Author Topic: Dual carb head - suggested length of custom manifolds?  (Read 1201 times)

Offline Fast 58 BSA

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My 67-1106 dual carb head is slowly rising from the ashes (after lots of welding and grinding...) and it is time to think about manifolds for my 389/689 monos. Per suggestions from the good members on this site I will run stock valves, the carbs are 1 1/8". I will have some manifolds made up, so the question is, what is the preferred/suggested length? I will run 8:5/9:1 pistons and 357 cam. Appreciate any thoughts and as, always thanks in advance for all the good help!
Telemark, Norway

'58 Golden Flash project bike - reborn as angry dual carb cafe racer/rgs rep

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Re: Dual carb head - suggested length of custom manifolds?
« Reply #1 on: 23.11. 2012 04:35 »
 G'day Fast58BSA,
                         depends on whether you want top end revs or lower down torque. For top end on the A7SS racer I made plates out of 1/4" alloy and kept the carbs close (2 x 32mm concentrics) with 2" velocity tubes. Now on the road as an A10 I have 1 1/2" manifolds with 2 x 30mm Kehins and 2" velocity tubes. I think the magic figure is 11" from valve to end of induction tract (tube or filter).
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
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Offline JulianM

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Re: Dual carb head - suggested length of custom manifolds?
« Reply #2 on: 23.11. 2012 08:05 »
Hi,
If you have the room, why not make a flange with a tube in the middle so you can mess around and do some real world testing?
Then when correct you can make it permanent.  I think the manifold length is more important than total length in all areas other than full throttle or close to full throttle.

Trouble is that the "perfect" length will only ever be at one particular rev point and may be far from ideal everywhere else.  The best generally is a good compromise position but where exactly is a shit load of calculations all based on rather a lot of informed guessing.  Testing is best!

I have 15" from valve to filter.
9.5" from carb outer end to valve and the rest is a long trumpet, but how this will perform is anyone?s guess until I try it.

good luck,
julian
55 Ariel HS mk1,
58 BSA Super Rocket Scrambler special,
60 BSA Gold Star, Jim hunter desert racer,
64 Norton N15CS,
65 Triumph TR6 SS,
66 Triumph Bonneville,
71 AJS Stormer,
71 BSA Rocket 3,
71 Laverda 750SF,
71 Laverda/Egli 750SF, Race bike,
73 Norton Commando 850,
74 Moto Morini 3-1/2 Sport,
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Offline Fast 58 BSA

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Re: Dual carb head - suggested length of custom manifolds?
« Reply #3 on: 26.11. 2012 13:13 »
Thanks for suggestions *smile*

Experimenting would be nice, but I am no alloy welder, so I would like to get it (close to) right the first time. I guess I will aim for around 11" total length, valve to carb inlet, and see how things work. Quick thought, Isn't a long intake only good for high revs? More velocity of mixture?

There are still way too many things to sort out before this bike will see the road. But it is important to enjoy the small steps (like broken head now having all its fins back!) *smile*
Telemark, Norway

'58 Golden Flash project bike - reborn as angry dual carb cafe racer/rgs rep

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Australia
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Offline Fast 58 BSA

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Re: Dual carb head - suggested length of custom manifolds?
« Reply #5 on: 26.11. 2012 19:13 »
Muskrat,
Thanks for the links *smile*. I'll check them out
Cheers
Telemark, Norway

'58 Golden Flash project bike - reborn as angry dual carb cafe racer/rgs rep

Offline Rocket Racer

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Re: Dual carb head - suggested length of custom manifolds?
« Reply #6 on: 26.11. 2012 20:33 »
In Mick Walkers BSA pre unit twins book on page 161 there is a picture of Hugh Andersons A10 running twin carbs. I recall seeing that in the mid eighties with extremely long inlets. As you say a race engine.
My A10 is running marine reinforced hose, which gives me some ability to vary, however my rigid frame is currently a limitation until I shorten the barrel which sould give me room to extend the inlets length further.

If you look at the off the shelf T140 type manifolds they are fairly short with a rubber hose.

For the road I suggest a shorter inlet would be fine, but having a rubber component is good for isolating heat and vibration from the carbs.

For looks you cant go past 10TT9's for bling  ;) but for real world use concentrics definitely.
A good rider periodically checks all nuts and bolts with a spanner to see that they are tight - Instruction Manual for BSA B series, p46, para 2.
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Offline JulianM

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Re: Dual carb head - suggested length of custom manifolds?
« Reply #7 on: 28.11. 2012 12:18 »
Morning.

Trumpet length is never the same for different engines but in general the consensus is Long for low speed and short for high speed running,
Just watch a variable trumpet system on a formula 1 race car and you will see them doing exactly that!
However you must realize that it's the gas velocity that matters, (cubic ft./min through a given orifice) therefore you will get very different results from the same trumpet from one engine to another depending bore / stroke ratio, cam choice, and especially engine displacement!

Saying that, The single most important area on any trumpet is the shape of the actual trumpet itself. We have found massive gains in flow by just re-shaping the bell.   Not only that but the pulse wave effect in the inlet really has only a very small effect on the engines most of us are playing with and is not really worth worrying too much about below 7000 rpm!  You would be much better advised to think about the ram effect of the incoming air, this has a very large effect on engines with fairly heavy cams and can significantly increase power by ramming the mixture in even when your piston is quite high up on it's compression stroke! Remembering that the dynamic pressure is proportional to the square of the inlet air speed, so by making the passage narrower or longer the dynamic pressure is increased.  That's why we don't just go for massive inlet ports and the biggest valves possible!

Julian
55 Ariel HS mk1,
58 BSA Super Rocket Scrambler special,
60 BSA Gold Star, Jim hunter desert racer,
64 Norton N15CS,
65 Triumph TR6 SS,
66 Triumph Bonneville,
71 AJS Stormer,
71 BSA Rocket 3,
71 Laverda 750SF,
71 Laverda/Egli 750SF, Race bike,
73 Norton Commando 850,
74 Moto Morini 3-1/2 Sport,
74 Bultaco Sherpa 350,
76 Beta Cross 50,
77 Moto Guzzi Le Mans Mk1,
80 Ducati 900 MHR,

Offline Housewiz

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Re: Dual carb head - suggested length of custom manifolds?
« Reply #8 on: 28.11. 2012 17:08 »
How about checking the measurements on the factory manifolds for a pretty good baseline to work from?  One member of this Forum must have a set?  Those boys at the factory must have spent lots of time/effort/$ to come up with the manifolds they sold to the public.

Although those boys/bikes were coming over here to go flat out at Daytona so maybe based on the above posted comments, those manifolds may have been too short for the street. 

This is a great thread for this rookie as I will be trying to sort all this out as well. 

Thanks,

Steve

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Re: Dual carb head - suggested length of custom manifolds?
« Reply #9 on: 28.11. 2012 18:53 »
 The originals were 1 5/8". Have a look here  http://atlanticgreen.com/a10dpm.htm
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7