How would this setup work on a 914 driver? Looking for solid performance across the RPM band with improved off idle response to get rid of the flat spot.

That being said my Mallory set to "12-28" runs like a dream.

I think the SVDA will require the manifolds to be drilled and tapped at all 4 "barrels". Then run vacuum line to a small manifold of sorts. The idea is to smooth out the vacuum pulses. John Connolly can probably elaborate...

.......................

Thanks guys. Are there any other dizzy options (non vac advance) that will provide solid drivability throughout the RPM band and good off idle response? I need to trash this 009 but don't want to get wild here with modifications.

This was written by a former GM engineer as a response to a similar question on a Camaro board:


As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren't fully-deployed until they see about 15" Hg. Manifold vacuum, so those cans don"t work very well on a modified engine; with less than 15" Hg. at a rough idle, the stock can will 'dither' in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15" Hg. of vacuum at idle need a vacuum advance can that's fully-deployed at least 1", preferably 2" of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8" of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it - they don't understand it, they're on commission, and they want to sell "race car" parts.

Has anyone tried using vacuum advance off the manifolds?

I'm running a stock L-Jet Dizzy (205AB) with the vacuum can disabled. When I bought my carbs and manifolds, the PO had installed vacuum ports in the manifolds which I removed and plugged.

I suppose I would need run vacuum lines to a 4 into 1 manifold and then measure the vacuum at idle, off idle, WOT and measure how much vacuum is needed to move the vacuum can. Does this sound right?

Interesting read. But not necessarily germane to the stock 914 engine--very few of these (and only the later ones) had A.I.R. systems ("smog pumps"), while the earlier engines did have ported-vacuum spark advance.

I suppose the Bosch and/or VW engineers may have just been kidding themselves, but it could also be that an SBC engine is not a VW Type IV engine.

--DD

But we're not talking about a "stock" engine after you put dual Webers on it. It seems to me that it calls into question the common wisdom of using a mechanical only advance distributor when switching to carbs. It also seems to point to the distributor as the source of alot of the driveability problems blamed on the Webers.

I doubt that Bosch and VW engineers were fooling themselves, they were doing exactly what the author said. That is engineering for emissions compliance not smooth or efficient running. Also remember the early (pre 75) 1.7, 1.8 and 2.0 engines had dual advance (manifold) and retard (ported) vacuum cans. The 75+ only had the ported retard connected.

Any way, I think I will use the FI distributor with Webers and manifold vacuum on my soon to be built 2056 and see what happens.

Dave: I did not understand your comment about Bosch VW. could you explain that for me?

However and old Peterson publication entitled "Basic Ignition and Electrical Systems" reviews the history of "Manifold" versus "Ported" vacuum. As they indicate, "Ported Vacuum" came about during the start of pollution controlled vehicles to reduce nitrous oxide emissions with lean air/fuel idle mixtures. They state that by using "Manifold Vacuum" the spark at idle will occur sooner and "less cylinder wall is exposed to the heat (of combustion), thereby lowering coolant temperature." Interpreting their statement, with more advance at idle, the combustion starts on the upper portion of the cylinder on the compression stroke. Flame front travels very slowly at the low cylinder pressure that occur when idling. Therefore the cylinder head and upper cylinder walls will absorb most of the combustion heat and the lower portion of the cylinder on the power stroke will be cooler. This transfers less total heat to the cooling system. In fact, on this 502/502 motor an additional ~ 10 degrees advance occurs at idle when the "Manifold Vacuum" port is connected to the distributor vacuum advance. This provides about 25 degrees advance in total when idling or when cranking! The engine idles considerably faster compared to when the "Ported Vacuum" port is employed. The throttle butterfly's have to be closed about 1/2 turn on the idle adjustment screw to maintain the ~900 RPM idle speed. All else being equal, less butterfly opening means less fuel consumed which equates to less total heat to transfer to the coolant.

