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MG MGB GT V8 Factory Originals Technical - Vacum Gauge Connection

Getting close to firing her up now!

But, can anyone tell me which side of my weber 500 I connect my vacum gauge to?

Is it the left (as you look at from the from the front) pipe, which my vacum advance is connected to, or the other, right hand side one?

How do you spell vacum, vacuum?

Thanks,

Liam
Liam H

Here in the US we spell it vacuum, but since a lot of word's spellings are differnt in the UK I'm not sure what you would use. Perhaps you take out the extra "U" to use in "colour". ;-)
Bill Young

Hmmph :-)

Anyways, after some considerable searching I found this web site which I thought was quite useful.

http://www.iwemalpg.com/Vacuum_gauge.htm

Cheers,

Liam
Liam H

You have two vacuum ports in front of the carb.
As you face the carb from the front of the car. the right side is is full vacuum, the one on the left is ported vacuum. Ported vacuum is for the distributor.
Many believe that the vacuum for the distributor should be conected to the full vacuum side which is a mistake. If you are checking the vacuum on your B then connect the gauge to the left port to get a vacuum reading at idle.

right port------vacuum gauge
left port-----distributor
Bill Guzman

"Many believe that the vacuum for the distributor should be conected to the full vacuum side which is a mistake"

Hey Big Bill,

You're a smart guy & I usually agree with you. You're on the left coast for sure on this one. Ported vacuum was invented for smog engines. Performance engines should run full manifold vacuum advance.
Carl Floyd

Carl, the same can be achieved by redoing the curve in the distributor and have a bit more with the vacuum pot when needed. If you have a real hot engine, then delete the vacuum and use only advance created by springs and weights. Such distributors are sold by many, such as MSD, ACCEL and....the list goes on.

To much advance at the low range rpm is not a benefit.

In a mild build engine, you should have around 10 initial degrees with about 24 degress at 3000 rpm.for a total of 34 degrees at 3000K Some engine require about 5-10 more degrees at hihger rpm all depends on the engine. Also some engines require a pot that retards the timing. Such engine are ones with blowers, nitro, turbo's.
Then there is the factor of cam timing, is cam advance or retarded.

Using full vacuum for the advance is the half A....way of doing things. You do not have control of how much advance you get with full vacuum. Not all vacuum pots are created equal. Some only advance 2--4 degrees

Some spring kits for the GM distributor come with a limiter for the vacuum advance pot and some pots are adjustable, so that it can be tailor to the amount of advance you can get when needed.

There is more on this subject, but I do not like to type this long post.

But there Myths that everyone believes and hold true to them and that is OK.

Bill Guzman

Morning Bill,

Hotrodders prefer manifold vacuum. My '63 Buick 215 came from the factory with the distributor advance hooked to full manifold vacuum. Since I haven't really played with the distributor, I connected it to the carb that way.

Don't know if this will all fit in one post, but I thought this was interesting:

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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.

Original Version
Unnamed Automotive Engineer
Carl Floyd

Carl, I believe you're slightly mixed up. On MGBs, ported vacuum was the setup of choice until 1971 when smog rules forced British Leyland to use ported vacuum to achieve emissions standards. Just check out a set of SUs. Through 1971 they offered a vacuum port - after that it went missing.

Think of it in terms of basic physics. If you want throttle response and a steady idle, use ported vacuum. You'll have no additional timing cutting in and out and no fluctuations based on unsteady vacuum signals There's also LESS advance when you try to start your car. That will improve the car's ability to start easily - NOT more timing which can cause the engine to buck.
Now to the physics part. The vacuum advance isn't magic. It takes time for it to move, and it takes time for the engine's vacuum levels to change. If you use manifold vacuum, you have at least a half second lost while the vacuum advance sheds its signal and reduces the engine timing. During that time your fuel mixture is wrong and engine power is lost. It takes a few more engine revolutions to get things back in order and by then, the whole thought of a throttle response is gone. Do you need the additional vacuum advance timing timing at idle? In most cases NO!
Jeff Schlemmer

Thanks for the very interesting discussion on both sides of the argument.

I think i will have to try it on both and see how I get on.

Liam
Liam H

Liam,

Paul Hunt did just that:

http://www.mgb-stuff.org.uk/

So did these guys:

http://www.highperformancepontiac.com/tech/vacuum_advance_tech/index.html

Jeff,

I don't think so. American cars didn't start using ported vacuum until the very late 60s/early 70s. MG engines don't count, I'm talking hotrod V8s with carbs. Besides, according to Des Hammill "Some Lucas distributors are specifically designed for use with SU or Zenith carburettors. The Lucas distributors vacuum advance is calibrated to accept vacuum from a specific position within the carburettor which is much less than inlet manifold vacuum."

I researched this quite a bit before I chose manifold vacuum. Yesterday, I went back & spent several hours searching for more info. I have discovered that "port vs manifold" is one of the biggest automotive controversies on the 'net! Seems most engines can be set up to run either way. Some prefer one over the other. Performance engines, especially cammed up engines, like manifold vacuum advance.

I wonder how many of our conversion brothers that have overheating problems are using ported vacuum. See quotes below:

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"Ported vacuum means the port is drawing vacuum "above" the throttle blades in the carburetor. This means that as RPM increases, vacuum increases and in turn, vacuum advance increases. This was fine on older applications with high lead fuel and other ancient engine designs. Using this set up today can cause detonation problems, overheating, and other grief."

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From a performance standpoint, most engines can usually benefit from running direct manifold vacuum instead of the ported vacuum to the distributor (all other ports on the carb are manifold vacuum), but hydrocarbon emissions will increase by doing so.

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"An 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."

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I have found that proponents of manifold vacuum advance include, David Vizard, Dave Emmanuel, & Des Hammill.
Carl Floyd

This thread was discussed between 10/04/2007 and 14/04/2007

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