benzino wrote:also, what pipe diameters did you try? maybe going to 2.75" was too big?
In my opinion you should not exceed the throttle body diameter ( 2.5" )
The general rules are:
1 ) Larger pipe diameter = power on top end, drop in torque on low end
2 ) Smaller pipe diameter = increased low end torque, drop in peak power on top end
3 ) Shorter pipe length = power on top end, drop in torque on low end
4 ) Longer pipe length = increased low end torque, drop in peak power on top end.
Increased diameter = lower air velocity = poor mixing of air in lower RPM but increased volumetric efficiency on the top end.
Decreased diameter = high air velocity = excellent mixing of air in lower RPM but decreased volumetric efficiency on the top end.
Personally I believe you would be best with a 2.5" intake pipe in the range of 24" in length between the throttle body and filter for the best mix of power and torque.
The intake designs were combinations of 2.5" and 3" tubing, I believe the best result was approximately 12" of 3" tube transitioning down to a 2.5" tube ( silicone reducer ) which was approximately 15" in length.
The 3" section was to match the cone filter and create a reserve volume, the 2.5" was designed to ensure decent intake velocity while maintaining the same diameter as the throttle body to reduce the risk of turbulence.
Some Formula SAE intakes manage VE numbers around 120% ( meaning they force 1.2L of air into a 1.0L cylinder )
Most of these intake pipes go from a primary throttle body diameter, taper slowly down to about 50% of Throttle body diameter over approximately 20" then taper back up to TB diameter over another 20"
The tapering increases the air speed dramatically and creates a unique resonance during a particular RPM band where it overfills the cylinders.
When it comes right down to it, the air coming in is smaller in volume and density than what is exiting out the exhaust.
If you don't have a 3" exhaust, there is not much point in having a larger diameter intake.
You need to ensure you don't create turbulence as the air comes to the throttle body, decreasing from 2.75 to 2.5" right at the TB will create a blockage of air due to turbulence.
Ideally you want a taper of roughly 7 degrees or less to reduce turbulence, unfortunately most silicone reducers are significantly higher than that.
This is why the 3" to 2.5" transition was done well before the throttle body to ensure that the air had some time to settle down before it hits the throttle body.