twin su's

[markattard]

Would anyone kindly know (and give some details) on how to install a twin su system of carburation?

Also could someone please advice on what power gain is obtained with such a system.

[nebogipfel]

FWIW I'd say don't fit them

You will gain nothing in power over a sensible size single carb and it will certainly use more fuel and not run so well.

I'm sure others with much more knowledge of these matters will be able to suggest the perfect carb if you supply your engine spec'.

Are you listening bmcecosse ?

The David Vizard book on tuning the A Series will give you gospel and verse but it is not an easy read if you don't have at least some engineering background

[bmcecosse]

Why do want to do this - what's your aim ? The best upgrade you can make to a standard car ( carb wise) is to fit the MG Metro inlet manifold along with an HIF 38 SU carb (both cheap on ebay). You can keep the standard exhaust manifold - just need to work up a slight sweat cutting the old inlet section off it with a sharp hacksaw. This will release an additional 5 bhp or so on an otherwise standard 1098 engine - without the hassles of fiddling with twin carbs which (if twin 1.25" SUs) would give about the same power increase. Note that most of the power increase comes from the inlet charge now going into the head COLD - on the standard Minor inlet it gets roasted by the co-joined exhaust and this reduces the volumetric efficiency. To get even more power you need to be looking at upgrading the cylinder head with larger valves and improved porting - this then allows the engine to suck in more air which it then turns into power. Better head with standard carb gives little improvement - the improved head breathing is strangled by the standard carb and so both need to be improved at the same time. Further power gains come from improved exhaust manifold and system - and then camshaft changes. My own 1098 engine has a larger valve head and the Mg manifold/HIF38 and it goes very well indeed - far better than standard. If your engine has already been upgraded to 1275 cc - then the same rules apply - but now you want the larger valve head from an MG Metro, same manifold but now with HIF 44 carb.

[plastic_orange]

There is nothing wrong with twin carbs properly set up - if you have them, then go for it, but if not, do what bmcecosse suggests.

Pete

[bmcecosse]

Twins are ok - it was the 'old' way. But they soon wised up and just fitted bigger single carb on a good inlet manifold. The engine only breathes air in to one cylinder at a time - and the cross-flow on the standard (ghastly as it turns out) BMC twin carb manifold is almost non-existant and in any case the rough edge caused by the cross flow tube ruins the main flow from the carb into the head. One mod which has been used successfully is to completely block off the cross tube with plastic metal - and then each carb gets a straight flow into it's port. However - the engine breathes 2134 in other words it pulls twice in quick succession on the first port, then twice on the second port. This causes flutter and uneven flow in the carbs - and so a good single carb (the HIF is a much improved SU carb) gets a much smoother flow with nice even pulses. It also costs less (one carb instead of two) and is easier to set properly. You can hand modify the needle profile without worrying about creating an exact copy for the other carb! You also don't have to frighten the wits out the insurance co by telling them you have fitted 'twin carbs' ! But as Orange says - if you have the twins sitting there - just use them.

[nebogipfel]

Several of the lads I know in the classic bike world have abandoned twin carbs for the sake of ease of maintenance, smoother running and economy.

Interestingly one chap has taken the twin Amals off his Commando and fitted a single SU. He says it performs just as well but the improvement in economy is amazing.

Adding to what BMC has said about the standard twin SU manifold as fitted to numerous 60's cars David Vizard is very scathing about it, saying it is just about the worst way a manifold could be constructed

As others have said, I suppose if you've got a pair sitting there ............. it will be interesting to see what you think of any power gain acheived and if there is much of a drop in fuel economy.

[bmcecosse]

And Burt Munro (The Fastest Indian - since bikes have cropped up) was always convinced a single carb was best!

[plastic_orange]

I've got 4 on my bike (not SU's though) - 132 rwhp from 893cc.

Pete

[PAULJ]

did you see phil63 titled midget head, perhaps you too need to talk

[KirstMin]

And if you were going to do it properly it would be advantageous to take it to a rolling road. Where of course setting up 2 carbs will cost you more money.

[plastic_orange]

I've never had a problem setting up twin SU's, but take the point that a single can do a similar job with slightly less hassle. A number of tuners are now using Bike carbs to great effect, but this would require an inlet manifold fabricated - may be an idea (if not already done) for an enterprising individual. You usually get 4 carbs for around £50 - £100, so share with a friend to reduce costs
Best place for SU spares nowadays:
http://www.burlen.co.uk/

Pete

[bmcecosse]

Cross flow 8 port head and 4 bike carbs works very well!
Twin SUs are fine to set up - provided you can use standard needles. Advantage of single carb is you can home modify the needle with a hand file!

[Kevin]

Also could someone please advice on what power gain is obtained with such a system.

To simplfy the above, none over a correctly set up single HIF easier to maintain and stay in tune and far fewer parts to look after .
What is your current set up and what are you trying to achieve.