idle vacuum vs manifold type

Patrick · Mar 9, 2004

A quick question that I couldn't find an answer for in the archives:
Would you expect the idle vacuum to be lower with a single plane compared to a dual plane? My vacuum is low (~11 inches in gear) for a cam that isn't too radical. I'm thinking of swaping my victor vortec for an RPM vortec. Thoughts?

(I've messed with the timing (ported or manifold) but it still seems to idle rougher than I would expect)

Specs: 355, vortec heads, victor vortec, crane cam (~230* @ 0.050, ~ .500 lift), 3310 holley, headers.

mr 4 speed · Mar 9, 2004

What is your base timing?

SS_Dave · Mar 9, 2004

I think all things being equal other than the intake, any air that reaches the intake must pass through the carb first, so I'm thinking the
vacume would be the same.
This is of course given the exact rpm, timeing carb etc.

Dave

Troy70SS · Mar 9, 2004

If it's idling rough and vacuum is low, I would suspect a vacuum leak. Have you checked manifold bolts, seals at the ends of the manifold, disconnected vacuum lines, etc. Just thinking out loud.

Troy.

Patrick · Mar 9, 2004

Guys,
I've messed with the timing - I have ~20 initial in park with the vacuum advance off. Checked for leaks around the carb - none found. Also have a PCV - when I disconnect it and plug the line, the idle quality is the same.
I could have a leak around the intake gasket - although I've tried spraying the edge of the intake to isolate a leak (none found). I guess there could be a leak that I can't access.

So, assuming no vacuum leaks, would you expect the idle quality to be the same between a single/dual plane manifold?

thanks for the ideas

TJC · Mar 9, 2004

You get a much stronger signal to the carb with a dual plane, as the plenum volume is split in half. The vacuum reading will be the same however between the two. So your idle would be more stable with the dual plane.

69SSRat · Mar 9, 2004

Patrick
The dual manafold will be better for street now the signal yes it will come up some but you need to do a vacuum reading if it is 11" then you need to check your vacuum advance.The number on it will tell you for what vacuum range it works.The advane should or could be moving at idle causing some of your problem are you running HEI?

Myself I am running only 6" vacuum and have full manafold vacuum at idle to the advance,which I change to one that was 2" lower in vacuum than the engine but this is to help the massive cam start at 12* and then pull to 16* for a better Idle quality

I am running the dual plane stealth manafold also

Patrick · Mar 9, 2004

TJC,
The manifold vacuum will be the same but the quality will be better? Meaning it won't be as choppy with a dual plane - is that what you mean?

66SSRat,
I checked the initial adv with the vacuum advance off (~20 deg in park). The vacuum advance is connected to port vacuum - not manifold. (A few months ago I had trouble having it idle well because the vacuum advance was unsteady). I'm using an HEI with adjustable vacuum advance (I have adjusted my curve to have ~16 deg mechanical advance - so I have ~ 36 deg mech adv at 3000 rpm)

Maybe I should recheck idle quality with the vacuum to manifold rather than port. If that doesn't help, maybe I'll replace the intake gasket.

How does that sound?

TJC · Mar 9, 2004

The idle will be more stable, and easier to tune. Do your idle mixture screws have any effect at the moment?? If you turn them all the way in, does the motor stall?

onovakind67 · Mar 9, 2004

Vacuum advance is just like mechanical advance. It has a starting point, a rate, and an ending point. You don't want to be idling on the vacuum advance curve as it will produce an unsteady idle. If you idle at 12inhg of vacuum, make sure your vacuum advance is all in by 12inhg. Lars Grimsrud has some good info on the specifications of various GM canisters.

http://www.corvettefaq.com/listing.asp?group=1&sub=7

Click on the vacuum advance control listing and open the file in wordpad.

Patrick · Mar 9, 2004

onovakind67,
Good link - thanks. Except I'm using a crane adjustable unit. I can't adjust the unit to be all in by a certain vacuum level, can I? I thought all I can do is adjust the rate and limit the total by use of a limiter plate...

onovakind67 · Mar 10, 2004

The problem I've found with the Crane is that the positive stop is on the wrong end. If you use the little eccentric and screw arrangement you will be limiting how far back the rod will go. If you weld a stop on the bracket that limits how far the vacuum will pull the rod, you can use the inner spring adjustment to fix when the curve starts and the welded stop to limit the travel. You can calculate the distance needed between the rod and the stop by using a little geometry. On a normal small distributor the rod end is about 1.5" from the crnter of the distributor. This scribes a circle that is 9.42" around, each degree is .026". Since one distributor degree equals 2 crank degrees, every .125" is about 10 crank degrees of advance.

We had a guy with a real nice 427 Cobra replica in for exactly this reason. His idle was very unsteady with manifold vacuum on the VA, and his car would stall when he put it in gear if he used ported vacuum. We welded a stop about 1/8" from the rod to limit the advance to 10° and lowered the adjustable spring so it was all in by 11" and his car ran much better.

Patrick · Mar 10, 2004

onovakind67,

Great info - I appreciate it. I'll work on it this weekend (hopefully) and post the results.

Greybeard · Mar 10, 2004

"We had a guy with a real nice 427 Cobra replica in for exactly this reason. His idle was very unsteady with manifold vacuum on the VA, and his car would stall when he put it in gear if he used ported vacuum."

A sign the End is near. A 427 Cobra with an auto trans.

onovakind67 · Mar 10, 2004

It's really amazing the lengths they go to look authentic. This was a 392" Ford small block with a AOD tranny in a 427 Cobra kit car, a very nice looking and complete car. The shifter looks like the original stick with the extreme forward tilt. The gear indicator is a small panel on the dash. It was a very solid, quick and quiet car but you sit crooked in the car, which takes a little getting used to.

idle vacuum vs manifold type

Patrick

mr 4 speed

SS_Dave

Troy70SS

Patrick

TJC

69SSRat

Patrick

TJC

onovakind67

Patrick

onovakind67

Patrick

Greybeard

onovakind67

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