Race Engine Forum UK

It is in relation to this in the theory section, I thought it best to start a new post.

viewtopic.php?t=679

I am confused in the calculations you square the diameter to work out the surface area. Should it not be this calculation?

Area = Pi times the radius squared (A=pr2)
So radius is half the diameter (ie to the centre - out) so for a 32 and 40mm throat.

32/2= 16
3.14 x 16^2 = 804.247719318987 and not 1089mm?

so 32mm = 804.25
40mm = 1256.64

But with turbulance being the biggest problem in this design how comes the flow calculation is not taking this into account?

Gas speed being fastest in the centre is where you have the biggest obstruction, ie the butterfly, the screws etc.

As a yardstick a 33mm diameter hole (broadly speaking) flows 102 cu ft/min at 10" depression. If your throttle body (or carb choke) is 40mm diameter it will flow more by the ratio of radius squared (^) ie:

33^2 = 1089 sq mm
40^2 = 1600 sq mm
1600/1089 = 1.47
1.47 x 102 = 150cfm
if the hole is smaller, invert the ratio.

Note that I am using the flow thru a 33mm (not 32mm) throttle bore to figure what it would flow if it was 40mm. Makes no difference whether you square the diameter or radius for the calculation, and what we're seeking is the ratio of the two under comparison, not the area itself. You don't need to work out the actual area to do the ratio. As far as theoretical losses are concerned mine is an empirical figure for the 33mm bore - taken from an actual flowbench test - not calculation. With that in mind you can pretty well ignore the effect of any turbulence from one size to another. OK?

GC

Thank you.

I understand now, sorry if I offended, feel free to remove.

Cristian

no need Cristian, I do make mistakes, it doesn't hurt for people to see how we can go about ID'ing and addressing them in a friendly way!

G