intake diameters

[Peter]

hi guys,
in the megamanual i read that with a 40 mm throttle bore , you can have up to 368 hp, with 4 throttle bores..
so why do we need for example 48 mm bores to make lets say 200+ hp on our engines?
cheers, Peter

[Roman]

Don't trust everything you read on the internet. 
Normally you step up when the carbs/throttle bodies are the limiting factor. You don't take them to the bitter end.
Check Jenveys FAQ:
http://www.jenvey.co.uk/Tech_QA.htm

[Peter]

hi guys,
and how do you decide if the are the limiting factor?
jenvey says 40 mm s are still good for 50 bhp per cilinder; maybe this is when the engine is boosted?
Its probably a whole other thing in real life right?

thanks, Peter

[Roman]

Read all:
Factors influencing size are; Power output, RPM, cylinder head design, cylinder capacity, position of the throttle body in the inlet tract and position of the injector.

Choice of bore size is a balanced compromise resulting from the following;
1) A larger bore leads to lower flow resistance, but obeying the laws of diminishing returns.
2) A smaller bore leads to better throttle control and response (never underestimate) and improved fuel mixing.
3) The system should be considered in total - from (at least) trumpet flange to cylinder and proportioned accordingly.

Basic references for BHP per cylinder, assuming ca 120mm from butterfly to valve head and a max of 9,000 rpm are;
Up to 30 - 30mm, up to 33 - 32mm, up to 39 - 35mm, up to 46 - 38mm, up to 51 - 40mm, up to 56 - 42mm
Up to 65 - 45mm, up to 74 - 48mm, up to 80 - 50mm, up to 87 - 52mm, up to 93 - 54mm.
These power figures may be increased by up to 10% in a purpose - designed and well proportioned system.
As butterfly to valve distance increases, butterfly size will need to increase in proportion to system taper and vice versa.

You have more than 120 mm from butterfly to valve, thus larger bore.
Add up to 10% in a well designed system - I hope you haven't made a bad design?

[Peter]

its just that i allready have dual 40 mm throttle bodies,
i thought they are probably too small if i compare them to other guys' setups,
but then i read this in the manual; then i got confused

[Roman]

Well maybe you won't get max power but you will get good throttle response and good torque. In a street engine that means more than peak power.

[Peter]

well, if they arent sold ,
i will surely give the'm a try...