MGExp

MGB & GT Forum

Engine More Responsive.

Moss Motors
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor

Tar Trekka Avatar
Tar Trekka Ross H
Sydney, NSW, Australia   AUS
1968 MG MGB MkII "Miss B"
Hi there,

My car has had 5 mm ID Rubber Tubing on the Vacuum Advance since I've had it.

The Bore of the Tag on the Carby is 2 mm.

I changed the Tubing to 2.5 mm I D.

An adapter was made from a 25 mm length of 5 mm O D Stiff Plastic Tube to connect the 2.5 mm I D Rubber Vacuum Advance Tubing to a 25 mm piece of 5 mm I D Rubber Tubing for the Distributor end.

The reduced Volume of the Vacuum Advance Tubing has made the acceleration much more responsive & smoother.

For the cost of a bit of Rubber Tubing, it may be worth a try on your MGB.

Tar Trekka.

Any day above the ground is a great day, especially "Driving Miss B".

. Hide banner ads & support this website by becoming a > Supporting Member <
spikemichael Avatar
spikemichael Platinum Member Michael Caputo
Ocean Shores, WA, USA   USA
1973 MG MGB "Freebie"
1973 MG MGB
1974 MG MGB "Spike"
1979 MG MGB "MegaBeanie"    & more
Actually it isn't the reduced volume it is that the tubing wasn't sealing well.

There is no flow of air through the tube it should be a dead end. It takes very little suction to create a change in pressure inside the tube.

Attaching a 5mm tube to a 2 mm port just won't provide that seal.



Michael J. Caputo
'79 RBB and '73 CBB owner with extensive experience in 12v Audio System design and installation.
Vendor of Regalia and Promotional Products. Forum Member with a warped sense of humor.


Member Services:
Calendar Photo Submission Deadline: July 31! Email tips@MGBCalendar.com for FREE PHOTO TAKING TIPS! You can always call me at 978-249-5760 PST about Weber DGV Carbs, the calendar or just to talk MGB!
Tar Trekka Avatar
Tar Trekka Ross H
Sydney, NSW, Australia   AUS
1968 MG MGB MkII "Miss B"
Hi there,

The Bore of the Tag of the Carby is 2 mm, but the O D of the Tag is 5.4 mm, so sealing was never the issue

Tar Trekka.

Ex-Calif Avatar
Ex-Calif Gold Member Dan D
Cincinnati, Ohio, USA   USA
1968 MG MGB GT "Bart - Yellow And Naughty"
1977 MG MGB "Red Betty"
2012 Jeep Liberty "Tank"
2014 Hyundai Accent "Skate"
If the ID of the tubing was always 2mm then where was the volume change?

It's possible the old rubber one was soft and colapsing and the new one is hard plastic line but fundamentally Michael is right - volume of the vaccum system has negligible effect.

But if you car is running better then congrats!



The goal - Reliable summer driver interspersed with mechanical tinkering...
Motto - "Driving fifty in the twisties..."
On Mods - It's your damn car - Do what you want. Haters gonna hate...
On SUVs - Drive your B like a soccer mom is texting her friends about how she wants to kill you...
Red Betty - http://www.mgexp.com/registry/GHN5UH418165
Bart - http://www.mgexp.com/registry/GHD4U146898G

. Hide banner ads & support this website by becoming a > Supporting Member <
Tar Trekka Avatar
Tar Trekka Ross H
Sydney, NSW, Australia   AUS
1968 MG MGB MkII "Miss B"
Hi there,

The I D of the Original Tube is 5 mm, the I D of the Tag is 2 mm.

The I D of the New Tube is 2.5 mm, so the volume in the New Tube is less.

Tar Trekka.

tvrgeek Avatar
tvrgeek Silver Member Scott S
Bowie, Maryland, USA   USA
1965 MG MGB
A little physics correction:

There is flow. Not much for not long, but it works by the diaphragm displacement so a couple of CCs do flow. It is dead end only under a static condition. Now my 123 that uses a strain gauge sensor only displaces a tiny volume, so it could be considered close to a dead end, +/- volume of the air in the tube that changes due to the pressure change. Minute for sure. Air is a compressible fluid, so by definition, there is flow.

