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TR6 1972 TR6 Vacuum Port

bailee2

Jedi Hopeful
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I installed Webers and now have an problem with a vacuum source.
Most TR6's had a Banjo fitting for vacuum but mine has a single vacuum barb for the Brake Servo off the intake manifold.
What I really need is a Banjo with two ports, one for the Distributor and one for the Servo.
Seems no one has a Banjo with this configuration.
A TR6 friend of mine said the later TR6's had a similar fitting.
But my search had netted zero.
Any help appreciated.
Thanks.
 

TR3driver

Great Pumpkin - R.I.P
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Are you installing a different distributor then?

The stock 72 TR6 dizzy only had provisions for vacuum retard, which you definitely do NOT want connected to manifold vacuum (which would retard the spark all the time).

By far the simplest approach is to simply leave the retard disconnected. It was only for emissions control, and not particularly effective at that.
 

poolboy

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Yep, do what Randall said. And BTW, the Banjo fitting was only found on the 73 thru 76 models. The addition of the 2nd nipple was for a Vacuum source for the Anti Run-on Valve to suck air out of the ZS carbs float chamber when the ignition key was turned to the "OFF" position.
You just need the kind of single nipple manifold fitting found on 69 thru 72 cars for the brake servo.
 

hondo402000

Darth Vader
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only problem is thats not where you need to hook up the vacuum advance for the distributor, it has to be on the filter side of the butterflys. I have the same issue and still have not figured out what to do except have Jeff at Advance dist recure my dist for centrifugal advance only

Hondo
 

poolboy

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From 72 on the engines came from the factory w/o vacuum advance, only retard, which as Randall said, you can live without. Once the engine speed gets over 1100 the retard is a non issue.
I don't think Jeff even has to make a recurve adjustment for omitting it. Having said that, if your dizzy hasn't been recurved since new I'd recommend it anyway.
 

TR3driver

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hondo402000 said:
only problem is thats not where you need to hook up the vacuum advance for the distributor, it has to be on the filter side of the butterflys. I have the same issue and still have not figured out what to do except have Jeff at Advance dist recure my dist for centrifugal advance only
One solution for that (assuming you do have a vacuum advance to hook up) is to use manifold vacuum plus one of those little 12vdc solenoids and a microswitch that closes it (blocks the vacuum) when the throttle is closed.

Friend of mine tried that on his TR4 with Webers, and said it improved highway fuel mileage by about 10%.

You can't get the same effect by recurving the centrifugal advance; because it doesn't 'know' anything about throttle position or engine load; and the optimum spark timing very definitely depends on load.

But then, if you were actually worried about cruise mileage, you wouldn't be using Webers in the first place, so it's all moot
grin.gif
 

hondo402000

Darth Vader
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I actually drilled 1/8 inch holes thru the weber carb housing on the filer side of the buttefly plate and put brass tubes in each hole and then ran vacuum lines to each one. Before you guys go off and say I ruined the carbs, I had no issue. only problem is since the DCOE are basically 6 one barrel carbs I only did 3 and I dont get enough vacuum to operated the advance on the dist, so once I get the painting done and the car back and running I will drill the last 3 barrels and see if that works, if not I will just plug the holes up. Just wondering if 1/8 inch is large enough or should I go 1/4

Hondo

Edit, not sure what I said above will actually work? First I need to know if the vac advance on my 71 TR6 is ported vacuum or manifold vacuum. Now that I am actually researching this I totally had is backwards, seems you want more advance on Idle when the Fuel to Air ratio is lean and when crusing and less vacuum advance when under load and acceleration. I was wrong and what I did will not work but more research is needed
 

