Heater bypass when using mini heated manifold

[metalmagpie]

That doesn't fully explain why nearly every manifold was heated. One explanation I saw was that the walls of the manifold are cooled by the petrol in the incoming charge evaporating. This caused petrol to condense on the walls of the manifold which then resulted in an overweak mixture when the throttle was opened suddenly. Sounds logical and there must have been a good reason for it before emission levels became so important. The inlet manifold on my old Vauxhall used to become as hot as the exhaust manifold after a long run and it performed well and economically.

[MarkyB]

Twin carb manifolds aren't well known for the quality of their design.

The dreadful truth is probably that all sorts of factors need to be taken into account to get the most out of exploding petrol in a very confined space.

Hence Oxygen sensors, temperature sensors and computers in cars.

Like a lot of things, they aren't as simple as you think they should be, or appear on the surface.

[bmcecosse]

I would say most twin carb manifolds are well designed - just not the BMC A series one ! But they are fitted to gain performance - and in most cases do that job well - and without heating the fuel. The heated manifolds are just an emissions thing. Previously of course - there was a hot spot from the exhaust - which robbed the A series engine of several bhp. The main power advantage of the 948 Sprite engine came not from the twin tiny SUs it carried - but from the fact the inlet manifold was not heated by the exhaust! Unfortunately it had the horrid cross feed pipe - which completely ruins the air flow - and so there grew up an aftermarket for better designed inlet manifolds without the daft cross feed pipe, and still not heated!

[metalmagpie]

In the 60's I fitted a Minnow Carb to my Traveller. It transformed it! More power, almost instant performance from start-up (no choke) and better fuel consumption. All together. It was said that it was all due to improved atomisation and no restrictions being required to to give a venturi effect: the butterfly did it all.
I have just purchased a Reece-Fish carburettor which is apparently the same design. I will be fitting it soon and see how it goes. No manifold heating required, or even desirable and it loves tortuous manifolds, it is said.
Yours, in hope of better things.....(always an optimist)

[MarkyB]

"loves tortuous manifolds"
How does that work then?

Even more interesting, where did you get the carb from?

[metalmagpie]

First, off e-bay, look for "Reece Fish" or "Minnow" or "Fish" in car spares. Most of them are for VWs and are configured as downdraught but the side chamber is rotatable to give side draught and they are largely self adjusting fuel wise.
They work by using a hollow butterfly spindle with multiple orifices into the throat. The shape of the butterfly constitutes the venturi and induces a mixture over the whole width of the choke tube which automatically adjusts as the spindle is rotated.
There is a pickup arm in the side chamber which moves with the accelerator and acts as an accelerator pump to give good acceleration.
If you put Reece Fish in "Ask" there is a lot of better info.

[Ratbag]

IIRC Oselli were selling their water heated HS6 manifold for the Mini 'A' series well before 'emissions' were heard of. Without it connected, mine used to ice up....

[bmcecosse]

Doesn't matter if the manifold ices up. The Fish etc carb was hugely promoted by Clive Trickey - famed Mini tuner and racer of the 70's. Can't see it liking a tortuous manifold though - manifold design is ALL about straight lines into the head to maximise air flow. Once an SU carb is running and the piston has risen to the top - the only air flow restriction is the slight venturi over the bridge - and the throttle spindle and butterfly. There are various schemes to minimise both - the 'bridge' should only be tampered with for out and out racing! The piston can be encouraged to rise more quickly by using a lighter spring (or even none at all) and thinner oil in the damper - but can then gasp horribly on sudden acceleration - but works well on the track! I believe there is a certain 'science' in setting up the Fish carbs just right - or they can be pretty hopeless. But I have never used one myself - so will be v interested to hear progress!

