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Twin SU carbs on your 2.25?

Started by NoBeardNoTopKnot, Jun 05, 2024, 12:47 PM

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NoBeardNoTopKnot

Saw this on eBay and had to smile. When you order an MGB and they drop a Series, and you've not got the heart to complain this is the one for you. 

It's not out of turn really, back in the day several outfits offered Twin SU conversions, this is one of them. I've got pix of those somewhere. For now there's this:

https://www.ebay.co.uk/itm/156237179508?mkevt=1&mkpid=0&emsid=e11021.m43.l3160&mkcid=7&ch=osgood&euid=9172f2275590420f9e69b01916805983&bu=43056896059&ut=RU&osub=-1%7E1&crd=20240602064258&segname=11021

Today's ACR HS6 conversion is top-of-the-hill, HNJ's a bit of lash-up but good enough.  Today we have a twin SU set-up, seller wants top-dollar? ... well, hey-ho... if your pocket-book allows, why not?

Bradley66

Quote from: NoBeardNoTopKnot on Jun 05, 2024, 12:47 PMSaw this on eBay and had to smile. When you order an MGB and they drop a Series, and you've not got the heart to complain this is the one for you. 

It's not out of turn really, back in the day several outfits offered Twin SU conversions, this is one of them. I've got pix of those somewhere. For now there's this:

https://www.ebay.co.uk/itm/156237179508?mkevt=1&mkpid=0&emsid=e11021.m43.l3160&mkcid=7&ch=osgood&euid=9172f2275590420f9e69b01916805983&bu=43056896059&ut=RU&osub=-1%7E1&crd=20240602064258&segname=11021

Today's ACR HS6 conversion is top-of-the-hill, HNJ's a bit of lash-up but good enough.  Today we have a twin SU set-up, seller wants top-dollar? ... well, hey-ho... if your pocket-book allows, why not?

ACRs isn't all it's cracked up to be , In practice, on a standard engine you wouldn't notice the difference between that and the HNJ conversion, and why not ? Because it's pointless.

Wittsend

People have enough trouble setting up and tuning a single carb. Twin carbs just doubles your troubles ???

If you want sports car performance, buy a sports car.


 :Denis

Peter Holden

#3
Back in the day whe I cas young and foolish I ran a mini Cooper for a while.  One of the first mods was to bin the twin SUs and fit 1 big one.  It went much better.

Peter

Alan Drover

Someone I knew fitted an SU HS6 to an 803cc Morris Minor engine. I can't remember how it went but I would say badly.
Series 3 Owner but interested in all real Land Rovers.
"Being born was my first big mistake."
"Ça plane pour moi!"

NoBeardNoTopKnot

#5
Quote from: Bradley66 on Jun 05, 2024, 05:21 PMACRs isn't all it's cracked up to be , In practice, on a standard engine you wouldn't notice the difference between that and the HNJ conversion, and why not ? Because it's pointless.

Mixing participles has me confused - I think I have your meaning:

1. The ACR is one casting - and water-heated - it has to flow better.
2. The HNJ elbow half does it.

Are they pointless?  There will be  differences. I have the ACR version. If unknown to me   the ACR was swapped for any HNJ jobby - until I opened the bonnet - doubtful I'd notice. The difference will be small. When i bought my ACR HS6  it was the only option. ACR stuff is good, there's no claim to be cheap, it's the proper job, I'd buy again.

I heard Minis like one SU over twins too. I get the idea twin-carbs on cooking-engines were as much about marketing blurb as performance. More to set-up and if there are gains, those gains are lost the moment they're out-of-wack. More to do to keep on top of it. For some, twiddling carbs is therapy..



LR never did get carbs right. This makes for a ready-market for carb conversions over the years. As now with HNJ, every man and his dog has made an SU elbow. I've seen APB; Lynx; Boxer; Spenwin;  home-brews.