TR3driver

Great Pumpkin - R.I.P
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hondo402000 said:
Edit, not sure what I said above will actually work?
I think it will work, but your vacuum signal is going to be much weaker than with a standard multi-cylinder manifold. And tying multiple barrels together isn't going to help unless you can come up with some kind of reed valve arrangement to block air flowing backwards through the ports that don't currently have vacuum. You'll probably need a recalibrated vacuum advance module to deal with the weaker signal.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:] First I need to know if the vac advance on my 71 TR6 is ported vacuum or manifold vacuum. Now that I am actually researching this I totally had is backwards, seems you want more advance on Idle when the Fuel to Air ratio is lean and when crusing and less vacuum advance when under load and acceleration.[/QUOTE]It was originally ported vacuum, meaning no vacuum signal at idle. This leaves the spark somewhat retarded from the theoretical optimum, which gives a more stable idle. Once the throttle is open slightly, it becomes manifold vacuum (which basically indicates engine load, high vacuum = light load).

The need for vacuum advance has little or nothing to do with mixture, it's primarily a function of manifold vacuum. When the manifold vacuum is higher, the pressure in the cylinder is lower and the fuel/air mixture burns slower. Without the vacuum advance, the peak cylinder pressure comes too late in the cycle, and less energy is transferred to the crankshaft.
 
D

Deleted member 8987

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Randall....
Basically manifold vacuum?

Isn't the port above the throttle butterfly?

This changes vacuum in the base of the venturi depending on load and throttle opening.
If it was manifold, then the minute you crack the throttle, there would be full manifold vacuum throughout the throat.

The specs on advance units, not necessarily Lucas, which lists start and stop vacuum and maximum advance, shows varying vacuum and varying advance.

Dave
 

TR3driver

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Not sure I understand your question, Dave. It is something of a simplification to say the port see manifold vacuum, but it's pretty close. The port does get nearly full manifold vacuum once the throttle plate opens far enough, which produces a fairly rapid change in timing.

Yes, the amount of advance varies with the manifold vacuum, but that is exactly what's needed. The manifold vacuum is what primarily determines the cylinder pressure before ignition (and in fact some fuel injection systems look at manifold vacuum to determine how much fuel to inject).

Here's a photo from the engine side of a ZS carb. (It's a Stag carb, but it's nearly identical to the original 71 TR6 carb.) (Photo taken by Miki Stojanovic, one of the Stag owners on the Stag Phorum.)
VacOff2.jpg


As you can see, with the throttle open, the vacuum port is on the manifold side of the throttle plate.
 
D

Deleted member 8987

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When I put a vacuum gauge on my port (not SU's), I get a varying in vacuum as I run it up.
It doesn't dump to full manifold vacuum when I crack the throttle.

Can't recall ever having done that on an SU, but I thought they worked the same.

Maybe it depends on how far above the butterfly the port is.

You dump full manifold vacuum onto your advance when you crack the throttle, it will ping like heck.

My particular LBC uses twin Solexes, BTW.
 

poolboy

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FWIW, I have some Vacuum readings taken from the manifold of a stock TR6 engine.

850 rpm idle = 19 in-Hg
2800 rpm steady cruising = 15
"hard" acceleration = 0-4
"normal" acceleration = 10
engine over-run = 24
 
D

Deleted member 8987

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Really didn't mean to hijack this thread, but:

The way ported works is like a sandblaster.
The air (velocity) moving across the "port" creates its own vacuum to pull the sand up, or, in this case, to pull the diaphram of the advance unit.

There is no "vacuum" in a sandblaster air hose.
It's the air moving across the port that causes the vacuum and lift.

If you put the port in the air cleaner, it would have no velocity.
It is the location in the venturi, at the place where there is velocity, that casues the vacuum at the advance, if I recall my auto theory 101 correctly.
 
D

Deleted member 8987

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One of my vehicles (street rod) has an OLD Pont-whack Bonneville vacuum gauge mounted on the dash.
Primary reason for using it is the DYMO tape stuck to the front that says 6.5".

That's the power valve in the Holley.
If I stay above 6.5", I stay outta the power valve.

That said, a vacuum gauge can tell you a lot about your car while running.
I have a big one (about 4") and a LONG hose, connect it up, stick it under the wiper.
 
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