Homemade single carb manifold; any good?

fiatfan · Dec 10, 2015

What´s the judgement on this piece? Had a friend weld it together for me a long time ago, had a 34mm Amal Mark II on it. Don't think it worked that well, could have been the carb as well :? .

Homemade single carb manifold; any good?

Tommy

mschmitz57 · Dec 10, 2015

I don't think it would seal very well between the ports without some way to secure it.
All the 2-1 manifolds I've seen have bolt holes inside the manifold to secure the 2 inner points.
Imagine that appendage shaking about on an idling Norton motor with only 2 studs to hold it on.

baz · Dec 10, 2015

mschmitz57 said:
I don't think it would seal very well between the ports without some way to secure it.
All the 2-1 manifolds I've seen have bolt holes inside the manifold to secure the 2 inner points.
Imagine that appendage shaking about on an idling Norton motor with only 2 studs to hold it on.

Some 2 into 1 inlet manifolds have all four screws some only have two I prefer the 2 bolt ones I just put some silicone in the 2 inner threaded holes no problem also my inlet manifold is made of steel I made it over 30 years ago to take a 32 mm concentric but now its bored out to 40mm with a tm 40 mikuni plus a k&n air filter never had a problem with air leaks or falling off plus no chance of injesting screws from inside the manifold cheers baz

Torontonian · Dec 10, 2015

Yes ,not good.

Rohan · Dec 10, 2015

The problem with angled manifolds like that is that they tend to bias the flow towards one cylinder.
Not much you can do about it either.

Other than cut the stub off, and weld it on straight....

dennisgb · Dec 10, 2015

It looks like the builder was trying to get a longer manifold which is a good thing.

Would be better if the two pathways were equal length though. Difficult to do on a Norton.

It should work IMO.

dennisgb · Dec 10, 2015

Rohan said:
The problem with angled manifolds like that is that they tend to bias the flow towards one cylinder.
Not much you can do about it either.

Hmmm...with only one cylinder at a time being on intake?

Rohan · Dec 10, 2015

dennisgb said:
Hmmm...with only one cylinder at a time being on intake?

With the carb pointing straight down one cylinders throat, which cylinder do you think this might favour most !!!

Some BSA twins had problems with favouring one cylinder, and that was with a symmetrical manifold.
The answer was to slant the carb very slightly towards the cylinder getting less.

V8's fed from one carb very rarely get even performance from all cylinders.
Thats why one carb per cylinder (or injector) gets soooo much better performance.
The gas flow doesn't have to dither changing direction, since it often has only milliseconds of flow anyway...

Some Benelli 4s and 6s toyed with only one carb, feeding into a common manifold.
But they found it killed much of the performance, ditch that idea..

British sports cars have been fitting twin SUs to the sportier models,
this has been known and understood for a very long time.
Triple SUs on the sports Jags.
6 twin-choke Webers on Ferraris etc !

dennisgb · Dec 10, 2015

Rohan said:
dennisgb said:

Hmmm...with only one cylinder at a time being on intake?

Click to expand...

With the carb pointing straight down one cylinders throat, which cylinder do you think this might favour most !!!

Some BSA twins had problems with favouring one cylinder, and that was with a symmetrical manifold.
The answer was to slant the carb very slightly towards the cylinder getting less.

V8's fed from one carb very rarely get even performance from all cylinders.
Thats why one carb per cylinder (or injector) gets soooo much better performance.
The gas flow doesn't have to dither changing direction, since it often has only milliseconds of flow anyway...

Some Benelli 4s and 6s toyed with only one carb, feeding into a common manifold.
But they found it killed much of the performance, ditch that idea..

So where does the fuel go when either cylinder is on intake? If it favors one then the other one must leave the fuel behind. That is impossible because there is a vacuum during intake. What you are saying just makes no sense to me. There could be a small difference in turbulence due to the shape of the manifold and the slightly different path length but it is so small as to be unapparent.

Rohan · Dec 10, 2015

Read a book on gas flow in inlet manifolds.
Of which there are many.
Its all about getting maximum fuel/air into each cylinder.

Gas flow in inlet manifolds is PULSED.

If you could watch it in slow motion,
there are long periods of nothing,
followed by brief moments of intense gas flow.

Its a lot more complicated than that, especially when multiple cylinders are involved.
Thats why folks can specialize, and make a fortune selling their Edelbrock or Offenhauser manifolds etc.

Or why (1950s) Triumphs or Nortons can offer their dual carb racing manifold kits, when the stock bike only had a single carb.

snail · Dec 10, 2015

I did this on a Bonnie about 1980. The cylinder with the "straight shot" got most of the fuel, , making the other one run very lean.
I "fixed" it by adding a sheet metal baffle which was adjustable from the outside. Without the (SU) carb, you could see the vertical baffle more or less bisecting the the inlet of the manifold. In effect, it became two unequal length tracts with considerable leakage between the 2.
I don't know what it did for performance ( Bunt hardtail frame !) but 100MPG was easy; good when you've only got a 1 gallon tank!

