try looking at www.4secondsflat.com, tech articles. Under carburetion there is Demon carb tuning 101. I think you can order it for ten bucks online and they'll e-mail it asap.

If you can solve this problem and get alot of other useful info it's a worthy $10.00 investment. Hope you get this solved soon as it drives me nutso when my engine is not working correctly.

I do have that little book. It has good info for sure. I guess i could go back and read it again. But he is the one who gave me the idea of disconnecting the vac adv. for flogging just because the vac adv. doesn't react fast enough to keep from possibly damaging the engine due to too much adv. when you hammer the loud pedal.

 

Yep, have the limiter plate set to 12 degrees adv. I originally had it hooked to manifold vacuum, but wanted to be able to disconnect it at the track, which then would require a retune, then tune again when I'm done at the track and ready to go home.

Why did you want remove the vacuum advance at the track?

Hmm. never thought of that before. Guess I always thought the idle sped up due to the more complete burn of the mixture.

Once you get close, changing the burn rate normally wouldn't vary the RPM much

Well, the LM-1 is out of the car right now. I took it out so that I wouldn't have to deal with it on the Power Tour. Had enough things to think about besides it

I hear that, try running a car on the street with all 8 Lambda's

I do have to idle the car higher to make sure it produces enough vacuum to keep the can "pegged."
I know that the vacuum at idle is higher with the extra timing than without it(at the same RPM) I gues due to the butterflies being closed more. I was thinking that perhaps the extra vacuum is pulling more fuel into the motor. Is that possible? Seems like it could happen.

Yes, with more vacuum it will draw more fuel...

And if I open the idle-eze valve to give it more air (to lean it out presumably), then it just wants to idle faster again, which makes it tend to roll-on when I kill the ignition.

Usually if you add more air, and the RPM increases it's another indication that we're too rich.

perhaps I need smaller idle feed restrictors? I guess I'm surprised that tuning this motor with the full time vac to the dist. is so much different than with the port vac source. My IABs are already larger than stock to lean the part throttle out, I have .078 and .076 bleeds (.070 is stock). MABs are .029", stock is .036"

Smaller IFR would most likely be the way to go.

Do you have any of your logs left you could look at from the Innovate system?

 

Why did you want remove the vacuum advance at the track?

Well, I've heard that the vac. advance takes a while to react after the motor goes to WOT, at which point you can have some brief detonation - it was said at that foursecondsflat.com paper that it could eventually hurt the motor.

Maybe a little too cautious, I don't know.

I hear that, try running a car on the street with all 8 Lambda's

NICE! I bet that's a mess of wires. whew! I wish I had the budget to allow me to do that. Would be so fun to learn more. Course, adjusting individual cylinder timing would make my head spin i believe. But getting the intake to more evenly distribute the mixture would be cool!

Yes, with more vacuum it will draw more fuel...

makes sense.

Usually if you add more air, and the RPM increases it's another indication that we're too rich.

Smaller IFR would most likely be the way to go.

Okay, that makes sense. Give the motor more of what it wants and it responds with more speed. Will try leaning out somehow. Now I need some IFRs. I have plenty of airbleeds, but no IFRs. And no brass set screws that small right now.
What happens if you solder them shut and redrill? Is that even possible or advisable?

Should I expect the motor to die if I introduce a vacuum leak, like the 3/8" port on the side, if the idle is set properly?

Do you have any of your logs left you could look at from the Innovate system?

yeah, tons. If you are wondering about AFRs, the idle was at about 13.5-13.8 IIRC, and the low speed and mid speed cruises both at around 14.7-15.6 or so depending on throttle position, etc, WOT was 12.2-12.5 depending on weather etc, a tad rich, but good for safety's sake I guess.

 

Well, I've heard that the vac. advance takes a while to react after the motor goes to WOT, at which point you can have some brief detonation - it was said at that foursecondsflat.com paper that it could eventually hurt the motor.

Maybe a little too cautious, I don't know.

On a street strip car, I'd set it to where the car runs the best on the street (from performance and safety) and let it be what it is at the track.

NICE! I bet that's a mess of wires. whew! I wish I had the budget to allow me to do that. Would be so fun to learn more. Course, adjusting individual cylinder timing would make my head spin i believe. But getting the intake to more evenly distribute the mixture would be cool!

More of a mess than you would want to know.. Add in all of the rest of the Data acquistion stuff

Okay, that makes sense. Give the motor more of what it wants and it responds with more speed. Will try leaning out somehow. Now I need some IFRs. I have plenty of airbleeds, but no IFRs. And no brass set screws that small right now.
What happens if you solder them shut and redrill? Is that even possible or advisable?

The IFR blanks are available through any of our dealers for your carb P/N 200083 #6, 32 TPI. You can solder and redrill too either way will work.

Should I expect the motor to die if I introduce a vacuum leak, like the 3/8" port on the side, if the idle is set properly?

Yes... or atleast start to want to die.


Bottom line is this give the engine whatever makes it happy. Don't be too concerned with a specific number or setting.