Ported Vacuum Question

[Doug Flaten]

Duke has mentioned ported vacuum in several postings recently. My impression is that it does not provide full engine vacuum. However, I do not know why it is different, e.g. is ported vacuum sourced above the throttle plates? Or does the "port" only operate under a certain range of vacuum?
Of the many vacuum sources on carburetors, is there an easy method to determine which of the fitting is ported and which is not? Or are all vacuum fittings on a carb "ported" vacuum.

[Joe L.]

Re: Ported Vacuum Question

Doug-----

A "ported" vacuum source is one that takes its vacuum from, basically, above the throttle plates. In that manner, there is no or little vacuum when the plates (throttles) are closed. Any vacuum source on a carburetor at which there is no vacuum with the throttle plates closed is a "ported" vacuum source.

Below the throttle plates or directly off the manifold is "manifold vacuum". This represents the full engine vacuum produced by the engine while it is running. It varies greatly according to engine operating regime.

[Duke W.]

Re: Ported Vacuum Question

"Ported" vacuum advance is an emission control strategy. As we've discussed before there is an optimum ignition timing for each and every engine operating condition - the full range from idle through part throttle at all revs, and WOT throughout the rev range. This amounts to an infinite number of conditions, but they can be broken down to no more than a couple of handfuls of discrete conditions with the optimum timing between each determined by interpolation. Then the initial, centrifugal, and vacuum advance are juggled to reasonably meet the established requirement, which is determined from testing. Medium performance Corvette engines generally work best with 20-25 degrees of TOTAL idle timing and 25-30 degrees for SHP/FI engines with the big OEM cams.

On pre-emission engines this total idle timing is achieved with the sum of initial timing and (hopefully) full vacuum advance, but we know that some SHP engines have vacuum cans that don't deliver full vacuum advance at idle vacuum, and these cans should be replaced with cans that do. If total idle timing is retarded from these optimum values, EGT increases (along with heat rejection to the cooling system as the exhaust gas passes through the exhaust ports, which is why less than the optimum idle timing can cause high coolant temperatures), fuel flow to maintain idle speed increases (less of the fuel's chemical energy is converted to useful work when more is being thrown out the exhaust), and idle quality suffers. The motivation to decrease total idle timing was driven by emission requirements because higher EGT will promote oxidation of HC and CO in the exhaust system, especially when accompanied by supplemental air injection, known to us as Air Injection Reactor or AIR.

If the vacuum advance signal port is above the level of the throttle blade at idle, there will be no vacuum signal to the vacuum can at idle, and this is the essense of "ported" vacuum advance. Probably the easiest way to test is to remove the vacuum signal line from the can and check to see if it shows manifold vacuum while the engine is idling. If not, your engine likely has ported vacuum advance, but if it has TCS (Transmission Controlled Spark, which disables vacuum advance entirely in all but top gear), check the line going to TCS solenoid from the vacuum source on the carburetor. If the carb is on the bench you could attach a hose to the vacuum advance nipple, blow air through it and see where it exits in the throttle bore - above or below the throttle plate.

Ported vacuum advance appears to have first shown up on '66 L-72s. My theory for this is that it was implemented to meet CA emission requirements, and the same version was sold in the 49 states to avoid having two configurations of this low volume engine. There is some controversy as to whether actual tail pipe standards existed for CA in '66-'67 and federal in '68 and early years. I believe tailpipe standards did exist, but only at idle and perhaps at a higher speed with no load, so the OEMs were only concerned with meeting tailpipe standards (if they existed) under a very narrow set of operating conditions. Beginning in the early to mid seventies (for sure by 1975), certification involved driving the car on a dynamometer through a complete driving schedule that lasted about 45 minutes, and essentially the same certification driving schedule is used today, although more rapid accelerations and use of the air conditioner to increase engine load is being phased in. The entire volume of exhaust gas produced during the test is collected in a huge bag and upon conclusion of the test, the emissions are sampled, converted to grams per miles and the amount of CO2 and emissions are used to compute the amount of fuel consumed, which is the source of the EPA fuel mileage ratings.

Other '66 Corvette engines for CA had AIR, but I don't know if they also had ported vacuum advance. Anyone have a '66 CA version engine with AIR who can check?

If anyone thinks my answer to Doug's question is to lengthy and detailed, I offer no apologies. Doug e-mailed me with the basic question, and since it's deserving of a more in depth discussion, I suggested he post it on the Board. In this day and age some issues just aren't simple enough for a one or two sentence answer.

Duke

[Mike M.]

Re: Ported Vacuum Question

duke-- beautiful response. mike

[Mike M.]

Re: Ported Vacuum Question

P>S>--Seems to me that Gary Whitting, owner of Quanta, has a 66 390 HP vet that was a K19 car. Give gary a call and see if my ancient memory is correct and he may be able to determine if the engine is ported vs non-ported.

[Brian Monticello]

Re: Ported Vacuum Question

Here's an example of a ported vacuum source off an old Carter AFB.
Brian

[William C.]

Re: Ported Vacuum Question

Intrestingly enough if the subject pictured is a '65 AFB, the car was built using manifold vacuum by design rather than ported vacuum.

