what is its main purpose? is it really needed? why do the racers not bother with them?
If you had a rotten manifold could you just cut the pipe off and weld the ends closed?
Do they still need it when fitting twin carbs like on Pembletons or Lomax type things or is it redundant?

just wondering....

Tim,
the main purpose of that pipe is to heat the part of the manifold directly under the carb, as this helps with vapourising the mixture.

As a liquid evaporates, it draws heat from its surroundings and in certain circumstances (most commonly with high humidity and temperatures slightly above freezing) this can cause carburettor icing if the preheat source isn't present.

So, both the UK race cars and those 'specials' with no preheat pipe are prone to carb icing, however the racers put up with it as at an ambient temperatures when icing isn't a problem, the cooler mixture will give more efficient cylinder filling and 'slightly' more power.

Most 'specials' stay indoors in cold, damp weather...

ken.

In my shed there's the remains of a brand new manifold. the previous owner has cut the inlet pipes around three inches from the cylinder head end, and removed the carb preheater pipes and exhaust manifold completely. All I've got left is a carb mounting block with two fuel supply pipes sticking out. However, what he's also done is fit oil pipe fittings where the preheat pipes used to enter, so I can modify an oil cooler circuit to heat my carb. This will be useful if I ever build an engine with shortened barrels, I suppose.

The real purpose is to have more torque at 3000 to 4000 rpm. It equals the pressure in both exhausts causing extra flow in the exaust, so more fresh air can into the engine meaning more torque.


But only by 3000 to 4000 rpm; at topspeed it increases a little bit of power thats why racecars don't bother (they suppost to work only at topspeed).

So now you van see that Citroen prefers more torque about mid rpm then topspeed with the 2cv

a swiss guy made a whole item of this at his site including making a valve between them, open at midrange rpm closed at topspeed.

And ofcourse it keeps the carburator from freezing like you say.

http://megasquirt-de.serviceline.ch/suc ... ex0001.htm it's in German


DieVorwärmung ist wichtig für Vergaserbetrieb im Winter, immer noch wichtig für Singlepointeinspritzung und wirkt als "Drossel" bei Multipoint-Einspritzung! Wer seine Ente nicht als Winterfahrzeug verwendet, kann es wohl auch bei TBI riskieren auf die Vorwärmung zu verzichten, auch hier treffen 2 gegensätzliche Phänomene aufeinander: Auf der einen Seite bedeuten kalte Ansaugrohre Kondensation (langsamere Gasannhme, Entmischung) auf der anderen Seite kann bei einem kalten Ansaugrohr mehr Luft (und somit auch Benzin) in den Motor gelangen. Als Anhalt kann man die 24h-Rennenten hernehmen die meines Wissens alle ohne Vergaservorwärmung fahren, allerdings auch ohne Interferenzrohr und ob das sinnvoll ist, bezweifle ich! Zumindest wenn man auch den unteren Drehzahlbereich nutzen will. Dieses Verbindungsrohr kann bei geringeren Drehzahlen das Laden/Entladen positiv beeinflussen. Auch ein Blick auf moderne BMW-Boxer-Motorräder zeigt, dass diese ein fettes(!) Verbindungsrohr zw. den beiden Auspuffrohren haben - allerdings weiter hinten!
Ich werde also die Vergaservorwärmung entfernen, sodass kein Wärmeübergang möglich ist, wohl aber ein Stück Rohr mit einem simplen Absperrventil einsetzen. Ist der Einfluss kaum zu merken werde ich das Verbindungsrohr komplett entfernen und die Löcher im Auspuffkrümmer für Lambda-Sonden nutzen.

but we all know thats crap and the exhaust serves no function...... its just a bit pipe



right wheres that link to a German tuner..... that quotes an American(?) engine builder.....

MEEoow

but yes all of the above! racers usually dont have standard exhaust, nor do they pootle about at 3000 rpm in the winter with a cold engine or have a choke fitted

a cold racer is a pig to drive

Citroen put oil-heating of the carb on very early GSs.

They soon gave it up as a stoopid plan - just compare the length of time before the exhaust gets hot to the amount of time before oil gets hot, especially on a cold morning, and you'll figure out why...

Citroen isn't Citroen when they could let it out they would, wouldn't they.

So it's not just a bit of pipe.

Even today's BMW (boxer engined) motercycles got this pipe, so it can't be just a littlebit of pipe.