Carbs

Well both kind of. They're really rated by choke size(s). Bigger choke (or more chokes) = more fuel. You need more fuel the bigger the cc, but if you want more power you'll need more fuel as well. For examlpe if you put a 30 single choke weber on a 3500cc rover V8 its gonna be starved of fuel which isnt good! However a 1000cc R1 (or whatever it is) revs to stupid RPM so needs a fair amount fuel (probably runs 4 carbs doesnt it?)

Normally by CFM or air the engine is going to demand.

This is dependant on CR, engine size, number of cylinders (if not 1 carb per pot) max rpm etc and several other variables.

Tim..

The car has a larger swept volume but the bike revs much higher, I would imagine they "consume" similar amount of air/fuel....-ish... ;)

CFM Only for American carbs.

All European and Jap carbs are choke size(s) in mm. eg. Solex 30-30 Z 2 CIT 329, Weber 32-34 DRTC etc are referring to choke (primary and secondary) size in mm. UK carbs used to be in inch sizes. Amal TT 1 1/2".

The carb model may not have the size in it but that's what they use.

-- Peter Hill Spamtrap reply domain as per NNTP-Posting-Host in header Can of worms - what every fisherman wants. Can of worms - what every PC owner gets!

Most carbs I've seen have been rated by cubic feet per minute, CFM, but you'll also want to jet the carb according to the application so it's not rich or lean.

AFAIK carbs often have numbers relating to CFM, I think this is mainly an American way of describing the carb, e.g. a Holley 390 is 390 cfm, a Weber 500 is 500 cfm, unless I've misread the datasheets.

Its a bit more complex than that when it comes to different setups. Take a bike/car that has nominally correct carb or throttle body, size is say 40mm on a bike (common on 4 cylinder 1100cc bikes) with the usual one per cylinder.

Then the same engine would "need" probably only a 40 to 45mm carb on a common inlet manifold with a single carb or throttle body.

Because on a four cylinder with four carb/throttle bodys each one only "sucks" for about a quarter of the time. The rest of the time the inlet valve is closed. With a common carb, the same size carb will have an almost continuous smooth flow. So now it flows 4 times as much...

With an 8 cylinder version of this motor you would need only two 40mm carbs.. on a common maniofold.

And with throttle bodies - one per cylinder, a bike 1100cc motor making

150bhp needs the same carbs as a 2 litre 150bhp car engine. The airflow through each carb in this case will be the same. So a set of big bike 40mm carbs will work great on a 16 to 2000 car motor if its a bit tuned up...

My younger brother fitted eight downdraft Exup 1000 carbs to his rover V8 in his sierra (hes a copycat!) and it was even jetted pretty close! But two of those carbs would be correct on the standard "SU" (AWFUL) manifold.

Thinking about what I said, it might give a bit of power improvement on a rovers SU manifold, to drill through the "wall" between the two carbs... But watch for water!

Carl Gibbs raved thus:

:: For examlpe if you put a 30 single choke weber on a 3500cc rover V8 :: its gonna be starved of fuel which isnt good!

Yeah, four downdraft twin 48's are the way to go here, I think :o)

On trumpets.

Burgerman raved thus:

:: And with throttle bodies - one per cylinder, a bike 1100cc motor :: making 150bhp needs the same carbs as a 2 litre 150bhp car engine. :: The airflow through each carb in this case will be the same. So a :: set of big bike 40mm carbs will work great on a 16 to 2000 car motor :: if its a bit tuned up...

Is there any advantage in fitting a set of 40mm bike carbs to a 2.0 motor (a Pinto, say) vs. a pair of twin sidedraft 40's? Would this change if we were talking about a 16v lump?

Actually, no!

A set of 40s and a set of 40mm bike carbs is the same thing.

No, same. Except the webers are easier to jet! And the bike carbs are CVs so if they are too "big" it will not die when you stand on the gas...

Forget valves. Think power. A 150BHP motor needs x amount of air however it achieves it.

Oh yes. Drooooooooooool :)

Maybe in terms of air flow. But not in terms of fuel control if the bike ones are CV or slide, let alone flatslide or smoothbore - all of which has a slide valve and a variable jet + idle circuit + main + secondary if CV. The fact I can't think of a fixed jet butterfly throttle bike carb like a Weber side draught must say something about the Weber design. Possibly the Bendix things they used to fit on Hogs but I never got a good look at one as they always hid the POS behind a big filter box. Great for WOT and competition but hard to set up for good progression on road as no variable jet and/or cutaway on slide? Shared float chamber suffers fuel surge giving rich mixture on one choke and lean on the other when braking or accelerating on longitudinal engine, even worse for cornering on transverse engines? Big and heavy so can't be hung off isolators resulting in fuel frothing and poor insulation from hot cylinder head so fuel density changes.

Proper carbs are smoothbores with a separate remote float chamber rubber mounted with a bottom feed direct to the jets, any possibility of fuel surge and you fit two float chambers or a swill pot to keep the fuel level constant at the jet(s). Failing a remote float then at least a float chamber that is concentric to the jet to minimise effect of fuel swill and a mount that isolates the carb from excess heat and vibration.

Think I'd prefer a rack of 4 Wal Phillips fuel injectors to a pair of Webers. Fit, synchronise, adjust fuel progression links between throttle and tap = done.

-- Peter Hill Spamtrap reply domain as per NNTP-Posting-Host in header Can of worms - what every fisherman wants. Can of worms - what every PC owner gets!