Twin Carb Advantage?

[Bidz]

Can anyone explain the advantage of running twin carbs, and if that's worth much on a standard 1098?

[panky]

A single 1 1/2" SU is preferable to the twin 1 1/4" for a normal road engine unless it is tuned and you are hammering it all the time. A resonance is set up in the inlet manifold by the pistons that acts on the carb, this is ironed out in the manifold somewhat with a single carb but twins are subject to twice that resonance and only work well at higher revs. Bigger single for me any time.

[bmcecosse]

There is NO advantage to the twins - except that the manifold is NOT heated up by the exhaust. You need to remember - each cylinder breathes independently of the others - and so each breathes through only ONE 1/1/4" carb... There is NO cross flow from one carb to the other port. Indeed the cross tube actually disrupts the nice straight flow from the carb into the port. There is a small advantage in blocking off the cross tube at each end - and nicely blending it into a smooth flowing tube. That done - there may be a small advantage overall - due to the cooler/more dense ingoing charge By far the best improvement (as mentioned above) is to fit a single 1.5" carb - ideally on an separate unheated alloy inlet manifold. Worth about 5 bhp on an otherwise standard 1098 engine.

[mike.perry]

No performance advantage but if you have a pair of twins lying around they will give the engine that classic 60s look[frame][/frame]

[kennatt]

Add on the constant balancing problems,stay with a 1.5

[alexmcguffie]

Its like the Irish man giving directions, it all depends where you're starting from If starting from a standard engine with stanbdard 1 1/4" carb then a pair of twins will give more flow and potentially more power. Obviously a single 1 1/2" is easier to set up and no balancing is needed but then its all down to what you have available. I've been running twin SU's(dontated free to me) on one of my Minors for 20 years (plus modified head and better exhaust) and they work just fine and return 50mpg on a good run.

I'm going to duck now before I get shot at

[bmcecosse]

That's just the point Alex - there is no improved flow...... The only slight advantage is the cold charge, and possibly easier flowing air filtration. There's no great mystery or hassle about setting them up - few minutes with a tube to the ear and job done.

[alexmcguffie]

Let me get my head around this then... 1 x 1.25 has the same potential flow rate as 2 x 1.25?

[bmcecosse]

Unfortunately yes. Because each cylinder only breathes through ONE carb - so it only sees the same 1.25" carb. There is no cross flow - and as I explain above - it's actually better to block off the cross tube at each end and 'streamline' the internal shape so the flow from the carb is directly into the port. There is then a slight benefit because that flow is in a nice straight line and not having to negotiate the S bends of the standard manifold. This is where the Weber DCOE carb gains an edge on normal twin SUs - the Weber manifold has no cross tube to interfere with flow. I ran twin 1.5" SUs on my 998 track Mini - on home made long 'Weber-like' manifolds. it ran very well indeed.

[alexmcguffie]

I'll trust my experts over your expertise

[Chipper]

Another snag with the twin carbs, not often mentioned, is that the throttle linkage setup is a bit 'loose' with them, which can result in a kangarooing effect if you're trickling along in traffic - the throttle response tends to be a bit jerky.

For this reason, I ditched my 1275cc Midget engine's twin SUs and went over to a single 1.75" HIF, which gives a far smoother and more progressive throttle, making it far easier to drive in traffic, yet no loss of power, and better fuel economy.

[philthehill]

Another snag with the twin carbs, not often mentioned, is that the throttle linkage setup is a bit 'loose' with them, which can result in a kangarooing effect if you're trickling along in traffic - the throttle response tends to be a bit jerky.

For this reason, I ditched my 1275cc Midget engine's twin SUs and went over to a single 1.75" HIF, which gives a far smoother and more progressive throttle, making it far easier to drive in traffic, yet no loss of power, and better fuel economy.

I would politely suggest that the problem with the kangarooing effect is more to do with the drivers foot than problems with the carbs. Once the foot starts moving the kangarooing problems start and it is then difficult to stop. If the carbs are set up properly, with the right piston damper springs fitted and the right viscosity oil in the damper tube you should not experience any kangarooing. Properly set up the engine will pull away from low revs in a clean and progressive manner.[frame][/frame]

[les]

I think I know what he means by 'loose', there is intentional play in the linkage but once the cable is pulled that play has no effect on driveability,( unless they are out of adjustment) the carbs are as one !

[Chipper]

Perhaps it's just the effect I noticed with the twin-carb setups I had on my Triumph Vitesse Mark 2 2-litre, 1275cc MG Midget engined Minor and an MGB GT then - all seemed a bit jerky when trickling along in traffic, no matter how much fettling I gave the carbs/linkages.

[philthehill]

All the cars I have owned, driven and fitted with twin SU carbs - including my Minor above with twin HD4 - 1 & 1/2" SUs on a 998cc engine have always pulled away cleanly and have never given me any problems even when driven at a crawl.
Yes there is a built in 'looseness' in the linkage but set up properly does not and should not effect carb operation.
If you think you have problems with two what about when four SUs are fitted.

http://sucarb.co.uk/news/loud/

http://sucarb.co.uk/news/endofprescott/

The car originally started out as a pre war Morris Minor and was eventually fitted with a Hudson straight 8 engine and driven by Peter Skinner of SU fame.

[welshrat]

I can only talk from personal experience, drove my mog with original twins (professionaly set up) for about 10 years, a few years ago switched to a Hif 45. Its like driving a different motor, quicker, flying up hills that were always a bit of a struggle and generally more responsive. I did however replace the exhaust system at the same time (from 1 inch to 2 inch) so I am guessing that this had an impact on increased power as well.

[Chipper]

I presume you mean the HIF44 (there were HIF38 and HIF44). I also have had one fitted to my Traveller since the mid 1990s and it's been excellent.

[welshrat]

Sorry HIF 44, been a long day, MG ZT alternator change on the drive, why do they make modern (ish) cars so complicated, 3 hours just to get it out. Not all that confident that I will get the new one back in! Give me a minor any day, ten minutes tops.

[Chipper]

Agreed, some modern cars can be a pain in that respect. I recall having to get a replacement alternator for my mum's Isuzu-engined Vauxhall Cavalier 1.7TD, which had some sort of oil pump on the back of it, unique to that engine - had to venture into a London breakers yard to get hold of one.

Which 2" exhaust did you fit, and has it affected the noise level much?

[panky]

A mate of mine has just got a programmable mandrel tube bender and is starting up making exhausts (he already makes body panels for Commer PB's), I can see if he's willing to make Minor one's if anyone is interested. He's making standard bore and 1 3/4" ones for Commers (which is plenty big enough for a Minor) mild steel or stainless - even experimenting with extractor manifolds

Here's an example of his Commer down pipe compared with a rusty original.[frame][/frame]