This can get very complicated and very technical.

Also very much depends on your application and how/where will be used.

OK, let's try and narrow it down.

If you are also thinking about side-draughts, it's usually used with old skool engines...

Say high performance streetcar, can be used in traffic at a push, but mostly used on weekend for track days/gymkhana, etc.

Will run on 95, head probably flowed, high compression, cam'ed to make power higher in the rev range, but not loose to much low end torque.

Something popular like, a Nissan L18, Ford 2L Pinto, Toyota 22R, VW 2L 8V.

Which sidedraught would you use, versus which throttle bodies?

I think first decide on a motor, as the motor will play a big role.

Also you need to determine what comp max you can run on 95 with that motor.

My race motor is 13:1 comp with throttles, but it wont run 95 (wont be able to) with close to 300deg spec cam, 8500rpm limit. (sr20) My throttles are made up to work with that combo.

43 ITB's are 43mm right through, 43 Siddes are choked down to probably 38mm or so depending on the engine and setup.

You can run much larger ITB's than siddes because the fuel injection is controlled by a computer and the fuel is injected at high pressure and properly atomised. It's not reliant on the vacuum an engine can can pull, which is why siddes or any carb has chokes, to create a pressure differential between the outside and the inside of the carb ie. vacuum which in turn pulls in the fuel.

If your siddes are to large for the engine it'll either not run, run really kak or run only at very high rpm. With ITB's as can be seen with the 1.6 4AGE's running 45mm this is not a problem. We've fitted 43 ITB's to a 1400 Nissan engine and it ran perfect, it was a bit nervous with very small throttle movements making large changes, but it ran well in traffic and on the road, you just had to get used to not giving as much throttle as you used to.

Valuable input here, thanks guys.
OK, I was hoping this could be some sort of general guide, for future reference, but will narrow down some more.

Engine choice: 2.0L 4-cylinder Datsun L20B @ 11.0:1 compression, 7500rpm redline

With this engine, Datsun competition cars used to run Weber 48's / Solex 50's and make 145kW@7200rpm, would 20V 45mm ITB's yield the same or better results, plus be more tractable, or do you need to run 50mm M3 ITB's?

45 ITB's would flow more and be more tractable.

Thanks Boksie

Would it flow much more? Enough that one might have to opt for 43mm throttles?

And what size injectors would you recommend?

Use the 45mm, they will be ok. I'd go for something in thee 350-450 range on injectors if you're just going to run unleaded. Avgas and racing fuel needs smaller injectors, but then you are good for both.

With that compression 45mm will be ideal.

Remember race motors run much higher compression, but can only run race fuel and be used on track.

What cam will you be running, plays major factor with the L20 motors.

My dad use to race those motors in his race cars back in the day, you would be amazed at the amount of work that went into these. Many small trade secrets...

They use to ports 4-5 heads at a time, check all of them on the dyno, one with the best gains they would cut up and make measurement for future porting...

The car needs to be very reliable, will be used for track days, gymkhana and club rallies, and I'd like it to run on 95, thus don't want to up the compression any more. Heads will be ported.
Got any suggestions on a cam, Mario?
Would cam choices also differ between sidies and ITB's?

I was am also looking at using ITBs on my BMW m10 4 banger .Got quite a bit of useful info from here
http://www.racehead.com.au/?page_id=52
http://www.racehead.com.au/?page_id=211