Triumphrat

ohiorider · 04-05-2008

Someone recently asked if there was a source for a manifold to convert the Bonnie to a single carb bike (a modern day TR6). I have a different question, and wondered if any of you have any thought about this particular design. The photo shows an old Volvo B16B or B18B engine with two constant velocity carbs, the SUs that were used on practically all Brit cars, as well as several series of the Volvos.

http://s115.photobucket.com/albums/n...alancepipe.jpg

The Volvo design ran a large diameter tube between the two intake manifolds. It was called a 'balance tube'. As you can see in the photo, it isn't a small piece of rubber tubing like some of us have used to tie the intake manifolds together via the vacuum ports, but a metal tube that was probably 3/4" inside diameter.

I never at the time questioned why it was there, aside from thinking that even when a carb wasn't feeding its primary cylinders, it could supply some mixture to the other cylinders. Sort of a primitive V-Boost!

I know there are some Brit car fans on the forum who have a good answer about the balance tubes. Wonder what a balance tube type manifold would do on a Bonnie twin carb engine?

Bob

CAPTDON · 04-05-2008

I asked about using SU carb(s) a while back & got booed off the block!! All said "Why use an old out dated carb"? I know the HD boys used to use the 2" SU w/ some good results. I posted another thread about using a Weber dual throat carb, & got some positive feedback. Someone here in Sweden did a setup that supposed to have some real good acel reports! I figured w/ an accel pump it would have to give some real good "off the line" results. Its a mater of making an intake that would fit a Bonne. I never followed it up but would like to! The dual throat Weber is a fantastic carb for peformance cars - so why not a Bonne? They put a pair on a 1600 cc car so why not a single on a 800 cc Bonne? Apparently it will work well, but gotta make a manifold that will work? If you get any info in that dept- let us all know? Sounds like something that might be a little special power booster! Pretty simple to keep tuned as well. I was an expert on SU's since most of my cars growing up were English & I kinda liked the ole SU. Granted today they might be considered OLD but a Weber isnt!! See what you can find!!

mikeinva · 04-05-2008

Well the first thing you have to think about is our bonnie motors might not be hot rods compared to jap bikes but they flow way more air per cubic inch then most cars.If you add up the intake valves there bigger then most hot rod small block chevys are.why would you want to hook the intake together the way they come you can adj them per cyl and give each what they need.I worked on alot of brit cars with SU carbs and i have to say they where some of the worst carbs ever put on a car.
weber made pretty good carbs but with the good flat slide carbs we have that you can put on these bikes who would want something thats not as good.
Get yourself some mikuni 42mm get them tuned and forget it.You will be happy you did they run so much better then the stock carbs.

SHILOH · 04-05-2008

I don't know what I would expect from a "balance tube" on the Twin engine. But I don't remember any of the "old su" carbed engines without them. As much as we enjoyed those carbs and cars, I don't recall anyone removing the balance tubes or blocking them.

It's a good question. Surely someone will know what to expect from a balance tube option on a twin.

ohiorider · 04-06-2008

Cap'n and Mike, what I was asking was ..... what was the purpose for the balance tube between the intake manifolds on older Volvos, MGs, Triumphs, etc. I wasn't proposing using SU carbs on our bikes (though I'm sure a pair off of a Bugeye Sprite would be about the right size).

What I was questioning was why all these cars used a relatively large diameter 'balance tube' between the intake manifolds. I think some of the older Britbikes may have used balance tubes, also ..... don't know for sure, since my old Beezer was a single carb, and the only Triumph I remember was a TR6 single carb model.

I do know that our Bonnies don't have a balance tube between left and right intake manifolds, unless it is internal to the head.

