Matching ports between carb & manifold

[JezWest]

I've been working on my carbs and when I went to put the new gaskets in between the carb and the manifold they looked to be too small. I looked a little more carefully and the throat on the carb is 40mm diameter (dual Dellorto DRLA40) but the manifold diameter is 48mm. Presumably Dellorto sold one size inlet manifold which could be used on all the different sizes of DRLA from 36-48? Or have I ended up with the wrong manifolds?

Much time and effort is put into matching the manifolds and heads so there is a nice smooth gas flow: but then I have this huge mismatch up at the carb: does this hurt performance..? Why put all the effort into the manifold/head transition when there's such a big step at the carb..? If I had a manifold with a 40mm diameter matching the carbs would I notice any difference..?

Does anyone else wish they paid more attention during thermodynamics lectures 

[Zach Gomulka]

Most manifolds are one size fits all. There are specific manifold sizes (40), bit they are hard to find.

[Neil Davies]

Does anyone else wish they paid more attention during thermodynamics lectures 

Yeah, I do! I always found it a bit tricky! You could use some JB weld to close up the ends of the manifolds and give them less of a step?

[Andy]

I think I read that having correctly sized manifolds will increase the torque out put.

Stateside Tuning sell the correct 40mm manifolds, maybe they will be able to help you more than I can.

[Bruce]

When the A/F mixture leaves the bottom of the carb and hits that giant toilet bowl, there is a big drop in pressure and air speed.  This causes the fuel to condense out of the vapour.
The manufacturers mostly sell only one size, with a 50mm hole to fit the biggest carb.  Do some searching, CSP sells correct 40mm manifolds.  CB used to supply small ID manifolds with their 36 and 40mm Dellortos, but not any more.  Swap meets turn these gems up on occasion.

[Gunter]

Most manifolds are one size fits all. There are specific manifold sizes (40), bit they are hard to find.

CSP makes inlet manifolds for sizes 40mm, 44mm and 48mm

for more info plse contact me gh@csp-shop.de

thanks

Gunter

[Zach Gomulka]

That's cool, Gunter. I didn't know CSP did that

[JezWest]

Well there you go! I learnt some things, thanks. I'll take a look around CSP's website again, always fun.

[Nick]

When the A/F mixture leaves the bottom of the carb and hits that giant toilet bowl, there is a big drop in pressure and air speed.

When air speed drops, pressure raises / the higher air speed, the lower pressure !


If you're interested in this stuff, have a look at the Bernoulli principle.

[Bruce]

....

[Gunter]

That's cool, Gunter. I didn't know CSP did that

thanks Zach , we actually do a lot 

[Gunter]

Well there you go! I learnt some things, thanks. I'll take a look around CSP's website again, always fun.

cool and if info or more info needed, just drop me a line

[Gunter]

When the A/F mixture leaves the bottom of the carb and hits that giant toilet bowl, there is a big drop in pressure and air speed.

When air speed drops, pressure raises / the higher air speed, the lower pressure !


If you're interested in this stuff, have a look at the Bernoulli principle.

cool Bruce 

[Nick]

Didn't want to be a "know-it-all", just point out the little bit incorrect detail 

[Bruce]

Right.
Either way, the giant bowl at the top of the manifold is a crappy design.

[CHR!S/DVK]

that Bernoulli principle is very interesting!

talking about pressure (or compression) there is however a difference between the dynamic and static kinds.

the wiki on the Bernoulli principle says:

Bernoulli's principle can be derived from the principle of conservation of energy. This states that, in a steady flow, the sum of all forms of mechanical energy in a fluid along a streamline is the same at all points on that streamline. This requires that the sum of kinetic energy and potential energy remain constant. Thus an increase in the speed of the fluid occurs proportionately with an increase in both its dynamic pressure and kinetic energy, and a decrease in its static pressure and potential energy.

a quick search further on wiki tells us this about static pressure:
In Aerodynamics, L.J. Clancy[7] writes: "To distinguish it from the total and dynamic pressures, the actual pressure of the fluid, which is associated not with its motion but with its state, is often referred to as the static pressure, but where the term pressure alone is used it refers to this static pressure."

a simplified version or Bernoulli principle reads:

static pressure + dynamic pressure = total pressure.
meaning that static and dynamic pressure are dependent on each other, because total pressure remains constant


since the dynamic pressure is dependent on the density of the fluid (or mixture, we're talking about air/fuel), this variable could change because air and fuel mix during the travel through the system.

the Bernoulli wiki tells us one more interesting thing:
The carburetor used in many reciprocating engines contains a venturi to create a region of low pressure to draw fuel into the carburetor and mix it thoroughly with the incoming air. The low pressure in the throat of a venturi can be explained by Bernoulli's principle; in the narrow throat, the air is moving at its fastest speed and therefore it is at its lowest pressure.


however, unregarded the things i wrote make sense or relevant, in a performance engine two things are preferable:

- short time of travel of the mixture from carburetor to head (so mostly high speeds)
- good mixture of the air and fuel

concluding, a narrow diameter of the manifold favors the speed of flowing mixture, but an area of wider diameter could enhance mixing of the air and fuel due to high pressure.


as you can see i did not give a closing conclusion, mainly since i try to steer this into a scientific discussion (and partly because i am puzzle as well right now)

[JezWest]

So I have this fuel/air mixture which leaves the bottom of the carb into this wider place. The speed of the flow drops and pressure increases.

This causes the fuel to condense out of the vapour.

Why..?

[Lids]

the gas laws.

[JezWest]

I knew it! Maybe one less beer at uni and thermo would have clicked more for me. Maybe I just completely missed the Dellorto lecture