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Throttle body bore diameter?

scsmith

Well Known Member
Why is it that throttle body bore diameter must be the best kept secret in aviation. I can not find the bore diameter of an RSA-5 fuel control/throttle body or for a Marvel Schebler M4-5 carburetor.

Anybody happen to know those?
 
I think you're right - a well-kept secret. I just checked the manuals I have for my Ellison 4-5 which should use the same bore and it has every dimension one could want *except the throttle body bore. If my intake scoop wasn't connected already, I'd measure it tomorrow with a pair calipers. Good luck, someone here must know.
 
Around 60mm IIRCC.

Thanks Ross,

I think that is right for the carburetor, and perhaps correct for the minimum throat diameter of the RSA-5, but I think the throttle plate is closer to 2.75"? (70mm?)

If I didn't have my induction system all connected up to it, I would just measure.
 
Bore - RSA - 5AD1

Steve - Here a couple of pics of mine, waiting to be installed on the -3 I'm building. Hope this helps. If more info is needed, just call - thx.

Inlet/Outlet "bore" = 2.313" +/- (by mechanical calipers)

David
 

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RSA-5 and FM150 bore diameter at the throttle plate and manifold flange is 2.3125"

RSA-10 is 3.4375"

An FM-200, as used on the 390-119, is 3.0625"

The RSA series has a significant bore reduction at its throat venturi, while the AFP units incorporate a relatively straight bore with a pedestal venturi. The difference is most obvious when comparing an RSA-5 with the FM-150. On AFP's Superflow bench, respective pressure drops at 1560 lbs/hr were 12.75" vs 10".
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RSA-5 and FM150 bore diameter at the throttle plate and manifold flange is 2.3125"

RSA-10 is 3.4375"

An FM-200, as used on the 390-119, is 3.0625"

The RSA series has a significant bore reduction at its throat venturi, while the AFP units incorporate a relatively straight bore with a pedestal venturi. The difference is most obvious when comparing an RSA-5 with the FM-150. On AFP's Superflow bench, respective pressure drops at 1560 lbs/hr were 12.75" vs 10".
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Seems like the pedestal venturi would contribute similar blockage that the throat venturi does. It would be interesting to measure the total cross section area at the max blockage point of the pedestal venturi and compare it to the cross section at the throat of an RSA-5.

The data does surely demonstrate less loss thru the FM-150, but I wonder if there might be another reason why?

Anyway, thanks Dan for documenting so here on VAF we can find it if we need it.
 
Plenum Inlet?

For comparison does anyone happen to know the bore diameter of the plenum inlet on the IO-360 and IO-390’s?

Skylor
 
For comparison does anyone happen to know the bore diameter of the plenum inlet on the IO-360 and IO-390’s?

The standard horizontal sump should be 2-5/16", like the RSA-5. The "cold air" manifold on the 390C and D is sized for an RSA-10.
 
390 Sunp

The standard horizontal sump should be 2-5/16", like the RSA-5. The "cold air" manifold on the 390C and D is sized for an RSA-10.

That’s interesting about the 390 C/D sump. Unless Lycoming used a different casting than the IO-360A sump, it doesn’t look like there’s room inside the sump to increase the bore 1” or so in diameter:


Skylor
 

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That’s interesting about the 390 C/D sump. Unless Lycoming used a different casting than the IO-360A sump, it doesn’t look like there’s room inside the sump to increase the bore 1” or so in diameter:

Same as the 360A. Ask Ron Moring about enlarging the flange. Apparently it can be increased enough to eliminate the adapter normally used to mount an FM-200, bore size 3-1/16". May not be able to fit an RSA-10.

It helps that the FM-200 bolt pattern is rectangular, 3.12 X 2.82. The RSA-10 bolt pattern is 3.375 square. Sketch below is to scale.

Beyond that, I dunno. Have not done it myself...but I'd like to.
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