Carbs and Balance Tube

61porsche

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Originally, carb'ed cars had an egr system that had a balance tube between the manifolds. The theory being that impulses from the valves closing, etc. can cancel bad the bad harmonics much like an extractor exhaust or crossover aids in a tuned exhaust sytem.

Yesterday, I tried it using vacuum hose. It seems to smooth the idle out.

Has anyone else observed this?
 

HB Chris

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Did you use the large fitting at the end of each manifold that is blanked off when EGR is removed? How did you fit a hose?
 

deQuincey

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i am interested, and i am in the possition of trying this, because I am using USA-spec manifolds, that included a 10mm diameter opening below the intake for the brake-servo (you can not see them in this pic anyway)



the only fault is that i need an appropriate adapter that fits in that conical hole (probably witworth thread ?)

Yes i had the intention of connecting one to the other because I was told by a Jaguar owner that it is done that way between the 2 six-cylinder blocks on the v-12 E-Type

so how did you do it Jerry ?

regards
 

Stevehose

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Oh that tube (#17 below?) - I took it off along with the rest of my old EGR stuff and capped the intake manifolds - so I should reinstall and cap the center piece where the old vacuum valve went? It's hanging in my garage as decoration now...

 

61porsche

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Carb balance tube

I just used a piece of vacuum hose to try it off the small openings. You don't need to use the big openings on the manifold just to see if the effect works for you. It's largely dependent on your state of tune alreadly which means your carbs, ignition, compression, etc.

It was a minor but definately noticeable difference for me as I tune by ear and sight besides all the other usual meters. I have the most solid exhaust note that I've ever had and when I did a drop test of each cylinder ( remove each spark plug wire in turn) it was a pretty dead even rpm drop.

I got hung up on the thought of using a flow meter on the secondary barrels to balance that side out better in order to see if I could get better vacuum signal to the primaries and get my secondaries opening quicker. Then the light bulb went off. BMW spent a lot of money and effort on the later Motronic manifolds to equalize the runners. The early balance tube must have had an effect as DQ says, Jags, etc. used the same principle.

In our haste to remove all the egr, something may have been overlooked.

It's worth a shot. If you see and feel the difference then you can approach the fix with proper fittings, hose, and clamps.

The only reason for the SS tubing was it was hooked to the exhaust and resultant heat. I'm not advocating that in any way.

Just hook the carbs together. I was pleasantly surprised; I hope it works for you too.
 

deQuincey

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can anyone identify which size and type of thread is in that holes of the manifold ???

is it a metric ? or other ?
 

MMercury

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can anyone identify which size and type of thread is in that holes of the manifold ??? is it a metric ? or other ?
Not having parts at hand, I would be guessing regarding sizes. But this would only be essential if you were making a semi permanent or rigid connection between the two manifolds. A simple rubber vacuum hose and connections between both manifolds is all you need to see if you will benefit from the arrangement.

Balance tube/s come in many shapes, sizes and compositions (metal, rubber and plastic). 61Porsche is merely describing a vacuum tube connection between the two manifolds and not necessarily a reattachment of a hard line connection. The fittings depicted in the illustrations for the EGR connection seem ready made for this; however, since the connection is to balance vacuum pulses - primarily at idle, other available/unplugged vacuum ports servicing the three runners on each manifold should work - even as a temporary measure. Aesthetics and interference with other working pieces are secondary considerations.

HTH.













 

deQuincey

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yeah you are right, it is a test only, but consider that now i have two capped holes in the front lower part of the manifolds, i only intend to put a small pipe and a rubber hose there

BTW: have you realised that in one of your engine pictures the manifolds are wrongly assembled ? rear manifold must be front one !

this remembers me that in my last visit to the bmw museum, i had to told a resposible guy that the windscreen wipers of the 3.0CS in the exhibition were wrongly mounted, the driver´s side one (the one with the small wind plate) was in the passenger´s side, :D:D (yeah, the car was NOT a RHD !)
 

