Twin carbs

[PAULJ]

Hi all Can anyone tell me the advantage or disadvantage to twin carbs. Once set up how much more power can be expected on a 1275.
Thanks

[chesney]

Twin carbs - OK if you want to spend a lot of time balancing them I understand.
A lot more effort in comparison to a single carb for not much more gain..

[PAULJ]

Thanks, my van is heavy !!! I hav an HIF44 on it. I have read that 1.5" carb is the most efficient but a Hif 38 not much better on the van. Im interested in what twin carbs would run like but are expencive to just try. Love to hear from anyone who has gone from 1,3/4" single to twin 1.25"s

[chrisryder]

If you're familiar with the Otto cycle, on a 4 cyl 4 stroke engine (unless you're looking at a Ducatti 'big banger') then only one cylinder is drawing charge it at any one point.

Each one will only draw in through one 1 1/4 carb...

Ok, twin carb inlet manifolds tend to have a 'bridge' between the two sides, but how much charge will come across that incredibly narrow tortuous pipe?

Twin's tended to be fitted to cars with packaging limitations... Mini, Midget... And certainly Mini's ditched them in favour of a bigger single carb later on...

Not to mention, the outlay of buying, servicing, and tuning, twice as many carbs.

On the plus side, they look lovely!

[bmcecosse]

We have moved on from twin 1.25" carbs -as above - NOTHING flows through the cross tube - so each port ONLY sees a single tiny 1.25" carb.... In fact - the cross tube spoils the flow - the best manifolds have NO cross tube. Your HIF 44 is by far the best option for a 1275. It could deliver 100 bhp (and more if tweaked) if the engine is otherwise capable. Next step is 2 X HIF 44 - on straight manifolds with NO crosstube...

[mike.perry]

I run a Midget 1275 engine in my Traveller with twin 1.25s, K&N pancake filters and a large bore LCB exhaust. Can't make any claims for performance as the engine is well used but it looks good[frame][/frame][frame][/frame]

[philthehill]

bmcecosse is right in that the balance pipe is not there to carry a fuel charge it is there to help equalise the depressions/pulsations generated in the inlet manifold tracts.
I run a 1380 A+ with two 45 Webers using all 4 chokes and there is NO balance tube between the inlet manifolds and for my engine it works well. I have run various 'A' series engines with twin SU carbs and have found some engines do seem to work better with a balance pipe and for others it makes no difference.

[IslipMinor]

A single HIF6/44 will produce over 100bhp and is very simple to maintain and adjust on the rare occasions it is required - the last rolling road session showed 112bhp at the flywheel on our 1380 with the HIF6 that has been fitted since getting the engine nearly 15 years ago . An update is due later this year, but I would not really expect much difference, as it is only the cam that has been changed to a Piper BP270 from a Kent MD276.

[PAULJ]

Thanks for all the info. I must admit Im now a bit stumped. As I said, my van is heavy 1.2 tonne's and looks like a brick but would have hoped to get mor than 55 MPH out of it. 1275 new rings and shells. compression is 12 bar all four pots. HIF44, BDL needle. large LCB exhaust Timing and tappets spot on. Givenn the weight and shape am I asking too much of this engine or would a cam change give me the pulling power I want ?. One advantage is I onle ever see traffic jams in the rear view mirror [frame][/frame]

[Sparticus]

haha that is the most incredible looking thing... I think you might be expecting a lot from it...especially with a caravan attached to your car

[PAULJ]

My wife and I Drove it from Somerset to IOM for the TT. Spent two weeks living in it . Great fun and now I want to go touring with a trailer and two motorbikes. I dont want to go fast. Im happy doing 55 but not with my foot to the floor

[IslipMinor]

Lots of questions, now I can see that it is not quite a standard 'van', and has a coefficient of drag way off the 'not good' end of the scale!

The standard 1275 head is the 12G940, so that is a good start. Is this what is fitted and have you done anything to it? What cam is fitted to the engine? What diff ratio do you have?

Do you know what revs the engine is doing at 55 mph - is the speedo reasonably accurate?

What is the gearbox? I hope it is from a Spridget, with the needle roller bearings for 2nd and 3rd gears - I would guess you spend a bit of time in the lower gears pulling quite hard?

As a broad spec, definitely clean up the head and open up the valve throats in the time-honoured way. I would think a 4.22 diff would be the highest, an MG Metro or Kent 266 cam would be the most to go to, and maybe not. If you have access to an HIF4/38 carburettor, I would be tempted to try it as it will bring the torque curve slightly lower, and may allow the engine to pull better.

[PAULJ]

Well the engine is from an 82 Ital. standard head, cam and carb. I cleaned up the head as best I could. re seated all the valves. New rings shells timing chain and oil pump. 3.7 diff. type 9 gear box. Here's the thing. Pulls well in first ,second and third. In third I can see 50 MPH on the sat nave Put it in forth and there is no engine note change just creeps up to the 55 and on a good day in 5th 60 MPH but thats hammering it. Ive no idea what its reving at, at 55 but sounds painfull. Having said that, It is a side exhaust which comes out just behind my ear. I do plan to go back to the 4.2 diff which I have under the bench and try a 38 carb. Im thinking of taking it to a dyno tester just to see what I am getting at the wheels. As you say it may just be because im pushing a brick.
What I would like is a twincam or anything else a bit more powerfull than what I have.

[IslipMinor]

A 3.7 diff + 5-spd gearbox is a tad OTT!! I have a 3.9 and 5-spd, and it pulls very well in 5th, but the weight and aerodynamics are somewhat different!

The HIF38 isn't going to help at this stage, but I would think that fitting a 4.22 diff should make a huge difference.

[PAULJ]

I fitted the 3.7 with 5 speed because I have that in my two door saloon and it drive very well. I will change the diff and report back. Thanks for the advice.

[daveyl]

I would think it's the noise factor that is the problem. I had a very similar set-up to yours and finally got myself a 4.2 diff. It has made the gearbox/drive much more flexible.

[Sparticus]

You can have my 4.2 diff off my traveller and i'll have your 3.7?

[Bazzalucas]

I don't dispute the claims for the single carb- since I have nothing to compare my experience to- but I will say the dual carbs on my 1275 are not hard to sync or maintain. Neither were the carbs on my MGBs, Triumphs (Strombergs!), or Datsun 240Z. I've always found the worries about that aspect slightly amusing.

[bmcecosse]

Yes the 3.7 is the problem with that weight and drag. Even with the 4.22 I doubt you will ever use 5th gear. The Ital engine cam is very soft.....but you don't then really want a high revving cam either. Actually a supercharger would probably be your best bet for good slogging torque.

[MarkyB]

I have the feeling that you are fighting the laws of physics to an extent, and the law of diminishing returns.
Even if you can get more power out of the engine it will be used up trying to push an essentially flat surface though the air.
I think you will actually be getting a low pressure area at the back which will help to hold it back too.

Quite a lot more power is unlikely to achieve much more speed.

How fast does it go on a steep downhill gradient?