Secondary School Question

[DocDirk]

I'm driving a BB 533 Ford engine with an 850 annular Might Demon carb, and I understand more astrophysics than I do about carburetors. It's been suggested my carb has mechanical secondaries and the car would perform better on the street with vacuum secondaries.

Would you please educate me on the advantages/disadvantages of one over the other in my or any other application?

Thanks!

Dirk

[Barnsnake]

With mechanical secondaries the amount that the secondaries open is controlled by your right foot.

With vacuum secondaries your right foot controls the ability for the secondaries to open, but their actual opening is controlled by a vacuum diaphragm. The diaphragm is "reading" the manifold vacuum and opening the secondaries as the engine "needs" the additional breathing capacity.

With mechanical secondaries and large carburetors it is possible to get a situation in which you have so much throttle opening that the velocity through the venturis drops so low the carburetor cannot meter properly. The vacuum secondary system addresses this issue.

The most common symptom of this problem is when you stomp the throttle at relatively low RPM and the motor bogs due to a lean condition, but if you stomp it at higher RPM it pulls fine. It can be addressed (to a degree) with a larger accelerator pump squirt, but that's not always enough.

Do you need to change? That depends.
Do you experience the problem described above? You have a lot of motor, but the cam profile and intake manifold design can have a bearing on how much breathing capacity the motor has at lower revs.

I hope this clarifies the situation a bit.

Jim