1/3 Scale Ford 289 Hi-Po

[peterl95124]

Peter, Charles, et. al.

For my last few engines I've been referencing the HMEM discussion thread that began here:

Determining a carburetor throath diameter | Home Model Engine Machinist Forum

D = K * sqrt(c*n)

where D is Venturi diameter in mm
K is a constant .65 to .90
c is cylinder displacement in cc
n is top rpm/2000 (for a 4-cycle engine)

"K is a constant .65 to .90" --- ahhh, there's nothing like an empirical fudge factor !!!


wondering how your .180 compares to 1/3 scale ?

 

[gbritnell]

The revelation to me was looking into the throat of the carb on my Holt engine. At idle there is such a small gap between the throttle barrel and the venturi bore you'd wonder how the engine would run at all. This engine has a 1.00" bore and 1.25" stroke. At that point I make all my carbs small and have never had any performance issues. Yo can always make the bore bigger but not so easy to make smaller

 

[mayhugh1]

"K is a constant .65 to .90" --- ahhh, there's nothing like an empirical fudge factor !!!


wondering how your .180 compares to 1/3 scale ?

A 650 cfm Holley 4 bbl (the largest you'd want to put on a 289) has a 1.125" Venturi. If you put 5krpm into the above equation, convert metric to imperial, and use a K of .7 you wind up with 1.02 inches for a needed Venturi. - Terry

 

[Charles Lamont]

The included angle of the outlet taper in my last revision was 16 degrees which is within the recommended range that I came across in my reading. It can be easily reduced to bring it within Charles' recommendation, but I'm wondering if these Venturi optimizations really apply to a rotating throttle valve in a carb. The theory seems to have been derived for optimum results at wide open throttle, but I wonder how much still applies at part throttle when the inlet and outlet geometries have changed so radically.

Good point. I would not argue with your 16°. The chapter on 'Turbulent Flow in Pipes' says the disturbance to the flow dowstream of an inefficient fitting like a globe valve may persist for for 50 or more pipe diameters before it sorts itself out. On the other hand, in the case of a straight, smooth bored system without obstacles the optimimum 'diffuser' angle is as little as 6° included.

 

[peterl95124]

A 650 cfm Holley 4 bbl (the largest you'd want to put on a 289) has a 1.125" Venturi. If you put 5krpm into the above equation, convert metric to imperial, and use a K of .7 you wind up with 1.02 inches for a needed Venturi. - Terry

OK, so 1/3 * 1.125" = .375" is the number I was looking for.
(thou I'm confused by what you said, is that 4 venturis at 1.125" ?)

getting back to scaling laws, the formula you gave has be be pretty much correct because you can re-work it to (throat)^2 ~~ displacement X rpm, which translates to throat area is proportional to air volume (per second), which means all carbs operate best at approximately the same throat air velocity regardless of size, and that's what I was getting at with my dimensional / scaling analysis.

so I'm guessing your throat is so small because you're not planning to operate at 3 x RPM (where scaling laws say you have to be for the same HP per Disp).

I tend towards the same model RPM as (what I'm guessing) you do, we don't want the model engine to "scream" (at super high rpm) but rather just "roar" (at rpms that sound like the full size or not too much more)

Pete.

 

[mayhugh1]

OK, so 1/3 * 1.125" = .375" is the number I was looking for.
(thou I'm confused by what you said, is that 4 venturis at 1.125" ?)

getting back to scaling laws, the formula you gave has be be pretty much correct because you can re-work it to (throat)^2 ~~ displacement X rpm, which translates to throat area is proportional to air volume (per second), which means all carbs operate best at approximately the same throat air velocity regardless of size, and that's what I was getting at with my dimensional / scaling analysis.

so I'm guessing your throat is so small because you're not planning to operate at 3 x RPM (where scaling laws say you have to be for the same HP per Disp).

I tend towards the same model RPM as (what I'm guessing) you do, we don't want the model engine to "scream" (at super high rpm) but rather just "roar" (at rpms that sound like the full size or not too much more)

Pete.

Pete,
The throat diameter doesn't linearly scale. You can't divide the full size Venturi diameter by the linear scaling factor to get the scaled value. The equation shows the diameter scales as the square root of the cylinder displacement. - Terry

 

[peterl95124]

Pete,
The throat diameter doesn't linearly scale. You can't divide the full size Venturi diameter by the linear scaling factor to get the scaled value. The equation shows the diameter scales as the square root of the cylinder displacement. - Terry

I think you're right, I think I messed up somewhere (wouldn't be the first time :-( !!!)

the reason its to hard to reason about this is Displacement doesn't scale linearly with Bore either, and I was attempting to relate throat bore to cylinder bore, and then you might or might not want to re-scale the RPM also.

in any event I still like the equation, because it implies same air-speed through the throat regardless of size, and I thank you for bringing it to my attention.

 

[mayhugh1]

I think you're right, I think I messed up somewhere (wouldn't be the first time :-( !!!)

the reason its to hard to reason about this is Displacement doesn't scale linearly with Bore either, and I was attempting to relate throat bore to cylinder bore, and then you might or might not want to re-scale the RPM also.

in any event I still like the equation, because it implies same air-speed through the throat regardless of size, and I thank you for bringing it to my attention.

I too am amazed that equation holds over such a wide range. The thread I referred to had several examples of running model engines where the throat diameter seemed to agree with the equation whether or not the equation was actually used or whether those diameters had been empirically determined. The only thing I haven't made my peace with is why the Perry RC carbs I have are so much larger (they're 5/16"). Anyway, I've started making chips on the design I last showed. - Terry