How does the leap from 2x38's to 4x40's even work?

[02Pilot]

The chart ONLY applies to individual runner carbs, for the reasons others have mentioned above. Carbs that feed a common plenum before the runners are not the same. Note also that the RPM shown is the power peak, not the rev limit. Unless you're building a race engine, you probably don't want 38mm venturis, as this will significantly reduce driveability at lower engine speeds.

Edited September 4, 2019 by Andrej

[Son of Marty]

The problem with these graphs and jetting chart is they come with out context, this venturi chart would only fit for a full race engine on the track it would be horrible on the street.

[AustrianVespaGuy]

9 minutes ago, Mucci said:

Are the Weber venturis replaceable in the DCOEs? What exactly is that doing? Off the top of my head I would think increasing air speed and therefore pulling more fuel? How is that helping tuning?

Now we're getting into the FUN stuff!  So yes, the venturis in DCOEs are removable and can be changed out for ones of different sizes, obviously a smaller diameter than that of the butterfly.  So let's take an example where you have a 40mm carb with a 38mm venturi as our baseline.  Now, swap that 38 for a 36mm venturi, what happens? Two things.  The more obvious one is that you have a smaller diameter airway that can't flow as much total air volume, thus limiting high RPM power output (as in that graph above).  The less obvious but advantageous change is that, since Mr. Bernoulli was correct and p1-p2=1/2rho(V2^2-V1^2), so the velocity of the intake air stream INCREASES with the smaller ventrui (A2 is smaller thus V2 is bigger).  And it's the air charge hitting the back of the closing intake valve with this higher velocity that sets up that key resonance in our independent runner system, which at the end of the day is what having individual throttles on each cylinder is all about!

Ok, so now let's go the other way and keep the 38mm venturis as that's the area we really need for our hypothetical motor's flow requirements, but instead let's jack up the carb size to 45mm.  We've still got a nice 7mm diameter taper to help speed up the airstream and top end flow, so all great there, but it's not completely without compromise.  The final velocity still isn't as fast as with the smaller 36mm chokes, and thus the overall air pressure in the system is higher, and now at low throttle openings, a small change in that 45mm butterfly angle makes for a MUCH larger change in area than the 40mm throttle did.  What this adds up to is poor throttle resolution off-idle and cruising.  Remember, when the airstream speeds up, the pressure drops, and you NEED this pressure drop to suck the fuel through the jets into the airstream.  So with a throttle plate that's too big, you just touch the pedal, suddenly get a huge opening, pressure goes up to atmospheric, flow kinda stalls, and now your cylinder isn't able to pull any fuel through the carb and into the combustion chamber -> sad times.

So in summary, and getting back to Toby's Suckety Suck Suck, big holes are great for sucking in a lot of air at high RPM, small holes are great to keep both air and fuel moving in the right direction when you're only sucking a little air, and the taper of the venturi is needed to be able to suck in the fuel along with the air.  The trick to tuning then is to balance all of that so things are big enough to flow for peak power, small enough to not make driveability crap, and the right about of taper to give you both good throttle response AND set up the resonance in the right RPM range, which is usually somewhere around peak torque (though this is also done by changing the length of the tract between the throttle plate and the back of the valve, but that's for another post!)

[Mucci]

Ahh! Great info AVG! Thanks for that. 

 

Even more questions...

 

- Can you even setup a DCOE 40 to have a 40mm venturi? Or is the 40mm number just the carb body diameter and not the actual air flow orifice measurement? In a bike carb, since they're fixed, a 40mm carb = a 40mm final airflow passage. Sounds like this is may not be the case here.

 

- What is the importance of the resonance against the closed intake valve? ...aside from the killer sound.

 

edit: After re-reading that a bunch of times I now understand the benefit / driveability of sequential 2-barrel carbs. Throwing DCOE 45's on a stock motor is like trying to suck up a pea through a paper towel tube. 

