I used to have a poster on my wall of a Bentley 4.5 litre (blower bentley)

That finned thing on the front is a Roots supercharger.

This is 1928 before fuel injection.

 

Always wondered how to size things for supercharge carb blow through..

 

Page 10 of this https://www.thesamba.com/vw/archives/manuals/solex_carb/Solex_Carburetors.pdf

has formulae.

 

 

For my 1263cc engine reaching 7500rpm and 700mmhg boost (G40 with RS1 work?) it will need 55mm carb barrel with 44mm chokes.

Yeah, I've got 11:1 compression ratio so decompression plate or different pistons, which changes cc, obviously.  But ballpark...

 

Using Python.

 

import math

0.41 * math.sqrt(315 * 7.5 * 4 ((760.0+700)/760.0))

 

=55.24

 

So, 55mm carb barrel (per cylinder or shared).

 

Page 22 then has formula for choke from this barrel size.

0.8*55.24

=44.19

(44mm choke)

 

There's some jetting that follows from the choke size. And it'll needs loads of experimentation for part throttle.

 

Maybe less boost would be sensible.

 

 

Edited 14 September, 20197 yr by dvderlm

Mikuni 50phh carbs are very rare.

Gene Berg does some big carbs, but $$$ and downdraft not sidedraft.

http://www.geneberg.com/cat.php?name=The Berg IDA Carb&cPath=240

 

 

So let me plug the numbers for a Lotus Esprit Turbo with 9.5psi boost (500mmhg) into a 2174cc 4 cylinder at 5000rpm

 

0.41 * math.sqrt((2174/4.0) * 5.0 * 4 * ((760+500.0)/760.0))

=55

0.8*55

=44mm

 

But it uses 40mm Dellortos with 36mm chokes. 

Intercooler? Turbo vs Supercharger?

Detuned?

 And I think it does 6000rpm for 160hp with DHLA40h(turbo)  or  211hp on DHLA45m ? in the high compression version.

Edited 14 September, 20197 yr by dvderlm

Lancia Y10 turbo 1049cc 81hp at 5750rpm.

Weber carb feeding 4 cylinders with 19mm primary and 23mm secondary choke which is same as a single 30mm I reckon.

Think it is 5psi  = 258mmhg boost.

0.41 * math.sqrt((1049/4.0) * 5.8 * 4 * ((760+258.0)/760.0))

=37.0

0.8 * 37

=29.6

so 30mm choke.

 

That matches. 

 

and trying the 5psi boost figure for my engine and conservative 5800rpm

0.41 * math.sqrt((1263/4.0) * 5.8 * 4 * ((760+258.0)/760.0))

=40.6

40 or 41mm barrel

0.8 * gives 32.5 or 33mm choke

 

But 33mm choke gives 8000rpm in n/a setup I reckon. aha, maybe the parasitic load of spinning the charger means inlet has to flow more..

Why do the yanks size their carbs using CFM?

https://performancetechnician.com/pdf/2014/06jun/Tuning-Carburetor-Equipped-Supercharged-Engine.pdf

 

 are the instructions actually for drawthrough?/

Yes, must be

https://victorylibrary.com/tech/supercharger/super-carb6.htm

distinguishes both methods

 A 1340cc Harley with supercharger has 32.5 pilot and 172.5 main jet in a single 42mm Mikuni carb, blow-through.

 

Edited 18 September, 20196 yr by dvderlm