What size DCOE

[Graham Perry]

I am sitting here spec'ing out various 'dream' engines for future installation in my car. Although I still haven't decided what to go for, it is certain that I will have to stay on Weber DCOE's and 3D for a year or so before upgrading to throttle bodies. Is there a ready reckoner re BHP output and the required Carb size, assuming a 4 cyl engine and twin carbs.

ie

up to 155 BHP I suspect 40's are the norm and 45's above that,

but at what point in terms of BHP, should one consider 48's and then 50's and larger.

 

Is Cubic capacity a factor in this or is BHP a good enough guide?

 

Edited by - Graham Perry on 29 Nov 2001 07:00:02

 

Edited by - Graham Perry on 29 Nov 2001 07:05:14

[oilyhands]

There is a standard scale / graph which will tell youi the venturi size for a given cylinder capacity and RPM where peak power is made have a look at it here. Once you have selected the venturi size , the correct carb size can be ascertained by multiplying by 1.25 and taking the next size up.

 

There is a more detailed explanatory article on sizing and jetting here and a jetting program that does the hard work for you here.

 

Oilydans

 

Edited by - oilyhands on 29 Nov 2001 07:44:18

[Graham Perry]

Thanks Oily, very interesting and useful . To what extent is something like this relevant to Throttle Bodies's ? Many people are running big BHP's through TB's with smaller Venturis than the equiv. DCOE chart would suggest as optimim (if venturi's are what they are called in TB's). I know they work completely different but the fundamentals of the orifice size cannot be much different, ie you can only get so much air through a certain sized hole.

[oilyhands]

I'd be really surprised if the TBS were smaller than the equivalent *venturi*, smaller than the equivalent barrel size yes. FWIW my ATMO Cosworth made an 240BHP plus on 42mm chokes in 48DCOEs this compares favourably with the 42mm DTH bodies used on the K series where the top spec engines are producing equivalent horsepower.

 

With a TB you dont need air flow speed to draw the fuel in , so the barrel/venturi size is less critical, there is still a point at which too large a barrel will be detrimental but it is not so clear a distinction as with a carburettor.

 

Oily