* * * Gasoline Engine VELOCITY STACKS * * *

All in all, velocity stacks can enhance the performance of your gasoline engine but there are some rules regarding their use. Its puropse is to straighten out the air flow to the carb thus lowereng the air resistance and disturbance passing through the venturi. If this happens properly you will see a more accurate tune, better throttle responce, and higher topend. However, this isn't always the case!

If you use a velocity stack to bring air in from OUTSIDE the engine cowling, you will likely see an improvement right off the bat. If you use it on a non-cowled engine, you should see, and likely will see an improvement (if its setup properly) but that isn't always the case.  
 
On side carb engines where the velocity stack is at 90 degrees across the air-flow, a standard velocity stack can actually disrupt the already crappy air-flow. In this case, a bologna slice stack is much better. Looking at the photo, you see a "standard" Zenoah velocity stack, and an aftermarket bologna slice stack. In order for the aftermarket one to work correctly, the HIGH SIDE of the slice needs to be on the leading side of the air-flow. In others words, the high side should be on the prop side of the engine. If you don't set it up this way, the stack will pressurize the venturi on the atmospheric side and will disrupt the fuel supply to the jets. NOT GOOD.
 
On rear carb engines the OEM style (bell shaped) works better but you won't see a massive improvement. But it will be much better than no stack at all.
 
Any time you install a velocity stack of any type, you really need to re-tune your carb to the new air-flow and check the performance compared to its original setup. If you don't see better overall improvement, you may be better off with the original setup (IE: no velocity stack).

BENCHMARKING:
A benchmark is nothing more than a "reference" to compare to. In the case of a testing a velocity stack, running your engine without a stack and documenting the RPM at various throttle settings will suffice. Then install the velocity stack, re-tune the engine and compare the results.

In the case of the bologna slice velocity stack I have found that having the angle torwads the prop really messes up the tune in flight. The reason for this is, the "ram air" effect. It just doesn't work too good on these carbs. Virtually all RC carbs are designed to operate at atmospheric pressure and the "ram air" disrupts this atmospheric flow.

BENCHMARK: Zenoah G26 with Pro Zinger 16x8 wood prop. 

  There was a significant improvement using the velocity stack with this particular application but this isn't always the case! Often times a velocity stack can actually degrade your performance. This happens when an engine is cowled in and the cowling disrupts the air flow.

mark.fuess@hotmail.com