Quoting another source; "Ported vacuum showed up when emissions became a factor - before that time, full manifold vacuum was provided to the distributor. Many emissions engines use a thermal vacuum control switch to deliver ported vacuum to the distributor until the engine reached operating temperature, then manifold vacuum to keep the engine cooler at idle."

Are you using “ported” or “manifold” vacuum to the distributor? “Ported” vacuum allows little or no vacuum to the
distributor at idle. “Manifold” vacuum allows actual manifold vacuum to the distributor at all times. Ported vacuum
was used as an emissions method control to retard timing at idle (by eliminated vacuum advance) in order to reduce
hydrocarbon emissions.

But we're not talking about a "stock" engine after you put dual Webers on it. It seems to me that it calls into question the common wisdom of using a mechanical only advance distributor when switching to carbs. It also seems to point to the distributor as the source of alot of the driveability problems blamed on the Webers.

I doubt that Bosch and VW engineers were fooling themselves, they were doing exactly what the author said. That is engineering for emissions compliance not smooth or efficient running. Also remember the early (pre 75) 1.7, 1.8 and 2.0 engines had dual advance (manifold) and retard (ported) vacumm cans. The 75+ only had the ported retard connected.

Any way, I think I will use the FI distributor with Webers and manifold vaccumm on my soon to be built 2056 and see what happens.

Alright, bumping this because I have a question...
I bought a 205aa distributor for my 2.0 with dual webers (it has an 009 now), and I'm not sure about the vacuum connections. The vacuum can on the 205 has two vac ports, as I understand it, one for advance and one for retard ( ). What do I do with the retard side, use it or plug it? Where would the retard side plug into? Will this operate as well as an SVDA (assuming both have petronix installed), or should I re-sell this 205 and pick one of those up instead? Do I need to use the vacuum from both carbs, or will one suffice?

TIA
Chris

as I recall, the retard port is hooked up to a vac port that is below the throttle plate, having an effect on fuel burn while the throttle plate is closed. There is a thread here regarding svda that may help you, do a search. John at aircooled.net helped me sort out my svda installation with L jet, worth a call or email to him...

if you use ported vacuum, as the throttle plate opens you will gradually advance. until you get to the point that you lose vacuum (anything above 1/3 throttle) then the vacuum adv will drop and the centrifugal adv will take over.
which is what most of us are after.

if you use manifold vacuum, with the throttle plate closed, vacuum will be super high and you will have full vacuum adv. then as the throttle plate is opened. vacuum is decreased as well as vacuum adv. which is not desirable.

what most of us want, is that lean cruise and low temp, at up to 1/3 throttle. that is how you get mpg and driveability.

.

That works for a lot of cruising/driving conditions, though some cars will spend some of their life in city driving/traffic and that 'cooler idle' with manifold vacuum along with even more fuel economy .... would be quite welcome.

Just putting more info out there, so owners can make a more informed decision .

i have twin spanish made brand new 40's on my 1.7 motor and the set up is like this


carb jet sizing

chokes 28mm
a vents 4.5
mains 115
airs 200
em tubes f11
idles 55
pumps 50
n valves 175

I also have an svda 034 with electric points distributor. The carbs do have a vac take off built in to them. I have a line of each which i join just before the distributor with a t piece

The only running issue i have is that sometimes it will stall on approach to a junction after a fast run, like 50+mph but i overcome that with a blip of the throttle first.

I haven't noticed a flat spot, in fact it launches pretty quick of the mark.

Read on the Samba or maybe somewhere else the SVDA isn't very happy with a high lift cam on carbs,something to do with vacuum issues,I don't know but it kinda makes sense,the latter Dellorto's only came with a vacuum pickup on one of the two carbs,not sure about the ones CB imported a while back,but with further research vacuum off one carb will do the job however have no experience of that.
Anyone on here have their svda running off one vacuum point??.
Issues with the latter Bosch distributers are about 80% of used ones have gummed up mechanical advance plates,the end result is normally flats spots and a slow return to the correct idle ,or in some cases little or no advance.
Interesting thread seeing what people have experienced....keep it going