 

I now see the issue with trying to choose what size motorcycle carbs to run. Since you can't swap venturis you're stuck with what you got. 

 

I've always wondered what the purpose of these double velocity stacks are on Yoshimura carburetors. Is this a way to run a larger diameter venturi (greater flow at WOT) but get more velocity at partial throttle (driveability)?

 

Edited September 4, 2019 by Mucci

[AustrianVespaGuy]

36 minutes ago, Mucci said:

Throwing DCOE 45's on a stock motor is like trying to suck up a pea through a paper towel tube.

This is an excellent analogy, well said!

37 minutes ago, Mucci said:

Can you even setup a DCOE 40 to have a 40mm venturi?

And no, this doesn't work, as then you just have a straight tube with no pressure drop, and with no pressure drop, you can't really suck any fuel in.  That said, it all depends on how the carb is marketed, as in a 40mm carb MAY actually have a 40mm venturi installed, but the nominal throat diameter and/or throttle plate are something larger, like 43mm or something.  But I'm pretty sure Webers don't work this way; the advertised size is the throttle area, and then the venturi is something smaller.

[Doug Riparetti]

11 hours ago, zinz said:

Waiting for Doug Riparetti to chime in ?

 

 

What would you like to know?

 

I have over 35,000-miles since installing the S14 back in December of 2017.

 

LOVE the sound and the performance of the 44mm Mikuni motorcycle carbs. 

 

They’ve been running the same set-up on 4-cylinders circle track cars since the early 90s  out here in California, and around Idaho, Utah, and Washington.

 

I've seen the Mikuni motorcycle carbs on the Ford Pinto motor, Renault, Datsun and Toyota’s (we ran the Toyota 3TC motor when I was racing). 

 

 

 

Edited September 5, 2019 by Doug Riparetti

[Mucci]

Is there some sort of calculation to help choose the correct size motorcycle carbs for a sporty daily driver?

 

Also, is there a reason you went with the individual cable Mikunis vs. a linkage bank with single cable?

Edited September 5, 2019 by Mucci

[Doug Riparetti]

1 hour ago, Mucci said:

Is there some sort of calculation to help choose the correct size motorcycle carbs for a sporty daily driver?

 

Also, is there a reason you went with the individual cable Mikunis vs. a linkage bank with single cable?

 I’m not aware of a calculation chart to determine the size of carburetors to go with. 

 

I went with these carburetors because of experience. My brother and I started racing with these carburetors at Madera Speedway in California back in 1997.

 

With over 20-years racing experience with these carburetors, I felt pretty comfortable about installing them on my S14. 

 

 I had (4) 36mm Mikuni motorcycle carbs on a stock low compression M10 motor, it ran pretty decent, but I never tried to 44s on it.

 

 I would not recommend the 36s, I would go straight to the 44s. 

 

 

Edited September 5, 2019 by Doug Riparetti

[Mucci]

Straight to the 44's on a stock M10?

[TobyB]

Quote

 Is this a way to run a larger diameter venturi (greater flow at WOT) but get more velocity at partial throttle (driveability)?

 

Yup- look for Constant Velocity (CV) carbs.

 

On cars, they're called SU's (Skinner Union)

and the venturi size is set by vacuum,

 

while on bikes, the venturi is usually controlled directly by the throttle cable.

 

They're not as free- flowing nor as good at atomization as a fixed venturi carb,

tho, so they lost favor in the emissions era.

 

As to venturi size, with, say, a 40 dcoe, you can go as big as a 36mm venturi.

It works fine at higher velocities, but starts to have issues at low airflows,

whereas a 45 dcoe with a 36mm has plenty of 'depression' at lower flows.

Which is another one of those "Whut???' things until you think that the

depression is caused by changes in  relative  airflow rates, not absolute rates.

So the air in the venturi's going the same velocity, but the air in the 

body's going much slower.

 

t

something about EFI...