Dual carb and distributor setup?, Looking for your feedback on this design Options

[NJ914Guy]

Car is headed into the shop this Friday. Pulling off the single Weber progressive carb and 009 distributor. My plan is to switch to Dual Weber 40's (on sale this week at AA) and upgrade the distributor to an SVDA 034 with an electronic ignition module (no points).

Engine is a stock 1973 2.0, no plans to convert back to factory FI.

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.

[SirAndy]

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.

You'll need fairly small venturis to create enough air-speed to get rid of any flat spots off idle ...


On my 2056 with 44IDFs, i had to go down to a 32mm venturi setup to dial out any flatspots. Bigger is *not* better in this case ...
(IMG:style_emoticons/default/shades.gif)

[yeahmag]

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...

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

[dirk9141973]

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

[s]The 1.7 stock dual 40 is about to large ? what about the dual 34 ? and what model Mallory do you reccomend is see 2 models for 914

[gothspeed]

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...

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

That sounds right, I have an SVDA 034 like NJ914GUY and will be running 44IDFs with a peppy camshaft .............. I was planning to tap all 4 intake runners to get the vacuum up for max vacuum advance actuation (IMG:style_emoticons/default/smile.gif) ............. however the website that sells that distributor says one can T into the ported vacuum of both carbs and it will work. I personally prefer manifold vacuum vs ported ..... (IMG:style_emoticons/default/shades.gif)

[Dave_Darling]

Manifold vacuum is very different from ported vacuum. Manifold vacuum is high when the throttle is closed (very high on overrun), and low when the throttle is open. Ported vacuum is low when the throttle is closed, high when the throttle is just barely open, and low again when the throttle is wider open.

In general, you use manifold vacuum for retard, and ported vacuum for advance. Definitely on the fuel injected cars; and probably also on the carb'ed cars.

--DD

[jaxdream]

The dual 40 IDFs that I have do not have the vacuum port on either carb . I suppose I could drill the port and attach a small brass tube , but haven't a clue as to the hole size that would be benifical , too big , too small ?? Anybody have any specs on port hole size ?? Vacuum advance would be a great option to use if one had a choice. I do agree that the manifold vacuum wouldn't be the best vacuum signal to use .

Jack / Jaxdream

[NJ914Guy]

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.

[yeahmag]

Mallory or stock. I preferred the Mallory.

[Grelber]

I have nearly this same combo on my 2.0L. (Weber 40's and electronic ignition w/ a hot coil). My engine builder had me source a 205 distributor out of an older 1.7L engine because it would throw all the way to 32* vs 28*. It has the vacuum canister, but it is not currently hooked up.

This setup for me pulls hard from off idle to redline. I really like it.

[jaxdream]

Yeah , the 205AA 3400 RPM @32* from a 1.8 is supposed to be a great stock type dizzy for use with carbs . I have also read threads from folks that reccomend the max advance be 28* and some say 32* .The little spec book that I have has variuos stock dizzys with varing max advance, ie;205D = 2900 RPM @27*,205A=3000 RPM @22* , 205 E + F 3000 RPM @27* , and 205 3000 RPM @24*, these cover the 1.7 , 1.8 . 2.0 with fi over a few different Mod Yrs specific to engine serial # range. I suppose one should go with what works very well with what particular engine you intend on using .

Jack / Jaxdream

[Root_Werks]

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.

Go find an "022" on thesamba.com classifieds, then get a pointless ign kit for it. You'll be happy. No vac ports to worry about.

As DD mentioned earlier, the carbs have to already have the adv vac ports or at least the boss that can be drilled out and the ports installed.

You're going to spend way more than it would to convert back to FI and in the end, FI would give you the same if not better power and much better fuel economy.

[gothspeed]

Yes, there is a difference between manifold vacuum and ported vacuum ...... I chose manifold vacuum for my engine for numerous reasons (IMG:style_emoticons/default/smile.gif)

Here is a good write up that should take the 'mystery' away from basic timing principles. I found this very simple straight forward explanation, to be quite helpful.

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.

[Root_Werks]

That's some good reading!

[Cevan]

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?

[jaxdream]

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?

I believe gothspeed mentioned that he's using it now or has . Check his post on this thread.
That sounds like a good start . Find out the specs on the dizzy as to when the vacuum comes in and ends , what rpm the centrifical begins and ends. The 205AB is not listed in my 1974 spec book . Should be some where to find info on it though.
Good Luck
Jack / Jaxdream

[mrbubblehead]

im running 40's and have no flat spots. currently im running an 009 distributor. i just clocked 34.1 mpg @ 75 mph average. im gonna go with the svda here pretty soon, i think i can hit the 40 mpg milestone. btw i have a 1.8 liter.

[Dave_Darling]

...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...

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

[JFJ914]

...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...

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) 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.

[gothspeed]

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.

+1 ... well said. This article is just one of a few that I read about ignition timing ..... others got into various technical details (dual plugs, repeated spark systems etc ), this article was concise and straight to the point .... (IMG:style_emoticons/default/smile.gif)

I am also building a 2056cc (still in progress) and will be running dual 44mm IDFs with a peppy, medium overlap web camshaft. The dizz I chose and bought is an 034, electronic ignition with vacuum advance can. I am confident it will give me what I need, for 'all around' performance use.

The engine combo will be tuned on the dyno, I will report my results (IMG:style_emoticons/default/sunglasses.gif)