Hey Guys,
I read 46mm SU's are too much for 4 cylinder engines. Why is that? I've seen Webers on nearly every second A-series motor and they have a 40/45mm butterfly for each intake port. how is it that a one 46mm carb can be too much for four intake ports? My little brother's playing with an A14 on an engine stand and I'd like to offer some older brotherly advice. We aren't sure what we're going to strap the motor in yet, but instead of ordering a set of webers, I thought he could use the two 46mm SU's lying around.

Most a-series engines running 40's and 45's are actually choked down to as low as 30 so it's all a bit of overkill really. People will probably say its too big but in my opinion a variable Venturi carb such as an SU should perform okay seeing as it will adjust accordingly.

Give it a try the larger slide may need a different oil so it responds quicker but give it a go.

My father and brother both vouch that the 38's work best for the motor, but his little A14 has 40mmx34mm(I think) oval ports, modified L16 intake valves and a 273deg camshaft. Hell if they can work for his A14, I might just source me an A12GX, stroke it with a 1400 crank, and jet the 46's for it. Can't be worse than 38's if correctly jetted can it? I mean we have the 46's here and if they provide even a slight improvement. . . .
Maybe I'm just too fussy, but I find the SU's a lot more attractive. Webers seem to make the engine appear to be more than it is

Weber carburettors and side drafts carbs of similar format will out do the SU carbs, their throttle response is much faster and their potential to perform at higher rev ranges is also much higher.

The dellorto dhla carburettor in my opinion is superior to the weber and Mikuni equivalents.

Big difference being that SU type operate on vacuum to operate only; being a constant velocity type carb whereas weber/dellorto/Mikuni Solex/ Holley etc have accelerator pump ccts built in to force the fuel in. It is a somewhat misleading to compare SU's V Weber Dellorto on an even playing field.

The advantage of an SU type carb is that the effective choke size constantly varies in size to maintain the [near uniform] velocity stream.

Delorto DHLA you say?...
And what if it were to be a low rpm/high torque motor like an A15 stroked to A17?

What about using Reed valves after the SU's? My grandfather used to believe in them heart and soul for his L20B

I am running twin 45mm side draught DCOE Webbers with a redline manifold on my A15 the engine has very mild work only a Cam and a little head work..

With them tuned right and a decent fuel pump (the one i had on it was just not working correctly)

I have had no issues at all and runs brilliant... blows away my mates A15 ute with single 40mm side draught

And nothing beats the sound of a Twin webber whilst driving

Yeah? 45mm DCOE with what size chokes? 34mm would be my guess.

Quote:
When that 46mm carburetor has large chokes (venturis), it will be too much for a 1.4 liter engine running at 6000 RPM. It might run well at full throttle, but part throttle will be less than optimum. Size the carb correctly and both full-throttle and part-throttle both will be excellent.

SU-type carburetors don't have traditional chokes, so it's not apples to oranges.

And it makes almost no difference if it is one carburetor feeding four cylinders, or twin-carbs or four carbs on the same engine. The sizing needs are nearly exactly the same. Because it's a four-stroke engine, one cylinder doesn't pull air through the carb continously.