[Brian Monticello]

Re: Ported Vacuum Question

I should have clarified - this is a '63 Carter AFB 3503 SA from a Buick.

[Everett Ogilvie]

Re: Ported Vacuum Question

The '65 396 carb also has ported vacuum - the L72 was not the first. I think Holley believed ported vacuum eliminated highly variable vacuum signals at idle - this may or may not have been part of a Holley emission strategy.

With ported vacuum carbs, what you often find is that many cars are in such a poor state of tune or setup (distributor and/or wires not installed correctly, timing off, carb issues), that the idle screw is adjusted so much that the throttle blades expose the ported vacuum source in the venturi. These folks think they have full vacuum, but in reality they don't.

[Duke W.]

Re: Ported Vacuum Question

I now recall previous discussions of this carb. The incidence of ported vacuum advance on pre-emission engines indicates some other engineering reason for it, but I can can only come up with highly speculative reasons.

Duke

[Doug Flaten]

Re: Ported Vacuum Question

Thanks guys,
I suspected ported vacuum was sourced above the the throttle plates. But I did not know why they would want to use a ported source over a manifold source. If an emission engine has ported vacuum advance, wouldn't the idle quality suffer? Or is it a case where a few degress is enough to raise the exhaust temperature sufficiently? Since HEI distributers typically have a large amount of vacuum advance, (at least they appear to when connected to a manifold source) are they usually connected to ported vacuum which would reduce the amount of vacuum advance? I suspect I am having mental block about optimized timing versus the timing of an emission engine. Do I need to lose the perception that you would try adjust another part of the timing in order to compensate for a ported vacuum source? Were the emission engines just flat out doggy compared to the same non-emission engine? In the 70's I always thought a great deal of the horsepower decline was due to SAE Gross and SAE Net rating. Apparently more of it may have been emission restricted than I remember.

[William C.]

Re: Ported Vacuum Question

At least a partial answer to your question is that early emissions strategies focused on running the engine hotter at idle to deal with certain types of emissions. Thus over the years we saw lowered initial timing and use of ported vacuum. the engine can be made to idle, it's just not as efficient, but most of the cars then used a AIR pump to add extra air into the manifolds to burn the excess fuel caused by the inefficient idle. I don't know the specifics, but that was the general idea. Depending on the year, loss of power was due to a variety of the tuning practices used to meet the emissions standards, which changed nearly (tougher to meet) every year. Things started to turn around once the Cat. converter was introduced (75) as it did much of the job of exhaust cleanup. but standards continued to tighten eventually driving the introduction of computer-controlled carbs and then TBI then EFI, which has contributed to the amazing performance of the C-4 and C-5 series cars.

[Duke W.]

Re: Ported Vacuum Question

Doug - Ditto what Bill said, but here's a bit more perspective. As far as idling is concerned achieving the optimum mixture and timing for the highest manifold vacuum will usually yield best idle quality. If idle timing is reduced via ported vacuum advance, quality will usually suffer.

Emission controlled engine timing maps are set up much differently than pre-emission engines. Emission controlled versions of basic fifties/sixties vintage engine designs usually had a lower specified initial timing and slower centrifugal curves. Total vacuum advance was usually more, but the signal line was "ported" to disable vacuum advacne at idle, and TCS was common on GM cars which only allowed vacuum advance in top gear. This is what it took to meet emission and fuel economy regs with crude technology, and that's why seventies vintage engines have bad reputations.

The screwball ignition maps would also make these engines prone to detonation on light acceleration or climbing hills. It was probably all the vacuum advance because if you accelerated harder the detonation would often go away. I remember someone being quoted that detonation was "the sound of fuel economy".
Seventies vintage engines were a kluge.

Because catalytic converters could clean up native exhaust gas that was relatively dirty, particularly with supplemental air injection, the OEMs were able to go back to more normal ignition and fuel maps with the advent of converters Driveability, fuel economy, and part throttle power improved, but top end power actually got worse because those early converters were terribly restrictive, and even the Corvette only got one, not two. Dave McClellan gave the lowdown on these "bead bed" converters in his book.

The best thing you can do with an emission controlled engine is to backdate the ignition map to an antecedent pre-emission engine. (assuming the car isn't subject to field emission tests.) In the case of the SBC, the '65 engines represent the best OEM ignition map, particularly the SHP/FI versions.

These '65 SBC specs are probably a good place to start for any wedge chamber engine regardless of make. Get all the centrifugal advance in as quickly as possible without detonation, rig up a full time vacuum advacne and make sure the vacuum advance is all in at least 2" below what the engine idles at, and the total vacuum advance should be a little less than one-half the optimum WOT total advance, which is the sum of initial plus full centrifugal, so on a SBC that typically has optimum total WOT timing of 36-38 degrees, the vacuum cans typically provided 16 degrees.

Duke

[Craig S.]

Dan Pepper has a 66 K19 L36 Coupe

Duke - Dan Pepper has a 66 K19 390 AC coupe...he just put back all the emissions gear. I bet he can answer your question...Craig