Would I do it today? No.....

dennisgb · Dec 10, 2015

Rohan said:
Read a book on gas flow in inlet manifolds.
Of which there are many.
Its all about getting maximum fuel/air into each cylinder.

Gas flow in inlet manifolds is PULSED.

If you could watch it in slow motion,
there are long periods of nothing,
followed by brief moments of intense gas flow.

Its a lot more complicated than that, especially when multiple cylinders are involved.
Thats why folks can specialize, and make a fortune selling their Edelbrock or Offenhauser manifolds etc.

Or why (1950s) Triumphs or Nortons can offer their dual carb racing manifold kits, when the stock bike only had a single carb.

Wow!

I worked as an engineer for 40 years...don't need you to TEACH me about flow.

Done talking to the resident expert on everything. Sorry I wasted my time.

Rohan · Dec 10, 2015

If you know so much, then why ask such basic questions ?
Multiple carbs appeared on engines a long time back, this stuff has been known for a l-o-n-g time

dennisgb · Dec 10, 2015

Rohan said:
If you know so much, then why ask such basic questions ?
Multiple carbs appeared on engines a long time back, this stuff has been known for a l-o-n-g time

Not worth it. You are the expert. I bow to your superior knowledge and now will block you so I don't have to see your worthless posts and attacks on others.

fiatfan · Dec 11, 2015

Ok, this was unexpected, didn´t mean to start a fight.... :shock: . Don´t like it, but we´re all different people, so "sh-t happens" I guess.
Back to my question;
I´m with "Baz" on the sealing issue, believe that could be solved.
About the angle; that´s obviously done for a reason, wouldn´t fit if pointed straight backwards.
"Snail"; This was also made for a hardtail, a rigid-operated original Commando frame.
The big issue here, does one cylinder get more than the other? It´s obviously not the most optimum way to do this, but it wasn´t built for a race bike, just tried to make it work with one carb. (had problems with my chromed, yes you read it right, CHROMED Amals. Who´d have guessed?) Didn´t know much about these things at the time. However the manifold is empty inside, creating a sort of chamber, wouldn´t that even out some of the pulses you talk about? Or is it too small?
Tommy

Rohan · Dec 11, 2015

dennisgb said:
worthless posts and attacks on others.

Sorting the wheat from the chaff ?
This isn't exactly rocket science, the very basics of gas flow in ports has been around for centuries - steam engines requiring similar.
As have most of the fundamentals of engines and engineering.

If by 'attacks' you mean pointing out basic errors in posts, better than all being lead up the garden path ?
Or, we could all grin and bear it with total nonsense, like other forums

P.S. Just to demonstrate the importance of correct inlet manifolds, there is a very famous example in Norton history, no less.
The dommie Model 7 of 1949 had a long bifurcated port in the iron head.

This was a total disaster (killed the performance), and delayed getting the new model to market while remedial action was taken.
The long split manifold was replaced with a short stubby little alloy U-shaped manifold.
Fixed all the problems.

More than "There could be a small difference in turbulence due to the shape of the manifold"...

Rohan · Dec 11, 2015

fiatfan said:
Ok, this was unexpected, didn´t mean to start a fight.... :shock: . Don´t like it, but we´re all different people, so "sh-t happens" I guess.
Back to my question;

Don't worry about it, this forum has its share of trolls that take offence with anything.
Nit pickers too, all of us...

I have a short stubby little 2-1 manifold for a Commando, could just make a version like that ?

Jim Comstock showed a large 2-1 that he made to suit a larger single carb, to keep much of the the performance (and more)
Not sure if he showed it fully, or said if they were for sale.
Getting air cleaners on them is always an issue, not much room in a C'do.

snail · Dec 11, 2015

Yep, the SU dashpot was chromed too!
My manifold was a "Y" shaped construction made from tube.In reality it was pretty much a plumbing fitting. Not sure about the air flow, but I got the impression that it was the inertia of the fuel droplets that was the problem.
No problem with an air filter, it didn't have one. It did have parts of an alarm clock as a throttle linkage!

At the time, it seemed like a good idea ( to a 19 yr old) but there are better ways of doing things today. Why didn't I have my machine shop and knowledge when I had that bike and a drag bike?

Rohan · Dec 11, 2015

snail said:
Why didn't I have my machine shop and knowledge

Knowledge is a wonderful thing - in 20/20 hindsight.

As more than a few have observed, if we lived life backwards, how simple everything would be.
Starting life with a machine shop could open so many doors...

Fast Eddie · Dec 11, 2015

You have a small carb, feeding a big chamber, feeding small ports. I am NOT a flow expert like other posters, but that seems to me that its likely to cause undesirable effects on gas velocity and pressure waves.

But as you say, if performance is not critical to you, give it a try and see.

Worst case it is doesn't work and you'll either have to fit a more proprietary single carb manifold or fit twin carbs.