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Twin 1 1/4 SUs https://www.ausmini.com/forums/viewtopic.php?f=2&t=59702	Page 1 of 2

Author:	850man [ Sat Dec 19, 2009 12:52 pm ]
Post subject:	Twin 1 1/4 SUs
I have just finished a full reco on these of for a customer. Going on a 1100 engine & will either have the original air filter box or HP filters fitted.

Author:	74snail [ Sat Dec 19, 2009 2:03 pm ]
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Very nice ! What I want to see is a set of triple 1 1/8 SU's with custom manifold on an 850 ! Just on those BMC log manifolds , they do have a reputation of not having the best flow rate, I know people have ported them , what is the best method ? .

Author:	GT mowog [ Sat Dec 19, 2009 3:01 pm ]
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74snail wrote: Very nice ! What I want to see is a set of triple 1 1/8 SU's with custom manifold on an 850 ! Just on those BMC log manifolds , they do have a reputation of not having the best flow rate, I know people have ported them , what is the best method ? . Probably to replace it with a non BMC / Leyland one ! Seriously though, I'm not too sure about the earlier factory manifolds, but the later ones (fitted to later MKII Cooper S and all Clubman GTs) are not so bad. They don't need a lot of 'port' work as such, it is mainly that the balance pipe is too big. Maybe some here has fitted a restrictor within? It wouldn't be too difficut as the ends of the balance pipes have removable steel plugs (welsh plugs infact) that give easy internal access. Nice reco job there 850man.

Author:	Mick [ Sat Dec 19, 2009 3:20 pm ]
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There is nothing sexier than a rebuilt set of twins on a mini imho... I hope you don't mind me including pictures of my kids in the thread

Author:	850man [ Sat Dec 19, 2009 5:25 pm ]
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Mick wrote: There is nothing sexier than a rebuilt set of twins on a mini imho... I hope you don't mind me including pictures of my kids in the thread I like the original fuel hose clamps, mmm nice

Author:	drmini in aust [ Sat Dec 19, 2009 5:43 pm ]
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Vizard reckons the major flow problem with all the BMC twin manifolds is the sharp edge (on the engine side) where the balance pipe meets each port. It needs a good radius put on it. This is less easy with the steel location rings being used, but can still be done.

Author:	MG Rocket [ Sat Dec 19, 2009 7:42 pm ]
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I think it was Morris who mentioned the idea of filling the balance tube with resin to down size the bore, but I don't remember how it was done.

Author:	drmini in aust [ Sat Dec 19, 2009 7:46 pm ]
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They removed the end caps, filled the whole tube with epoxy? resin, and then drilled it out once set.

Author:	Morris 1100 [ Sat Dec 19, 2009 7:49 pm ]
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drmini in aust wrote: They removed the end caps, filled the whole tube with epoxy? resin, and then drilled it out once set. The result was a nice straight inlet runner with a balance tube of about 1/2" dia from memory. Just a slight radius on the entry to the balance tube and it was all done.

Author:	drmini in aust [ Sat Dec 19, 2009 7:52 pm ]
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The problem is not only the size. It's the VERY poor shape where it meets the ports.

Author:	Morris 1100 [ Sat Dec 19, 2009 7:59 pm ]
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drmini in aust wrote: The problem is not only the size. It's the VERY poor shape where it meets the ports. I only saw the resin filled manifold once about 25 years ago. A racer was selling all his old Mini stuff and I picked up a few bargains like a 648 cam but I left a few things that I didn't need at the time like a Cooper S block with a 648 roller bearing cam and the resin filled manifold! I should have bought the lot.

Author:	GR [ Sat Dec 19, 2009 8:49 pm ]
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HI All Once again BMC did get it right with the size of the balance pipe,yes you can fill it in to get better flow and some times it will get you some more power, but its there for a reason, first of all the balance pipe should be 1/2 the dia of the inlet port, the reason for this is that when you run big camshafts with su carb you have to brake the strong pulse up returning to the carb because it will make the piston flutter/pant bounce up and down,call it what you like but at low speed it will flood the engine. Now you can go to big with the balance pipe but this cam be over come by placing a restricter between the to carbs with a smaller hole in it, this small hole can alter the power and torque of the engine through out the reve range so some times you have to play to get it right,but BMC have all ready sorted this out for you, as they run the 649,sprint and supa sprint with this manifold in the works cars and it worked. Now if you have a look at the big 6cyl healey their balance pipe is about 1.5 ins in dia with the manifold blocked in the centre with a 3/8 hole in it they run 2x 2ins SU,good one BMC you nailed it again. Graham Russell

Author:	drmini in aust [ Sat Dec 19, 2009 9:02 pm ]
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Graham, does the Maniflow steel twin SU manifold with the very small balance pipe give any more power than the BMC manifolds? Or, is it just marketing hype?

Author:	MG Rocket [ Sat Dec 19, 2009 9:03 pm ]
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Lets say you run a much longer manifold about 8''. Would the pulse be much weaker by the time it got to the carby...which in turn would allow a smaller dia balance pipe?

Author:	GR [ Sat Dec 19, 2009 9:21 pm ]
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Hi Kev Yes the mainflow does work better with small cams but not with big ones, I redesigned the 1.3/4 mainfold for maniflow which Keith CALVER uses on his historic mini i made it longer and bigger in dia with a bigger balance pipe but it could do with a bigger one yet i think, when we did a back to back on the two manifolds it went from 121hp to 143hp but it's a tight fit with the master cyl. Graham Russell Hi Mr rocket yes the longer runner does help a little bit but you still need the balance pipe.

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