Port taper and choke size

[superbike]

A question
Is there any basic rule of thumb choosing the choke size in a port ?
In the ducati engines choke sizes are 82-87 percent of the valve sizes depending on the model.

[Guy Croft]

Chris, hi

Here is some info that I worked out some years back from extensive dyno tests, I published it in the GC Workshop Manual in '95 and it seems about about as close to the 'right answer' as you'll find.

Choke size:
Valve throat diameter should be at least 95% of choke size and may be up to 120% larger.

Carburettor size:
The carb diameter should be 1.12-1.2 x choke diameter

Port narrowest section (nominal diameter):
This section should be no bigger in diameter (or equivalent if not round) than 73-85% of the carburettor diameter (not the choke).


I hope it helps you.

GC

[superbike]

GC
when you say the Valve throat diameter( lower part of the seat into port) should be at least 95% of choke size and may be up to 120% larger. What choke are you talking about ? throttle body ?

[Guy Croft]

That's for carbs, Chris, throttle bodies don't have/need chokes because they are injecting. If you want to know about throttle body size - work off the port narrowest section bit and see what it says.

You did ask about relative sizes to chokes, ie: carburettor venturis, yes?

GC