3rd picture: 45mm efi throttle bodies

lucky last...

hey this brings up another question,

last week i think i asked what the science was with the different lengths of velocity stacks, and someone replied saying the shorter they were the more power they produced up top in the rev range, the longer they were the more power they produced at the bottom of the rev range...

cuong's pics, alan's pics, and the pics i have of serious S20 powered race machines all show LOOONNNGGG velocity stacks - the person who said the above got it bakwards, right? longer the stack the higher the peak power...

another thing while i am at it...

if we compare say a 40mm high velocity stack vs a 60mm one,

does changing from one to the other merely MOVE the peak power in the rev range to a higher level without actually changing the HP figure, or does it actually make more power because the air/fuel travels a greater distance inside the stack and is therefore more accelerated when it enters the combustion chamber and therefore maybe also more of it enters the CC when the valve opens meaning a higher HP figure...

in short: does longer make more power, or are they merely for tuning your carbs to be in sync with your aftermarket cam's higher operating range...

-e

Well you see it is like this. On carbureted engines, the longer the intake tract after the butterfly, the more torque, see it allows for higher velocities thru the venturi by creating more vacuum at throttle opening due to the longer tract between the butterfly and the intake valve. Velocity stacks are sort of misleading, they are not really designed to increase velocity rather to straighten the airflow before it reaches the carb jets....its side benifit is that changing the length will create minor increases or decreases in air volume....very minor, but when tuning carbs to perfection these differences can be used to help final tune. This is called "Tract Length Tuning" On fuel injection engines, this is negated due to having a pressurised fuel spray as opposed to "sucking" out of float bowls. This is aloso why throttle body injectors dont have venturii's. The Air horn (velocity stack) on injection bodies is used to straighten the airflow before the butterfly, and for show.

Did I also mention that tract length tuning is almost considered a black art...usually the confines of an engine bay removes the possibility of true tract length tuning. It was lost when fuel injection was first used in formula one....they no longer needed the velocity stack lenght and then covered the engines....art lost.

thanks russell

that's exactly the kind of answer i was looking for...

-e