Also consider the "buffer" function of a restricted flow. The smaller hose might slightly slow the rate of advance. The restriction may reduce the pulsations that Jeff discovered rips apart the diaphragm. Managing the rate of advance with tube size, length and even restrictions into damper cans was SOP back in the 70's for emissions. However, the slowing of the advance change usually decreased the engine dynamics, so I suspect the new hose performance was not due to it increasing the friction and slowing the rate of change. The volume of the hose is insignificant as far as what the engine can supply, so I doubt just the volume changed the performance. Complicated one-way valves were used to make rate or increase vs decrease different. Remember the map of hoses on an late 70's car? what a mess!

I bet it was actually leaking.



Cogito ergo sum periculoso

Fred Winterburn Avatar
Ripley, Ontario, Canada   CAN
-Ross is correct that it would make the vacuum advance more responsive. Half the ID corresponds to 1/4 the volume in the tubing. For the pressure in the tubing to equalize to the reduced pressure in the manifold, air must be evacuated from the tubing. The more volume, the longer it will take to evacuate because the restriction at the manifold remains the same. Taking the concept to an extreme, imagine a 5 gallon air receiver between the manifold and the diaphragm. The receiver would be at the average manifold pressure, and the diaphragm would hardly move, and when it did move, the resulting movement would be delayed.
-- If the tubing is made too small, then friction comes into play that would then slow the diaphragm response, so there is a balance that Ross seems to have obtained. The copper tubing on my Morgan vacuum advance is probably no more than 1.5mm ID. Fred edit:changed Tar to Ross


In reply to # 3571974 by Tar Trekka Hi there,

My car has had 5 mm ID Rubber Tubing on the Vacuum Advance since I've had it.

The Bore of the Tag on the Carby is 2 mm.

I changed the Tubing to 2.5 mm I D.

An adapter was made from a 25 mm length of 5 mm O D Stiff Plastic Tube to connect the 2.5 mm I D Rubber Vacuum Advance Tubing to a 25 mm piece of 5 mm I D Rubber Tubing for the Distributor end.

The reduced Volume of the Vacuum Advance Tubing has made the acceleration much more responsive & smoother.

For the cost of a bit of Rubber Tubing, it may be worth a try on your MGB.

Tar Trekka.

Any day above the ground is a great day, especially "Driving Miss B".



'Anyone who likes liver, can't taste it'
'If you want to repair car electrical systems successfully, learn Ohm's Law'.



Edited 1 time(s). Last edit at 2017-08-13 11:50 AM by Fred Winterburn.

Ex-Calif Avatar
Ex-Calif Gold Member Dan D
Cincinnati, Ohio, USA   USA
1968 MG MGB GT "Bart - Yellow And Naughty"
1977 MG MGB "Red Betty"
2012 Jeep Liberty "Tank"
2014 Hyundai Accent "Skate"
We've had this debate before...

No conclusion...

Agree to disagree...



The goal - Reliable summer driver interspersed with mechanical tinkering...
Motto - "Driving fifty in the twisties..."
On Mods - It's your damn car - Do what you want. Haters gonna hate...
On SUVs - Drive your B like a soccer mom is texting her friends about how she wants to kill you...
Red Betty - http://www.mgexp.com/registry/GHN5UH418165
Bart - http://www.mgexp.com/registry/GHD4U146898G

. Hide banner ads & support this website by becoming a > Supporting Member <
Fred Winterburn Avatar
Ripley, Ontario, Canada   CAN
It only matters because air is compressible. If you compress a gas to 1/2 its volume you double the density and the pressure (Boyle's law). Conversely, if you expand the gas to twice the volume, the density is halved and so is the pressure. To get any movement of the diaphragm which has atmospheric pressure on the one side, the pressure in the pipe leading to it must reduce in pressure. The only way for that to happen is to remove air, but in a quantity volumetrically speaking, much larger than if the tubing were filled with a liquid. That's why the size of the tubing matters when it doesn't with a liquid. It's also the reason that vessels are pressure tested using a liquid rather than a gas. The pressure drop on a failure during a test with a liquid is instantaneous and the vessel is unlikely to blow up. If air is used, the stored energy is much greater to obtain the same pressure and on a failure the breach could lead to catastrophic results. Fred

In reply to # 3572200 by Ex-Calif We've had this debate before...

