Twin carbs on a 1275

Post by **Sidney'61** » Sat Jul 04, 2009 11:15 pm

Hi,
I run a 1275 with a single carb
How much difference would the twin carb set up generally make to performance or enconomy?

Not like I can afford to do it, just wondering...

Post by **Peetee** » Sat Jul 04, 2009 11:35 pm

Current thinking suggests negligable performance increase and greater fuel consumption.
If it's for the 1275 then, having known the car before your ownership, I can confirm it would be a pointless exercise. And the 948 needs a better cylinder head as a frst step.

Post by **Sidney'61** » Sat Jul 04, 2009 11:56 pm

Thanks Pete.
After a quick clean up and top up of the carburettor today I noticed a considerable increase in power! (maybe I should have done that months ago

) so the single carb will do me nicely, I'm just being inquisitive

Post by **bmcecosse** » Sun Jul 05, 2009 8:51 am

Just depends on size of the single carb - and how it's set up.
Fully modified 1275 will work very well with twin 1.5" SUs - even twin 1.75" for top power. But generally - a single 1.75" HIF44 on a decent alloy manifold will work well enough - and still give reasonable economy.

Sun Jul 05, 2009 2:23 pm

as I recall from reading the tune your a series book.. the advantage of twin carb comes from the improved inlet design that is possible. a twin carb on its own is no better than a single carb because half of the time it is sitting around doing nothing! Apparently some after market twin carb manifolds perform worst than the standard manifold single carb..

they do look nice though..

Post by **mike.perry** » Sun Jul 05, 2009 9:09 pm

Twin carbs look good under the bonnet, with 3 branch exhaust and pancake filters, but if you are no good at setting them up (me) then stick with a decent sized single. I'm sure the engine went better when I had the HS4 on it. Only took it off because the standard exhaust kept dropping off the Marina manifold.

Post by **bmcecosse** » Sun Jul 05, 2009 9:28 pm

I just posted this (in answer to a similar Q on tthe Mini Forum)

"I agree - it's not so bad on a 998 - but a 1275 really struggles on twin HS2s. The balance pipe on the standard manifold is more hinderance then anything - so effectively - remembering that each cylinder draws individually - then each port draws through only one small 1.25" HS2 carb. Fit a good single HIF38 with 1.5" bore on a decent alloy manifold - now each port can draw through 1.5" carb - much better"
In fact - the standard BMC twin SU manifold is just about the worst possible manifold you can get !

Post by **mike.perry** » Sun Jul 05, 2009 9:50 pm

Can I use an HIF 44 instead?

Post by **bmcecosse** » Sun Jul 05, 2009 10:11 pm

If the 1275 has been modified- it will be even better. On a standard unit - it may be a little bit overkill - the 38 may give better fuel economy with only very little loss of max power potential.

Post by **Kevin** » Sun Jul 05, 2009 10:36 pm

Next time we meet Mike you can have a go in mine same engine as your Traveller with an HS4 and set up properly.

Post by **Peetee** » Mon Jul 06, 2009 8:03 am

Can I use an HIF 44 instead

You could but the small benefit would only be aparent if the whole intake system was matched to suit. The '38 is more than up to the job if matched to a free flow filter, water heated intake and healthy cylinder head.
With any change the best results almost always come from a rolling road set up. Expensive, but the only way to have your cake and eat it.

Post by **bmcecosse** » Mon Jul 06, 2009 8:05 am

Only if the rolling road operator knows anything about A series engines - very few and far between. You would need to look for a Mini specialist.