Any thoughts on Twin SU carbs for a 1098?

[Moggie94]

Has anybody here tried to put twin SUs on a Minor 1000?
What are the results/ problems with doing this and what other changes were needed to make it work?

I would imagine the cylinder head and exhaust might need altering too for any major gains...

[irmscher]

Why use two when one will do

[Moggie94]

Because there is only so far you can go with a single SU and I plan to go over that boundary

[Alex'n'Ane]

Because there is only so far you can go with a single SU and I plan to go over that boundary

Have a look on the mmocym fb page, there was a long discussion yesterday. Irmscher is right, Also why do you say you will be "going over that boundary" A single su can go up to 1 3/4" inch (standard is 1 1/4") and that is plenty for even a 1275. So unless your going non a series?! Then a single su will be best! What were you planning on this engine??

[autolycus]

I've owned A-series engined cars with both single and twin SU carbs, and recall that an MG1100 I looked after for many years (twin SU) ran sweetly and economically, but I've not done back to back tests to compare them, let alone attempted to optimise needles and other settings for different configurations.

I think we should remember, however, that greater engineers than, dare I say it, even those here present, have chosen the twin SU route over a single carb. Rolls-Royce fitted a single carb to the Silver Dawn in the late forties, but twin SUs to the near-identical Mk VI Bentley engine where power mattered more than it did on the Silver Dawn. And Rolls-Royce knew a thing or two about engine development. MG used twin SUs for many years, with some notable success.

"Those who cannot remember the past are condemned to repeat it".

Of course, if setting up twin SUs is too much of a challenge...

Kevin

[bmcecosse]

Let's see all the details of this 'over the boundary' 1098 engine - can hardly wait......
And yes - the first step for your 1098 is a single 1.5" SU which on a decent manifold will out perform twin 1 1/4" SUs easily. Even BL eventually realised that and gave up fitting twins, going the large single route on the 1275GT/1300GT etc etc. MGs only had twin carbs because they were 'sports' cars.......and back in the day -it was de rigueur to have 'twin carbs' cf the Bentley mentioned above...... Of course the RR engine mentioned was hopelessly undercarbed - so any increase in air flow was going to help.
And of course - as I think most of us know by now - the essential way forward for a 1098 engine is a much better cylinder head, and remember it must not be revved over 6000 -ever.

[SGTBILKO]

Wasn't BLs change to single carbs for the go-faster versions of their cars at least partly due to cost-cutting (one carb is cheaper than two)? Otherwise the tried and tested way to go seems to be as bmcecosse suggests, an improved cylinder head. It's all been done before and David Vizard wrote a rather good book about it.

[bmcecosse]

Exactly - and in that book he explains the little understood fact that each cylinder breathes independently - the BMC twin carb arrangement with it's hopeless crosspipe manifold ensures that each port still only breathes through ONE small carb - there is no worthwhile crossflow in the pipe - in fact - it actually disrupts the straightline flow from each carb into the port. The actual advantage of the BMC twin setup is that the manifold is NOT HEATED, and so nice cool/dense charge gets to the ports. If the twins fed into some sort of plenum chamber they might work rather better flow wise - but throttle lag would be awful... The larger single carb ensures each port has access to the larger bore of that carb, with the obvious disadvantage of the twisty path from carb to port - hence the comment re 'good' inlet manifold - which obviously should NOT be heated....

[dunketh]

Why make things more complicated than they need to be?
As for 1100cc power, I dont think a carb will prove your bottleneck.

[IslipMinor]

Because there is only so far you can go with a single SU

Yes, but how far do you want to go? Our 1380 is on a single 1 3/4" HIF6 SU and produces 112bhp on a rolling road, with a 'good' inlet manifold (Titan), which is both flowed and heated!

Lots of debate and opinions about heat or not; the only back to back comparison I have is from many years ago with a non-standard twin SU setup on a Sunbeam Rapier I had. The cold/warm up performance needed lots of very careful playing with the choke, which disappeared after adding water heating to the manifold.

One day I might block the heating off to the Titan and see if I notice any difference? Next time I do a rolling road session I will see if I can do a back to back comparison on power.

[bmcecosse]

The water heating is nothing in comparison to the exhaust heating that the standard cast inlet unfortunately provides.... The difference is not likely to be significant re water on/water off. If I ran my car all year round I would probably connect the heating in winter and discard in summer. The charge in the cylinder is heated to ~ 600 C by the compression before the spark ignites it anyway - the only difference is in the density of the charge being sucked into the engine. In winter - water heating may just prevent fuel 'pooling' in the inlet manifold - so may help economy slightly.

[mike.perry]

For that classic 60s tuned engine under the bonnet, you can't beat a pair of twin carbs with a decent exhaust system. You may not get any extra power out of them and they may be tricky to set up but they sure look more impressive and in keeping with the period than a single HIF[frame][/frame][frame][/frame]

[chrisryder]

on the topic of 'to heat or not to heat', i happened to be browsing the vizard bible the other day and to paraphrase: 'i would keep the manifold plumbed in all year around as there are great advantages in winter and imperceptible disadvantages in summer'

he suggested that heating the manifold to coolant temp makes little difference in a hot engine bay that will be having head head conducting through to the manifold anyway.

mine is now reconnected for the winter.

[bmcecosse]

That engine shot with the 'old skool' twin carbs looks real 60's - until you notice the alloy rocker cover..........
I hadn't read that in Vizard - must be a later version than mine..

[chrisryder]

the digital photo with '2009' in the corner doesn't help the 60's vibe either...

i'll admit that the quote is from the 'economy' chapter, so i'm not sure whether it's a general statement, or with a view purely towards economy.

[bmcecosse]

Ahh - not read the economy section! But yes - it should help economy slightly as I suggested by eliminating any pooling.

[mike.perry]

[quote="bmcecosse"]That engine shot with the 'old skool' twin carbs looks real 60's - until you notice the alloy rocker cover..........
I believe alloy rocker boxes were available in the late '60s, not so sure about the K&N filters though.

[bmcecosse]

Or the alternator..........

[mike.perry]

Alternators fitted to police Minors, therefore period

[Moggie94]

If I could just bring it back on topic, how would it be if I set twin carbs to deliver the same amount of fuel and air that one carb did previously?


The performance would be the same but surely the better flow would help economy