Tri-carb intake manifold balance tube size

[HealeyPassion]

Gentlemen,
Regarding the factory tri-carb intake SU manifolds ....does anyone know why the factory changed from the .75" balance tube to the 1" balance tube during the production run?

I have always assumed (never a good thing), that it was done because the .75" balance tube version was causing problems (tuning?...running?) with the triple SU's and the larger 1" balance tube was needed to fix the problem. Since the factory (okay, works) didn't generally make running changes to a production model (except at the end of production when they were using up old parts or integrating new parts... and neither of these conditions applies to the tri-carb models).

I'll bet someone knows!

Cheers,
Steve

 

[red57]

I may be wrong, but I seem to remember reading somewhere that is had to do with smoothing the vacuum pulses to help the SUs work better, ie smoother piston lift with less fluttering. I run a triple HD6 set-up using the DW manifolds and their balance tube is about .650 id. I too am curious about the tube size adn looknig forward to others comments.
Dave

 

[Duane_Rhynard]

Gentlemen,
Regarding the factory tri-carb intake SU manifolds ....does anyone know why the factory changed from the .75" balance tube to the 1" balance tube during the production run?

I have always assumed (never a good thing), that it was done because the .75" balance tube version was causing problems (tuning?...running?) with the triple SU's and the larger 1" balance tube was needed to fix the problem. Since the factory (okay, works) didn't generally make running changes to a production model (except at the end of production when they were using up old parts or integrating new parts... and neither of these conditions applies to the tri-carb models).

I'll bet someone knows!

Cheers,
Steve
View attachment 43621

Steve,

According to the parts list it was changed commencing with engine 29E-H929. I have a 1961 BT7 project with engine number 29E-H686 (German Export), which according to the heritage certificate was built June 12-14 of 1961, and it has the smaller tube. In Clausagers book (Original Austin Healey) he states the change was at engine 29E-H929 as well, in June of 1961, along with asbestos insulation on both sides of the carb heat shield.

Thanks,
Duane

 

[Brinkerhoff]

I have both versions in the shop now coincidentally and was wondering the same thing. The same hose clamps were used and are only just barely large enough to fit over the short rubber hoses on the large version. The earlier car has the side shift gearbox other than that they are the same.

 

[HealeyPassion]

Dave,
I recently got a DWM tri-carb intake for my new Healey project and I was surprised to see that it was the smaller version rather than the later larger balance tube. That's what got me wondering about this.

I have the smaller balance tube on my EFI car (it's an original Healey triple manifold), but the EFI doesn't play by the same rules as the old SU's so I've never had a problem or really worried about it before.

Thanks for any and all input,
Steve

Steve

 

[Marvin Gruber]

I have a tri carb set I picked up years ago but couldn't tell you what size balance pipe it has. Will dig them out next week and measure.
Marv

 

[HealeyPassion]

Since I'm really interested if anyone knows what factory inside knowledge caused them to change from a .75" balance tube to a 1" balance tube (a 25% increase in volume) I decide to ask some experts.

I exchanged multiple notes with Denis Welch Motorsports, since after all they sell the recreation with the .75" balance tube. But, turns out they don't know why the factory (okay, works) changed from the .75" to the 1". So one may wonder why they (DWM) decided to go with the .75" rather than the 1", which was the later version.... seems no one knows.

I sent a query to Steve Norton of Cape International in the UK... a super nice guy I had the pleasure of meeting and chatting with during the 2009 RETURN TO BONNIVLLE. He has helped me by providing a number of things on my works tribute car... some not generally available.

Here's what Steve N. said:
"The parts book lists these as Adaptor – balance pipe, the first part number was AEC2053 which covered engine # 101 thro 928 (engines 889, 890 & 892 are identified separately?).
Engine 929 thro 5799 used part # AEC2118.

Engine 929 was built in June 1961 hence dating the change, which I thought might alight with homologation or technical reasoning but I haven’t found anything. Increasing the size of adaptor may just have created a larger volume of vacuum?............to improve breathing with some minor efficiency gains although I haven’t seen any facts to support this?

I’m not sure if this helps, but its good to debate these subjects every now and then."

So, can anyone add to what Steve N. says?

Cheers,
Steve

 

[red57]

Steve, I don't have any answers but possibly adding to the question....

I still think it has to do with SUs specifically (never noticed balance tubes on cars equipped with triple Webers). It seems to me that the Ruddspeed triple SU set up from the '60s used an even smaller balance tube but I never had one so could be wrong on that.

However, have you noticed that there is a 'balance port' (for want of a better name) inside the log manifolds for twin SUs? The wall that separates the front and rear chambers has a hole in it connecting the two halves - I just looked inside an AEC 862 and an AEC 964 and they both look the same at about 1/2" (kinda hard to measure without removing the casting plug at the rear). So, has this hole size ever been changed as part of any production changes? If so, why?

Please continue to post any new info you discover.

 

[HealeyPassion]

Okay, more input... the MOSS catalog AHY-120 on page 18 in a note on the top left side of the page says "No 35 pipe diameter was increased from 3/4" to 1" to mitigate vapor lock problems with the rear carb." (I can't believe I never saw this before... but I hadn't.)

Now I ask smarter engineering guys than me (since I'm not a real engineer)... hopefully on this forum.... does that make sense to you? My limited knowledge base says that vapor lock is the result of boiling fuel in the carburetor.... so how does enlarging the vacuum balance tube size between the three separate manifolds "mitigate" the vapor lock issue for the rear carb??

Is this the real answer?... or has MOSS just made this up so people would stop asking them why there were two sizes for the vacuum balance tube on triple carb engines?

Steve

 

[ch47insp]

I have a 1962 BT7 with the larger size balance tube installed with a fitting attached on the top. My investigation revealed that one possible explanation could be that during this time frame, a brake vacuum assist servo was available as an option and that the larger balance tube was installed for that reason. My car did not come to me with a servo installed and I do not see any signs that one was installed, but maybe this was the first step for a PO.
Just another twist.

 

[steveg]

Since this is a nitpicky conversation...the difference between the two sizes is determined by the areas of their circles: .75" D = .441 sq"; 1" = .785 sq" -- almost twice the area. pi*radius squared

My DMD manifold has only a 3/8" balance tube.