Extending the carb out?

[LohringMiller]

RPM = 108K ( 1st hamonic ) 97K ( 2nd harmonic ) 54K ( 3rd harmonic ) / L ( inches )

So examples are 1st Harmonic 6000 rpm = 18",and 12,000 rpm = 9"

2nd Harmonic 6000 rpm = 16.16" and 12,000 rpm = 8.08"

3rd Harmonic 6000 rpm = 9" and 12,000 rpm = 4.5"

The length of the intake tract through the carb to the piston equals 108,000 divided by the RPM for the first harmonic.

Sorry, the formulas aren't the way I would have written them. I would have written L = 108,000 (1st harmonic) / RPM etc.

We don't get close to the third harmonic lengths even at over 20,000 rpm in gas engines. (54,000/20,000=2.7 inches) The intake tracts are around 1" long. Still, there is some effect with different length isolator blocks.

Lohring Miller

 

[johnsteltzer]

I dont understand the math but I know that my motor ran great this is the second one I have built they both run awesome to me all I can say there is a gain in performance when you atomize fuel better in any motor my theory is when the carb is in the stock location it is there because that's where it cut weeds the best I am using it to race and I think it atomizers the fuel better it is not coming right out of the carb and sticking to the piston rod and all the other stuff but dont knock till you try it thanks

 

[Dennis Somers]

John I'm glad you are happy with it and that it works for ya, and I'm NOT trying to change your mind.

I too have a 632HP S/B street eng. with two 660 center shooters on a tunnel ram that pulls HARD from 3500 to 8000+. However, the intake dynamics of a 4 stroke race eng. at any RPM, are far different from a pressure/pulse 2 stroke firing every revolution. Flow IS flow, and I understand what your vision is. But your ration-ale for atomization is off.

The fuel is atomized by the addition of air right at the venturi. After that, the heavier components of the mixture ( gas/oil ) are trying to separate. The longer and more convoluted the path, the more time/surfaces aid in that separation. That, together with the intake reversion on every cycle, combine to reduce velocity and increase the separation.

Yes, we have tried it! And it runs. But not as well as it can. Just my building experience since 1961. ENJOY IT!

 

[johnsteltzer]

Sounds good I see what you are saying and I understand it but my motor ran just fine but did zenoha put the carb there for convince are did they know what

You know they didn't build this motor to race they built it to cut weeds

 

[Dennis Somers]

In the utility engine business, Zenoah and all the rest of them, want to build the most power in the smallest/lightest package ( that's what sells ). To that end, they make everything to be compact, and get the best compromise of power/torque in an end use friendly configuration. The intake, as well as all the other systems are not race perfect by any stretch. We, as builders/modifiers, have to recognize the potential and the limits of the entire engine. And there is little to nothing, that can't be tweaked for our purpose. As Mercury Racing says, the race is never over. LOL

 

[johnsteltzer]

I heard that has mercury racing ever experimented with moving the carbs away from the motor

 

[Dennis Somers]

I would be VERY surprised if they hadn't, on their experimental race engines. But they are reed induction and carb distance doesn't have the same effect as piston port. Keep in mind, their production motors also have to fit under compact hoods, run quiet, and the 2 strokes don't have tuned pipes. And they are relatively moderate RPM machines. Commercial outboards don't make anywhere near the 4 1/2 + HP per cubic inch that our little guys do.

 

[johnsteltzer]

I see what you are saying