Pix: ACR single;  twin SU Spenwin;  twin SU by Boxer:

Craig T

#6
A lot of American V8 builders make the mistake of thinking a bigger carb creates more power.
The smallest Edelbrock or Webber 4 barrel carb is 500 CFM (Cubic feet / minute) Holley however make a 390 CFM carb and that tends to run smoother on a small V8 like the Rover 3.5 litre.

The larger the carb the slower the air passes through it so too big and the fuel doesn't get atomised correctly. The ideal fuel mixture is on the verge of a vapour when it enters the cylinder but a large carb can cause fuel droplets to form again on the walls of the intake manifold and cause misfiring, spitting back through the carb, bore wash etc.

My Chevy V8 has 8, 50mm chokes on it, one for each cylinder. I can only get away with that as it's fuel injected so the fuel gets mostly atomised upon injection under high pressure. If they were carbs I'd probably be down to 40mm or so chokes to get it running right.

Sometimes bigger isn't always better.

I guess twin carbs on a 2286 engine would need smaller SU's than the 1 3/4" versions commonly fitted to the V8's

Craig.

Wittsend

#7
My Mini went like stink with an 1½" SU (donated from my MG) on a water heated inlet manifold  :gold-cup
(with a 4-into-1 straight through exhaust, gas flowed head following David Vizard's guidance)


:tool-box

NoBeardNoTopKnot

#8
There's a whole heap of perennial boy-racer  myths that never get debunked. Ever bigger-zhorsts won't usually work either.

For most of these myths, we're required to consider major manufacturers and their teams of automotive-development engineers idiots. Whilst I'm on, I'll add braided brake-hose to the fallacy list.

My single HS6 is 1 3/4".

Old Hywel


Myles (Mr Fox) Davison

Quote from: NoBeardNoTopKnot on Jun 06, 2024, 04:58 PMThere's a whole heap of perennial boy-racer  myths that never get debunked. Ever bigger-zhorsts won't usually work either.

For most of these myths, we're required to consider major manufacturers and their teams of automotive-development engineers idiots. Whilst I'm on, I'll add braided brake-hose to the fallacy list.

My single HS6 is 1 3/4".

Quite off-topic, but as an ex-Caterham owner, my favourite quote was when one of the technical staff (Jez Coates I think) was asked "What's the advantage of the new aero-shaped (in place of round-tube) wishbones, he replied "I don't know - ask the marketing department!" :cheers

NoBeardNoTopKnot

#11
Whilst this is a carb thread, I've gone full closed-loop gas injection.

Some of you might be familiar with the Youtuber 'Number 27'. He's not far from me and is interested in doing a video feature on my set-up. If you're familiar with his channel you'll know this departs from his usual format.

Number 27

My SU HS6 is still there, only these days its use is more akin to a throttle-body. I've gone this way because I can. Like twin-carbs whilst there are gains it follows the law of diminishing returns. It has to be just about as grunty and as cheap to run as any LR petrol 4 pot ever gets.

Seems likely Number 27 will  shoot this summer..  it'll be more about what I've done than the vehicle it was done to. If it comes off you'll see this on his channel.



JohnR2

Quote from: Peter Holden on Jun 05, 2024, 06:29 PMBack in the day whe I cas young and foolish I ran a mini Cooper for a while.  One of the first mods was to bin the twin SUs and fit 1 big one.  It went much better.

Peter
Yes, but the twins look so much cooler!

NoBeardNoTopKnot

#13
Yup, don't care what they say - twins do look good. With the stock ZenSolWeb offerings on mine I have the hand off the seller of those twins. Just because...

What does that bracket do?

Wittsend

Quote from: JohnR2 on Jun 07, 2024, 09:01 AM
Quote from: Peter Holden on Jun 05, 2024, 06:29 PMBack in the day whe I cas young and foolish I ran a mini Cooper for a while.  One of the first mods was to bin the twin SUs and fit 1 big one.  It went much better.

Peter

Yes, but the twins look so much cooler!


Yes, but those in the know, know there is little or no gain and less hassle in setting up. ???

:cheers-man