[metalmagpie]

I wonder sometimes what the air/fuel mixture in a manifold moves like: it must be pretty violent in there! First inlet valve 1 opens: mixture rushes forward to that cylinder, then stops and reflects as the valve closes. Next up is No 3, the mixture flow reverses, violently I would guess, to fill that cylinder. No 4 is next, not such a problem. Then No 2 with another reversal of flow. It is hard to imagine how it all succeeds especially with the reversal of flow occasioned when a valve closes the movement abruptly and reflection occurs.
Early cars had the Carburettor at one of the manifold so that the flow through the latter was all in one direction, maybe they had something, but then early engines were all long stroke and low rev and everything occured in a much more stately fashion.
I saw a manifold on e-bay just now which comprised an ox-bow loop with the carb at the apex. I think it was someones idea of the straightest smoothest path practicable. It could work as long as the reflection from one branch passed the carb just as a valve on the other branch opened. But that suggests is would have a very narrow rev range and that is not what is wanted.
I suppose common designs are more concerned with cost and simplicity of manufacture and just accept that the flow inside is just chaotic.
The Fish claims to create a mixture in which the fuel is much better mixed and so moves more uniformly in the manifold. The design eliminates any restriction bar the butterfly and uses multiple "jets" across the butterfly. Certainly my experiences of it were favourable and setting it up was simplicity (I didn't have a rolling road) The car was extensively used for rallies and road trials as well as being my everyday vehicle. The carb had come off an Austin 1100 which had similar use with some success.
If my Fish ever arrives, it's been a week coming now, I shall revisit the improvements I found in the '60s and report.

[bmcecosse]

Yes - the breathing (and exhaust) system of the A series is what limits it's power compared to say a Ford BDA unit! The firing order of 2134 does show there are consecutive pulls on the first branch - then the same on the second branch - hence the use of 8 port (and sometimes 7 port) heads which ease the problem considerably. There is a 'ram effect' on the second cylinder in each pair - especially when using a twin carb setup - which gives uneven mixture distribution on the cylinders. I am keen to hear how the carb performs - Trickey certainly couldn't see past them ( yet slightly later, Vizard doesn't mention it in any depth) - in fact I think almost all the 'single choke' Mini racers used them - although some used 45 DCOE Weber with one choke blanked off.

[Ratbag]

Doesn't matter if the manifold ices up.

Certainly did matter in my case - ran like a pig until the hot water was piped up.....

[bmcecosse]

I wonder how all the folks with twin carbs and unheated manifolds manage.........

[Ratbag]

I wonder how all the folks with twin carbs and unheated manifolds manage.........

...perhaps by pulling warmer, less moisture-laden air from somewhere? (another option I explored with some success).

Phil.

[Kevin]

I wonder how all the folks with twin carbs and unheated manifolds manage.........

Good job there are not many on Moggies then.

The firing order of 2134 does show there are consecutive pulls on the first branch

The proper sequence is 1342 by the way.

[MarkyB]

I wonder how all the folks with twin carbs and unheated manifolds manage........."

According to;http://en.wikipedia.org/wiki/Carburetor_heat

This cooling has two causes: the air pressure drops due to the expansion caused by engine suction and the increase in speed through the butterfly valve (in part-throttle operation) which drops the air temperature; and the liquid gasoline being introduced into the airstream must evaporate, and the heat of evaporation is extracted from the airstream, cooling it"

So splitting the flow between two venturis and the greater surface area of two carbs is probably enough to negate the effect.

[metalmagpie]

1342 is only "proper" when you start at 1! I have seen manifolds with a division so that the mixture should split to serve either 1-2 or 3-4.
1432, as ford do it can also be re-written as 3241 which is not much better than BMCs layout. I guess 4 carbs and very straight short manifolds as motorcycles do it, would be much nearer to ideal but much harder for "mr average" car owner to live with.

[bmcecosse]

" The proper sequence is 1342 by the way. "

Or - 3421 or 4213 or 2134......... All the same Kevin! I listed it my way to demonstrate the two consecutive pulses per inlet port drawback of the A series 5 port head.

[Kevin]

1342 is only "proper" when you start at 1!

Which is how all engines are numbered number one is always first.

I guess 4 carbs and very straight short manifolds as motorcycles do it, would be much nearer to ideal but much harder for "mr average" car owner to live with.

Well believe it or not that idea is not new as it was tried in the 60's and wasn't ideal, I may have a photo of the set-up if I can remember where it is, if I can find it do you want me to post it up.

[MarkyB]

As icing isn't an issue with twin carbs why bother with four?
Or have we moved away from talking about water heated manifolds?

[Kevin]

My reply to metalmagpie was about the 4 carbs he mentioned and if he wanted to see a setup I would have posted a seperate post to show them (if I can locate the set-up) out of interest.