Bob

mikeinva · 04-06-2008

bob they did that so if the carbs where not dead on you wouldnt know as much.The old carbs with vac slides where kinda on one day off another.if they didnt have the tubes a 4 cyl. motor would shake your guts out if carbs where off.Iwas one of the poor suckers that got stuck working on mg and volvo back in the 70s and early 80s .most mechanics hated them things but the where fun when right.
I wish you lived closer one ride on my bike with the 42mm carbs and you would be hooked.
I dont think the guys that are hooking the vac line from one intake to the other on syock carbs are doing there selfs any good it keeps the slide from giveing each cly. what it needs when it needs it.(that is if vac slides work like they are supose to i still dont trust them)

ohiorider · 04-07-2008

Mike, that makes sense. I remember how I used to struggle to keep the old Volvo 444 carbs balanced. I got so frustrated with the slightly worn SUs, I attempted to make my own downdraft manifold and install a set of Holley downdrafts. Wow .... if we think we have jetting issues with the Bonnies! Needless to say, the Holleys came off, and the SUs went back on.

When I had the SUs right, they were fine. But about every couple of weeks, because of out of balance carbs, the softly mounted engine and tranny would begin shaking right to left as I gave it the gas, to the point where the long floor shift would physically whip back and forth. So, it was back to using either (a) a tube to listen to the carbs, or (b) a pair of "L" shaped wires placed in the carb dashpots. When those were aligned, you were good to go, at least for a couple of weeks.

But I loved that car.

Bob

ivar · 04-07-2008

Such "balance tubes" are used on some 2-stroke snowmobile engines, where they are called boost bottles. Had one on my 120HP 700cc Polaris twin w/ 2 keihin PWK 39 carbs. It did nothing for top end power but cleaned up th bottom/mid some. Took it off b/c it ripped holes in my carb boots.

The figures below are from a Yamaha 110hp 700cc 2-stroke triple.

Quote:

Engine Boost Bottle Theory

Boost Bottle theory is that the boost/equalizer /tube/bottle is a passageway between intake manifolds with a fixed volume, usually the CC of one cylinder or larger, a (momentary) storage container for pre-atomized fuel/air mixture.

Since the fuel/air mixture is at a high velocity while traveling through the intake manifold, guess what happens when the reeds suddenly close? The mixture has inertia and physics would suggest the fuel/air mixture has to dissipate this inertia, through a tube maybe?

This inertia (pulse) forces the pre-atomized mixture that's already in the tube from the previous cycle into the next intake manifold that's reeds are just opening and ready to receive the fuel/air mixture, this is like a little supercharger to each cycle. This all happens so fast at higher rpm's that it is most effective at lower and mid-range where you are on/off throttle.

It takes venturi velocity to atomize the raw fuel from the main jets. The tube/bottle has a charge of pre-atomized fuel that is instantly ready when you go from off to on throttle. It assists in filling the cylinder for a fraction of a second until the carburetor venturi gets enough velocity to fully atomize the available fuel. The advantage is that it brings up the torque and horsepower sooner in the bottom of the rpm range. These have been Dyno tested and proven to work, they show 10hp low end, 4hp at mid rpm, and a 2 hp increase on top end.

SHILOH · 04-07-2008

Quote:

Originally Posted by mikeinva

bob they did that so if the carbs where not dead on you wouldnt know as much.The old carbs with vac slides where kinda on one day off another.if they didnt have the tubes a 4 cyl. motor would shake your guts out if carbs where off.Iwas one of the poor suckers that got stuck working on mg and volvo back in the 70s and early 80s .most mechanics hated them things but the where fun when right.
I wish you lived closer one ride on my bike with the 42mm carbs and you would be hooked.
I dont think the guys that are hooking the vac line from one intake to the other on syock carbs are doing there selfs any good it keeps the slide from giveing each cly. what it needs when it needs it.(that is if vac slides work like they are supose to i still dont trust them)

I always wonder, suspected that it have something to do with carb variation.

Yeah, I could adjust the SU perfect Sunday afternoon, and Monday morning things were different. I wonder, but haven't the Zenith and now the CVK's, etc gotten rid of most of the SU problems (maintenance, maintenance, maintenance which I think really was the end result of the brass plunger/oil assembly used for damping).

mikeinva · 04-07-2008

looking back i think what made them so hard to keep right was that cheap ass metal they cast stuff out of back then and the rubber we used back then .just look back at most of the alum. motors of thoses days you couldnt keep them from leaking oil or anti freeze.Now a days alum engines are great.