Right, I've been thinking again...always a dangerous thing

Been doing a bunch of reading lately on induction, including a lot of Vizard's stuff. He (and others) go to great lengths explaining why twin's are not simply an automatic improvement over a single, how each port is only seeing one carb at a time etc etc

So the question then seems obvious: if a 1 3/4" is the SU to go for, for a largish A series beasty (ie enough air flow), why then are twin 1 3/4's not the automatic choice? This should be the best of both worlds: a straighter shot at the port, combined with the air flow of the 1 3/4". Why mess about with 1/4 or 1/3?? I would have thought the twin 1 3/4 would achieve much if not all of what Vizard was trying to achieve with his VS1 inlet (ie more carb per port).

Discuss at your leisure

I'm now going off to try and figure out why twins use such vastly different needles when they are in effect just being used as two singles

twin 1" 3/4's or hs6's/hif44 would be the bomb

i'd prefer twins over a single because you get a better manifold with twins, so your middle cylinders dont run leaner than the outer ones.

I am building a set of twin SU 1.5 inch for my re13 cam, 1360 engine, ported head etc etc

more torque down low, bling bling under bonnet

just remember you need to make sure they are in very good condtion as most carbs these days are 30 plus years old.. if they are worn out they will not give you the performance you are seeking

The problem is that Minis idile quite low compared to other engines and worn carbs show up at low idle

I am waiting to run my engine in before dyno tune / twin carb instalation

I also have a set of twin 1.75 to try one day

B

I used to run twin 1 1/4s on my 1330, the engine ran nice enough, but it was undercarburetted, I reckon it needed 1 1/2s.

Either way it runs a crap load better with the HIF44, but I like twins, if I wasn't into superchargring I'd go with twin 1 1/2s or Weber/Dellorto.

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1964 Morris 850, 1330 Supercharged - 81.8hp atws.
1975 Leyland Mini S 1100S powered - Nice and reliable.
1977 Leyland Mini LS - Project LS-T

i think a lot of people steer away from twins because of setup balance and like kiwi said wear and tear can cause problems. ive Managed to get a set of twin 1 1/2s on a 1310 set up with good results. The main thing is that the carbs are in good condition and linkages are not messy.

We run a pair of HS4s on Barney (1293A+) with a Tighe 104 cam and Toyota Corolla rockers.
It pulls great from 1800 to 7000.
Not near as quick as my 1360 beast with the 45 Dellorto, but much more civilised to drive.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

The problem with two large carbs is that you don't get high enough gas velocity at low engine speeds to provide decent torque. You need high gas velocity to better mix the fuel with the air, and with low velocity you're just not getting the best torque out of the engine at low RPM.

It'll scream at high revs, but you'll get nothing down low.

If you plan to run your engine at high RPM (e.g. on the track) then do it but if it's a street engine you'll lose a lot of flexibility.

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Simon

The adventures of an owner builder in the Tallarook Ranges

it wouldnt be worse than a single 1" 3/4 though would it as only one cyl is going at a time?

You won't lose as much as you think- unlike a Weber etc, an SU is a constant depression carb. At low revs it just won't lift the pistons high enough to drop gas velocity.
And fuel economy is still good, for the same reason.
I find the twin HS4s just as good as HS2s down low, but lots better from about 4500 up.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Actually, that's a really good point.. the SU is better at compensating for oversize for that reason.

Twin HS4's for that engine would be about right, but twin 1-3/4" (can't remember the SU designation for those) would be overkill, surely..

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Simon

The adventures of an owner builder in the Tallarook Ranges

The same would apply- just even less piston lift at low rpm.
I think TK runs twin HS6s, he says flexibility is fine.
I agree they are probably overkill on a street motor.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

agree 1.75 twins a little big for the street

plus needle selection a litlte harder as no comon base line to use hence twin 1.5 su's

Also think the twins will be a good fit with the re13 cam..

Time will tell, have driven a cooper S with twin 1.5su went well no issues fun to drive
went beter than my 1293 with 1.25 Su's

Cheers

B

I can't get the smile off my face.....
Twin 1-3/4" with a decent inlet manifold match ported to the head and good filters,then set-up on dyno and jetted is just the best.

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aha it was U I was thinking of, not TK.
Ta for the pic.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

I have found that manifold design is the critical thing with both single and twin set-ups with spring type and oil viscosity in piston in how quickly the piston lifts to throttle responce. Piston / chamber seal is another factor to check.
I have used single 2" S.U.'s on 1100 engs with a "Y" shaped manifold so that the manifold split is near carby. I have found there is little difference in performance to twin 1 1/2". What ever the air velocity is it is a matter of setting up the carby to suit.

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Research is the difference between speculation and investment. Anyone who copys some one else will always be second
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