Question:
Could you give me some advice about the change in power characteristics after installing an airbox. The result - Top speed slight increased but slight decreased torque at slow and middle rev than before (used open air filter with long air horns). Now changed short air horns, I hope to get a top speed and useful torque at low rev. When I installed an airbox, I thought to get ram effect and more power.
Answer:
Shorter air horns will normally reduce the lower to mid range torque, but potentially increase upper bhp, this is provided that the air horn length is optimised for the engine in the first place for all round characteristics.
An air box can have a dramatic effect on performance.
Unfortunately this effect can vary from installation to installation e.g. a
change in cam design, compression ratio, air horn length can sometimes have a
negative effect and sometimes even a positive effect. The only way to discover
what is best is to try every combination possible.
To prove ram effect this can only be done either in a pressurised dyno cell and then measuring the air pressure inside the air box when the car is travelling at speeds. The ECU will also have to be able to compensate for changes in air pressure within the air box. Sometimes you may actually find that the engine itself when driven at speed had negative pressure producing loss in performance, but once the air box is fitted you were able to maintain atmospheric pressure which means that you have a net gain as opposed to gaining positive pressure and a gain over what you would see on a rolling road in a static condition.