C2 SB, ported vac smooth, manifold vac spikey

[Steve D.]

When I connect a vacuum gauge to a fitting on the manifold or to the manifold vacuum fitting on the carburetor, the needle is very jumpy. However, when I connect to the ported vacuum fitting on the carburetor, the needle is very steady. Is this the expected response?

Steve

[Stuart F.]

Re: C2 SB, ported vac smooth, manifold vac spikey

The ported fittings tend to dampen the vacuum signal. Do you also see the same relative mean signal value?.

Stu Fox

[Duke W.]

Re: C2 SB, ported vac smooth, manifold vac spikey

You haven't provided enough information to even hazzard a guess.

What year and engine? OE equivalent or modified? Carb manufacturer and number? Is it the original OE carb, a functionaly equivalent OE replacement or a "one size fits all" type replacement.

Provide the actual vacuum numbers/range @ engine speed from both non-ported and ported connections.

At idle a ported vacuum reading should be zero.

Duke

[Steve D.]

Re: C2 SB, ported vac smooth, manifold vac spikey

The body is 63, the engine is not a matched set: block is Tonawanda replacement (65), the heads are 67, cam is LT-1, no emission control, carburetor is Edelbrock 600 CFM (pt no 1406).

I had been reading vacuum posts in the archives, and curosity lead me to hook up the gauge. On the ported fitting, vacuum is zero at idle and all non-zero readings at higher RPM are rock steady; no needle quiver at all. By contrast, when hooked to manifold vacuum, the needle is jumping rapidily over a range of 2 to 3 degrees (i.e., 12 - 15 inches at idle).

[Duke W.]

Re: C2 SB, ported vac smooth, manifold vac spikey

Okay, now we're getting somewhere, but you forgot to state the idle speed (which I underlined for emphasis in my previous post), or more precisely - the range of idle speed since there is likely some variation in idle speed as there is with manifold vacuum while the engine is idling.

Duke

[Joe C.]

Re: C2 SB, ported vac smooth, manifold vac spikey

The body is 63, the engine is not a matched set: block is Tonawanda replacement (65), the heads are 67, cam is LT-1, no emission control, carburetor is Edelbrock 600 CFM (pt no 1406).

I had been reading vacuum posts in the archives, and curosity lead me to hook up the gauge. On the ported fitting, vacuum is zero at idle and all non-zero readings at higher RPM are rock steady; no needle quiver at all. By contrast, when hooked to manifold vacuum, the needle is jumping rapidily over a range of 2 to 3 degrees (i.e., 12 - 15 inches at idle).

SNAFU.
Only the mildest of camshafts will cause an engine to idle with a rock steady vacuum reading. Some fluctuation is normal, especially with a cam such as the LT1. If the fluctuation is rhythmic, and varies as the speed of the engine, this could be an indication of a leaky valve.
Joe

[Stuart F.]

Re: C2 SB, ported vac smooth, manifold vac spikey

Which brings us back to my point about the ported fitting, which I take to mean an orifice, will probably dampen the signal. Now whether this would be true with say a bad valve or other fault I don't know. I have little experience troubleshooting internally sick engines - never had any.

Stu Fox

[Joe C.]

Re: C2 SB, ported vac smooth, manifold vac spikey

Which brings us back to my point about the ported fitting, which I take to mean an orifice, will probably dampen the signal. Now whether this would be true with say a bad valve or other fault I don't know. I have little experience troubleshooting internally sick engines - never had any.

Stu Fox

"Ported vacuum" is not an indication of absolute pressure within the intake manifold, but rather an airspeed signal sourced from a Pitot tap just beneath the throttle blades. A Pitot tube measures fluid velocity, and so, "ported vacuum" is a relative term..............not the same as manifold vacuum............but is a general measure of throttle position. At "closed" throttle (which is actually "cracked" or throttled open) the air speed signal is too weak to be measured by the Pitot tap. When the throttle plates open to the point where the signal can be measured by the tap, that is the point of HIGHEST "ported vacuum", because that is the point of highest velocity past the Pitot tap. High manifold vacuum, small flow cross sectional area translates to high velocity since:

Flow = Rho x velocity x cross sectional area, where rho is a constant based on the fluid whose flow is being measured.

As the throttle plates are opened further, manifold vacuum begins to drop, and cross sectional area increases, which means that velocity past the Pitot tap decreases, as does "ported vacuum".

"Ported vacuum" is not an indicator of absolute manifold pressure and therefore is not a valid tool for determining the condition of an engine.

Joe

[Duke W.]

Re: C2 SB, ported vac smooth, manifold vac spikey

Pitot tube? A "ported" vacuum source simply means that the "port" is above the throttle blade at idle, so it is not exposed to manifold vacuum. As the throttle is opened the throttle blade moves above the "port" and exposes it to manifold vacuum.

Duke

[Joe C.]

Re: C2 SB, ported vac smooth, manifold vac spikey

That's right. Here is an excellent explanation:

http://www.lbfun.com/warehouse/tech_..._explained.pdf

[Stuart F.]

Re: C2 SB, ported vac smooth, manifold vac spikey

On Steve's original post he spoke of "fittings" and my comments were addressing that aspect, assuming all knew about the ported spark advance concept. The fitting I believe he spoke of has a small hole in it. I got one to see what they looked like and considered it during my experiments of this past year. I dismissed it out of hand as not being worthy of my true SHP L-76 63 vintage. In tests, I found it did dampen the vacuum signal - so what, right?

Stu Fox

[Duke W.]

Re: C2 SB, ported vac smooth, manifold vac spikey

On a four cylinder engine that idles slowly one will often find that the needle fluctuates quickly over a small range because manifold vacuum is cyclic with one cylinder aspirating every 180 degrees. This can also be the case on a V8 engine with a 180 degree manifold.

Note that on an OE AFB there are two small slots, one to each side of the isolated 180 degree manifold, on the base of the carb that lead to the VAC port, so vacuum fluctuations are less.

Pinching the hose slightly (not enough to completely close off the passage) creates a restriction that will damp (not dampen) out these fluctuations and lead to a more steady reading on the gage.

A damper (such as an automotive "shock absorber", a resistor in an electical circuit, or a restriction in a fluid circuit) provides damping that damps variations.

Duke

[Stuart F.]

Re: C2 SB, ported vac smooth, manifold vac spikey

Duke;

Right you are my friend. You caught me slipping back to using street talk again, and I should know better as a retired Tech Writer. No excuses. If I were to tender one, it would probably be that my degree came by way of U of NM and Maryland - part timing as I played tricks on the bad guys in SEA. But, I guess you confirmed my observation of what an orificed fitting will do. Have a great day.

We're having a bad T.S. here right now and the house next door got hit, car alarms are going off and all hell is breaking loose. So, guess I'll log off with my surge protector.

See Ya;

Stu Fox