The idea of SU's is to provide a combination of performance WITH DRIVEABILITY. Rather than snap the throttles wide open, resulting in a rapid loss in velocity of air flowing through the carbs (as in webers), they only allow as much air to enter the engine as it can handle.

It is my understanding that the accelerator pump mechanism on a weber is an attempt to overcome this problem that doesn't exist in an SU, rather than an advantage over the SU's
Not really, as air velocity is way down until the engine reaches higher RPM. You don't want extra ar / fuel until the engine can handle all this extra air and fuel.

In effect, the SU's design offers a vaiable venturi. Hence, you don't have to worry about whether a 40 or 45 is better , as it varies between virtually nothing and 38mm for the Hitachi-SU's.
Fair point, but if you have the ability to make the needles (ie have a lathe or have access to someone with one), this could be seen as an advantage. You can make your own custom needles to get the mixture right all across the rev range. My SU's have custom needles, and the difference is amazing.

Just my understanding of how the two different carbs work, I stand to be corrected

It should also be noted that I don't claim SU's to be better than webbers, just different. Probably more driveable on a slightly tuned car, but the webbers will undoubtedly offer better performance ON A CAR THAT CAN TAKE ADVANTAGE OF THEM (ie more highly modified).

p.s. as much as I like SU's , I will be fitting 44mm Solex carbs to my next engine (opens new can of worms)

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1970 ex Group A Rally P510
1971 P510
1972 180B SSS
1965 SP310 Fairlady
1966 SP311 Fairlady

Dave,

Ahh.. I see what you mean. I was thinking that the throttle controlled the air and the venturi area controlled the fuel. Gotcha now.
Meh, carbies all suck anyway, EFI your arse!

Dave

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USA Daily: 2014 Nissan GT-R, very minor modifications for the track
USA Project: 1978 280Z, minor suspension upgrades, VK56DE conversion in progress. SOLD
AUS Race Car: 1973 240Z, L28ET, Autronic, GT35R. SOLD
AUS Project: 1972 1600, 3200km old S15 SR20DET, ground up rebuild. SOLD

amen to that brother.

testify

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frankenstein projects R us.

The Republic of Western Australia -
Propping up the Nation's Economy since 1901.

Argh! 2 Dave's!! (The Helix & "Dave").

"Dave" - you da man. Agree 100%.

My old man used a HIF6 on his Mini, and set the needle up on a lathe. Worked very, very well. Ran like a dream... until it rained / got very cold. Then it ran like a dog with 3 legs! Carbs aren't too flash at that kind of thing, SU's or otherwise.

So yeah, I'm my planning on EFI for my datto.

Well, being a bit "old school" (or maybe just plain old ), carbies are one thing I'm OK with.

The following may help (or amuse). Taken straight from a Datsun Fairlady manual.

The suction chamber is installed on the upper side of the throttle chamber, in which the suction piston operates vertically.
On the top of the suction piston, load of the venturi downflow is transmitted through the suction hole and the underside of it passws to the open air through the air hole and the air cleaner.
The suction piston automatically makes vertical movement by the balance of the load works on the top and weight of it and the strength of the suction spring.
When the throttle valve is widely opened and much air is sucked in, the load on the top of the suction piston increases and makes the venturi wedely open. When the air is little, the load is small, them the venturi also opens little.
Weight of the suction piston and strength of the suction spring are selected so as the venturi opening will meet with any running conditions of the engine.
In order to heighten acceledration, the oil damper is provided the suction piston rod and prevents the piston from an abrupt opening.

Makes it all perfectly clear

anyway, they also added diagrams, which are worth 1000 words of Japlish.

The manual also says about the accelerator pump on Solex carbs (Jap copies of webers)

When accelerator pedal is pressed down sharply, the fuel to the main system cannot follow the increase of the airflow and the mixture will thin its density temporarily.
The Accelerator pump is the device ... to prevent the temporary thinning of the mixture.

_________________
1970 ex Group A Rally P510
1971 P510
1972 180B SSS
1965 SP310 Fairlady
1966 SP311 Fairlady

But back to poor old bloob, who's probably wondering what he's started !

Most of what Datsss73 said in a very early reply was good advice.

Mild head work (bigger valves, mild porting of the ports and matching manifolds to intake ports, along with a mild cam), APPROPRIATE carbs (I'd go sith SU's or a single downdraft weber, or even a single sidedraft weber), 2 - 2 1/4 inch exhaust, extractors should all combine to make the old L20B perform quite nicely.

They respond pretty well to sensible modifications, which can be done fairly cheaply.

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1970 ex Group A Rally P510
1971 P510
1972 180B SSS
1965 SP310 Fairlady
1966 SP311 Fairlady

Nothing like a good debate to educate everyone, myself included.

I think we could do with a "Carby Theory" in the Technical section of the Forum.

Comparing SU's to webers.... is like oranges and apples. I've had both, if you know how to keep SU's tuned and they are in good nick they are probably a cheap and chearful solution. If you dont have a good ear or bubble guage, piss em off and get webers, once they get tuned u never have to touch em.

Only downside for twin 40's for me is the airflow is not enough to make them run properly until i get onto my cam. with a single 45mm I would have better drivabilty and fuel economy but would loose top end. Perfect for a street car with no CAM.

And dont even get me started on induction noise. NOTHING BEATS TWIN WEBERS... I'm hoping for a simlar sound from my quad TB's

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i totally agree
twin 40's on full noise
just thinking about it makes me wanna get rid of the turbs and dong the carbs back on
although i am getting used to spooling and turbo flutter, it just doesnt put the fear into traffic the same
p.s. as the thread starts and to make the worlds biggest argument
bang for bucks = carby turbo
reliable and costly = hi comp engine
"so thats what opening a can of worms sounds like"

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