Stevehose

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I did this mod today and it is definitively smoother at idle as well as throughout the rev range.

deQ: the manifold thread is M10 x 1.0 (the side with the copper crush washer on it), the the other side of the piece that the old EGR pipe screws onto has male threads that a 3/8" brass compression cap wil fit on (compression fittings have finer threads that match this, other 3/8" plumbing pieces like flare etc won't fit), this is what I had on it until I put on a reducer fitting which would then accept a fuel hose so I could see if Jerry's suggestion was going to work for me:



I bought copper crush washers at Napa and a 3/8" ID fuel hose and clamps. Hooked it all up and noticed immediate improvement in idle and overall smoothness through all gears and revs. Because the engine is smoother it runs "quieter" with less effort, especially when starting from a stop. Awesome suggestion Jerry.

Since I fortunately saved my EGR piece after removing it last year (now I know why I never throw any old parts out), I decided to remove the fuel hose and brass reducer and go ahead and cap the center fitting and reattach the original metal tube:



So now it is permanent. If your metal tube piece is long gone, the above fitting/fuel hose setup will be more than adequate. Worth a try by all means, my engine runs even better now.

can anyone identify which size and type of thread is in that holes of the manifold ???

is it a metric ? or other ?
 
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deQuincey

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thanks steve,

i was considering it to be M10, but i didn't know about the 1mm thread

as long as i do not have any egr-system pipes, i will adapt a small terminal in order to use a fuel hose, just like you show in your picture

your experience sounded nice, i am eager to try

regards
 

HB Chris

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Instead of the EGR fitting, can the vacuum port above it on the carb base be used that was also part of the EGR system? This pic shows it plugged with a screw.
 

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Stevehose

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I think Jerry said in his post that it is ok to do so - unless the larger diameter of the manifold egr tube has anything to do with it.
 

61porsche

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Ideally, from the manifold. Mine has extra small ports so that's what I tried. But anything below the throttle plates ( unported) would work.

I have two sets of manifolds. One was plugged at the fitting, the other had the tubing cut off and silver soldered closed past the first 90 degree bend. One was leaking slightly. So when I do mine permanently, I'll use the 90 degree tubing bends, hose and pipe/ tubing.

One other word on balance- indiviually balance the carbs for best vacuum. In other words, without the balance tube. Then put it back.

I've still got to work on those secondary air flows to balance that side.

I've not yet gone beyond trying to tune it better/ finer, but I tune a lot.
I do recommend everyone go back over all the carb screws/ body.

Time to stab that Porsche engine back today now that it's all shiney again and without the squirrel s/ acorns that decided to nest in one of the exhaust /heads. Pesky varmints.:shock:

Steve, I did go for an extended drive last week. The balance tube did calm things down at highway speed; the engine ran smoother at speed and was noticible ( quieter and smoother) in the cabin if you're really picky.

The engine runs smoother, definately. That's my story.:-o
 

Stevehose

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For sure it runs smoother and quieter - in particular at a stop light, starting from stop, and revving it up high in 1st and 2nd gear was a little "raw" before, now it is much cleaner as it heads toward 4k. Took it out again today to make sure I wasn't dreaming - even runs smoother when cold idleing with almost no rpm fluctuation and a cleaner sounding exhaust note.

Went to the BMW dealer to order a valve cover gasket - the sales staff came streaming out to look at the oddity parked by the parts dept!

The balance tube did calm things down at highway speed; the engine ran smoother at speed and was noticible ( quieter and smoother) in the cabin if you're really picky.

The engine runs smoother, definately. That's my story.:-o
 

deQuincey

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+1

great idea to interconnect the manifolds !
smoother run at low revs and a better transition
thanks jerry for your continous research !

regards
 

David

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can anyone identify which size and type of thread is in that holes of the manifold ???

is it a metric ? or other ?
Updating this for posterity.

BMW had two, possibly three different threads for this part. My US 74 uses a M10x1.0. However, I didnt have the EGR part and tried to source it from the usual sources with no luck. I pulled the part from two different coupes at Autobahn Dismantler today. Both were M12 (sorry, I dont know the thread). I spoke with Carl Nelson who said there were three variations. All appear to have the same part number. I'm now looking at Grainger,etc for a suitable part.
 
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