No conclusion...

Agree to disagree...



'Anyone who likes liver, can't taste it'
'If you want to repair car electrical systems successfully, learn Ohm's Law'.

Ex-Calif Avatar
Ex-Calif Gold Member Dan D
Cincinnati, Ohio, USA   USA
1968 MG MGB GT "Bart - Yellow And Naughty"
1977 MG MGB "Red Betty"
2012 Jeep Liberty "Tank"
2014 Hyundai Accent "Skate"
Yes and the counter theory is that the soft 5mm id tube expands and collapses changing the volume and reducing the responsiveness in the system.

We all agree (I hope) that the pressure is equal throughout a closed system.

It's the difference in changing the pressure in a steel chamber vs. a balloon.

The rigid small diameter line does not flex as the soft rubber one does.



The goal - Reliable summer driver interspersed with mechanical tinkering...
Motto - "Driving fifty in the twisties..."
On Mods - It's your damn car - Do what you want. Haters gonna hate...
On SUVs - Drive your B like a soccer mom is texting her friends about how she wants to kill you...
Red Betty - http://www.mgexp.com/registry/GHN5UH418165
Bart - http://www.mgexp.com/registry/GHD4U146898G

. Hide banner ads & support this website by becoming a > Supporting Member <
tvrgeek Avatar
tvrgeek Silver Member Scott S
Bowie, Maryland, USA   USA
1965 MG MGB
It is not a closed system. One side is variable displacement, the other an "infinite" vacuum source.

I might note, modern cars tend to use rigid tubing with rubber ends. I am sure this is more expensive than just rubber tube, so it must have some effect.



Cogito ergo sum periculoso

barry s Avatar
barry s Silver Member Barry Stoll
Alexandria, VA, USA   USA
1972 MG MGB GT
1974 MG MGB
1976 Triumph TR6
1980 MG MGB
I suggest that at least 2/3 of those on this forum barely can keep up with the discussions of routine maintenance and common faults/repairs. Yet some (e.g. Fred and Scott) insist on 'theoretical' discussions of interest to only a few. I wish Skye would create a special page for them so that they can argue to their heart's content.

OldDuffer Avatar
OldDuffer Silver Member John S
Eugene, Oregon, USA   USA
1970 MG MGB "Ella B"
1979 MG MGB "Mae B *SOLD*"
Seems like a lot of trouble for Skye to go to when one can simply choose to not read a given thread.



I was addicted to the hokey pokey but I turned myself around.

riley1489 Avatar
riley1489 Gold Member Bruce H
Great White North, QC, Canada   CAN
1953 Jaguar XK120
1959 Riley 1.5 "King George"
1973 MG MGB
The early cars were fitted with a small diameter metal line. The later cars were fitted with a small diameter rigid plastic line.
The common denominator being a small diameter line.

B



Check your ego Amigo!

DrewM Avatar
DrewM Silver Member Drew Maddock
74 MGB roadster, Southern California, USA   USA
"I suggest that at least 2/3 of those on this forum barely can keep up with the discussions of routine maintenance and common faults/repairs. Yet some (e.g. Fred and Scott) insist on 'theoretical' discussions of interest to only a few. I wish Skye would create a special page for them so that they can argue to their heart's content."

Ah no, please, half the fun here is listening to people debate technical issues even if I don't understand them. Please don't make this all about which brand of oil is "best" or how to replace a broken wiper blade. As far as I'm concerned this is the technical forum.



Drew Maddock, So. Calif.

. Hide banner ads & support this website by becoming a > Supporting Member <

To add your reply, or post your own questions

Members Sign In   or   Create an Account

Registration is FREE and takes less than a minute!


Having trouble posting or changing forum settings?
Read the Forum Help (FAQ) or contact the webmaster





Join The Club

Sign in to ask questions, share photos, and access all website features

Your Cars

1977 MG MGB

Text Size

Larger Smaller
Reset Save

Sponsor Links